1

Dry Machining and Minimum Quantity Lubrication K. Weinert1 (1), I. Inasaki2 (1), J. W. Sutherland3 (2), T. Wakabayashi4

1Dept. of Machining Technology, University of Dortmund, Germany 2Faculty of Science and Technology, Keio University, Yokohama, Japan

3Dept. of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, Michigan, USA 4Faculty of Engineering, Kagawa University, Takamatsu, Japan

Abstract Modern machining processes face continuous cost pressures and high quality expectations. To remain competitive a company must continually identify cost reduction opportunities in production, exploit economic opportunities, and continuously improve production processes. A key technology that represents cost saving opportunities related to cooling lubrication, and simultaneously improves the overall performance of cutting operations, is dry machining. The elimination of, or significant reduction in, cooling lubricants affects all components of a production system. A detailed analysis and adaptation of cutting parameters, cutting tools, machine tools and the production environment is mandatory to ensure an efficient process and successfully enable dry machining. Keywords: Machining, Environment, Minimum Quantity Lubrication

1 INTRODUCTION A change in environmental awareness and increasing cost pressures on industrial enterprises have led to a critical consideration of conventional cooling lubricants used in most machining processes. Depending on the workpiece, the production structure, and the production location the costs related to the use of cooling lubricants range from 7 - 17% of the total costs of the manufactured work piece [1, 2, 3, 4]. By abandoning conventional cooling lubricants and using the technologies of dry machining or minimum quantity lubrication (MQL), this cost component can be reduced significantly. Besides an improvement in the efficiency of the production process, such a technology change makes a contribution to the protection of labor [5, 6, 7] and the environment [8, 9]. The reduction of substantial exposure to cooling lubricants at the work place raises job satisfaction and improves the work result at the same time. Furthermore, an enterprise can use economically-friendly production processes for advertising purposes, which leads to a better image in the market (Figure 1) [10, 11, 12, 13]. Analyzing and understanding the cutting process mechanisms is a key issue in developing an economical and safe dry machining process. Beyond the adoption of this new machining technology, the construction of machine tools and their peripheral equipment must also be considered [14]. Industrial practitioners will only be willing to accept dry machining technology when comprehensive solutions exist. Thus, results for a large variety of work piece materials and common production methods are essential to prove the superiority of this innovative machining technology [15, 16]. The implementation of dry machining cannot be accomplished by simply turning off the cooling lubricant supply. In fact, the cooling lubricant performs several important functions, which, in its absence, must be taken over by other components in the machining process. Cooling lubricants reduce the friction, and thus the generation of heat, and dissipate the generated heat. In addition, cooling lubricants are responsible for a variety of secondary functions, like the transport of chips as well as the cleaning of tools, work pieces and fixtures. They provide for a failure-free, automated operation of the production system. In addition, cooling lubricants help to

provide a uniform temperature field inside the workpiece and machine tool and help to meet specified tolerances [14, 17].

DRY MACHININGDRY MACHINING

laws and regulations

decreasedcoolant cost job satisfactionimage gain

environment

leakage flow

DRY MACHININGDRY MACHINING

laws and regulations

decreasedcoolant cost job satisfactionimage gain

environment

leakage flowleakage flow

Figure 1: Benefits of dry machining.

2 MINIMUM QUANTITY LUBRICATION In many machining operations, minimum quantity cooling lubrication (MQCL) is the key to successful dry machining. Any move to manufacture functional components under dry machining conditions depends on an understanding of MQCL as a system, whose individual components – feed technology, MQCL media, parameter settings, tools, and machine tools mutually affect the operation of all of the others (Figure 2). All of the components in the MQLC system must be very carefully coordinated in order to achieve the desired outcome, which is optimal, both technologically and economically [18].

Fluids

- fatty alcohol- synthetic esters

...

Machine Tool

- MQCL supply- upgradability

...

Settings

- oil flow- air flow

...

Tools

- internal feed- external feed

...

Equipment

- int. / ext. feed- 1 or 2 channels

...

EconomicDry Machining

with MQCL

Fluids

- fatty alcohol- synthetic esters

...

Machine Tool

- MQCL supply- upgradability

...

Settings

- oil flow- air flow

...

Tools

- internal feed- external feed

...

Equipment

- int. / ext. feed- 1 or 2 channels

...


with MQCL


with MQCL

Figure 2: Minimum quantity cooling lubrication system

(MQCL) [19].

2.1 Definition The primary functions of a cooling lubricant in wet machining operations are to cool, to lubricate, and to remove the chips. As a rule, emulsions or straight oils are generally used, depending on the manufacturing operation and machining task involved. Emulsions possess excellent heat transfer characteristics because of their high water content. Straight oils excel when a high degree of lubricity is required. Both media guarantee efficient chip transport. When compressed air is used instead of a cooling lubricant, the lubrication benefit of the fluid is lost. The coolant effect is much less pronounced than with water or oil. Water and oil are also superior to air in terms of chip transport characteristics. In MQCL operations, the media used is generally a straight oil, but some applications have also utilized an emulsion or water. These fluid media are fed to the tool and/or machining point in tiny quantities. This is done with or without the assistance of a transport medium, e.g., air. In the case of the former, the so-called airless systems, a pump supplies the tool with the medium, usually oil, in the form of a rapid succession of precision-metered droplets. In the case of the latter, the medium is atomized in a nozzle to form extremely fine droplets, which are then fed to the machining point in form of an aerosol spray [20, 21].

Minimal Quantity Cooling Lubrication(MQCL)

normal consumption per machine hour:10 – 50 ml MQCL medium

Minimal QuantityCooling(MQC)

Minimal Quantity Lubrication

(MQL)

Emulsion (Water+Oil)

Water cp,water = 4.18 kJ/kgKAir cp,air = 1.04 kJ/kgK

Oil cp,oil = 1.92 kJ/kgK

Medium

Emulsion

Oil

Air pressure

Cooling

excellent

good

little

Lubrication

good

excellent

no

Chip removal

excellent

good

little








(MQL)


(MQL)





Oil cp,oil = 1.92 kJ/kgK Oil cp,oil = 1.92 kJ/kgK

Medium

Emulsion

Oil

Air pressure

Cooling

excellent

good

little

Lubrication

good

excellent

no

Chip removal

excellent

good

little

Medium

Emulsion

Oil

Air pressure

Medium

Emulsion

Oil

Air pressure

Cooling

excellent

good

little

Cooling

excellent

good

little

Lubrication

good

excellent

no

Lubrication

good

excellent

no

Chip removal

excellent

good

little

Chip removal

excellent

good

little

Figure 3: Definition of Minimum Quantity Cooling Lubrication (MQCL).

Within the context of dry machining, the term MQCL is generally used to refer to the supply of the cooling lubricant in the form of an aerosol. Depending on the type and on the main function of the fluid medium supplied, a distinction can be drawn between minimum quantity lubrication (MQL) and minimum quantity cooling (MQC) (Figure 3) [19, 22]. When oils are used as the fluid medium, the emphasis is on their good lubrication properties. Their function is to reduce friction and adhesion between the workpiece, the chip and the tool. As a result, the amount of friction heat generated is also reduced. Consequently, the tool and the workpiece are exposed to less heat than they would be if the machining operation was performed completely dry (Figure 4) [23]. The direct cooling effect of the oil/air mix is of minor importance due to the low thermal capacity of oil (cp,oil = 1.92 kJ/kgK) and air (cp,air = 1.04 kJ/kgK), and also to the small amount of the medium involved. Given the very slight cooling effect of the oil/air mix, the use of oil as a medium is regarded as a MQL strategy. Emulsions and water are used much less frequently than oil as media in MQCL operations. Generally speaking, they are used only when it is essential to cool the tool or the part more efficiently than is possible using oil. Operations in which emulsion, water or air (cold or liquid air) are used are referred to as minimum quantity cooling (MQC) operations since the emulsion provides a considerably lower level of lubrication than oil, yet more than water and air, which provide no lubrication at all. In contrast to minimum quantity lubrication (MQL), minimum quantity cooling (MQC) has, until now, been a seldom used, and therefore largely unexplored, component of the MQCL technique among industrial users. However, the minimum quantity cooling technique can make a major contribution to the solution of thermal problems affecting the tool and/or the part in dry machining operations [19].

Process: Drilling Material : Ck45 (AISI 1045)Tool: HC-P + TiN Diameter: D = 11.8 mmFeed rate: f = 0.20 mm

twist drill

thermoelectriccouples closeto major cutting edge

400

300

250

200

150coolant

MQL

dry

40 60 m/min 1000

°C

Cutting speed vc

Tool

tem

pera

ture

ϑm

ax

0

Process: Drilling Material : Ck45 (AISI 1045)Tool: HC-P + TiN Diameter: D = 11.8 mmFeed rate: f = 0.20 mm

twist drill

thermoelectriccouples closeto major cutting edge

400

300

250

200

150coolant

MQL

dry

40 60 m/min 1000

°C

Cutting speed vc

Tool

tem

pera

ture

ϑm

ax

0

Figure 4: Tool temperatures in drilling

with different cooling lubricants [24, 25].

2.2 Supply Systems A distinction is drawn in the minimum quantity lubrication technique between external supply via nozzles fitted separately in the machine area and internal supply of the medium via channels built into the tool (Figure 4). Each of these systems has specialized individual areas of application. In applications involving external supply, the aerosol is sprayed onto the tool from outside via one or more nozzles. The number and direction of the nozzles in conjunction with the spray pattern, which depends on the

nozzle arrangement, play an important role in the quality of the outcome. This technique is used in sawing, end and face milling, and turning operations. In the case of machining operations, such as drilling, reaming, or tapping, external supply of the medium is appropriate only up to length/diameter ratios of l/d < 3. When the l/d ratio is larger than this, the tool may have to be withdrawn several times so that it can be wetted again, resulting in a considerable increase in the overall machining time. An external supply may also present problems in the case of machining tasks requiring the use of multiple tools with widely varying lengths and diameters. In operations of this nature, the orientation of the supply nozzle(s) must be adjusted either manually or with the assistance of positioning systems that are coupled to the machine control unit to position the nozzles axially or radially, depending on the length and the diameter of the tool, or to rotate them through a certain angle. An external MQL supply may also be vital when the tools involved in the operation do not have any internal cooling channels [26].

Supply-Systems

External MQL-feed

InternalMQL-feed

1-channel 2-channel

Injector

1-channel

2-channel

Oil

air feed

no pressurevessel

Oilchamber

air feed

oil feed

Source: Bielomatik, Link, Steidle, Unilube, Vogel

Supply-Systems

External MQL-feed

InternalMQL-feed

1-channel 2-channel

Injector

1-channel

2-channel

Oil

air feed

no pressurevessel

Oilchamber

air feed

oil feed

Oil

air feed

no pressurevessel

Oilchamber

air feed

oil feed

Source: Bielomatik, Link, Steidle, Unilube, Vogel Figure 5: MQL-feed systems [19].

An internal supply of the medium via the spindle and tool, is beneficial in drilling, reaming, and tapping operations with larger l/d ratios, since this ensures that the medium is constantly available close to the cutting edge, regardless of the tool position. For similar reasons, this also applies to tools with very different dimensions. In deep hole drilling operations, the large l/d ratio makes an internal MQL-supply indispensable. There are additional advantages of an internal MQL-medium supply in that concerns related to nozzle positioning errors are eliminated, and the integration of the MQL-supply within the machine tool means that the machining area is not cluttered by supply pipes. With respect to internal MQL-supply, a further distinction is drawn between the so-called 1-channel and 2-channel systems. When the 1-channel option is used, the aerosol mixture is formed outside the spindle, and the single channel acts as a feed route for the mixture. In the case of 2-channel systems, oil and air are fed separately through the spindle. The air-oil mix is then produced directly ahead of the tool. The principal requirement in each of these variations is that a sufficient quantity of the medium is available at the machining point when the cutting operation begins.

2.3 MQL-Media In conventional machining operations that use a flood coolant supply, cutting fluids have so far been selected mainly on the basis of their primary characteristics, i.e., their role in influencing cutting performance. A great number of investigations have recently been performed to evaluate cutting fluid performance [27, 28, 29, 30, 31, 32, 33, 34, 35]. The investigations take on greater meaning because of growing interest in environmentally acceptable applications of cutting fluids [36, 37, 38, 39]. In near-dry machining operations with MQL supply, however, secondary characteristics as biodegradability, oxidation stability and storage stability, are even more important because the lubricants must be compatible with the environment and chemically stable under long-term usage when there is a very low consumption rate [40]. With this in mind, some description of lubricant performance regarding secondary characteristics is important. This will be followed by a discussion of MQL-media in terms of their primary characteristics. The most important measure of the environmental compatibility of lubricants is their biodegradability. In general, the base stocks of cutting fluids are mineral oil or polyalkylene glycol and those base stocks do not have high biodegradability. Because of their high biodegradability characteristics, therefore, vegetable-based oils have normally been used for MQL applications [41]. Synthetic esters provide a wide range of biodegradability depending on their combined molecular structures of acids and alcohols. If one compares the degrees of biodegradability of various synthetic esters, then monoester, diester and polyol ester can be regarded as biodegradable [42]. Since polyol esters may probably have suitable viscosities for MQL machining, several polyol esters have been examined as lubricants [43]. When an MQL system is used, the lubricant may adhere to the interior and exterior surfaces of the machine tools in the form of a thin oil film. Since such a thin oil film can easily oxidize and form sticky substances, MQL lubricants should be stable against thin film oxidation. In order to simulate this, thin oil films were left under atmospheric conditions at 70 °C for 168 hours. After this heating test, the change in the molecular weight was measured by a gel permeation chromatography (GPC) analysis. The test oils were fully synthetic polyol esters A, B, and C having viscosities of 19, 25, and 48 mm2/s at 40 °C respectively, and vegetable oil D, whose viscosity was 36 mm2/s. All these lubricants had a biodegradability of more than 98 % by the CEC-L-33-A-93 test [42]. Figure 6 shows the increase in molecular weight for the four oils [40]. From prior studies it was observed that if the molecular weight of the oil film increased by more than 10 %, it felt sticky to the touch. In this case, the molecular weight of vegetable oil D increased by 65 %, and the film felt very sticky. In contrast, there was no significant change in the molecular weights of polyol esters A and B, and little change for polyol ester C. Lubricant containers are often stored outside, and the temperature in the containers can rise to as high as 70 °C or even higher. Since an MQL system consumes very little lubricant, the lubricant must remain stable for a long time under such conditions. In order to simulate this storage situation, each oil was placed in a glass bottle at 70 °C for 4 weeks. After this test, polyol ester A presented no significant change in viscosity, total acid number (TAN) and odor, whereas vegetable oil D indicated considerable increase in viscosity and TAN : a gluey material had formed near the bottle cap and a peculiar odor was observed.

Increase in molecular weight %

Veget. oil D

Polyol ester C

Polyol ester B

Polyol ester A

0 20 40 60 80

Weight Avg. Mol. Weight MwNumber Avg. Mol. Weight Mn

Increase in molecular weight %

Veget. oil D

Polyol ester C

Polyol ester B

Polyol ester A

0 20 40 60 80

Weight Avg. Mol. Weight MwNumber Avg. Mol. Weight Mn

Figure 6: Results of oxidation stability of thin films [40]

Based on the reported test results, synthetic biodegradable polyol esters were found to be superior to vegetable oils. In terms of secondary characteristics, biodegradable polyol esters were identified as the preferred lubricant for MQL machining [40]. The tapping test is recognized as a standard screening method to evaluate the cutting performance of fluids, and the tapping energy efficiency provides a sensitive and accurate measure of performance [27]. Figure 7 shows the tapping energy efficiency in MQL cutting with the above-mentioned polyol esters and vegetable oil [40]. For comparison, this graph also presents the results for the case of conventional flood supply of a water-soluble coolant (emulsion type) and air-cooled dry cutting. The performance of MQL using polyol ester A was found to be almost equal to that of conventional cutting, a very small amount of this synthetic ester provided effective lubricating action near the cutting zone.

70 75 80 85 90

Tapping energy efficiency %

Polyol ester A

Polyol ester C

Polyol ester B

Vegetable oil D

Conv. cutting(Emulsion 10%)

Dry cutting

70 75 80 85 90

Tapping energy efficiency %

Polyol ester A

Polyol ester C

Polyol ester B

Vegetable oil D

Conv. cutting(Emulsion 10%)

Dry cutting

Figure 7: Results of tapping test [40].

Figure 8 shows surface finish roughness results for turning steel at a cutting speed of 200 m/min [91].

Surface roughness Ra

0 0.5 1.0 1.5µm

Dry

Neat type oil

Vegetable oil D

Polyol ester A

(a) cemented carbide tool(b) cermet tool

Surface roughness Ra

0 0.5 1.0 1.5µm

Dry

Neat type oil

Vegetable oil D

Polyol ester A

(a) cemented carbide tool(b) cermet tool

Figure 8: Surface roughness from turning tests.

These results indicate that the cutting performance in MQL machining with polyol ester A is superior to that of dry machining. In addition, even compared with the

conventional flood supply of neat type oil (that includes a sulfur-based extreme pressure additive), the synthetic polyol ester provides excellent performance for both cemented carbide and cermet tools. On the other hand, MQL machining with vegetable oil D is not always advantageous. Fatty alcohols and synthetic esters (chemically modified vegetable oil) are the media most commonly used in MQL applications (Tables 1 and 2). The medium selected depends on the type of supply, the material involved, the machining operation, and subsequent finishing operations required by the part (e.g., annealing, coating, painting) [44, 45].

Table 1: Characteristics of MQL Fluids.

Synthetic esters Fatty alcohols Chemically modified

vegetable oils Long-chained alcohols made from natural raw

materials or from mineral oils

- good biodegradeability - low level of hazard to water - toxicologically harmless

- high flash and boiling point with low viscosity

- very good lubrication properties

- good corrosion resistance

- inferior cooling properties

- vaporizes with residuals

- low flash and boiling point, comparatively high viscosity

- poor lubrication properties

- better heat removal due to evaporation heat

- little residuals

Source: Fuchs Petrolub AG At the same level of viscosity, fatty alcohols have a lower flash-point than synthetic esters. Although they do have a cooling effect, this effect is very slight, since they evaporate relatively quickly. In contrast to synthetic ester, their lubricating effect is very moderate. Fatty alcohols are. therefore, used more frequently in machining operations in which the cooling effect is more important than lubrication. Operations conducted on gray cast iron, in which the graphite deposits within the material help to provide lubrication, are examples of these applications. Because they evaporate readily, fatty alcohols have the advantage that the parts being machined remain largely dry.

Table 2: Main areas of application of MQL Fluids.

Synthetic esters Fatty alcohols Application for machining technologies if

- primarily reduction of friction

- high surface qualities are demanded

- adhesive work piece materials (build-up edge, apparent chips)

- low cutting speeds and high specific area load

- lubrication of supporting and/or guiding rails

- primarily heat removal - examples are: sawing,

turning and milling of gray cast iron, machining of cast aluminum alloys

Source: Fuchs Petrolub AG Synthetic esters are frequently used in machining operations and with materials where, by ensuring good lubrication, the primary objective is to reduce the levels of friction and adhesion between the tool, workpiece, and/or

chips,. Drilling, fine-boring, tapping, and thread-forming operations conducted in steel and aluminum materials are examples of such operations. A synthetic ester has a high boiling temperature and flash point, and a low viscosity. As a result, it evaporates more slowly than a fatty alcohol, and this leave a thin film of oil on the workpiece which serves to resist corrosion. In addition to these characteristics, synthetic esters are bio-degradable and are classified as Water Risk Category 1 because of their good toxicological characteristics.

2.4 MQL Design of System Components The internal MQL supply strategy demands tools with internal cooling channels (Figure 9). In the case of drilling tools, there is a further limitation in that the minimum tool diameter must be approximately 4 mm in order to guarantee a sufficiently high level of tool rigidity. For this reason, an external MQL supply is generally employed where smaller tool diameters are required. However, tool manufacturers offer customized solutions, which permit the MQL medium flowing through the spindle to be fed to the outside via the tool holder and then to be fed to the machining point along the perimeter of the tool in the longitudinal tool direction (Figure 10) [19, 46].

Source: Gühring,Kennametal, Prototyp Source: Komet JEL Dihart

MQL-exit in the tools flutefor wetting the flute withMQL and for supporting

the chip removal

Drills and taps

MQL

radial MQL-exit forsupplying the cutting edgewith lubricant and for thesupport of chip removal

Face mill 45°

MQL

Source: Gühring,Kennametal, Prototyp Source: Komet JEL Dihart

MQL-exit in the tools flutefor wetting the flute withMQL and for supporting

the chip removal

Drills and taps

MQL

radial MQL-exit forsupplying the cutting edgewith lubricant and for thesupport of chip removal

Face mill 45°

MQL

Figure 9: Tool design for internal MQL supply [19].

Tool holder for machining centerswith internal MQL supply andtools without internal coolinglubricant channels

Source: EmugeSource: Chiron-Werke

Additional nozzle forexternal MQL supplyin a machining center

Figure 10: Possible MQL supply in machining center.

Machine tools suitable for dry machining operations should have one internal and/or one external MQL supply, depending on the product range to be manufactured and on the machining operations required. Machining centers employing tools with very small diameters should be equipped with both supply systems. Since the aerosol droplets settle very slowly and can, therefore, remain in the workspace for a long time, the machine tool should be encapsulated, if possible and should be equipped with a mist extraction (exhaust) unit. MQL is essential in many dry machining operations involving a wide range of material-process combinations (Table 3). In terms of materials, this applies particularly to dry machining operations conducted on aluminum and on

wrought aluminum alloys. In terms of the processes involved, MQL applies to hole-making operations virtually irrespective of the material concerned. Sawing is a classical area of application for the MQL technique. Turning and milling operations involving steel and cast iron materials can usually be performed completely dry by virtue of the high capacity for resistance to thermal wear of the coated carbide tools currently available.

Table 3: Application areas for dry machining and minimum quantity lubrication [47].

Material Aluminum Steel Cast iron

Process ca

st

allo

ys

wro

ught

al

loys

high

-allo

yed

bear

ing

stee

l

Free

cut

ting,

qu

ench

and

te

mpe

red

stee

l

GG

20 to

G

GG

70

Drilling MQL MQL MQL MQLdry

MQLdry

Reaming

MQL MQL MQL MQL MQL

Tapping

MQL MQL MQL MQL MQL

Thread forming

MQL MQL MQL MQL MQL

Deep hole drilling

MQL MQL MQL MQL

Milling MQLdry

MQL dry dry dry

Turing MQLdry

MQL dry

dry dry dry

Gear milling

dry dry dry

Sawing

MQL MQL MQL MQL MQL

Broaching

MQL dry dry

Efficient and reliable production of holes in steel and aluminum materials and mastery of the subsequent operations without the use of a cooling lubricant are keys to any attempt to achieve a blanket implementation of a completely dry machining strategy. In the case of operations such as drilling, fine-boring, reaming, and tapping, minimum quantity lubrication remains essential in virtually all applications. Qualitative figures relating to the requirement for air and MQL media in various machining tasks are given in Table 4. The danger that the hot and highly formable chip will become lodged in the drilling flutes increases with the drilling depth, and, thereby, the area of friction between the chip, hole wall, and tool increases. Whereas a minimal level of lubrication is generally essential for the production of holes in aluminum, holes with l/d ratios up to 3 can be produced completely dry in steel materials when the tool substrate, coating, and geometry have been optimized in accordance with the special requirements of dry machining. Deeper holes require the use of an MQL medium. When machining is conducted in steel in

minimum quantity lubrication mode using tools which are suitable for dry machining, holes with l/d ratios of 5-7 can be produced without any problem.

Table 4: Cutting parameters for different machining operations with MQL supply.

Face Milling F51 01050 /01 ae=50 mm SiN34 (z=4)

ap=1.0 mm

Drilling XV85 07460 (z=1) 02 D=11.8 mm P25M-TiCN/TiN(a)

l/D=3.5 P40-TiAlN(i)

Reaming 565.55-ASG12 /03 D=12H7 Cermet-TiAlN

L/D=3.5

Drilling V30 72600 (z=1) 06 D=26 mm P25M-TiCN/TiN(a)


Boring M03Speed07 D=26.5H7 M03 00113

L/D=1.6 CBN

Thread MGF-M1410 milling K10-TiCN

M14x2 (z=4)

Thread Tomill SR11 milling 16x25x2 (z=5)

M24x2 K10-TiCN

Cutting parametersN.

Source: Komet JEL Dihart

700 0.80

225 0.15

225 0.25

250 0.15

500 0.05

100 0.28

100 0.28

vc[m/min]

OperationTool /

Cuttingmaterial

f[mm] MQL Air

Face Milling F51 01050 /01 ae=50 mm SiN34 (z=4)

ap=1.0 mm

Drilling XV85 07460 (z=1) 02 D=11.8 mm P25M-TiCN/TiN(a)


Reaming 565.55-ASG12 /03 D=12H7 Cermet-TiAlN

L/D=3.5

Drilling V30 72600 (z=1) 06 D=26 mm P25M-TiCN/TiN(a)


Boring M03Speed07 D=26.5H7 M03 00113

L/D=1.6 CBN

Thread MGF-M1410 milling K10-TiCN

M14x2 (z=4)

Thread Tomill SR11 milling 16x25x2 (z=5)

M24x2 K10-TiCN

Cutting parametersN.

Source: Komet JEL Dihart

700 0.80

225 0.15

225 0.25

250 0.15

500 0.05

100 0.28

100 0.28

vc[m/min]

OperationTool /

Cuttingmaterial

f[mm] MQL Air

The improved performance in MQL-aided drilling operations relies on two important effects. Firstly, the lubricating effect of the MQL medium results in a dramatic reduction in the level of wear due to friction and adhesion in the area around the heel of the twist drill. Secondly, chip formation and removal from the drill flutes is improved very substantially by the cooling and lubricating effect of the air-media mix. In contrast to the long-compressed spiral chips produced in operations conducted purely in a dry mode, the chips which form in operations conducted in the MQL mode are shorter and can be removed from the hole with ease [48, 49]. Minimum quantity lubrication is a central element in the dry machining strategy. It is vital in many machining operations. This applies particularly to machining operations conducted on materials such as aluminum alloys, which are highly susceptible to tool adhesion. MQL is essential in processes involving high levels of friction and adhesion, such as drilling, tapping, fine-boring, and reaming and is also indispensable when the surface quality must meet exacting requirements. 3 TRIBOLOGY Friction and wear of cutting tools often detrimentally limit the performance of cutting processes. Process improvement may be achieved by understanding these mechanisms. The complexity of a machining process makes it difficult to systematically analyze the friction and wear mechanisms at the active areas of the tool. Therefore, detailed knowledge of the tribological interactions in the contact zone is of particular importance to understand, to control, and to economically design a machining process.

When analyzing cutting processes, the tribological mechanisms are determined by the contact partners, as well as the environment and the controlling parameters [50]. Corresponding to the common description, a tribological system is characterized by a basic body, as the element subjected to wear, and a counter body, whose wear is not decisive for the course of the process [51]. Based on this definition, the cutting tool represents the basic body and the workpiece the counterpart in the tribological system. Furthermore, the interfacial element and the environmental medium are relevant for the behavior of the tribological system. Reaction products, abrasive materials in the contact zone, and cooling lubricants also influence the system's behavior. The cutting speed and the feed rate are additional variables which influence the friction and wear mechanisms of the system (Figure 11).

Workpiecehardnesstoughnessstructure...

Cuttingparameters

• cutting speed• feed• depth of cut• ...

Contactconditions

• direct stress• shearing stress• temperatures• ...

Toolpropertiescoatingsurface...

InterfacialelementnonecoolantMQL...

Surface damage

Tribochemicalreactions

Adhesion

Abrasion



Cuttingparameters


Cuttingparameters


Contactconditions


Contactconditions






Surface damage

Tribochemicalreactions

Adhesion

Abrasion

Figure 11: Cutting process as a tribological system [14].

The friction of the contact partners inevitably leads to wear, which changes the shape of the basic body and the counterbody. The main wear mechanisms present in a cutting process are abrasion, adhesion, tribochemical reactions, and surface ruin. The wear mechanisms can occur individually or in combination. The resulting forms of wear may be grooves, scratches, material deposits, reaction products, and surface cracks. To ensure a high performance with satisfactory process reliability, all elements of the tribological system have to be adapted to the specific requirements of dry machining processes. The adaptations particularly affect the cutting material and the tool coating (basic body), the workpiece material (counterpart), and the MQL medium (interfacial element).

3.1 Cutting Materials Especially in dry machining processes cutting edges and guiding pads are subject to high mechanical, thermal, and chemical loads. To ensure a good performance and a high wear resistance, the cutting materials have to fulfill certain requirements regarding their physical properties. Figure 12 illustrates an ideal cutting material, combining properties like high hardness, good toughness, and chemical stability. However, these requirements represent opposing properties so that an optimal and universal cutting material is not realizable from a technological point of view.

High hardness and resistance

to pressure

High toughnessand critical stress

intensity factor

High hot hardness

High thermalfatigue limit

High chemicalresistance

UniversalCutting Material


to pressure


to pressure


intensity factor


intensity factor

High hot hardnessHigh hot hardness





UniversalCutting Material

Figure 12: Optimal cutting materials for dry machining.

Cemented Carbides Cemented carbides are the cutting materials most commonly used in modern machining applications. In a powder metallurgical process a metallic hard material is sintered at high pressures and temperatures with cobalt as binder. The properties of cemented carbides are mainly based on the ratio of tungsten carbide to cobalt binder and the grain size of the compound. In general, the finer the grain size, the less cobalt is used and the more wear resistant the material becomes [52]. By reducing the grain sizes of the tungsten-carbide powders to submicron grain (0.5 – 0.8 µm) and ultra-fine grain (0.2 – 0.5 µm), cemented carbides for challenging machining operations, e.g. dry machining of high-alloyed steel or high-strength materials, became available. On the one hand, these cutting materials possess high strengths at elevated temperatures. On the other hand, the very fine and homogeneous structure of the cutting materials leads to satisfactory tensile strengths. Thus, even very small tools and thin wedges can be manufactured with good cutting edge stability [53]. The differences in the wear behavior of cemented carbides were investigated when machining stainless steel with minimum quantity lubrication. Such a steel is characterized by high alloy content and an austenitic-ferritic structure. The cutting materials used in the investigation were fine grain, submicron grain, and ultra-fine grain cemented carbides with identical geometry and a PVD-TiAlN-coating (Figure 13).

mm

Cutting volume Vc

Material: 1.4462 (AISI F51)Insert: CNMG 120408Cut. speed: vc = 100 m/minFeed: f = 0.15 mmDepth of cut: ap = 1.0 mmCL concept: MQL

HC3-TiAlN(ultrafine grain)HC2-TiAlN(submicron grain)HC1-TiAlN(fine grain)

Material: 1.4462 (AISI F51)Insert: CNMG 120408Cut. speed: vc = 100 m/minFeed: f = 0.15 mmDepth of cut: ap = 1.0 mmCL concept: MQL

HC3-TiAlN(ultrafine grain)HC2-TiAlN(submicron grain)HC1-TiAlN(fine grain)

Max

imum

flan

k w

ear V

Bm

ax

0 100 200 300 400 5000

0.1

0.2

0.4

cm³

HC1HC1 HC2HC2

HC3HC3

Figure 13: Influence of the grain size on the performance

of cemented carbides [54].

The application of fine grain cemented carbide led to a tool breakage after a cutting volume of 300 cm³. The cutting insert showed large surface cracks on the clearance face as a result of a mechanical overload. The application of the submicron grain carbide ensured a continuous increase in the flank wear at a significantly lower wear rate and the end of tool life was reached at a cutting volume of 400 cm³. The best performance was observed with the micro grain cutting insert. Even after a cutting volume of 400 cm³, no cracks had occurred and only a small level of flank wear was evident. The high strength and hot hardness of the micro grain carbide cutting material resulted in excellent process operation.

Cermets For some dry machining applications coated cemented carbides provide insufficient material properties at elevated temperatures. In these cases cermets can be applied as cutting materials. While cermets have a similar microstructure to cemented carbides, hard components like titanium/tantalum carbide/nitrides are added to a binder matrix of cobalt and nickel [53, 55]. The microstructure of cermets leads to a higher hot hardness compared to conventional cemented carbides, thus making it possible to use higher cutting speeds. Due to ceramic components, cermets have an outstanding chemical stability against oxidation and tribochemical wear, as well as a reduced affinity for diffusion [56]. A major criticism of cermets has been their reduced toughness, making them susceptible to sudden cracks. With improvements in manufacturing technology it is now possible to produce nitrogen containing cermets with toughness comparable to conventional carbides. To judge the relative merits of cermets, an HSC-drilling process on tempered steel was examined (vc = 225 m/min; f = 0.10 mm) with short hole drills and an internal MQL-supply. Conventional cemented carbide inserts reached the end of tool life at a drilling length of 4 m due to a high flank wear of the outer cutting edge. With cermets as cutting material neither the outer nor the inner cutting edge showed significant wear at equivalent drilling lengths. The difference in the behavior of the two tool materials may be attributed to differences in their hot hardness. This leads to a reduced mechanical load on the cutting edges due to a thermal softening of the workpiece material in the shear zone [16].

Cutting tool: Short hole drill Cutting parameters:Workpiece: Ck45 (AISI1045) vc= 225 m/minDiameter: D= 25 mm f = 0.1 mmDepth: l = 25 mm Lf= 3.3 mCL concept: internal MQL-supply

Cemented Carbide(HC-P25)

Cermet(HT-P20)

Inner Cutting EdgeOuter Cutting Edge

Cutting tool: Short hole drill Cutting parameters:Workpiece: Ck45 (AISI1045) vc= 225 m/minDiameter: D= 25 mm f = 0.1 mmDepth: l = 25 mm Lf= 3.3 mCL concept: internal MQL-supply

Cemented Carbide(HC-P25)

Cermet(HT-P20)

Inner Cutting EdgeOuter Cutting Edge

Figure 14: Different wear behavior of cemented carbide and cermets [57].

Ceramics The high temperatures generated in dry machining, especially at high cutting speeds, necessitate the use of

tools that can withstand the highest temperatures and provide a long tool life. For machining of gray cast iron and hardened steels, ceramic cutting materials are applicable. Basically, ceramics for cutting purposes can be either aluminum oxide or silicon nitride ceramics. Due to the high hardness at elevated temperatures and the reduced resistance against thermal shocks, ceramics are often used without a cooling lubricant supply. A major drawback of ceramics is their reduced toughness, especially for machining operations with an interrupted cut, like milling. But SiC-whisker-reinforced ceramic composites and submicron aluminum oxide ceramics enlarge the possible fields of application [16]. With the development of aluminum oxide ceramic tools advancements in dry machining could be made [58]. The basic characteristic of the new cutting material is the use of very pure (99.99%) and submicron (0.22 µm) grain size aluminum powder. The fine powder is sintered with virtually no binder at low temperatures, resulting in a wear and fracture resistant ceramic cutting material. In turning and milling tests of gray cast iron and carbon steel the performance of the new ceramic cutting material was evaluated and compared to conventional ceramic materials. The new cutting material produced significantly smaller wear rates than the conventional aluminum ceramic. Ceramics with large grain sizes had a rugged surface resulting from intercrystalline fracture and consequent dislodgement of grains. In contrast, the wear of the new ceramic tool was based on transcrystalline fracture. In the turning tests, the differences in the wear behavior lead to the conclusion that not only the higher hardness but also the higher bond strength between the grains is responsible for the superior performance of the new tool material [58]. Face milling tests conducted with the new tool material permitted significantly higher feed rates. While the conventional ceramic failed at a feed rate of 0.25 mm/tooth, the new material showed good performance up to 0.50 mm/tooth. The worn surfaces were very similar to the results of the turning tests. In addition to improve wear resistance, the milling test results also illustrate the high fracture resistance to thermal and mechanical shocks of the submicron ceramic tool.

Cubic Boron Nitrides Similar to ceramics, a typical application of cubic boron nitrides is dry turning and fine boring of gray cast iron and hardened steels. For the manufacturing of cutting tools, the base material, cubic boron nitride (CBN), is combined with a ceramic, sometimes even with a metallic binder. The resulting compound has excellent material features, like very high hardness and chemical wear resistance up to extreme temperatures [59]. In hard turning operations the mechanical energy is almost completely transformed into heat by forming and friction processes. The extremely high temperatures in the cutting zone lead to a thermal softening of the workpiece material. Thus, hard turning operations are conducted dry, using temperature resistant cutting materials like CBN or ceramic.

Diamond The hardest cutting material available is polycrystalline diamond (PCD). Carbon serves as the base material and their specific atomic grid structure provides technical diamonds with several extraordinary properties. The performance of PCD cutting tools is based on their high

strength, low coefficient of friction, small thermal expansion, and high resistance to chemical corrosion. But the application of diamond tools is limited to temperatures less than 600°C because graphitization occurs above this temperature [53]. PCD is mainly used for machining light metals based on aluminum, magnesium or titanium. Especially for applications in the automotive industry, PCD is the state-of-the-art cutting material for dry machining of engine and chassis components. Due to the chemical affinity of carbon and iron, the machining of steel materials with diamonds results in high wear rates.

3.2 Coatings The tribological behavior of cutting tools for dry machining can be improved using modern tool coatings. They help to compensate for the functions of the cooling lubricant so that the contact characteristics and the wear progress of a cutting tool can be improved. Depending on the expected wear mechanisms, coatings have to fulfill various requirements [60]. Coatings have to reduce material removal, as a result of abrasive wear (hard coatings) as well as adhesive wear, by serving as a barrier layer between the cutting and workpiece materials. The thermal load of the cutting material is reduced by thermal barrier layers with low thermal conductivity. The sliding behavior on flank and rake faces may be improved by low friction coatings [14, 61, 62].

Multilayer Coatings Generally, tool coatings can be classified by the method of deposition, e.g., CVD (chemical vapor deposition) and PVD (physical vapor deposition), and by their composition (mono-layer or multi-layer coatings). While mono-layer coatings consist of a single coating type, multi-layer coatings are composed of numerous layers of the same, or combinations of different, coating types [63, 64]. The aim of a multi-layer coating is to combine the positive characteristics of different coatings. Furthermore, the thin layers of multi-layer coatings lead to a favorable distribution of stresses. If a crack in the surface of the coating occurs, the crack energy is relieved by crack deflection and branching. In contrast to this, mono-layer coatings provide little resistance to crack growth. Crack peaks can easily extend to the surface of a tool and large coating areas can break out (Figure 15) [16].

Mono-layer Coating

Small obstruction of crack extension; Crack peaks run

to substrate-surface

Multi-layer Coating

Reduction of crack energy by crack deflection

and branching

Mono-layer Coating

Small obstruction of crack extension; Crack peaks run

to substrate-surface

Multi-layer Coating

Reduction of crack energy by crack deflection

and branching

Figure 15: Different crack extensions in mono-layer and

multi-layer coatings [16].

From a technological point of view the possible number of layers is almost unlimited. However, the thickness of the coating system leads to an increase in the cutting edge radius. Since a sharp cutting edge is of particular importance for many dry machining applications, thin layers are often favored.

Nanolayer Coatings Advancements in coating technology have made it possible to reduce the thickness of single layers. By minimizing the microstructural or spatial scale to nanometer dimensions, the properties and performance of modern tool coatings may be improved significantly. Multilayer nano-coatings on cemented carbide tools can be selected from a wide range of material systems. While the thickness of each layer is only a few nanometers, the total thickness of the coating can be in a range of 2-5 µm. Thus, a nanolayer coating system will consist of hundreds of layers, e.g. hard/solid lubricant or hard/tough materials [64, 65]. When dry machining an AISI4140 steel, a nanolayer coating of 100 bilayers of 13A B4C/18A W showed a dramatic reduction in the flank wear compared to an uncoated and a conventionally TiC-Al2O3-TiN and TiAlN coated cemented carbide tool. In addition, dry drilling tests of a difficult-to-machine Ti-6Al-4V alloy with a solid-lubricant nanolayer coating (400 bilayers of 80A MoS2/Mo) on a HSS-drill were conducted. The nanolayer coating led to a 33% reduction of the torque and no evidence of failure at the end of tool life of the uncoated drill [66].

Supernitrides Recent developments in coating technology have made it possible to produce new and innovative coating systems. Based on a sputtering technology using high ionized plasmas, high performance hard coatings, called supernitrides, were developed. Supernitrides belong to the group of nano-composites and combine the high chemical stability of oxide layers with the mechanical properties of hard nitride coatings. In contrast to conventional PVD processing, this production process makes it possible to produce conducting or insulating TiAlN-coatings with extremely high aluminum content [67]. The improved wear behavior of supernitrides compared to PVD-TiAlN coatings was demonstrated in a set of dry milling tests on 42CrMo4V steel (Figure 16). Although the number of cuts resulting dropped as the cutting speed increased for both tool materials, the nano-structure and the increased AlN content of the supernitride coating resulted in a larger number of cuts at every cutting speed. The bottom part of the figure shows the difference in the number of cuts resulting when using supernitride coating and the conventional TiAlN coating [68].

Workpiece: 42CrMo4V Cutting parameters:Cutting mat.: HC (K05-K20) f = 0.2 mm; hcu = 0.12 mm Coolant: none axy = az = 3 mm

Figure 16: Cutting performance of supernitrides [68].

Self-lubricating Coatings In dry machining operations the lubricating function can be partly compensated for with soft coatings, also called self-lubricating coatings. Typical self-lubricating coatings are MoS2 or amorphous WC/C (Figure 17). These types of coatings are usually deposited on top of a hard coating, like titanium-aluminum-nitride. Self-lubricating coatings improve the run-in behavior of a tool and reduce the friction between the cutting tool and the workpiece. As a consequence, the cutting forces and the process heat generated can be reduced [53, 69]. When drilling tempered steel with an MQL supply, a self-lubricating top coating of WC/C on a TiAlN-coated cermet substrate led to a significant improvement of the wear resistance. Although the soft coating was worn down after a small drilling length, the hard coating below demonstrated a good run-in behavior. The rough surface is smoothed, protecting the cutting tool from adhesive wear [16].

Removal ofWC/C-Layer

SmoothedTransition Zone

Cutting tool: Short hole drill Cutting parameters:Cutting mat.: Cermet (P20) vc= 225 m/minCoating: WC/C+TiAlN f = 0.1 mm; Lf = 3.0 m Workpiece: Ck45 (AISI1045) D= 25 mm; l = 25 mm CL concept: MQL

Removal ofWC/C-Layer

SmoothedTransition Zone

Cutting tool: Short hole drill Cutting parameters:Cutting mat.: Cermet (P20) vc= 225 m/minCoating: WC/C+TiAlN f = 0.1 mm; Lf = 3.0 m Workpiece: Ck45 (AISI1045) D= 25 mm; l = 25 mm CL concept: MQL

Figure 17: Wear Behavior of WC/C-TiAlN-Coatings [16].

CBN coatings The application of suitable cutting materials is one of the prerequisites for dry machining. Cubic boron nitride (CBN) is the hardest material known suitable for the machining of ferrous materials. The current development of a CBN coating for cutting tools, combining the advantages of coating and of CBN, is of great importance for the steel manufacturing industry. Compared to widely used sintered polycrystalline cubic boron nitride (PCBN), physical vapor deposition (PVD) CBN coatings on cemented carbide substrates have a number of advantages, regarding the geometrical flexibility, sharp cutting edges without chamfers and negative rake angles, no diffusion of binder material, as well as cost efficiency. Recently, the first successful cutting tests with CBN coated indexable inserts machining heat treated CrNiMo steel, spherulitic cast iron and hardened CrV tool steel were carried out. In comparison with other tool coatings, the workpiece roughness generated by CBN coated tools is much lower. Furthermore, the cutting forces, which are significantly lower than those of CVD TiCN coatings, are similar to those of PVD TiAlN coatings. However the film thickness, film adhesion, and geometry of the substrate may be further optimized. Experiments were carried out without any cooling fluid because of the thermal stability and conductivity, as well as the low coefficient of friction, of PVD CBN coatings [70].

CVD-diamond Diamond cutting tools are often the only choice for machining such materials as non-ferrous metal alloys, metal matrix composites and fiber reinforced plastics. In addition to the solid cutting material PCD, which is manufactured in a diamond synthesis, diamond films made in a CVD process are being used more and more [71]. The nanocrystalline and microcrystalline CVD diamond films help to reduce abrasive wear due to their inherent hardness [72]. Compared to PCD, CVD-diamond not only offers outstanding properties but also has high geometrical flexibility with respect to the tools to be coated. Based on the thickness of the coating, it is possible to distinguish between CVD-diamond thin films and thick films, which are usually deposited on a cemented carbide substrate. In dry turning tests of hypereutectic aluminum silicon alloy CVD-diamond thick coatings showed outstanding performance. The high resistance to abrasive wear and thermal loads leads to a long tool life. While no significant build-up edge on the face could be detected, adhesive material deposits on the flank were found to be characteristic. The PCD cutting materials do not possess the performance capacity of the CVD-diamond thick coatings. Due to the soft metallic binder matrix, the PCD is sensitive to abrasive and thermal loads [73].

50

CDE

min

30

20

10

0

CDM

Tool

life

TVB

0.2

PCD002

PCD010

PCD025

Outer diameter turning

Machine tool: Workpiece mat.: Cooling lubricant:Traub TNS 30 AlSi17Cu4Mg Dry machining

Process parameters: Tool geometry:Feed f = 0.1 mmDepth of cut ap = 0.5 mmCutting speed

vc = 600 m/min vc = 800 m/min vc = 1200 m/min

γ0 α0 λs κr εr rε5° 6° 0° 75° 90° 0.4 mm

Outer diameter turning

Machine tool: Workpiece mat.: Cooling lubricant:Traub TNS 30 AlSi17Cu4Mg Dry machining

Process parameters: Tool geometry:Feed f = 0.1 mmDepth of cut ap = 0.5 mmCutting speed

vc = 600 m/min vc = 800 m/min vc = 1200 m/min

γ0 α0 λs κr εr rε5° 6° 0° 75° 90° 0.4 mm

γ0 α0 λs κr εr rε5° 6° 0° 75° 90° 0.4 mm

Cutting material

Figure 18 : Dry turning of AlSi17Cu4Mg with diamond.

Face milling and tribological tests, have shown the potential of CVD-diamond thin coatings during the dry machining of a hypereutectic aluminum silicon alloy [74]. However, first experiments with CVD-diamond coated taps under dry conditions and with the use of MQL revealed that insufficient film adhesion to the substrate and cutting edge roundness currently limit the application range of CVD-diamond thin- film tools [75, 76].

3.3 Workpiece Materials The chemical composition and the microstructure of workpiece materials have a significant influence on the applicability of dry machining and MQL. The reduction of the cooling lubricant supply tends to affect the abrasive wear in a positive way. Due to high temperatures, the chip and the workpiece material in the cutting zone plastically soften, thus facilitating the pressing of hard particles into the chip or the work material. As a result, the cutting edge is protected from the abrasive wear caused by the hard particles [16, 47].

In contrast to this, adhesive tool wear is of particular importance in dry machining processes. The high process temperatures ease mechanical interlocking and diffusion due to the ductility of the workpiece material. The diffusion mechanisms are supported by the temperatures, as well as by the better contact conditions. The workpiece materials commonly used in modern machining applications reveal differences in their suitability for dry machining with respect to the adhesive wear. The driving force for dry machining technology has been the automotive industry, including their major suppliers. The workpiece materials used by the companies are mainly gray cast iron and aluminum cast alloys, which yield low cutting forces and temperatures [77]. But with respect to workpiece material adhesion, wrought alloys and highly-alloyed stainless steels make the highest demands on the process design (Figure 19) [16, 78].

Suitabilityfor Dry Machining

Increasing AdhesiveTool Wear

Steel Materials

Ferritic / Perlitic Austenitic

Ni-, Ti-, V- and/or Ni-contentferrite-content rises decreases

Aluminum-Alloys

Cast- Wrought-Alloys Alloys

Si + Al2O3 Mg + Si decreases decreases

Brass-AlloysMagnesium-Alloys

Gray Cast Iron

Toughness / Ductility

Suitabilityfor Dry Machining

Increasing AdhesiveTool Wear

Steel Materials



Steel Materials



Aluminum-Alloys



Aluminum-Alloys




Gray Cast Iron



Gray Cast Iron


Figure 19: Suitability of different workpiece materials

for dry machining [16].

3.4 Fluids It has been well recognized empirically that supplying some fluid to the vicinity of contact between the tool and the workpiece could facilitate the machining operation so that the functions of cutting fluids have also been the main subject of early investigations. At higher cutting speeds, since the tool undergoes wear because of increased temperature, it is important that the cutting fluid acts as a coolant. As cutting speeds lower, the lubricating properties of the fluid become more prominent, easing the flow of the chip up the tool rake face. The main functions of cutting fluids are, therefore, cooling at relatively high cutting speeds and lubrication at relatively low cutting speeds [79]. However, a distinction between high and low cutting speeds is rather ambiguous and, actually in most cases, both cooling and lubrication are, to some extent, performed by cutting fluids [80]. For example, the cooling action of cutting fluids is ordinarily understood to be the ability to remove generated heat but may also include the capability to reduce heat generation with the aid of the lubricating properties of the fluids. When a cutting fluid acts as a lubricant at the chip-tool interface, the problem is the mode of its access to this boundary. With a capillary phenomenon through micro-apertures between the chip and the tool, with a pressure

difference between the inside of the apertures and the atmosphere or with a pumping action caused by a relative vibration between the chip and the tool, the fluid may arrive at the vicinity of a zone where the tool carries out cutting. In all cases, however, the fluid must penetrate to the interface between the tool and the chip in a direction opposed to the motion of the chip flow. With this in mind, access from the sides of the tool or from the flank face side may also be considered. Another mode of penetration that has been proposed is lubricant diffusion to the interface between the tool and the chip through the plastically deforming material within the primary shear zone [81] or through fissures in the chip [82]. On the other hand, cutting fluids may approach the vicinity of the cutting zone in a vapor phase. From this point of view, the pioneering work of Rowe and Smart found a lower cutting force in oxygen than that in a vacuum when cutting ferrous materials, emphasizing the important role of gaseous oxygen as a lubricant [83, 84, 85]. Some experiments using carbon tetrachloride as a lubricant showed that both the vapor and the liquid were far more efficient than oxygen [79], and that the vapor was effective at relatively higher cutting speeds [86]. More recently, the action of various gas-phase lubricants in machining of an aerospace aluminum alloy was investigated [87, 88, 89]. In particular, it was found that the rate controlling step of gas-phase lubrication is the reaction, rather than the transport, of the lubricant molecules [89]. This fact is very important because the reaction of a chemical with a freshly cut, thus highly active, metal surface is normally considered to be very rapid, but, nevertheless, lubricant particles can be transported even faster than such a rapid reaction rate through the interface between the tool and the chip. This view is acceptable and, probably in practical cutting, fluids may evaporate due to high cutting temperature and can readily penetrate, against the chip flow motion, to some extent, deep through a network of micro-capillaries existing between the tool and the chip. Furthermore, perhaps in the case of MQL machining, small particles of the lubricant should evaporate extremely easily, compared with cutting fluids in the case of flood fluid supply. The tribological behavior of lubricants has been demonstrated to be influenced by their adsorption characteristics on a freshly cut metal surface [90]. If the above vapor phase lubrication is expected in MQL machining, there is, therefore, a possibility that the tribological action of MQL lubricants is related to their adsorption phenomena on the freshly cut metal surface. In order to investigate fundamentally the adsorption characteristics of synthetic esters on a freshly cut metal surface, the adsorption activity is measured using a controlled atmosphere machining apparatus [91]. In this experiment, cutting of a metal specimen is conducted in a vacuum chamber and two kinds of gas components can individually be introduced into the chamber at a certain constant value of pressure. Since the vapor pressure of the practical synthetic MQL lubricant ester is not high enough to introduce it into the chamber, methyl propionate is used as a model ester and n-hexane is also used as a model hydrocarbon. Figure 20 compares the adsorption activity of methyl propionate with that of n-hexane. Methyl propionate shows relatively high adsorption activity, whereas n-hexane shows no significant adsorption. This is the expected result because hydrocarbons have no polar group to adsorb on the metal surface.

0 0.1 0.2 sec-1 0.4Adsorption activity

Methyl propionate

Methyl propionate+ O2

n-Hexane

0 0.1 0.2 sec-1 0.4Adsorption activity

Methyl propionate

Methyl propionate+ O2

n-Hexane

Figure 20: Values of measured adsorption activity [91].

It is considerably interesting that the adsorption activity of methyl propionate is increased if oxygen is present, suggesting that oxygen can enhance the adsorption ability of ester. This situation may possibly be very similar to the behavior of a lubricant in MQL cutting because, even near the cutting point, the lubricant particles are surrounded by a large amount of air containing oxygen. Figure 21 illustrates schematically the difference between (a) the conventional flood supply and (b) the MQL supply. In MQL cutting, the reactivity of the lubricant ester is intensified by atmospheric oxygen, leading to the formation of a robust and tribologically effective lubricating film. The modified version of this apparatus is now available and the investigations have just been initiated in order to understand the influence of the lubricant adsorption activity on the actual cutting behavior [92].

(a) Conventional supply (b) MQL supply

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Cutting fluidAir (oxygen) +

lubricant particles

(a) Conventional supply (b) MQL supply

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Workpiece

Tool

Cutting fluidAir (oxygen) +

lubricant particles

Figure 21: Schematic illustration of the difference between

(a) conventional supply and (b) MQL supply [91].

4 MACHINING PROCESSES Different machining processes set varying demands on the amount of cooling lubricants needed for secure and satisfactory machining results. To implement dry machining, aspects like heat generation, clearance of chips, and kinematic conditions have to be considered when designing the process [14, 47, 93]. The classical dry machining applications are sawing and milling. Figure 22 shows the influence of the machining process on the cooling lubricant supply. The interrupted cut ensures short breaking chips, a good chip clearance and a cooling of the cutting edges. Due to the usually good accessibility of the cutting zone, turning operations are also suited for a cooling lubricant reduction [94]. Since the continuous cut of ductile materials can lead to long and unfavorable chips, the geometry of the cutting inserts, e.g., chip breaker, and the cutting parameters play an important role. For processes with a geometrically defined cutting edge, hole-making operations like drilling, tapping, and reaming are often hard to accomplish without at least some cooling lubricant. Due to the covered cutting zone

high process heat may be generated and the chip removal out of the hole is crucial. Furthermore, the cutting speed decreases with the diameter of the drill, which leads to different loads on the outer corner and the center of the tool. The high demands on the hole quality and the different loads at the cutting edges and the guide rails in reaming operations require an accurate analysis and adaptation of the machining process [47, 95, 96, 97].

machining processeswith geometrically

defined cutting edge

ReamingDrillingTurningMillingSawing

LappingHoningGrinding

Requirementof coolinglubricant

supply

Drymachiningpossible


undefined cutting edge


defined cutting edge

ReamingDrillingTurningMillingSawing

LappingHoningGrinding

Requirementof coolinglubricant

supply

Drymachiningpossible


undefined cutting edge

Figure 22: Influence of the machining process

on the cooling lubricant supply.

The high energy density, the inaccessible cutting zone and the extremely high quality demands make it very difficult to reduce the supply of conventional cooling lubricants in machining processes with a geometrically undefined cutting edge. However, some investigations point out the possibilities and potentials of MQL supply in grinding processes.

4.1 Geometrically Defined Cutting Edge

Milling Based on the kinematic conditions, dry milling can be considered as a state-of-the-art machining technology for a wide spectrum of workpiece materials. However, in some applications the workpiece material or a special machining task requires a detailed analysis and adaptation of all process components and parameters to achieve economical dry machining [47, 98, 99, 100]. Dry milling of trapezoid slots into the shank of a rock drill made of 34CrNiMo6 (AISI4340) represents such a challenging application (Figure 23). Due to a small slot width of 2.55 mm, all tool modifications had to take place on a small scale.

Tool

life

[m]

TiN TiAlN (monolayer)

Workpiece: 34CrNiMo6 Cutting parameters:Cutting mat.: HC (K30F) vc = 80 m/min; vf = 600 mm/min

25

20

15

10

5

0

40

35

30

25

20

15

10

5

0

40

35

30

Tool coating

z = 2; f = 0.03 mm z = 3; f = 0.02 mmz = 2; f = 0.03 mm z = 3; f = 0.02 mm

Figure 23: Tool life in dry milling [101].

General machining problems occurred in the form of work material adhering to the cutting edges and rough surfaces on the slot wall machined in up-cut milling, leading to reduced tool life. Submicron and ultra-fine grained cemented carbides combined with a TiAlN multilayer coating resulted in a form cutter that could withstand the high mechanical loads found in the process. Especially when increasing the feed per tooth to values ≥ 0.03 mm, a hard substrate had to be applied. In considering the geometrical design of the form cutter, an increase in the number of teeth from 2 to 3 resulted in a significantly longer tool life. For a secure milling process the chip flute size must be sufficiently large, which limits the maximum number of teeth. Further modifications are related to a load adapted design of the cutting edges using finite element simulation [101].

Turning In current industrial practice, many turning operations can be performed dry or with an MQL supply. Besides the classic fields of hard turning of bearings and chuck components [102, 103], drive shafts and rotationally symmetrical forging parts can also be machined without a coolant in large scale production [104]. Since modern cutting tools usually provide a high hardness at elevated temperatures and with high performance coatings, cutting materials for dry machining applications can be considered as standard tools [105]. Recent developments in the production of cemented carbides and coatings allow for the turning of high-alloyed, stainless steels with MQL. Designing the machining process for a fitting bolt made of compound steel (SAE329), required modification of the cutting insert and the cutting parameters in response to the changed cutting conditions (Figure 24) [106].

Workpiece: fitting bolt M24 X 80Material: X8 CrNiMo 27 5 (SAE329)

workpiece cutting tool

externalMQL-supply

Workpiece: fitting bolt M24 X 80Material: X8 CrNiMo 27 5 (SAE329)

workpiece cutting tool

externalMQL-supply

Figure 24: Turning high-alloyed, stainless steel with MQL.

Applying standard cutting tools, the high mechanical and thermal load of the dry machining process resulted in tool breakage after 3 fitting bolts were machined. The heat and high pressure on the corner of the cutting edge during rough machining especially restricted the performance of the process. Taking into account the problems detected, the cutting material was replaced with a submicron grained cemented carbide with a hard and temperature resistant nanolayer coating based on AlN/TiN. In addition, the depth of cut at the bolt head was reduced and the cutting speed and the feed rate were increased to ensure constant processing times. The results showed that the modification of the cutting material and the machining strategy enabled an economic turning of high-alloyed steel with an MQL supply (Figure 25).

Workpiece mat.: X8 CrNiMo 27 5Cutting speed: vc= 80 m/minFeed rate: f= 0,25-0,32 mmCL concept: extrenal MQL

Cutting mat.:HC-M30 (CVD-multilayer)Depth of cut: No. of partsap,max= 3,0 mm n=3

Cutting mat.:HC-M25 (PVD-nanolayer)Depth of cut: No. of parts ap,max= 1,6 mm n=15

Workpiece mat.: X8 CrNiMo 27 5Cutting speed: vc= 80 m/minFeed rate: f= 0,25-0,32 mmCL concept: extrenal MQL

Cutting mat.:HC-M30 (CVD-multilayer)Depth of cut: No. of partsap,max= 3,0 mm n=3

Cutting mat.:HC-M25 (PVD-nanolayer)Depth of cut: No. of parts ap,max= 1,6 mm n=15

Figure 25: Adaptation of the dry turning strategy [106].

When turning the fitting bolts using MQL, all quality characteristics entirely met the requirements of the wet machining process. With the exception of the first two parts from the set-up, all fitting bolts were within the diameter-related tolerance limit of 25j8. Regarding the surface roughness, all machined fittings were significantly smaller than the allowed maximum of 16 µm. The lubricating effect of the MQL medium makes it possible to produce very good surface finish (Rz ≈ 6.3 µm) with a small scatter [106]. Another example of a difficult-to-solve turning operation is the production of a wrought aluminum alloy casing. The task is especially challenging since a high volume of the raw workpiece material is removed (about 90% of the solid bar feed material). Figure 26 shows the raw material and the shape of the finished part.

raw material finished part

• turret with 8 tools andindividual MQL-supply

• MQL-adapted tool design• significant increase of

cutting parameters• additional compressed air

to ensure chip removal

raw material finished part

• turret with 8 tools andindividual MQL-supply

• MQL-adapted tool design• significant increase of

cutting parameters• additional compressed air

to ensure chip removal Figure 26: MQL-Turning of an aluminum casing [107].

Applying the standard tools and cutting parameters from the wet machining process results in significant problems concerning extremely long flow chips and severe adhesion. The large amount of internal machining results in bad surface quality and a significant increase in the temperature of the workpiece [107]. The solution of these machining problems was found through several steps. In the first step a retrofit MQL-supply system was applied that allowed for using individual setups for every tool. For the cutting material, cemented carbides with an adapted geometry and a TiB2

coating were applied to reduce adhesion. In addition, the cutting parameters were increased significantly (vc ≤ 950 m/min; f ≤ 0.5 mm; ap ≤ 5 mm) to improve chip breaking and to reduce the heating of the workpiece. The tool holders were modified by eroding cooling channels into them to ensure an optimal MQL supply and additional channels for the supply of compressed air. The compressed air and a tapered pre-machining of the internal contour provided for secure removal of the hot chips. The modifications created a reliable process capable of machining the aluminum casing. All adapted tools produced only short breaking chips. Material adhesion to the tool was minimized using a smooth coating and an optimized MQL supply. The compressed air and the specific machining strategy resulted in effective removal of the chips [107]. Drilling Cemented carbides are cutting materials favored in the manufacturing of high performance drills. Based on the continuing trend to dry HSC and HPC applications, more and more tool manufacturers are modifying standard tools to meet the challenging requirements in dry machining. Representative modifications include sharp and stable cutting edges to realize small cutting forces, an adapted design of the cooling channels to allow for a secure transport of the MQL aerosol, wide chip flutes that open to ensure a reliable removal of chips, and a smooth tool surface to reduce friction and adhesive wear (Figure 27) [108, 25]. The modification of the tool and an increase in the cutting parameters causes significantly higher flow velocities of the chips. Thus, the contact times between the tool and workpiece may be decreased and the process performance improved. In HPC applications, the feed speed may be two to three times higher than for a conventional machining process.

MQL-adaptedtool design

Reduction ofadhesive wear

Properties ofcutting material

Geometry ofcutting tool

MQL-adaptedtool design

Reduction ofadhesive wear

Properties ofcutting material

Geometry ofcutting tool

Figure 27: Design of drills for dry machining [16, 47].

Measurements of the torque and the cutting forces emphasize the potential of these kinds of drills (Figure 28). Neither the feed force nor the torque shows a highly dynamic behavior. Forces and torques are nearly constant for the whole drilling depth. Thus, the chip formation and the chip clearance can be controlled to ensure a high process reliability. Increasing the cutting speed leads to a slight increase in the feed forces. A similar behavior can be detected for the drilling torque. Due to the higher temperature level, the cutting processes proceed more easily. Thus, the energy needed for forming and separating the material only increases by a small amount. As a drawback, the dynamic behavior of the force and torque measurements increases.

0 0

s

1

2

kN

-10 1 2 3 4 5 6 8

cutting time t

3

1

2

3

Nm

5

-1

5

feed

forc

e F f

torq

ueM

d

vc=200m/min

Ff

Md

vc=100m/min

Ff

Md

Material: AISI 1045 Feed : f = 0.3 mmCutting mat.: HC Diameter: d = 19.8 mmCL concept: internal MQL Depth: l = 29 mm

0 0

s

1

2

kN

-10 1 2 3 4 5 6 8

cutting time t

3

1

2

3

Nm

5

-1

5

feed

forc

e F f

torq

ueM

d

vc=200m/min

Ff

Md

vc=100m/min

Ff

Md

Material: AISI 1045 Feed : f = 0.3 mmCutting mat.: HC Diameter: d = 19.8 mmCL concept: internal MQL Depth: l = 29 mm

Figure 28: Feed force and torque in dry drilling [109].

In series production dry machining using high performance drills may offer the possibility of reducing the production times or of increasing the tool life. Figure 29 illustrates the results of an experimental investigation of the drilling of flanges made of tempered steel without any coolant supply [109].

Material: AISI 1045 Diameter: D1= 10.6 mm Cutting mat.: HC-TiAlN D2=1 2.5 mmCutting speed: vc1= 71.5 m/min Feed rate: f= 0.3 mm

vc2= 89.4 m/min Coolant: none

3000

0

500

1000

1500

2000

2500

num

bero

f bor

e ho

les

wet machining

dry machining

1080

2760

workpiece

drill

Material: AISI 1045 Diameter: D1= 10.6 mm Cutting mat.: HC-TiAlN D2=1 2.5 mmCutting speed: vc1= 71.5 m/min Feed rate: f= 0.3 mm

vc2= 89.4 m/min Coolant: none

3000

0

500

1000

1500

2000

2500

num

bero

f bor

e ho

les

wet machining

dry machining

1080

2760

workpiece

drill

Figure 29: Dry drilling of flanges in series production.

Applying a two-step twist drill with a submicron grain cemented carbide substrate and a TiAlN coating without any cooling lubricant, the tool life can be increased significantly by 150% compared to a conventional process with the cutting parameters kept constant. The improved performance of the tool can basically be ascribed to the reduced levels of thermal shock [109].

Deep Hole Drilling Even deep hole drilling operations with a length-to-diameter ratio much larger than three can be conducted using a supply of compressed air or MQL through the cooling channels of the tool. Examples of such applications can be found in the automotive industry, and include the machining of bore hole systems into engine blocks made of gray cast iron or cast aluminum using single edge gun drills [3], as well as machining of crank shafts made of tempered steel with twist drills [110]. Regarding deep hole drilling of gray cast iron the choice of the cutting parameters and the design of the tool is crucial for a satisfactory machining result (Figure 30). Investigations of the tool lifetime reveal that economic dry machining is technologically possible. At a cutting speed of 80 m/min the wear of the guide pads indicates a distinctive minimum. In addition, the wear behavior can be

improved by rounding and polishing the transition from the chamfer to the guiding pad [14].

Cutting tool: Workpiece mat.: Cutting param.: single gun drill GG-26 Cr f = 0.03 mm form C Diameter: l = 250 mm HW-K10/20 d = 7.0 mm Lf = 2000 m

40 60 1000

10

20

30

µm

50

wea

rof g

uide

pad

εi

m/min chamferedrounded

0

5

10

µm

20

roundedpolished

condition of guiding padcutting speed vc

compressed airmineral oilnative oil vc = 63 m/min

guide pad 1guide pad 2

Cutting tool: Workpiece mat.: Cutting param.: single gun drill GG-26 Cr f = 0.03 mm form C Diameter: l = 250 mm HW-K10/20 d = 7.0 mm Lf = 2000 m

40 60 1000

10

20

30

µm

50

wea

rof g

uide

pad

εi

m/min chamferedrounded

0

5

10

µm

20

roundedpolished

condition of guiding padcutting speed vc

compressed airmineral oilnative oil vc = 63 m/min

guide pad 1guide pad 2guide pad 1guide pad 2

Figure 30: Wear reduction in dry deep hole drilling [14].

In contrast to the results mentioned above, wear resistant cutting materials and coatings as well as an internal MQL supply are mandatory for machining aluminum cast alloys and steel materials. TiAlN hard coatings with an MoS2 lubricating top layer lead to good machining results. Concerning the MQL supply, the efficiency of MQL fluid is of particular importance. Synthetic esters are especially good at helping to prevent material adhesion on the cutting tools and to improve the quality of the hole.

Reaming Reaming operations which rely on guide-rail-assisted, one or two or multi-blade reamers offer excellent opportunities for minimum quantity lubrication [111, 112] (Figure 31). When guide-rail-assisted tools are used, the guide rails are affected by the cutting force and provide support for the bore hole, thus limiting the deflection. The friction partners, guide-rail and hole wall, must be lubricated and the chips have to be transported reliably from the hole. Thus, the generation of heat, the level of wear sustained by the guide-rails and the detrimental effect on surface formation may be minimized. It is important that the MQL medium be fed into the tool via internal channels. The pattern of exit openings for the MQL medium is the decisive factor for ensuring continuous lubrication of the guide rails and reliable chip transport.

Source: Heidelberger Druckmaschinen

4015

080

00.

10.

62.

0

GG20

2015

045

00.

20.

21.

0

9SMn28k

Wet machining in seriesproductionDry machining with MQLin series productionHigh performance machining(future dry with MQL)

0.5

1.5

2.0

1.0

mm

feed

f

0100200300400500600

800m/min

cutti

ng s

peed

vc

0100200300400500600

800m/min

cutti

ng s

peed

vc

0

0.5

1.5

2.0

1.0

mm

feed

f

0

Source: Heidelberger Druckmaschinen

4015

080

00.

10.

62.

0

GG20

2015

045

00.

20.

21.

0

9SMn28k

Wet machining in seriesproductionDry machining with MQLin series productionHigh performance machining(future dry with MQL)

0.5

1.5

2.0

1.0

mm

feed

f

0100200300400500600

800m/min

cutti

ng s

peed

vc

0100200300400500600

800m/min

cutti

ng s

peed

vc

0

0.5

1.5

2.0

1.0

mm

feed

f

0

0.5

1.5

2.0

1.0

mm

feed

f

0

Figure 31: High performance reaming with MQL.

The enormous potential of dry machining is illustrated particularly clearly in reaming. The technology of fine-boring was optimized over a long period of time with the single objective of achieving very high levels of precision and surface quality. In conventional wet-machining reaming operations, the majority of cutting parameters are therefore set at low levels. New reaming tools designed as a part of the development of dry machining and HPC make it possible to achieve cutting speeds which are several times higher than those recorded for conventional wet-machining operations. As demonstrated by the example of high performance reaming techniques either already applied in series production or projected, dry machining and machining with MQL offer enormous potential for increasing efficiency and reducing costs [113, 98].

Threading Both thread production and drilling are extremely important operations. The tools are exposed to very high levels of mechanical and thermal load in these operations as a result of ploughing, friction and adhesion [114]. A number of coated, geometrically optimized, HSS tools are available on the market for dry tapping operations on cast iron, steel and aluminum materials. It is essential to implement a minimal lubrication strategy when these tools are used [115]. The same applies to thread-forming operations. The application of this technique is standard industrial practice in most operations producing threads in aluminum materials. Investigations indicate that this technique is also suitable for tapping operations conducted on ductile steel materials. Dry machining with minimal lubrication is suitable, too. Tapping and thread- forming operations must be conducted at comparatively low cutting speeds due to material-related and process-related restrictions (Figure 32). tapping

M10

thread milling

thread forming

tc = 5.8 svc = 8 m/minn = 260 min-1

MQL

tc = 4.5 svc = 10 m/minn = 320 min-1

MQL

tc = 0.8 svc = 80 m/minn = 3315 min-1

dry

M10

M10time [s]

Nm

-5-10

0

10

torq

ueM

dto

rque

Md

1.

600.10

25Nm

-100

time [s]

5Nm15

-5-10

0

600.

1.

torq

ueM

d

time [s]

tapping

M10M10

thread milling

thread forming

tc = 5.8 svc = 8 m/minn = 260 min-1

MQL

tc = 4.5 svc = 10 m/minn = 320 min-1

MQL

tc = 0.8 svc = 80 m/minn = 3315 min-1

dry

M10M10

M10M10time [s]

Nm

-5-10

0

10

torq

ueM

d

time [s]

Nm

-5-10

0

10Nm

-5-10

0

10

torq

ueM

dto

rque

Md

1.

600.10

25Nm

-100

time [s]

torq

ueM

d

1.

600.10

25Nm

-100

1.

600.10

25Nm

-100

time [s]

5Nm15

-5-10

0

600.

1.

torq

ueM

d

time [s]

5Nm15

-5-10

0

600.

1.

torq

ueM

d

5Nm15

-5-10

0

600.

1.

torq

ueM

d

time [s]

Figure 32: Tools, torques and cutting parameters

in thread production [19]

Thread milling is an efficient alternative. Not only do the carbide tools used in this operation ensure higher cutting rates along with an enormous reduction in manufacturing time, but they also eliminate the need for minimum quantity lubrication. Furthermore, combined tools and processes such as the thriller (drilling, chamfering and thread milling with one tool) or circular thrilling (thread hole, thread and chamfer with one tool at the same time)

allow for a reduction in the number of tools used and, correspondingly, in the number of tool changes and non-productive times. In each of the process or tool options, the same tool produces the thread hole and the thread. This means that one less tool and one less tool change are needed than in a thread-milling operation where two different tools are required [116]. The strategy of replacing conventional processes with circular processes, opens up new areas of application for dry machining. The time saved is considerable. The cutting speed of the tool is de-coupled from the feed when tapping or thread forming is replaced by thread milling. Cutting speed and feed become freely selectable. They can be optimized separately to suit the application at hand., The discontinuous contact in thread-milling operations is preferable to the continuous contact between the tool and the workpiece in tapping and thread forming. This is a significant move in the direction of a switch to dry machining. Aluminum alloys are among the most interesting materials in terms of dry machining. The high levels of thermal conductivity associated with these alloys permit them to absorb a considerable amount of heat from the machining process. Combined with their pronounced thermal expansion properties, this results in considerable changes to the shape of the part and to an intensification of problems related to chip formation as a result of their low melting and fusion point. Many aluminum alloys are prone to stick to the tools, to clog up the chip spaces, and to form apparent chips depending on the machining temperature. When these operations are carried out under a completely dry machining strategy, these characteristics often result in significant deterioration of part quality, perhaps even going so far as to result in tool failure due to fracture. Successful dry turning, drilling, reaming, thread-drilling or end-milling operations conducted on aluminum alloys are, therefore, reliant on the implementation of a minimum quantity lubrication strategy, as well as on tools with suitable multi-layer coating systems [19].

Broaching Broaching is a highly efficient machining process often utilized in the high-volume production of high-precision components with complex geometry. Short processing times and robust machine tool techniques make the process very attractive for selective cutting operations like internal tooth systems or bearing carrier. Negative aspects of the process include its limited flexibility and high tool costs. In broaching operations, almost all of the process technology and the associated machining performance are defined by the tool. For this reason the tool plays a decisive role in the whole process. Especially in the case of dry machining, tool design becomes very important, with special emphasis placed on improving cutting material and specific tool coatings. Applying a standard tool designed for a wet machining process causes problems when machining under dry conditions (Figure 33). Especially the insufficient stability of the cutting edge leads to a disruption of the cutting material. This fact is underlined by a significant increase in tool wear at the clearance face, which is more than three times higher than the tool wear that occurs in the corresponding process using a conventional coolant supply. In addition, the absence of coolant and the non- adapted tool geometry leads to increasing thermo-mechanical interaction with the workpiece. As a result, built-up edges appear and the surface roughness increases.

flank

wea

rVB

Material: Ck45 (AISI1045) Cutting path: 100 mCut. speed: 30 m/min Chip. thickness: 80 µm

020406080

100µm140

tool/coating/coolant

050

100150200250N/mm

300


spec

. cut

. for

ce k

c


4

6

8

µm

12

surfa

ce ru

ghne

ss R

zStandard-Tool, uncoated, with coolantStandard tool, TiN-coated, without coolantStandard-Tool, TiN-coated, with coolantDry machining tool, TiN-coated, without coolant

flank

wea

rVB

Material: Ck45 (AISI1045) Cutting path: 100 mCut. speed: 30 m/min Chip. thickness: 80 µm

020406080

100µm140


050

100150200250N/mm

300


spec

. cut

. for

ce k

c


4

6

8

µm

12

surfa

ce ru

ghne

ss R

zStandard-Tool, uncoated, with coolantStandard tool, TiN-coated, without coolantStandard-Tool, TiN-coated, with coolantDry machining tool, TiN-coated, without coolant

Figure 33: Broaching of tempering steel with different tool

designs and cooling strategies [117].

Based on the results described, a dry broaching tool with a high quality substrate material and a multi-layer-coating combined with enlarged chip angles and chip spaces was developed. Testing of the adapted tool demonstrated a harmonic cut deviation, improved chip formation and a decrease in noise emissions. The improved performance of the dry broaching tool can basically be attributed to the design of the rake and clearance angles, as well as the reduction of friction associated with the tool coating (more heat went into the chip). Furthermore, the powder metallurgic high-speed steel provided for a highly stable cutting edge and a coating that resisted adhesion.

spec

. cut

. for

ce k

c

Workpiece mat.: Ck45 (AISI1045)Chip. thickness: 80 µmCoating: TiAlN multilayerCutting path: 100 m

050

100150200

N/mm

12

cutting speed vc

vc = 15 m/minvc = 30 m/minvc = 45 m/minvc = 60 m/min

cutting speed vc

surf.

roug

hnes

s R

z

8

µm

10

9

300

spec

. cut

. for

ce k

c

Workpiece mat.: Ck45 (AISI1045)Chip. thickness: 80 µmCoating: TiAlN multilayerCutting path: 100 m

050

100150200

N/mm

12

cutting speed vc

vc = 15 m/minvc = 30 m/minvc = 45 m/minvc = 60 m/min

cutting speed vc

surf.

roug

hnes

s R

z

8

µm

10

9

300

Figure 34: Dry broaching with different cutting speeds

[117].

Figure 34 illustrates the influence of the cutting speed on the performance of the dry broaching tool. It can be seen that better surface roughness and lower cutting forces can be achieved at higher cutting speeds. This behavior is mainly based on the improvement of the cutting conditions at high broaching speeds, whereby the friction at the clearance face is reduced. Furthermore, high cutting speeds ensure short contact times and higher temperatures in the cutting zone, thus benefiting the machining process. The chip volume rises and the coloration turns from metallic silver to blue. But recent test results here revealed that the maximum flank wear is doubled to about 38 µm as the cutting speed increases from 30 to 80 m/min. Nevertheless, the limit of 40 µm is not reached, up to a cutting length of 150 m [117].

Gear Shaping Gear shaping is a continuous forming process in metalcutting gear manufacture. Basically three separate movements characterize the machining process. These

are the cutting movement of the tool, the rotary movement of the cutter and workpiece, and the radial movement of the cutter. Regardless of how the operation is defined, there are basically three infeed processes, which may be distinguished based on the radial movement. When plunging without rolling movement, the radial infeed is affected without a rotation movement of tool and workpiece. In the case of plunging with a rolling movement, the infeed rate of the tool and the infeed depth are achieved with a simultaneous turning of the tool and the workpiece, whereby the rolling angle covered is typically less than 180°. A helical process with degressively controlled radial feed corresponds to a situation in which the infeed rate and infeed depth are changed continuously over a number of workpiece revolutions. For gear shaping, cutting force measurements help to analyze the loads occurring on the cutter for equal production rates, but under different machining conditions (Figure 35). In conventional gear-cutting processes (plunge with/without rolling), the load increase in dry machining is between 15% and 25%, depending on the cutting parameters used. For helical-degressive-radial infeed, dry cutting means that an increase in cutting speed causes a significantly smaller increase in main cutting force (approx. 5%). This is due to the high rolling feeds, with only short contact times between the tool and the chip. In addition, the proportion of frictional work in chip formation is small. The “lubricating” function of the cutting fluid only plays a minor part under these contact conditions. The cutting force measurements reveal that the helical- degressive-radial infeed technique is more suitable for dry machining than the conventional infeed processes. This technique is, therefore, used in the subsequent investigations.

50 60 70

05

1015

%25

load

incr

ease

indr

y m

achi

ning

cutting speed

m/min

Cutting param.: z1 = z2 = 42 Material: 16MnCr5 m = 3 Cutting mat.: ASP30w = 20 mm Coating: TiN

Plunge with/without rollingHelical process with degressive controlled radial feed

50 60 70

05

1015

%25

load

incr

ease

indr

y m

achi

ning

cutting speed

m/min

50 60 70

05

1015

%25

load

incr

ease

indr

y m

achi

ning

cutting speed

m/min

Cutting param.: z1 = z2 = 42 Material: 16MnCr5 m = 3 Cutting mat.: ASP30w = 20 mm Coating: TiN

Plunge with/without rollingHelical process with degressive controlled radial feedPlunge with/without rollingHelical process with degressive controlled radial feed

Figure 35: Cutting force comparison in dry machining

[118].

For standard titanium nitride (TiN) coated tools an increase in cutting speed from 60 to 75 m/min (25%) increases crater depth, regardless of lubrication conditions (Figure 36). Interpolating the values to a crater depth of 100 µm, the cutting speed increase serves to reduce tool life to 30% of the original level. It is also clear that, for the same cutting speed, there is very little difference between the tool life achieved in dry machining and in the conventional cutting fluid application. For a specified tool life, dry machining always results in a crater depth less than that which occurs in wet machining.

number of tooth gaps made

crat

er d

epth

0

20

40

60

µm

100

60 120 180 240 300 360 420

Cutting param.: z1 = z2 = 42; m = 3; a = 20°;vc = 60 m/min; roll feed = 3DH/div

Gear width: 20 mm F/E = 3 6 spirals Material: 16MnCr5 Cutting mat.: ASP30

vc = 75 m/min with cutting fluid (oil) dry machiningvc = 60 m/min with cutting fluid (oil) dry machining

vc = 75 m/min

vc = 60 m/min

number of tooth gaps made

crat

er d

epth

0

20

40

60

µm

100

60 120 180 240 300 360 420

Cutting param.: z1 = z2 = 42; m = 3; a = 20°;vc = 60 m/min; roll feed = 3DH/div

Gear width: 20 mm F/E = 3 6 spirals Material: 16MnCr5 Cutting mat.: ASP30

vc = 75 m/min with cutting fluid (oil) dry machiningvc = 60 m/min with cutting fluid (oil) dry machining

vc = 75 m/min

vc = 60 m/min

Figure 36: Influence of cutting fluid and cutting speed

on crater depth [118].

Recent developments in coating technology have produced major benefits in cutter wear resistance. Innovative layer systems (commercially standard hard layers (CrN, (Ti,Al)N, Ti(C,N) gradient) and new types of coatings (Ti(C,N)Multilayer, MoS2 and TiN+MoS2 coatings)) have been examined to determine suitability for the specific requirements of the gear-shaping process. The aim of the experimental study was to achieve a major increase in cutter life. For identical cutting conditions, the different standard coatings (Ti(C,N) gradient, (Ti,Al)N, TiN, CrN) result in only minor differences in tool life. There is only a slight difference in crater depths between these coating materials for a tool life of 450 tooth gaps cut. The positive wear characteristics of the “combined layer” TiN+MoS2 are comparable to those of Ti(C,N) multilayer-coated tools. The improvement in coating methods means that dry machining results in doubling the tool life, compared to using standard layers [118].

4.2 Geometrically Undefined Cutting Edge

Grinding Several investigations reveal that even grinding processes have the potential for significant reductions in the consumption of conventional cooling lubricants [119, 120, 121, 122, 123, 124]. A clear example is the surface grinding of hardened steel with MQL. In the investigation hardened steel 16MnCr5 (SAE 5115) was used, which is often applied for high loaded parts, like gears or shafts. For pendulum grinding Figure 37 shows the resulting normal forces as a function of the depth of cut and specific material removal rate for different cooling conditions. To obtain reliable information from the grinding process, each experiment was repeated eight times and the results presented are an average of these tests. The results show that the normal force increases degressively as a function of the specific removal rates (higher depth of cuts) for the flood coolant supply. Maximum normal forces of more than Fn = 900 N are observed at a specific material removal rate of Q’w = 14 mm³/(mm s), which corresponds to a depth of cut of ae = 140 µm. The resin-bonded corundum wheel is characterized by a dense structure and high bonding hardness that results in a fast loading of the wheel topography by metallic chips. This effect leads to thermal damage of the workpiece, such as

cracks or white and dark etched areas, according to short-time metallurgical impact on the surface layer. As is evident, when applying MQL under the same grinding conditions thermal damage occurs at lower specific removal rates for Q’w < 1 mm³/(mm s). In comparison to the flood coolant supply, the normal force increases more quickly when using MQL.

010

0 1 2 3 4 5 6 7 8 9 10 11 12 15mm3

mm s

2030

4050

6070

8090 130

µm150110

100 120

depth of cut ae

specific material removal rate Q‘w

0100200300400500600700800

N1000

norm

afo

rce

F n

flood coolant supply(emulsion 3%)

minimum quantity lubrication (ester)

thermal damages

thermaldamages

Material: 16MnCr5 (SAE5115)Hardness: 58 HRCCut. speed: vc = 30 m/sFeed speed: vf = 6 m/minGrind. wheel: A80 Q4 B vft

vc

Pendulumgrinding:

010

0 1 2 3 4 5 6 7 8 9 10 11 12 15mm3

mm s0 1 2 3 4 5 6 7 8 9 10 11 12 15mm3

mm smm3

mm s

2030

4050

6070

8090 130

µm150110

100 120

depth of cut ae

specific material removal rate Q‘w

0100200300400500600700800

N1000

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afo

rce

F n

flood coolant supply(emulsion 3%)

minimum quantity lubrication (ester)

thermal damages

thermaldamages


vc

Pendulumgrinding:


vc

Pendulumgrinding:

Figure 37: Influence of coolant supply on normal force for different depths of cut and material removal rates [125].

Further investigation of dry grinding of the case-hardened steel for the same experimental grinding conditions showed thermal damage as well as higher grinding forces; the workpiece roughness surrendered at even lower specific material removal rates of Q’w < 0.5 mm³/(mm·s). The reason for this process behavior becomes evident through scanning-electron-microscopy investigation of the chips. In wet machining, relatively long chips with different width and thickness appeared, suggesting good chip removal and sufficient porosity in front of the acting grains. With MQL and dry grinding, the chip shapes changed significantly and were characterized by great quantities of fragmented chips, indicative of a deterioration of the chip removal because of wheel loading [126]. In Figure 38, the normal force in relation to the work speed is shown. With the flood coolant emulsions applied to the contact zone, the normal force remains nearly constant and no thermal damage is observed. In comparison, the use of MQL leads to a continuous increase in the normal forces with a great deviation in the results, perhaps due to reduced process stability with this coolant supply. At the same time, thermal damage occurs at a low removal rate of Q’w < 0.5 mm³/(mm·s). Comparing the results it becomes evident that the normal force is more sensitive to the depth of cut than it is to the work speed, and, therefore, a reduction in the depth of cut is a more effective strategy for achieving a damage free workpiece surface than a decrease in the work speed. In both cases, the specific material removal rate, the equivalent chip thickness, as well as the forces, are reduced. A lower work speed also causes an increase in the time affecting heat generation, whereby any reduced level of heat generation is partially offset. Altogether in pendulum grinding with MQL a maximum removal rate of Q’w = 0.5 mm³/(mm·s) or Q’w = 0.25 mm³/(mm·s) is achievable without thermal damage to the workpiece surface layer [125].

MQL withester

flooding withemulsion 3%

specific material removal rate Qw

0 0.25 0.50 0.75 1.25

0 1.5 3.0 4.5 7.50

50

100

150

N

250thermal damages

m/minwork speed vft

mm3/(mm s)

norm

al fo

rce

F n

Material: 16MnCr5 (SAE5115)Hardness: 58 HRCCut. speed: vc = 30 m/s Grind. wheel: A80 Q4 BDepth of cut: ae = 10 µm vft

vc

Pendulumgrinding:

MQL withester

flooding withemulsion 3%

specific material removal rate Qw

0 0.25 0.50 0.75 1.25

0 1.5 3.0 4.5 7.50

50

100

150

N

250thermal damages

m/minwork speed vft

mm3/(mm s)

norm

al fo

rce

F n


vc

Pendulumgrinding:


vc

Pendulumgrinding:

Figure 38: Influence of coolant on normal force depending

on work speed and material removal rate [126].

5 MACHINE TOOLS AND PERIPHERAL EQUIPMENT Dry machining not only requires a technological adjustment of the machining process but also new standards for machine tools and equipment that supports the machine tool [146]. The omission of the cooling lubricant functions, like cooling, temperature control, chip flushing, transport of chips, cleaning, and conserving, requires alternative solutions, which can only be achieved through appropriate design of the machine tools. The adaptation of machine tools to meet the demands of dry machining must be looked at separately for old and new systems. On existing equipment, constructional changes usually require much effort and high expenditures. If the costs for rebuilding are not economically feasible, frequently the application of an MQL-supply system and a housing for the working area represent reasonable and necessary modifications. With the development of new machine tools, the possibility of making substantial design changes is greater, since the requirements for dry machining or MQL can be considered from the beginning. However, the boundary conditions of the manufacturing structures existing in practice have too many differences to find a single comprehensive solution. Therefore, it is necessary to consider transfer line, machining centers, and special purpose machines separately (Figure 39).

Requirements of Dry MachiningRequirements of Dry Machining

Large Scale Production

System

Large Scale Production

System

FlexibleProduction

System

FlexibleProduction

System

Special Purpose Machine

Special Purpose Machine

Modification of Existing Machines

Modification of Existing Machines

Realized ConceptsRealized Concepts

Figure 39: Requirements of dry machining in different manufacturing structures [14].

The demands placed on machine tools suited for dry machining in mass production and flexible manufacturing systems can be separated into different categories. One of the most important issues is chip removal from the working area to avoid chip clusters and to minimize the heat buildup in machine tool components. In addition, temperature compensation, MQL integration, as well as safety measures, are important aspects that have to be considered [127, 128].

5.1 Machine tool design With respect to chip removal, it is important that hot chips do not transfer kinetic and thermal energy to workpieces, fixtures, and machine tool components. A basic requirement is to have a steep design of the working area and an effective chip clearance from the working area. To ensure high productivity manual chip removal has to be avoided. An adapted working zone should exhibit cover plates, sloping surfaces, and steeply positioning sheet metals. In addition, cover plates with a smooth and wear resistant surface and hanging or tilted workpieces discourage chip buildup [130, 131, 132]. The chip handling and the chip transportation are facilitated by a totally enclosed working area and an automated chip conveyor (Figure 40, Figure 41) [133].

Chip Fallby Gravity

VerticalInversion Jig

Hanging Workpiece

Chip ConveyorSteeply Standing Sheet Metals Cover Plates

Closed Working Area

Figure 40: Design for improved chip removal [129].

(a) (b)© Grob© Krauseco/Mauser

(a) (b)© Grob© Krauseco/Mauser

Figure 41: Workpiece/spindle arrangement with different

machine concepts: (a) hanging (b) lying.

If vertical surfaces cannot be realized in the machine tool, automated cleaning and extraction systems help to avoid the accumulation of chips. Meanwhile, many different exhaust systems exist to remove the chips as fast as possible. The changes needed to accommodate dry machining processes do not only affect the interior design of machine tools. Other components of a manufacturing system have to be modified and the peripheral equipment has to be adapted to the specific needs. Figure 42 shows some exemplary design elements to avoid chip accumulations and heat flow into the machine tool components.

Horizontal Z-Axis

Pallet Changer

Sheet Metals

Tool Magazine outsideof Cutting Space

Guide WayBlanking Plates

Suspending Workpiece

Mineral Cast Machine Bed

External LoadingStation

Figure 42: Machine tool components for dry machining.

Besides a housing for the working area and extraction of chips, dust, and particles, the handling of emissions has to be considered. Filters and separation systems are needed for separating and cleaning reasons. Ideally, these components are operated with fully regenerative materials. Considering the health protection of the workforce, housings should limit the noise load to 75 railways (A) [134, 107]. The selection of appropriate construction materials for high-performance machine tools is of particular importance. There is increasing use of reaction resin concrete as an alternative construction material. To account for higher thermal loads and high power densities of driving motors and spindles, composite beds are frequently employed for machine tools for finish and hard-turning operations and dry machining applications [135, 136]. Several commercial machine tools have been developed that are adapted to the needs of dry machining. Typical characteristics are an automatic handling of the workpiece and the integration of internal MQL-supply systems (Figure 43).

(a) (b) Figure 43: Common dry-machining centers.

5.2 MQL system integration With respect to an MQL supply, the integration and the control of MQL systems is a basic requirement for machine tools suited for dry machining. Dosing quantities of air and fluid, as well as tool related data, have to be stored in the numerical control unit. Because of frequent tool changes in an automated production system it is necessary to ensure short response times of the MQL-units [137, 138]. Lubricant drips and releasing air pressure during a tool change have to be avoided. Short response times, switching on impulse for the metering units, and the withdrawal of the aerosol from the cooling ducts are essential [14, 127, 128]. Generally, the external MQL supply can be regarded as entirely dependent on the metering unit. Limitations of an MQL supply are generally associated with poor accessibility to the cutting zone, which is, in turn, dependent on the positioning of the nozzles relative to the

cutting tool. For the majority of automated cutting applications an internal MQL-supply through the spindle is favored. Due to the many interfaces in the chain between the MQL-supply system and the outlets of the tool, an adaptation of the whole system is necessary [134]. One of the main obstacles is the centrifugal force of the spindle rotation on the oil-air-mixture. The aerosol may separate and accumulate inside the spindle. Uneven cross sections and high revolutions impair an effective transport of the mixture [20]. In addition, the cross sections of the cooling channels affect the flow velocities [16]. For the MQL systems the exact and efficient lubrication of the cutting tool is crucial. Systems are needed that can provide properly dispersed, fine droplets to the tool outlets. With single-channel-supply systems the aerosol has to cover a long route, and the aerosol components may separate or the droplets coagulate. A two-channel- supply system allows a variable adjustment of the droplet size, a secure transport of the mixture, and short response times. [139]. An MQL system usually has to supply many different tools, which require different quantities of oil and air. Based on this fact, a further requirement is the complete integration of the MQL system into the numerical control of the machine tool [130, 133].

5.3 Chip Removal In turning processes the chips usually fall into the chip conveyor because of the accessible cutting zone. In contrast to this, milling applications can lead to severe chip or particle contamination of the working areas. Because of the interrupted cut, small chips are generated and dispensed by the revolving tool. As a result, chips can easily deposit on pallets, jigs, or work pieces. In these cases, exhausting systems can be applied, which remove the chips close to the cutting zone [14].

Table Extraction

System

Ring Nozzleswith Brushes

NozzleFlange

Flexible Nozzle Tubes

Effectiveness

Construction Unit Size andPositioning Range

Table Extraction

System

Ring Nozzleswith Brushes

NozzleFlange

Flexible Nozzle Tubes

Effectiveness

Construction Unit Size andPositioning Range

Figure 44: Examples of exhauster options for milling [14].

Several investigations have focused on the collection of chips in drilling applications. The proposed concepts basically differ in terms of the interface between tool and workpiece and the strategy of chip removal. Some exhaust hoods eject the chips in a specific direction in the working area, whereas other concepts exhaust the chips, via hoses, directly out of the machine tool. However, all suction hood constructions have to face the basic problem that the chips must be seized and exhausted by the collection element, whereby the process should not be interfered with. This requires a very careful construction and realization of the collection element as it relates to air

and chip guidance, as well as an optimized tool for chip collection. The relevant criteria for the definition of the constructional characteristics of exhaust hoods are based on necessary technical requirements. The main demands are the interface between the machine and the tool, a small construction volume, compatibility with the tool changer, a building brick system (interfaces), and suction power suitable for the machining process. Figure 45 illustrates different present-day exhausting-system solutions [134].

(a) (b) (c)

Figure 45: Common exhauster systems: (a) drilling, (b,c) milling [129].

Recently, a system was developed which collects and exhausts the chips with a spring band spiral. The system not only seizes the chips reliably, but also extracts the MQL aerosol. Figure 46 shows an MQL application with extreme mist formation. Applying identical machining parameters, the spring band spiral leads to a clean machining environment, in which chips and aerosols are completely removed, without contacting the machine tool components [147].

(a) (b)(a) (b) Figure 46: Removal of chips by a spring band spiral

system (a) not operating (b) operating.

In order to implement automated operations on machining centers, the tool and the exhaust hood are integrated together into the spindle. The exhaust hood element has to fulfill different requirements than the tool to ensure an effective and economical operation. The hood element must be held by a torque support at the spindle since it must not rotate during the machining process. Furthermore, the hood must be coupled with the tool holder by an anti-twist device, whereas the hood element must be completely decoupled from the tool holder during machining. To use the spring band spiral for machining centers a modular construction from several standard engineering components was built. A fast and simple change of the system as well as a reliable operation is important. Figure 47 shows the inserted exhaust system in a machining center with a telescoping spindle. During the change procedure in the tool magazine the exhaust hood is held by three ball thrust pieces implemented in a ring on the tool holder. With a change to the machine spindle the balls are released from the tool holder and the torque support takes over the adjustment of the exhaust hood. When the exhaust system is removed from the spindle, the hood is again released by the torque support and now the shaped ball thrust pieces take over the position fixing of the hood on the tool holder. During the machining

process there is no contact between the rotating tool holder and the non-rotating exhaust hood [147].

exhausthood spindlespring band

spiraltorque

supportexhaust

hood spindlespring bandspiral

torquesupport

Figure 47: Built-in system on machining center.

Current investigations of chip collection systems focus on the development of reliable and easy-to-maintain extraction systems. These investigations have emphasized that one must take into account technical requirements, such as identifying the causes of possible failures and improving chip handling and the reliability of the operation, in order to achieve an industry-friendly design [107]. Compared to aluminum or gray cast iron, many risks are associated with dry, or even MQL, cutting of magnesium. If process temperatures higher than 450°C occur, the chips may ignite. A fire of magnesium chips or dust may reach approx. 3000 °C. Moreover, the migration of chips or dust away from the machine tool or chip collection area could have a fuse effect and spread the fire to other locations. Furthermore, magnesium particles smaller than 500 µm present an imminent explosion danger [140, 141, 142] An important task in dry magnesium machining is to reduce the quantity of chips that reside within the workspace of the machine tool [144, 145]. Related to this measure, the generated chips must be removed from the machine tool as fast and reliably as possible. Achieving this goal reduces the heat inside the workspace, and increases the accuracy of the workpiece. The risk of machine damage will also be reduced, since the quantity of magnesium chips in the workspace will be small and a potential fire would have fewer consequences [143]. One chip removal concept is based on a cyclone separator, which extracts chips and dust from the machine tool. A briquetting press directly connected to the chip removal system compacts the extracted material as soon as a sufficient amount of chips has been collected (Figure 48).

loaded air inlet

Machine tool workspace

chipcollector briquetting press

funnels

spindle pallet

briquette outlet

cyclone separatorcleaned air outletloaded air inlet

Machine tool workspace

chipcollector briquetting press

funnels

spindle pallet

briquette outlet

cyclone separatorcleaned air outlet

Figure 48: Design of the chip removal system.

To reduce adherence of chips to the system components, walls and funnels are made of plain stainless steel. With these measures, the amount of heat stored in the chips that is transferred to the machine tool is reduced. In addition, to assist with chip collection, the workspace should be shaped with steep walls and the shape should follow the envelope of the machine motions [128]. Measurements of the dust emission were performed at the outlet pipe by a certified association during milling and drilling operations of magnesium alloy AZ91HP. The cutting parameters were chosen to produce larger levels of dust than under typical machining conditions; the average mass concentration of dust per cubic meter of air measured was 1.5 mg/m3, with a maximum of 3.1 mg/m3. Both of these values were significantly under the maximum permissible value for the locally applicable occupational safety standard (10 mg/m3). Figure 49 gives an overview of the assembled machine prototype.

cycloneseparator

briquettingpressFunnels

cleaned air outlet

loadedair inlet

cycloneseparator

briquettingpressFunnels

cleaned air outlet

loadedair inlet

Figure 49: Overview of the machine prototype.

The briquettes produced using the process above behave like massive magnesium parts, which means that the chips are not flammable in their pressed form. Hence, the chips that leave the system represent little, if any, fire hazard. Due to the continuous operation of the suction system no accumulation of chips takes place in the machine tool or the production environment. In addition, the cyclone separator avoids the problem of fine and explosive dust since no filter mat is needed for separation. The briquettes also require much less storage volume than the corresponding uncompacted chips because of very high compression factors up to 11. Numerous machining tests were performed with the machine tool prototype developed. During the tests the safety of the system for machining magnesium under dry cutting conditions was verified.

5.4 Temperature control As mentioned previously, the removal of hot chips, which contain about 90% of the heat generated in a cutting process, is a key issue with respect to avoiding a significant heating of workpieces and machine tool components. However, if the heat transfer cannot be avoided, it is important to know the amount of heat generated during the machining process. Significant thermal expansions of the workpiece and the tools have to be detected, e.g., integrated laser measuring systems, and compensations immediately implemented through the numerical control system. However, this issue and associated system changes are not just related to dry machining; they apply equally well to virtually all machining systems [16, 146]. Other measures that serve to keep thermal deviations of the workpieces on a small scale are thermal stabilization before and after the machining process, and an adapted machining strategy. A modified strategy or process plan that is focused on avoiding carrying over thermal

distortion from one process to another would make possible machining operations with high cutting volumes, long contact times, or high levels of friction rates at the end of the machining process plan. If possible, high precision operations should to be placed at the beginning (Figure 50) [130, 133].

• removal of chipsinside working area

• temperature compensation

• work piece tempering• corrosion protection

of work pieces • temperature resistant

machine bed material• machining strategy• …

Handling of Work Piece:

Figure 50: Requirements on workpiece handling [16].

5.5 Fire and explosion protection The omission of coolants in dry machining may require specific safety precautions. During the machining of light metal alloys with MQL, an explosive mixture of dust and fine particles may be produced. Because of the high process temperatures an ignition source may be present, especially under aggressive processing conditions. Although an ignition of the airborne aerosol mixture is unlikely, measures should be taken to protect machine tools and other capital equipment, but more importantly the workers' welfare [148, 149]. State-of-the-art technology for the machining of magnesium alloys in a conventional wet machining process employs a fire and explosion protection system. Basically, these system components can be transferred to MQL applications. Due to the danger of explosion, the enclosed workspace has to withstand the pressure that would be created from an explosion. To prevent the catastrophic bursting of the machine tool, pressure relief flaps are integrated into the working area. Active protection measures can be realized by suitable fire-extinguishing systems. Optical and thermal sensors monitor the machining process and automatically trigger the extinguishing system. At the same time, a machine alarm announces the fire to the service personnel [150]. In addition to the protection of the machine tool itself, peripheral components, like extraction systems and chip containers, must also be considered, since chips and dust are not bound by a medium. In magnesium machining, for example, dry chips should be stored in closed chip containers to prevent ignition. Or, as has been described previously, a briquetting process may be used to mitigate the risk associated with chip fire and explosion. 6 FUTURE TRENDS AND DEVELOPMENTS Cutting Tools and Coatings Modern machining technologies like dry machining and MQL, as well as HSC and HPC, have driven enormous cutting-tool developments over the past decade. Wear and heat resistant cutting materials and coatings were, and still are, the key to high-performance machining. It can be expected that the trend to apply fine to ultrafine grained cemented carbides will continue. Currently, improvement appears to be limited by sintering process technology. Nevertheless, investigations on a laboratory scale are focusing on the development of cutting materials

with nano-structures, whose properties will exceed those of currently available cemented carbides [54]. Powder metallurgically produced cutting materials, like cemented carbides, cermets, CBN and PCD, gain their properties from the sintering process. Since machining performance depends on these properties, optimization of cutting-material manufacturing represents an extremely important area for investigation. As an example, the production of gradient or textured structures have to be mentioned; these structures allow for tool design adapted to specific loads [151, 152]. Further developments are to be expected in terms of tool coatings. Besides the optimization of nanolayer coatings and supernitrides, CVD-diamond and CBN coatings will gain in importance. However, many cutting-tool manufacturers are turning away from thick and complex multilayer coatings and are instead focusing on thinner and more powerful films [151]. The varying boundary conditions of machining processes and the application of new and difficult-to-cut workpiece materials often require special tooling solutions. Due to the challenging tasks, a trend to tailor-made cutting tools is evident and will continue. This is especially true in mass production, where often less than 10 % of the production are standard cutting tools. MQL-Supply With respect to MQL supply several problems still have to be studied. Certainly, MQL technology offers tremendous potential in terms of improving overall process performance. Up to now, the requirements for MQL fluid media, providing excellent lubrication, continuous spraying, and a cleaning of the machine tool, have not been fulfilled. Furthermore, the MQL-supply systems have to be further developed to minimize the necessary lubricant amounts. This requirement is directly related to the need to optimize and standardize the interfaces spindle - tool holder - cutting tool. An other important development focuses on the monitoring of the MQL aerosol. Investigations of the aerosol flow have shown that, with increasing rotational speed, the direction of the flow is influenced and a vortex forms. Based on these results, the tool design and, especially, the position of the outlets of the coolant channels have to be modified. Measuring and understanding the two-component aerosol flow of MQL processes is necessary for further and broader applications of this technique. Reliable and steady lubrication under any process condition has to be achieved to ensure a successful dry-machining process. The selection of applicable generator types, the design of the coolant channels, and the process control require knowledge of the temporal and quantitative behavior of the aerosol at the rotating tool tip (Figure 51).

InternalAerosolSupply

Guide Pad

Outlet

Mono BlocReaming Tool

514 nm350 mW

CC

D v

iew

AerosolOil Drops

Coolant Outlet

Cutting Edge

InternalAerosolSupply

Guide Pad

Outlet

Mono BlocReaming Tool

514 nm350 mW

CC

D v

iew

AerosolOil Drops

Coolant Outlet

Cutting Edge Figure 51: Light sheet for aerosol analysis at tool tip [153].

The results of laser light sheet visualization and corresponding tribometer experiments help to understand and control the effects often experienced when using MQL systems. In addition to providing important insights into the process design, the results of the investigations will be useful in the development of an aerosol sensor to be integrated into the machine tool [153].

Machine Tool Design Considering the developments over the past years, dry machining and MQL are technologies that have to be considered in the design of new machine-tool systems. While large-scale production systems are focused on single workpieces, materials and machining operations, small and medium scale industries are especially challenged with a huge variety of machining tasks. Thus, difficult-to-cut materials and high-precision machining operations may still require a conventional coolant supply. However, further advancements in the field of cutting tools will help to increase the share of machine tools specifically suited for dry machining (Figure 52). Nowadays, machine tools that meet the requirements of dry machining are commercially available. Although these machine tool concepts provide for good chip clearance, the cleanliness of the working area is still a major issue in dry machining and MQL applications. Thus, developments have to focus on improved chip and dust extraction, as well as on adapted cleaning techniques.

Figure 52: Existing and future design of machine tools.

The possible generation of high process temperatures demands a temperature monitoring of the machine-tool components and the workpiece. Thermal deformations have to be detected, calculated and compensated for via the numerical control system. Increasing the productivity of manufacturing processes may cause conventional machine tools to reach their physical limits. Modern machine-tool concepts focus on spatial multi-axis kinematics or combined constructions of rod kinematics and conventional Cartesian concepts. Due to improved dynamic behavior and higher velocities these kinds of machine tools are well suited for dry machining applications [154, 155, 156, 157]. 7 SUMMARY The reduction of cooling lubricants in the modern cutting technologies of dry machining and MQL has led to significant advancements in machining technology. Today, many machining processes and workpiece materials are produced by applying modern cutting tools and coatings, adapted tool designs and machining

strategies, as well as optimized machine tools. These high-performance system components ensure economic and highly productive processes, slightly reducing the production times of wet machining processes and improving the workpiece quality significantly. Dry machining operations, mainly applied in high-volume, large-scale industries, like automotive manufacturing, still require special solutions. However, it is envisioned that the increasing number of industrial applications and the ongoing research activities in the field of dry machining and MQL will support and ultimately result in the expansion of these modern high-performance technologies to small and medium-sized manufacturers. 8 ACKNOWLEDGMENTS The authors would like to thank the following persons who have contributed to this paper (in alphabetical order and with CIRP members denoted by *): Dr. A. Aranzabe, Prof. E. Brinksmeier*, Prof. K.-D. Bouzakis*, Dr. R. Bueno*, Prof. H. Chandrasekaran*, Prof. J. Fleischer, Prof. U. Heisel*, Prof. F. Klocke*, Dr. J. Leopold, Prof. S. Malkin, Prof. T. Matsuo*, Prof. R. Neugebauer*, Prof. J. Schmidt, Prof. E. Uhlmann*, Prof. V. C. Venkatesch*, Prof. M. Weck*. 9 REFERENCES [1] Schirsch, R., Thamke, D., Zielasko, W., 1998,

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