1

ABSTRACT

In machining of industrial components, surface quality is one of the most

specified customer requirements. Major indication of surface quality on machined parts is

surface roughness. Dry machining is making inroads onto many shop floors, as it

eliminates or greatly reduces coolant use, often cutting costs and providing a healthier

working environment.

Finish hard turning using Cubic Boron Nitride (CBN) tools allows manufacturers

to simplify their processes and still achieve the desired surface roughness. There are

various machining parameters which influence the surface roughness, but the effect of

machining process parameters on surface roughness during hard turning have not been

adequately quantified.

In order for manufacturers to maximize their gains from utilizing finish hard

turning, accurate predictive models for surface roughness and tool wear must be

developed.

During the hard turning powerful interactions among process parameters like

surface roughness, speed and depth of cut affect surface roughness of the product. It is

not possible to discover interactions by changing only one factor at a time. Proper design

of experiments (DOE) will reveal interactions that can help to achieve breakthrough

improvements in process efficiency and product quality. Factorial designs would enable

to study the joint effects of the factors on a response.

An L27 orthogonal array is used for experimental layout. The necessary

experimentation is carried out using hard turning concept and the surface roughness for

all experiments was recorded. The results are further analyzed using statistical techniques

to find out the influence of selected process parameters in controlling the desired surface

finish. In the present investigation ‘MINITAB’ statistical program is used for developing

and analyzing the experimental results.

1

CHAPTER 1

1 INTRODUCTION

1.1 HARD TURNING

As the title indicates, this work focuses on machining hardened steels with Cubic

Boron Nitride (CBN) cutting tools. The name itself implies “hard turning” is the

process if turning hard material.

Rotational turning” may sound like a redundancy, because turning already involves

rotation. The work piece spins. However, rotational turning is a process that adds a

brand new rotating element. A special tool pivots to sweep its long cutting edge

across the work piece surface. The result is turning so smooth that it can compete

with grinding and polishing. Rotational turning is a dry process that does not require

the costs associated with coolant. Meanwhile, rotational turning’s cycle times tend to

be twice as fast as those of hard turning.

Hard materials are defined somewhat arbitrarily, but a consistent threshold of 45HRC

(Rockwell C scale hardness) seems to exist. Typical materials for which hard turning

may be a potential machining process include heat treatment and case hardened

steels. These steels constitute an important class of engineering materials due to

improve strength and wear resistance compared with other materials. Due to stringent

dimensional and surface requirements, these materials have traditionally been

machined to finished geometries by abrasive processes such as grinding. However

2

recent improvements in machine tool technology (specifically the rigidity and

precision of modern CNC lathes and the advent of ceramic cutting tools have made it

possible to remove material from hardened steel components by turning and milling).

Turning is a type of machining process which is shown in the figure 1.1 on a manual

lathe and again from a side view in figure 1.2.In this process, material is removed by

sliding a hard cutting tool through a softer material and forming a chip. The main

process variables in this case are the cutting speed, the feed rate and the depth of cut.

Cutting speed is either given as the rotational velocity of the work piece or the linear

tangential velocity of the work piece at the tip of the cutting tool, feed rate is defined

as the linear distance that tool traverses during one rotation of the work piece and

cutting depth is the radial engagement between the cutting tool and the work piece.

Hard turning differs from conventional turning of softer materials in several key

ways. Because the material is harder, specific cutting forces are larger than in

conventional turning and thus the engagement between the cutting tools and the

workpiece must be limited. At the small cutting depths required, cutting takes

place on the nose radius of cutting tools, and the tools are prepared with chamfered or

honed edges to provide the stronger edge geometry that is prone to premature

fracture. Cutting on a chamfered or honed edge.

3

(fig 1.1) Example of turning process(from tvent 1977)

equates to a large negative effective rake angle, when neutral or positive rake angles

are typical in conventional machining. The large negative rake angles yield increased

cutting forces compared to machining with positive rake angles, and also induce

larger compressive loads on the machining surface. Higher temperatures are also

generated in the cutting zone, and because cutting is typically done without coolant,

hard turned surfaces can exhibit thermal damage in the form of micro structural

changes and tensile residual stresses.

4

(fig 1.2) Side view of the turning process

Cryogenic Hard Turning:

Cubic boron nitride (CBN) and polycrystalline cubic boron nitride (PCBN) inserts

have traditionally been the tools of choice for hard turning applications. However,

many shops contemplating hard turning are turned off by their high price. A new

extreme-temperature coolant method has been developed to offer longer insert life,

faster cutting rates and more affordable hard turning insert options.

The liquid nitrogen raises insert hardness, which significantly reduces the thermal

softening effect that an insert may experience as a result of hard turning’s inherent

high cutting temperatures. The steep temperature gradient between the chip/tool

interface and insert body also helps remove heat from the cutting zone. In addition,

the significant cooling maintains insert edge integrity to prevent “smearing” a part’s

hot, compressed surface layer, thus providing a quality surface finish.

5

Unlike CBN and PCBN, ceramic inserts tend to wear

unevenly and are prone to fracturing when hard turning

dry or with water- or oil-based coolants. Increased

fracture toughness resulting from low-temperature liquid

nitrogen cooling provides more predictable, gradual flank

wear for ceramic inserts, as well as increased cutting

speeds up to 200 percent.

(fig 1.3)

Hard turning has many potential advantages compared to grinding, and several are a

direct result of the way in which material is removed by the two processes. A

significant advantage of the turning process is that cutting is done with tools that have

a geometrically defined cutting edge. This allows many different parts to be machined

with same cutting tool by changing the relative path between the tool and the work

piece. On modern CNC lathes, many tools are mounted in the turret of the machine,

and the computer-controlled machines can produce a variety of part geometries with

only a handful of different tools. In grinding, where the individual grit geometries are

random, the overall shape of the grinding wheel must be modified to produce

different parts. This is typically addressed either by stocking a different wheel for

Liquid nitrogen insert

cooling extends insert

life and allows greater

use of low-cost ceramic

inserts for hard turning

operations.

6

each part, or by reshaping the wheel into a new in to a mew geometry each time a

new part is required. Both methods are time consuming, either due to changeover

time or dressing time (the process of reshaping the wheel. The small sizes of typical

grits also reduce the potential engagement between the grits and work piece thus

limiting the size of the chips that can be removed. This generally leads to small

material removal rates compared to turning, although some grinding operations are

done with wide grinding wheels to improve the volumetric removal rate.

By eliminating the need to change or redress grinding wheels, hard turning offers

increased flexibility by significantly reducing the setup time. For a typical setup, all

that is required is changing of the collet or chuck in the machine and loading a

different computer program in the control. The combination of increased flexibility

and improved material removal rates is becoming more important as manufacturers

tend toward production strategies that minimize the inventory and batch sizes.

Furthermore, because the process is more flexible and productive, fewer machine

tools are required. The cost of lathes is substantially lower than grinders (as an

estimate, the cost of a good CNC lathe and associated tooling maybe 1/2 to 1/10 the

cost of modern CNC grinder). Turning is also a more efficient cutting process than

grinding, so less energy is required to remove the same volume of material. This is

due to the small engagements between abrasive grits and the work piece in the

grinding, and the associated plowing that contributes to significant energy losses.

Finally, hard turning has the additional benefit of environmental friendliness because

cutting is done typically dry. This eliminates the cost and the environmental impact

7

of using cutting fluid and disposing of grinding sludge. Given the potential

advantages of hard turning is not being used more extensively.

First, it is a relatively new technology. It is only over the last couple of decades that

improvements in machine tools and development of ceramic inserts have made this

technology a viable alternative. Furthermore, because hard turning is so young

compared to grinding will take some time to overcome. Also, there are applications

where hard turning is not currently good enough to replace certain grinding

operations. A good example of this centre less grinding, which is a process for

grinding a cylindrical work piece where the part is not constrained to a geometric

center of rotation (which typically results when holding in a chuck or collet).The

process allows improved roundness, and existing fixturing methods on lathes will not

allow hard turning to complete with center less grinding when roundness tolerances

are tight (especially for compliant parts). Aside from special cases, the biggest

limitations on further implementation of hard turning are concerns about surface

quality and un acceptable life of expensive cutting tools.

Due to these concerns, there is a need to develop a better understanding of the effects

of process conditions on the wear behavior of cutting tools and the resultant surface

quality that can be obtained by hard turning. The potential advantages of hard turning

are attractive to many manufacturers’ tools to make more educated decisions about

selecting process conditions and enabling further implementation of this technology.

8

A properly dialed-in hard turning process can

deliver surface finish of 0.00011 inch,

roundness of 0.000009 inch and diameter

tolerance of ±0.0002 inch. Such performance can

be achieved on the same machine that "soft"

turns the part prior to hardening, maximizing

equipment utilization.

(fig 1.4)

Part—Though 45-Rc material is hard turning's starting point, hard turning is

regularly performed on parts that are 60 Rc and higher. Commonly hard-turned

materials include tool, bearing and case-hardened steels. From a metallurgical

standpoint, materials with a small hardness deviation (less than two Rc points)

throughout the cutting depth allow the best process predictability.

In some cases, a part's size or geometry simply does not lend itself to hard turning.

Parts that are best suited for hard turning have a small length-to-diameter (L/D) ratio.

In general, an L/D ratio for unsupported work pieces should be no more than 4:1, and

The goal in hard turning is to

deliver at least 80 percent of the

heat out with the chip in order

to maintain part thermal

stability.

9

it should be no more than 8:1 for supported work pieces. Despite tailstock support for

long, thin parts, high cutting pressures would likely induce chatter.

Machine—The degree of machine rigidity dictates the degree of hard turning

accuracy. Most machines made in the last 15 to 20 years have sufficient rigidity to

handle some hard turning applications. In many cases, a machine's overall condition

is more of a factor than its age. Even an old, well-maintained manual lathe can be a

candidate for hard turning. However, as required part tolerances get tighter and

surface finishes get finer, machine rigidity becomes more of an issue.

Maximizing system rigidity means minimizing all overhangs, tool extensions and part

extensions, as well as eliminating shims and spacers. The goal is to keep everything

as close to the turret as possible

Process—Because hard turning delivers the majority of cutting heat out in the chip,

examining the chips during and after the cut will reveal whether or not the process is

well-tuned. During a continuous cut, the chips should be blazing orange and flow off

like a ribbon. If cooled chips essentially disintegrate when crunched by hand, then

that demonstrates that the proper amount of heat is being carried out in the chip.

1.2 WET MACHINING

Metal working fluids in manufacturing processes is viewed as undesirable for both

economic and environmental reasons. Every year manufacturers consume millions of

gallons of metalworking fluids. Metalworking fluids have an considerable affect on

manufacturing costs and environment. Even more important is the fact that OSHA

10

(Occupational safety and health administration) and the EPA (Environmental

protection agency) consider metal working fluids to be detrimental to the

environment. These fluids contaminate the air causing maintenance and the employer

health problems. Also, at the end of fluids useful life it must be disposed properly.

Machining parts with metal working fluids puts an enormous burden on

manufacturing companies and environment Manufacturing companies need to realize

the costs and environmental issues involved

with the use of metal working fluids and move to environmentally and cost conscious

manufacturing practices.

Due to increasingly strict environmental laws aimed at controlling the health hazards

and pollution, the costs of metal working fluids use in manufacturing processes is

rising substantially. Therefore, the elimination of metal working fluids in

manufacturing processes can be significantly economic incentive

11

Wet machining(fig 1.5}

1.3 DRY MACHNING

Dry machining is no longer a utopian dream in the metal working industry.

Manufacturing companies all over the world are currently examining the question

whether metal working fluids are really needed in machining process and if so, to

what extent. While the need for dry machining may be apparent, issues including the

perceived as inability to cut dry and change over cost, resulting dry machining being

perceived as impractical by most manufacturers. How ever, this is not the case, high-

speed dry machining is possible with most manufacturing process.

12

(fig 1.6) Test cut with PCBN cutting tool integrate correctly, manufacturing can realize improve work piece accuracy, reduced

manufacturing cost, and other related benefits associated with high speed dry

machining.

Recent research reveals that trend in manufacturing is to minimize or eliminate the

use of metal working fluids in manufacturing processes. Dry machining has the

potential to reduce environmental pollution, health hazards, and costs associated with

the use of metal working fluids. However, to pursue dry machining, one has to

13

compensate for several beneficial effects of metal working fluids without using them.

Removal of metal working fluids in manufacturing processes can use a variety of

machining problems related to heat, tool life, and chip removal. In dry machining, the

functions of metal working fluids must be assumed by alternative methods. The

challenge of heat dissipation without coolant requires a completely different approach

to manufacturing. Special tooling utilizing high performance coatings, heat resistant

materials and through spindle air are required. By examining the manufacturing

processes capable of dry machining, it becomes apparent that the key is a balance

between advanced metal cutting strategies, special tooling and the machine tool

specifications.

1.4 WET OR DRY MACHINING

For a number of years, manufacturing professionals have explored the potential of dry

machining. Driving the interest in dry machining is often related to the costs and

health issues associated with the use of coolants in most manufacturing operations.

The total cost of coolant is upwards of five times the cost of cutting tools.

There a number of factors that needs to be considered before moving to a dry

machining environment or even MQL machining (Minimum Quantity Lubricant).

What if we look at the use of coolant from the point of view of the tool? Instead of an

all or nothing decision regarding the application of coolant in a shop or on a specific

machine, what happens if we consider eliminating the use of coolant on certain types

of tools in order to improve that tools performance?

14

The key to exploiting the potential of turning off the coolant for tools is based on

recognizing what factor heat is having on the operation. Simply put heat is the friend

of tools, but too much heat is an enemy. Though a combination of energy (generated

primarily from the spindle/cutting speed) and tool geometry, the cutting zone reaches

a temperature where the yield strength of the work piece material is approached and

separation, or a chip, can be efficiently formed. The more heat in the cutting zone, the

easier the chip will separate. Too much heat though and not only does the work piece

want to separate, but the tool as well.

Here are suggestions about when to use coolant and when to cut dry:

Threading: Single point threading tool producing standard thread profiles often run

with significantly longer tool life when they are run without coolant.

Grooving: shallow grooving operations such as snap ring grooves are frequently

improved with out coolant.

Index able milling tools: because of the frequent thermal cycling due to being in and

out of cut nearly all index able milling tools preferred to run without coolant when

possible.

Short contact time finish turning operations : when the tools in contact time is short

and the material is not especially sticky tools will generally run longer when produce

improved surface finishes when they are run dry.

15

1.5 Problems of wet machining:

1) Environmental hazards: Used coolant is from machining processes is always

harmful to both environment and human health. Chemical substances that provide

the lubrication function used in the machining process are toxic to the

environment if the coolant and cutting fluids are released into soil and water.

2) Health hazards: the chemical substances used in coolant cause serious health

problems to workers who are exposed to the coolant in both liquid and mist form.

3) Contamination: Some cutting fluids stain or contaminate the workpiece thus

affecting the surface finish.

4) Increasing costs: The cost of using coolant is increasing as the number and the

extensiveness of environmental laws and regulations are increasing.

5) Management costs: The maintenance and the management costs are also

increasing because of chemical disintegration of some coolants.

6) Disposal: This is another main problem concerned with the coolants. Disposing

into environment leads to many hazards as discussed above.

7) Treatment costs: The used coolant should be treated and then released into the

environment. This treatment process is an additional burden. According to a

survey coolant, coolant management & coolant treatment costs account for 16 to

20% of the manufacturing costs.

1.6 Advantages of dry machining:

1) Increases tool life by eliminating thermal shocks created by flood coolant

16

2) Eliminates coolant purchase and disposal costs.

3) Tool life increases because coated carbide, ceramics, cermets, cubic boron nitride

(CBN), and polycrystalline diamond (PCD) are all brittle, they are susceptible to the

chipping and breaking caused by thermal stresses—especially those found in face-

turning and milling operations—that can be aggravated by the introduction of coolant.

4) For continuous cuts, the high tool tip temperatures that occur in dry turning serve

to anneal (soften) the pre-cut area, which lowers the hardness value and makes the

material easier to shear. This phenomenon is why it is beneficial to increase the

speeds when cutting dry.

5) A chip formed through a properly configured hard turning process takes with it 80

to 90 percent of the heat generated (cutting zone temperatures can reach 1,700°F). If

such a blazing chip would contact straight-oil coolant with a low flash point, the

process could literally go up in flames.

Parameters Conventional

wet

High speed dry Improvement

Surface feet/min 250 5000 2000%

Revolutions/min 160 3200 2000%

Inches/min 32 80 250%

No of inserts 10 5 50%

17

Tool life 1600 6000 375%

(Tab 1.1}Improvement in performance for conventional wet turning to high

speed dry turning operation.

CHAPTER-2

2 LITERATURE SURVEY

2.1 Turning to hard turning:

The vast majority of hard components used in the automotive industry are machine to

final geometrical form after hardening. Currently, grinding is the pre dominant

18

method for finishing these parts, which includes bearings, Gears, shafts, and pinions.

However, thanks to improvements in machine tool rigidity and development of CBN

and PCBN cutting tools, hard turning is gaining ground as a cot effective alternative

to grinding.

Hard turning is performed on the materials in the 45 to 60 HRC range using the

variety of tipped of solid cutting inserts. Since its production in the mid 80’s, the

process has dramatically increased in popularity and the sales of CBN cutting tools

are dealing in hundreds of millions annually. Clearly, more and more manufacturers

are recognizing the advantages of hard turning. But due to the cost of CBN cutting

tools, many continue to view it as an expensive process.

2.1.1 Is hard turning more expensive??????????

While CBN cutting tools ca cost up to 10 to 20 times more than conventional tools,

studies have shown them to be 10 to 300 times more effective in terms of overall

productivity and tool life. In part, these finding are based on the tool cost per parts

analysis. For a better understanding of economic benefits of hard turning, it helps to

consider a few factors that are some times overlooked by the accounting department.

These include tool change time, setup time, cycle time, machine maintenance, part

quality and original machine cost.

19

Part of the cost effectiveness of the hard turning may be attributed to the machine tool

itself. A grinder is a much larger investment than a CNC Lathe, it is typically one half

to one third of a grinders cost. Also, CNC Lathes are much more flexible in terms of

machining capabilities. Tool changes can be made in less than 2 mins , without the

production time losses necessary for a wheel change. This flexibility allows fast, cost

effective production of small batches of parts.

2.1.2 Benefits of hard turning:

1) Low maintenance is also a benefit , as worn CBN tools may be quickly removed

and replaced with new inserts, and do not require truing or dressing to maintain the

cutting profile.

2) CNC lathe also takes less floor space than grinders, do not require flume systems.

3) It does not require coolant.

4) Since hard turning removes metal “peeling” a softened chip from the work piece,

coolant is generally not recommended. This helps to keep costs down while

eliminating the environmental damage caused by coolant use.

5) Dry machining also reduces the time and money spent on government regulated

chip disposal and reclamation process.

6) Although grinding is known to produce good surface finishes at relatively high

feed rates, hard turning using CBN inserts can produce better surface finish and

significantly higher metal removal rates.

7) Although the process consists of small depth of cuts and feed rates, estimates of

reduced machining time are as high as 60% for conventional hard turning.

20

8) Studies have shown that by using the right combination of insert nose radii, feed

rate, or the new wiper technology, hard turning can produce a better surface finish

than grinding.

9) The fact that multiple hard turning operations may be performed in single

chucking rather than multiple grinding setups also contributes to high accuracies.

However, there is still much debate surrounding the overall surface integrity of hard

turned parts.

2.2 Hard turning vs. Grinding:

1) Material removal rates are higher in hard turning than in grinding.

2) The experimental results showed that intermittent hard turning can produce

surface integrity which is good enough for replacing the grinding process.

3) Machining time is reduced in Hard turning compared to Grinding.

4) The fatigue life of hard turned surfaces is better than that of ground surfaces.

5) The turned surface has a longer life than the ground one with equivalent surface

finish.

6) Hard turning generates less heat in the workpiece than grinding, due the CBN’s

ability to put most of the heat into the chips -- not into the workpiece.

7) Traditional grinding, in contrast, creates extreme heat that requires coolant and

may cause surface imperfections.

8) Shorter cutting time, less tool change time make the hard turning process a faster

process than grinding.

21

HARD TURNED SURFACE GROUND SURFACE

(Magnified 2000 times) (Magnified 2000 times)

(Fig 2.1) Surface obtained by hard turning and grinding.

22

2.3 A surface caveat:

Hard turning affects the surface microstructure by generating residual stress patterns

and over hardened surface zones, also known as white layers. These thin, rehardened

layers are typically followed by an over tempered region just below the white layer.

Due to the structural alteration of the material, the rehardened layer appears white in

an optical micrograph, and the tempered region appears dark.

Research has confirmed the existence of white layers on both hard turned and ground

surfaces .Although they are commonly associated with residual tensile stress, white

layers may also indicate residual compressive stresses. Either way, the cause of

white layers and the effects they have on finished workpiece is not completely

understood.

Some studies suggest that the use of cutting coolant helps in eliminating white layers,

while others indicate that it has no effect. There is also some evidence that tool

conditions affect the white layer formation. If a white layer forms during hard

turning, it is typically because a dull insert causes too much heat to be delivered into

the part. It is most commonly formed on bearing steels and is most problematic for

parts like bearing races that receive high contact stresses. Over time, the white layer

can delaminate and lead to bearing failure.

23

Overall, new tools are more likely to produce undamaged surfaces, with the white

layer increasing with tool wear. This may result from the heat generated by friction

between the tool and the workpiece as the flank land increases, or by higher plastic

deformation caused by increased friction.

In recent years, the unknown surface integrity of hard turned parts has also caused

some reluctance to use hard turning as a finishing operation on critical surfaces.

However recent advances in cutting tool technology are eliminating these

perceptions. In particular, CBN inserts have proven to produce as good or better

tolerances than conventional grinding processes and reduce machining time up to

90%.

24

CHAPTER-3

CUTTING TOOL MATERIALS FOR MACHINING PURPOSE

1) CUBIC BORON NITRIDE (CBN)

2) CERAMICS

3) CARBIDES

4) CERMETS

5) POLYCRYSTALLINE CUBIC BORON NITRIDE (PCBN)

3.1 CBN Cutting Tools:

Cubic boron nitride cutting tools (CBN) have CBN, which is a synthesized material

exceeded in hardness only by diamond. However, unlike diamond cutting tools, CBN

cutting tools are stable at temperatures up to 1,800 degrees F for machining hardened

ferrous or super alloy materials of Rc 45 or higher and for machining some cast irons.

Polycrystalline cubic boron nitride cutting tools (PCBN) feature PCBN blanks, which

are manufactured from CBN crystals using a high-temperature, high-pressure process.

CBN cutting tools have crystals, which are either sintered with a binder phase or

25

integrally bonded to a tungsten-carbide substrate. The binder phase, usually a metallic

or a ceramic matrix provides chemical stability that enables the qualities of the PCBN

to be used in high-speed machining operations. By varying the binder phase and the

percentage of CBN crystals, PCBN cutting tools can be made in a wide range of

grades and in a variety of shapes and sizes for turning, boring, facing, forming,

milling, grooving, reaming, parting, and fly cutting applications. Because PCBN

cutting tools maintain their cutting edge, they impart excellent surface finishes to

parts, while maintaining close tolerances and high productivity rates.

Higher CBN content in specific grades of cutting tools provide increased fracture

toughness and resistance to abrasion. Cutting tools that have grades with more than

70 percent CBN content exhibit high thermal conductivity, excellent abrasion

resistance, and exceptional toughness. High-content CBN cutting tools have grades,

which are primarily used for machining case-hardened and through-hardened steels,

super alloys, chilled cast iron, sintered metals, pearlitic gray cast iron, hard coatings,

hard facing materials and work hardening materials.

CBN cutting tools with grades less than 69 percent CBN offer low thermal

conductivity, low diffusion wear and chemical inertness. Cutting tools with these

grades are often used for finish cuts on hardened steels even with interruptions, hard

powdered metals, nitride steels, and hardened cast iron.

CBN cutting tools can run dry for clean machining processes to save coolant,

maintenance, and disposal costs while reducing the potential for environmental and

health impacts.

26

CBN cutting tools are available as solid, full-face, or tipped inserts. Tipped CBN

cutting tools use CBN inserts, which can be economical and reliable for a wide range

of roughing and finishing applications, and some applications require a solid or full-

faced insert.

As discussed earlier CBN cutting tools machine hardened steels with apparent ease

because, using relatively high surface speeds, heat is generated at the point of cutting,

so the CBN cutting tool cuts locally softened material. The heat is carried away by the

swarf, which becomes brittle and harmless and CBN cutting tool, which has high

thermal conductivity. If a light cut is required, however, a tool with high CBN content

conducts too much heat away from the shear zone and the condition for efficient

machining are not achieved.

Low CBN tools can therefore keep sufficient heat at the cutting point to enable the

optimum cutting conditions to be achieved when a light cut is taken. In most cases,

even when very light cuts are required, low CBN tools employ negative rake

geometry to provide a strong edge. Due to the nature of cutting, however, cutting

forces are still very small. Low CBN can be used to provide a productive and cost

effective alternative to grinding. Tolerances achieved are comparable but machining

time can be dramatically reduced.

27

CBN Inserts

Mini Tips Flip Tips Standards

Inexpensive.

In many

shapes and

grades for

shallow cuts

Cost-effective. Two cutting

edges for finish cuts. Flip

Tips is a trademark of J&M

Diamond Tool, Inc.

A wide

assortment of

shapes and

grades

Large Tips  Solid CBN Full Face

For deeper

cuts. -

Available on

most standard

inserts

Cutting edges on both

sides. Indexable.

Cutting edges.

Full top surface

Indexable.

28

(fig 3.1) Figures of different CBN inserts.

Materials recommended for cutting with CBN (Tab 3.1)

Alloy steel (45-68 RC) Ni HardDie steel (45-

68 RC)Rene

Carbon tool steel (45-68 RC)Forged

steelStellite

Ductile

iron

Moly chrome steel rollsNodular

ironColmonoy Incoloy

High speed steel (45-68 RC) Grey iron Waspoloy Monel

Chilled cast ironInconel

600

Meehanite

iron

Grade Application

1000 Most cast iron

2500

Continuos cutting hardened

steel and slight interrupted

cuts.

3000 Hardened steel ( severely

interrupted cut )

29

5000 Nodular iron - Ductile Iron

6000 Super alloy, Ni/Co base alloys

8200Continuos cutting hardened

steel and slight interruptedcuts

30

Recommended Use of CBN

Grades

Material CBNGrades

Alloy

steels ( 45-

68 RC )

2500, 3000,

8200

Carbon

tool steels (

45-68 RC )

2500, 3000,

8200

Die steel

( 45-68 RC

)

2500, 3000,

8200

High speed

steel ( 45-

68 RC )

2500, 3000,

8200

Chilled

cast iron1000

Nodular

cast iron5000

Ni Hard 1000, 6000

Forged

steel2500, 8200

Moly

chrome

steel rolls

6000

Inconel

6006000

Rene 6000

Incoloy 6000

Monel 6000

Recommended Speeds and Feeds

MaterialSpeed

(SFM)

Feed

Rate

(IPR)

Depth

of

Cut

(inche

s)

Carbon

Steel

200 -

500

.008

Max

.020

Max

Bearing

Steel

200 -

500

.008

Max

.020

Max

Alloy Steel200 -

500

.008

Max

.020

Max

Die Steel160 -

350

.008

Max

.020

Max

Tool Steel160 -

350

.008

Max

.020

Max

High

Tensile

Cast Iron

200 -

500

.060

Max

.020

Max

Chilled

Cast

Iron

130 -

260

.032

Max

.020

Max

Grey Cast

Iron

2000

-

4000

.020

Max

.020

Max

Powdered

Metal

500 -

650

.016

Max

.020

Max

Inconel500 -

650

.006

Max

.020

Max

Rene 42500 -

650

.006

Max

.020

Max

31

(Tab 3.2) (Tab 3.3)

3.1.1 Advantages & Cost effectiveness of CBN cutting tool:

“Hard turning” tool steels 45Rc to 60Rc

1) Speeds of 200sfm to 600+sfm (surface feet per minute).

2) Chips load of .002 to .020 IPR (inch per revolution)

3) Depth of cut .002 to .200 inches

4) Good surface finishes.

5) Interrupted cuts –no problem

6) Solid top CBN inserts eliminate the problem of Mimi-Tip melt off.

Chilled Cast Irons & Grey Cast Irons

1) 300% increased productivity Vs. carbide

2) Wear life 5 to 10 times TiN coated carbide

3) 200% wear life in finishing cuts Vs. ceramics

4) Operations include turning, milling and boring.

3.2 CERAMICS IN MACHNING:

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There are many applications for ceramic materials due to hardness and increased wear

resistance resulting from ionic or covalent bonding. A growing area for application of

ceramic materials is in machining, where cutting operations are increasingly replacing

traditional abrasive processes for finishing hard metals, ceramics, and inter metallic

compounds.

Machining processes, which are defined as shaping of a part by the removal of

material, are used to produce metal parts in countless applications. The definition of

machining encompasses three subsets of processes that can be called abrasive

processes, cutting processes, and non traditional machining processes.

Design engineers continually desire new materials with improved strength, hardness,

thermal characteristics, and wear resistance. However, the same properties that make

these materials desirable also make them difficult to machine. Two examples are

hardened steel alloys that provide strength and wear resistance in automotive

applications, and nickel-based super alloys that maintain high temperature strength in

aerospace applications. Because these materials are often used in applications where

some degree of precision is required, at least one of the machining processes are

required after casting or forging to achieve desirable dimensional accuracy and

surface finish. As an example of a typical application with a steel alloy, processing of

a ball bearing for the automotive industry will be discussed. To withstand repetitive

contact stresses while achieving acceptable fatigue lives, bearings are typically made

from steel alloys with significant amounts of carbon to allow hardening by heat

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treatment and chromium to provide corrosion resistance. Typically processing steps

include: casting or forging to obtain a cylindrical shape, rough turning the steel in the

soft state to approximate dimensions, heat treating the part to obtain the desired

properties (hardness and toughness), grinding to finished dimensions, and possibly

super finishing to improve surface quality.

Ceramic cutting tools have made it possible to combine rough turning and grinding

into a single turning process after heat treatment, which offers substantial cost

benefits. There are many types of ceramic tools that have been developed for

machining hard materials, but the tools can be much more expensive than traditional

steel tools, and the life of ceramic tools can be prohibitively short if the wear behavior

is not understood. To provide supporting information relative to this research, the

processing, applications, and wear of several ceramic cutting tool materials will be

discussed.

3.3 CARBIDES

To allow machining at higher cutting speeds (and increased production rates), carbide

tools were developed in 1930s (kalpakjian 1997). These tools now consume an

estimated 70% of the machining market. Because the tools are typically pressed and

sintered from ceramic powders (often with cobalt binder material), they are

sometimes called sintered carbides or cemented carbides. There are two basic subsets

of carbide tools : tungsten carbide (WC) and titanium carbide (TiC), WC tools being

the most prevalent.

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Pure WC is very hard, but also brittle. To improve toughness, WC powder is mixed

with 5-15% cobalt (weight percentage). Hardness and wear resistant can be improved

by reducing the grain size of the WC particles, which are typically in the range of 0.5-

5 micro meter. An optional grain size and cobalt percentage must be determined to

allow the hardness and toughness required for a particular cutting operation.

Even at relatively low cutting speeds around 150 ft/min (45 m/min), WC-Co tools

form significant craters behind the cutting edge because cutting temperatures can

exceed 1000 c, and steel work piece materials can absorb WC in solid solution. To

reduce cratering, 5-25% of titanium carbide (TiC) can be added to WC-Co tools. TiC

has very low solubility in iron, and thus acts as a barrier to cratering by the diffusion

of WC. TiC is also harder than WC, so addition of TiC improves abrasive wear

resistance in addition to improving the chemical stability.

3.4 CERMETS:

Since the 1920s, cermets have been an integral part of the metalworking industry.

However, in the past few years they have enjoyed a surge in popularity, thanks to new

technology, which has expanded possible cermets applications. Traditionally used

just for semi finishing to finishing operations, newer cermets have increased

toughness that makes them comparable to some carbides, while their good shock

resistance promotes good performance for some interrupted cuts. Origins The word

cermet is derived from the terms ceramic and metal. A cermet is a hard material based

on titanium carbide or titanium carbonitride cemented with a metal binder. The first

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series of cermets developed in the late 1920s were based on titanium

carbide/molybdenum carbide with a nickel binder, and were characterized by low

bending strength and high brittleness. However, in the 1960s improvements were

made with the addition of molybdenum to the binder.

This increased cermet toughness. A few years later, the addition of metal

carbonitrides contributed to improved wear resistance, thermal shock resistance and

decreased plastic deformation. In 2003, new micro grain cermet grades were

introduced, including a PVD-coated super micro grain cermet whose coating provides

greater thermal stability and the ability to handle higher cutting speeds. The micro

grain structure of these cermets contributes to a doubling in the bending strength,

while the fracture toughness remains comparable to other cermets. This new

generation of cermets also is capable of handling interrupted cutting operations that

before weren't feasible.

Charecteristics

Characteristics of cermets include high wear resistance, low reactivity with most

work pieces and long tool life. They produce excellent surface finishes, and maintain

tight tolerances over their life span. Higher cutting speeds may be used with cermets,

especially for semi finishing to finishing operations, because of their high wear

resistance.

High Wear Resistance

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Cermets' high wear resistance is contributed in part by their high hot hardness, which

is greater than most cemented carbides. Cermets also feature low reactivity, as they

are more chemically inert than tungsten carbide. The low affinity to the work piece

means that edge buildup and cratering are not significant for cermets, as with carbides

resistance than previous cermets, thereby allowing the use of coolant when necessary

for a wide range of applications. It is not mandatory to use coolant when machining

with cermets; however, coolant is generally applied for three reasons: cooling,

lubrication and chip evacuation. Due to their good wear resistance, welding resistance

and high hot hardness, cermets provide excellent surface finishes even when dry

machining. In addition, numerous different chip breakers promote smooth chip

evacuation without the use of coolant. Chip breakers are the molded or ground

patterns on an insert that break up the chips formed by the machining process into

manageable pieces. Long chips can damage the work piece, or be dangerous to the

operator.

Applications

Traditionally, ideal applications for cermets have included finishing and semi

finishing at higher speeds, lower feeds and cutting depths. Although turning on CNC

lathes is the most common cermet application, Swiss-type machining is also ideal for

cermets, since they hold their size and maintain tight tolerances.

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Grooving and threading applications also are compatible. Cermets also may be

considered for finishing milling operations, and they offer better finishes and

dimensional control than carbides. Significant tool cost reduction may be achieved

when milling with cermets as they can run at higher cutting speeds than carbides, and

their tool life is longer. In some cases, they can eliminate grinding or polishing

operations. In addition, the new generation of micro grain cermets can also handle

interrupted cuts when turning due to their superior toughness. When specifically

pertaining to the mold industry, cermets are used in milling molds and dies for

applications like vehicle transmission pieces, brake drums and rotors, rear differential

housing, body panel molds, etc.

Comparison of cutting material properties (fig 3.2)

Disadvantages of Cermets:

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The cermets known to the state of the art have either various binder contents at the

surface, which can be recognized by their spotty appearance, or have a tendency of

attachment of the binder to the sintered substrate, which leads to changes of the

composition in the contact zone because of the reactions related thereto. Further

disadvantages of the cermets known to the present state of the art are a partially high

surface roughness, as well as poor attachment of the applied wear-resistant layers due

to the increased binder content in the surface. The mentioned disadvantages show

particularly clearly that the cermets can not be used as cutting inserts in machining

processes.

3.5 CUTTING MATERIALS

All cutting operations require tool materials that can with stand the difficult

conditions produced during machining. There are primarily three problems all cutting

tools face wear at cutting edge, heat generated during the cutting process, and thermo

mechanical stock. Characteristics that allow tool materials to stand up to cutting

process include hardness, toughness, wear resistance, and chemical stability. In

general, increased hardness improves wear resistance but is associated with decreased

toughness. Depending on machining conditions and work piece properties, different

degrees of hardness and toughness are required.

3.6 HIGH SPEED STEELS

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High speed steels (HSS) are important to discuss due to their extensive use in metal

cutting and to demonstrate the need for ceramic tools. High speed steels have the

lowest hardness and highest fracture toughness for general use tools. The primary

reason high speed steel tools are not used more extensively is that they soften

significantly at temperatures above 500 c, as shown in figure. This softening behavior

limits high speed steels to relatively low cutting speeds on softer materials, and has

caused the need for carbide and ceramic cutting tools that maintain hardness at

elevated cutting temperatures.

(fig 3.3)Hot Hardness of several cutting tool materials (from Kalpakjian1997)

3.7 SURFACE INTEGRITY

If hard turning is to replace any grinding operation it must be capable of producing

surfaces of acceptable quality. This includes both the surface topography (surface

finish) and surface integrity, which is achieved when “the surface of a component

meets the demands of a specific stress system and environment”.

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There are two major types of surface damage that can be caused by hard turning. The

first white layer, which has generally been assumed to result from temperatures

generated at the work piece surface that exceed the austenizing temperature of the

material, followed by rapid cooling . The second type of damage is the formation of

undesirable residual stress profiles at, and just below, the work piece surface .

Mechanical loading, plastic flow, and phase transformation can affect residual

stresses, but negative effects are primarily due to elevated temperature during

machining. Thus the two types of damage (white layer and tensile residual stresses)

are related and have generally been investigated together.

It is generally believed that generation of white layer requires both excessive heat at

the work piece surface and subsequent rapid cooling. Heat generation is attributed to

large amounts of energy generated in the shear region during chip formation and to

the frictional energy between the tool flank and work piece surface. However,

experimental results disagree about the source of rapid cooling. Tonshoff performed

hard turning experiments with and without coolant and found the white layer

magnitude was identical, indicating that work piece self cooling, and not coolant,

must be responsible for quenching of the work piece surface. This argument is

reasonable because the heat affected zone is small in hard turning, and because the

cutting velocities are large enough that the contact time between the tool and work

piece is minimal. Therefore, it is possible that the bulk piece material acts as a heat

sink and draws heat from the surface to create a self cooling effect. However, it has

41

been found that cutting with a worn tool produced white layer, but that a similar

cutting conditions with the application of coolant resulted in undamaged surfaces.

Many researchers have paid considerable attention to the generation of white layer

because it appears similar to thermal damage generated in grinding that is referred to

as “grinding burn”. To determine the structure of white layer, we used an X-ray

technique to determine the separate structures of bulk work piece material and the

white layer region. The results showed that for 16 MnCr 5 steel hardened to 60-62

HRC, the bulk material was composed of approximately 75% martensite and 25%

austenite. The whole layer consisted of only 30% martensite and almost 70

%austenite.

Griffiths (1987) reported three situations where white layers have been generated :

surface subjected to significant rubbing and wear), surfaces that see similar conditions

resulting from pin-on disk testing, and the surfaces that undergo certain machining

processes . In addition to machining conditions, material properties affect white layer

generation. Formation of all white layers to heating and quenching of the materials,

and concluded that chemical composition of the material affects the transformation.

Several publications have proposed that white layers may have increased hardness

relative to the bulk material. Others have reported nearly identical hardness in the

white layers compared with the bulk material.

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To discuss the effects of hard turning on residual stresses, the surface influences of

hard tuning compared to grinding should be mentioned. Compared to grinding, the

force components are large, particularly the thrust force, which is generally larger

than cutting force in hard turning. If the tool loading is thought of as a Hertzian

contact, the maximum compressive stress induced in the work piece occurs at a depth

approximately 0.7 times the contact area of the tool. Because the contact area is larger

than grinding (a single grit) and load is increased, larger residual compressive stresses

that penetrate deeper below the work piece surface result in hard turned components.

As expected and the depth of residual stresses are a function of tool geometry and

process conditions.

Unlike residual tensile stress, reasonable levels of compressive stress are desirable.

Based on the residual stress caused by mechanical loading only, hard turned surfaces

should exhibit increased fatigue life compared to ground surfaces. However, the

undesirable tensile stresses generated by heat are super imposed on the compressive

stress. As tool flank wear increases, so does the frictional energy

Between the tool flank and work piece, as well as the depth of the compressive stress

induced by mechanical loading. Thus, increased tool wear results in larger tensile

stresses near the surface, followed by steep stress gradients with a larger compressive

stress further below the surface. The stress pattern with les overall change was

generated by a tool with very little flank wear compared to the other stress pattern,

which was generated with a significantly worn tool.

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