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Oct 30, 2014

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Dry Machining and Minimum Quantity LubricationK. Weinert1 (1), I. Inasaki2 (1), J. W. Sutherland3 (2), T. Wakabayashi4 1 Dept. of Machining Technology, University of Dortmund, Germany 2 Faculty of Science and Technology, Keio University, Yokohama, Japan 3 Dept. of Mechanical Engineering-Engineering Mechanics, Michigan Technological University, Houghton, Michigan, USA 4 Faculty 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].laws and regulationsenvironment leakage flow

DRY MACHINING DRY MACHINING

image gain

decreased coolant cost

job satisfaction

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].

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

Tools - internal feed - external feed ...

Fluids - fatty alcohol - synthetic esters ...

Economic Dry Machining with MQCL

Settings - oil flow - air flow ...

Machine Tool

- MQCL supply - upgradability ... Figure 2: Minimum quantity cooling lubrication system (MQCL) [19].

Tool temperature max

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 Quantity Cooling (MQC) Emulsion (Water+Oil) Water cp,water = 4.18 kJ/kgK Air cp,air = 1.04 kJ/kgK Medium Emulsion Oil Air pressure Cooling excellent good little Oil cp,oil = 1.92 kJ/kgK Minimal Quantity Lubrication (MQL)

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 Tool: HC-P + TiN Feed rate: f = 0.20 mm 400 C 300 250 200 150 0 0 40

Material : Ck45 (AISI 1045) Diameter: D = 11.8 mm

dry MQL

twist drill

coolant

60 m/min Cutting speed vc

thermoelectric couples close to major cutting 100 edge

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

Lubrication Chip removal good excellent no excellent good little

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

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, rea