Trent E.M., Wright P.K. Metal Cutting

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LUBRICANTS 333

flank wear less than water alone. There is evidence to demonstrate that the water, by its action at

the interface, modifies the flow close to the tool edge and changes the mechanism of tool wear to

severe attrition.

10.3.9.3 Cutting steels at medium speeds

König and Diederich

demonstrate that water-based cutting lubricants can greatly reduce the

rate of cratering wear at medium cutting speeds. This is particularly true when cutting steel

deoxidized with aluminium at high cutting speed using a steel-cutting grade of carbide. Penetra-

tion of the fluid or its vapors into the rear end of the contact area on the rake face promote the

formation of relatively thick protective layers of oxide at the interface.

10.3.9.4 Cutting steel at high cutting speed (greater than 175 m min

-1

)

Figure 10.18 compares the maximum rake face temperature near the rear of the crater when

machining low carbon steel a) for dry cutting and b) for coolant supplied from above the cutting

process (say in Figure 2.1). The application of a coolant is beneficial in low speed cutting opera-

tions but of doubtful value in high-speed turning of iron and steel.

Once again the inability of the coolant to significantly reduce tool temperatures at speeds

greater than 175 m min

-1

is predominantly due to the conditions of intimate contact, or seizure,

that occur over the major part of the chip-tool contact area. This part of the contact area is not

accessible to the water-based lubricant. While the coolant can slightly penetrate to the friction

region at the rear of the contact length, and also into the area created by the ragged edge of the

chip, these effects do not greatly influence temperature, cutting forces or tool life at the higher

speed.

FIGURE 10.17 Wear on rake face of carbide tools when cutting steel at 30 m min

-1

(100 ft/min) at low

speed.

(a) In air; (b) with mineral oil lubricant; (c) with water lubricant; (d) with jet of oxygen directed

under chip in rake face

334 COOLANTS AND LUBRICANTS

FIGURE 10.18 Reduced effect of a water-based coolant as speed is increased above 175 m min

-1

10.4 CONCLUSIONS ON COOLANTS AND LUBRICANTS

10.4.1 General observations

Understanding of the action of coolants and lubricants in metal cutting is still at a rather prim-

itive level. The conditions are very complex, and it is still best to rely on experience and testing

in the selection of cutting fluids. The results of such testing are shown in Table 10.1. The actions

of lubricants in other engineering applications are of rather little value in guiding the selection

and development of cutting and grinding fluids.

This is because of the conditions of seizure

that act close to the tool edge.

10.4.2 Trends towards dry cutting

It is well known that the use of lubricants and coolants is under substantial criticism at the time

of writing. It is feared by health professionals that long term exposure creates health hazards.

These may range from minor yet unpleasant skin irritations, to respiratory problems, and to the

more serious concerns about skin and other types of cancer.

Sheng

and colleagues have reviewed such data and related them to possible changes in the

process plans for machining. It is possible to re-organize the process planning sequence and

reduce the amount of fluid used for certain features on a machined part.

Another strategy is to “target operations in the factory where cutting fluids may be of not much

help or may even be making things worse”. Some of the results in the last few sections may well

have a significant impact on today’s focus on “dry machining”. Some examples include:

• In Sections 10.3.9.1 and 10.3.9.2, it was shown that carbide tools and high speed steel tools

often behave poorly at low speeds in the presence of water-based lubricants. The evidence

indicates that attrition wear takes place on the cooler tool surfaces. This is despite the pres-

ence of some lubrication effects. A cooler tool surface, attached to seized work material, pro-

motes attrition and this outweighs any lubrication.

CONCLUSIONS ON COOLANTS AND LUBRICANTS 335

• Figure 10.18 shows the reduced effectiveness of coolants in high-speed cutting (of some

workmaterials). It is quite possible that very high speed cutting is best done dry for this rea-

son in any case. Fluids might create no benefit and even leave ceramic tools vulnerable to

craze-cracking.

• Section 10.2 showed that a much smaller amount of coolant directed accurately at a “hot

spot” can be effective. Although more expensive to engineer, this has great potential in reduc-

ing the volume of fluid used. A “mist coolant” spray similarly directed has been used com-

mercially to solve difficult problems, as in the cutting of nickel based alloys.

Observations such as the above may eventually allow machine tool operators to first reduce,

and then cease the use of coolants, without feeling that they have been unduly pressurized by

government regulations to do so. They might even find beneficial cost reductions by some of the

strategies suggested above.

To investigate dry machining, Malshe et al.,

machined aluminum A390 (aluminum-silicon

alloy with 17% Si) using both polycrystalline diamond (PCD) and CVD diamond coated tools.

They installed a particle counter over the lathe spindle to measure potentially injurious emis-

sions from the tool surfaces. As might be expected, for both tool materials, the overall contribu-

tion to the mass of the overall emissions was low and was significantly lower than the proposed

NIOSH standard for metalworking fluids. The PCD tool showed more emissions than the CVD-

coated tool. This phenomena was attributed to the significant difference in the surface roughness

of the cutting edge between the two tools. For the CVD tool the total emissions were 0.06 g/m

while the PCD tool had a rate of 0.1 g/m

Tool wear in particular is greatly influenced by the atmosphere or environment. It is interesting

to note that in dry cutting, air itself plays a subtle role. The oxygen in the air has a lubricating

effect. This is because the oxides formed at the end of the chip-tool contact length, reduce sei-

zure between the tool and the freshly cut metal. However, the oxygen accelerates notch wear at

the ragged edge of the cut. The nitrogen in the air tends to reduce this notch wear. Trends

towards dry cutting should build upon such research findings. In many situations, health-hazard

lubricants can be replaced with less dangerous inert gas-jets.

Of course, at medium speeds, the water-based lubricants have a very beneficial effect. They

reduce crater wear by diffusion or plastic deformation of the cutting edge. Given a gradual

reduction in the use of cutting fluids in any given factory, these operations should be kept under

coolant control for as many years as possible. Furthermore, difficult operations such as broach-

ing and gun-drilling must rely on lubricants.

10.4.3 Recommendations

10.4.3.1 Creative solutions that avoid cutting fluids

The first practical objective should be to look more broadly at the “dry machining” challenge.

Throughout Chapters 5, 9 and 10, several design issues and metallurgical effects have been

described that offer creative solutions beyond the use of coolants/lubricants. These include:

19-23

• The use of modified tooling designs that might reduce tool temperatures and thus reduce the

need for coolant. Figure 5.20 shows such an effect based on controlled contact tools. In other

cases, careful selection of chip breaking grooves may reduce tool temperatures and also carry

336 COOLANTS AND LUBRICANTS

the chip away more easily.

21-23

This can often prevent the workpiece from overheating and

distorting.

• The use of “free cutting” steels with the correct grade of carbide. An extremely important

finding is described in Sections 9.7.3 and 9.7.4. The MnS inclusions interact with the mixed

crystal carbides to give “internal lubrication” effects. By sharp contrast, the effects with the

straight WC-Co grades are by no means as effective. A factory can make good use of this

result by being meticulous in the choices of work material and tool material.

• The use of “calcium de-oxidized” steels where possible. Section 9.13.2 summarized the

important effects of the silicates shown in Figures 9.31 and 9.32. Once again the formation of

such layers is “very tool material dependent”. These effects show some other options for

eliminating coolant use when machining steel.

• Of course, if the new strategy does involve a switch from a standard steel to a “free cutting

grade” it cannot be done without careful consultation with the original part designer. As diffi-

cult as “machinability” might be, no compromise can be made on the final component’s

fatigue life, fracture toughness or corrosion resistance. Integrated CAD/CAM systems that

allow designers and fabricators to work cooperatively are therefore important software tools

for such decision making. And as mentioned above, such systems can also reconsider the

detailed process planning steps. For some components, where design features overlap, cool-

ant can sometimes be reduced if the machining sequence is changed.

10.4.3.2 Target operations that are, in any case, best done dry

The second practical objective should be to “target” those operations in a factory that are best

done dry or at no loss without coolant. It has been shown above, and in the recent research

“dry machining” that many aluminum alloys and cast irons can be machined dry with no loss of

performance. Moreover, many operations with ceramic tools are best done dry.

10.4.3.3 Focus on tooling “hot spots” with pointed jets and mist coolant

The third practical step is to point the coolant nozzle only onto the “hot spots” of tools, or to

use “mist coolant”. This reduces the volume of fluid used. The results from such studies are

shown in Figures 10.1 through 10.5.

10.4.3.4 Inert gases and cryogenic possibilities

The fourth practical step is to consider jets of inert gases for some applications where notch

wear of tools is a problem. Also, Hong and colleagues

have shown that liquid nitrogen can be

directed through small channels in the chip-breaker pad and into the rear of the chip-tool contact

area. This has a very beneficial effect when machining titanium alloys. It cools the chip-tool

interface, curls the chip away more readily, improves surface finish on the bar and allows speed

to be increased.

10.4.3.5 Reserve the regulated amounts of coolant/lubricant for the extreme operations

Hopefully once these “more obvious targets” have been addressed it will bring a particular fac-

tory’s annual consumption underneath the regulated limits. As a fifth suggestion, this will then

REFERENCES 337

allow some lubrication - at a regulated limit - to be used in the more difficult operations. Such

extreme processes include broaching and tapping operations, and those steel cutting operations

at medium speeds where the use of cutting fluids still appears essential.

10.5 REFERENCES

1. Morton, I.S., I.S.I. Special Report, 94, 185 (1967)

2. Cookson, J.O., Tribology International, 5th Feb. (1967)

3. Morton, I.S., Industrial Lubrication and Tribology, July/August, 163 (1972)

4. Smart, E.F. and Trent, E.M., Proc. 15th Int. Conf M.T.D.R., p. 187 (1975)

5. Wright, P.K., Trans. ASME, Journal of Engineering for Industry, 100, (2), 131 (1978)

6. Kurimoto, T. and Barrow, G., Proc. 22nd Conf M.T.D.R., P. 237 (1981)

7. Kurimoto, T. and Barrow, G., Annals C.I.R.P.

31 (l), 19 (1982)

8. Hau-Bracamonte, J.L., Metals Technol.

8 (2), 447 (1981)

9. Mazurkiewicz, M. et al., Trans. A.S.M.E., J. Eng. for Ind.,

111 (7), 7 (1989)

10. Rowe, G.W. and Smart, E.F., Brit. J. App. Phys.,

14, 924 (1963)

11. Wright, P.K., Metals Technol.,

8 (4), 150 (1981)

12. Dearnley, P.A. and Trent, E.M., Metals Technol.

9, (2), 60 (1982)

13. Trent, E.M., I.S.I. Special Report,

94, 77 (1967)

14. Childs, T.H.C. and Rowe, G.W., Reports on Progress in Physics

36 (3), 225 (1973)

15. Childs, T.H.C. and Smith, A.B., Metals Technol.,

9 (7), 292 (1982)

16. Opitz, H. and K

Önig, W., I.S.I. Publication, 126, 6 (1970)

17. KÖnig, W. and Diederich, N., Annals of C.I.R.P. 17, 17 (1969)

18. Malkin, S., Grinding Technology, Society of Manufacturing Engineers, Dearborn, MI.

19. Bauer, D.J., Thurwachter, S., and Sheng, P.S., Trans. NAMRI/SME, 171 (1998)

20. Malshe, A.P., Taher, M.A., Muyshondt, A., Schmidt, W.F., Mohammed, Hafez, and Moham-

med, Hazek, Transactions of the NAMRI/SME, 26, 267 (1998)

21. Klocke, F., and Eisenblatter, G., Dry Cutting - State of Research in the Conference on High-

Performance Tools, Dusseldorf, Germany, p. 159, November 1998

22. Hong, S. and Santosh, M., “Effects of liquid nitrogen as a cutting fluid in the machining of

titanium” International Journal of Machine Tools and Manufacture (also see Hong’s video-

tapes - Columbia University, New York, NY., 10027)

23. Jawahir, I.S., Dillon, O.W., Balaji, A.K., Redetzky, M., and Fang., Proceedings of the CIRP

International Workshop on Modeling of Machining Operations, held in Atlanta, GA., Pub-

lished by the University of Kentucky Lexington, KY, 40506-0108. p.161, (1998)

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CHAPTER 11 HIGH SPEED

MACHINING

11.1 INTRODUCTION TO HIGH SPEED MACHINING

Recently, there has been renewed interest in machining at very high cutting speeds. Some

economic and materials science issues are thus reviewed in this chapter.

The first section outlines some of the reasons for the renewed interest in high speed machin-

ing. Twenty or even ten years ago, it would not have seemed reasonable to machine very large

structures from a solid monolithic slab. However, modified component designs - shown in the

lower photograph of Figure 11.1, eliminate costly and unpredictable joining operations in the

factory.

1-3

The behavior of workmaterials at high rates of strain is then reviewed. This is fundamental to

an understanding of high speed cutting.

4-40

Sections 11.6 to 11.8 describe some specific experiments on stainless steel, AISI 4340, alu-

minum aerospace alloys, and titanium alloys. These materials have different responses to high

speed machining based on their material properties.

The last section discusses tool wear and tool life. A major question is whether or not adia-

batic heating can lead to dynamic recovery or even localized melting in the shear zones of cut-

ting. It was once hoped that such processes would occur and make a significant enough

difference to the workmaterial properties that the local stresses on the tool would be reduced. So

far, the evidence is that forces and stresses on the tool edge do not greatly reduce as speed is

increased to very high values.

Beyond 200 m min

-1

most materials show a “flattening out”, or a very minor reduction, of

the force vs. speed curve. In fact, there is far too little information on the type of tool wear that

may be expected. If high speed machining is to be successful, then a study of tool wear mecha-

nisms and data on tool life should be major considerations in future research work on this topic.

340 HIGH SPEED MACHINING

11.2 ECONOMICS OF HIGH SPEED MACHINING

New machining research programs at The Boeing Company provide an interesting motiva-

tion for high speed machining. The following example shows how design and manufacturing

keep changing to suit a rather complex interaction between a) the availability of innovative man-

ufacturing techniques and b) new economic conditions.

Inside the ceiling of a plane, structural members that resemble “giant coat-hangers” are

spaced across the plane at intervals to give it torsional stability. Today, most are made from

many conjoined pieces. This arrangement is shown in the upper photograph of Figure 11.1. By

contrast, newer component designs - shown in the lower photograph of Figure 11.1 - eliminate

costly and unpredictable joining operations in the factory.

Thomas

has observed that such manufacturing innovations, flowing-back into the design

phase, must be the new way of organizing the relationship between design and manufacturing.

Similarly, Ayres

, provides the succinct definition of computer integrated manufacturing (CIM)

as the confluence of the supply elements (such as new computer technologies) and the demand

elements (the consumer requirements of flexibility, quality, and variety).

Using Ayres’ definition it can be observed that the new innovations, or the new supply ele-

ments include:

• Improved cutting tool technology and an understanding of how to control the accuracy of

very high speed machining processes

• The availability of stiffer machine tools and very high speed spindles

• More homogeneous microstructures that give uniformity in large forging slabs

• The ability to carry out comprehensive testing, and show that these one-piece structures are at

least if not more reliable than multiple-piece structures.

Meanwhile the new demand elements include:

• Escalating costs of joining and riveting operations which can only be semi-automated. Spe-

cifically these operations often require manual fixturing of the workpieces

• A preference for eliminating multiple fabrication steps which always demand more setup, fix-

turing, documentation and extra quality assurance

• General pressures on the whole of the airline industry, since deregulation, to cut costs and yet

to improve the safety and the integrity of the aircraft.

These trends introduce a great deal of complexity into the design and manufacturing process.

The conclusion to be drawn is that no single component or manufacturing process should be

analyzed and optimized in isolation. There will always be something that can be improved, sim-

plified or made cheaper if design and manufacturing are viewed from a slightly wider system

perspective.

How do work materials behave when the cutting speed is raised as high as 3,500 m min

-1

(11,500 ft/min) for aluminum alloy wing panels?

What are the forces on the tool?

What is the effect on tool life?

These are some of the questions answered in this chapter.

BRIEF HISTORICAL PERSPECTIVE 341

FIGURE 11.1 Integrated product and process design allows this aerospace component to be completely

machined from the solid as shown in the lower photograph (Courtesy of Dr. Donald Sandstrom, The

Boeing Company)

11.3 BRIEF HISTORICAL PERSPECTIVE

In the 1930s, Trent

and others provided clear metallographic evidence for adiabatic shear

zones in high strain rate deformation processes. Figure 5.6 shows such a shear band that

occurred during a high strain rate impact experiment.

In 1957, Cottrell

addressed the Conference on the Properties of Materials at High Strain

Rate with the following succinct summary: “If the rate of plastic flow is sufficiently high, there

may not be time enough to conduct away the heat produced from the plastic working, and the

temperature may rise sufficiently to soften the deforming material...intense rapid flow becomes

concentrated in the first zones to become seriously weakened by this effect.”

In 1964, Recht

developed a model for catastrophic shear instability in metals under

dynamic loading conditions. He noted that in certain combinations of temperature and stress,

the slope of the stress-strain curve could become zero or even negative. This would especially

be the case if thermal softening dominated over strain hardening.

342 HIGH SPEED MACHINING

These general observations by materials scientists were the inspiration for machining experi-

ments that hoped to capitalize on such effects. Among them, Siekmann

and Kececioglu

both

reported their high speed cutting tests in 1958. Kececioglu’s work is remarkable for its thorough-

ness on the shear strain rates and shear flow stresses in metal cutting. This paper is recom-

mended for a wealth of experimental data on shear zone widths and primary shear zone stress

values. Of particular note is that the primary shear zone average shear stress remains relatively

constant over a rather large range of cutting speeds. To illustrate this point, the authors have

replotted Kececioglu’s results in the upper curve of Figure 11.2.

FIGURE 11.2 Comparison between Kececioglu’s machining tests and Campbell and Furguson’s high

strain rate Hopkinson bar data for the yield stress of mild steel at various temperatures in degrees Kelvin

(subtract 273 for degrees Centigrade)

During the late 1960s and throughout the 1970s, experiments with projectiles and ultra high

speed cutting were conducted by several research teams including Recht’s. The reader with a

deep interest in high speed machining is especially referred to the work based on Arndt’s

Ph.D.

thesis at Monash University with R.H. Brown. The extensive citation list in Arndt’s 1973 paper

probably contains all the European, Japanese and U.S. work pertaining to high speed machining

up until that time. The experiment at Monash used a military Bofer’s gun to fire the workpiece

passed the stationary strain-gaged tool. Cutting forces were measured at different speeds up to

ballistic values. These experiments were further reinforced during the 1980s in a large, U.S. Air

Force funded program led by General Electric, Inc. The results relating to material properties

and chip formation are given by von Turkovich, Komanduri, Flom and colleagues.

10 -15

The high speed machining experiments up to that point held out a fascinating promise. The

basic questions were: i) can the very high speeds lead to dynamic recovery or even local melting