The machining process : cutting

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The machining process : cutting

Citation for published version (APA):

Hutchins, P. (1988). The machining process : cutting. (TH Eindhoven. Afd. Werktuigbouwkunde, Vakgroep Produktietechnologie : WPB; Vol. WPA0638). Technische Universiteit Eindhoven.

Document status and date: Published: 01/01/1988

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THE MACHINING PROCESS CUTTING

wI-BY PATRICIA HUTCHINS

WPA 0638 OCTOBER, 1988

Eindhoven University of Technology

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THE MACHINING PROCESS : CUTTING

According to J. Kaczmarek, the process of cutting used in

the machining of materials is done with cutting tools. These

tools have surfaces called the rake face (or cutting face) and

the flank (Fig 1.).

In the cutting process, wear takes place on the cutting

tool. Kaczmarek describes this result as the wearing action of a

tool, the ability to lose volume and mass gradually during the

cutting process(5:76). Thomas J. Drozda and Charles Wick discuss

that the wear of the tool may be caused by abrasion, adhesion,

diffusion, chemical processes, or oxidation(2:144-145); Kaczmarek adds to this list mechanical forces(5:75).

According to Drozda and Wick, abrasion occurs when hard

particles from the chip slide on the tool face and remove tool

material. Adhesion occurs when two pieces of material are

brought together by high temperature and high pressure.

Diffusion occurs when atoms move from an area of "high atomic

concentration" to an area of low atomic concentration. The

chemical processes that can cause wear begin when the tool and

workpiece are used "in an environment of suitably active

chemicals." Oxidation results at very high temperatures which

cause the structure of the tool to weaken{2:144-145). The

mechanical forces that lead to wear mentioned by Kaczmarek, may

be mechanical abrasion, ultimate strength wear, or fatigue

strength wear{5:75).

As said earlier, in diffusion, atoms move from one lattice

point of high atomic concentration to one of low atomic

concentration, and Paul H. Black points out that solid-state

diffusion is a cause of flank wear and crater wear in cutting

tools(I:127). Drozda and Wick define flank wear as a form of

abrasion that occurs between the workpiece and the flank of the

tool (Fig 2.). The rate of flank wear increases as the cutting

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2

,

L _ _~_~_~,,::::_,..-..,, __

fIG. 't. c.~oSS S"Er..""(loN of \AlOU,", t"~f!>\OE j1IOL S Kco"",AlC> J:LANt. w'''A..

A14D c.R"~ll. IIVttAt\.· O"S~,p L.4HE " ORIG-INA" ou"t"L'NE.

Also shown in Fig 2., is crater wear, defined by Drozda and

wick as a combination of adhesion, abrasion, and chemical

diffusion that occurs on the rake face of the tool(2:3.1-2).

According to Black, as the chip moves over the rake face, i t

rubs on the face of the tool, thus creating a crater; Black

believes the following method is how a crater is formed:

( l ) "As the chip glides over the cutting face of the tool, i t is at high temperature as a result of heat generated at the shear

plane and frictional heat from its passage over the tool face.

Local adhesion of a pair of contacting asperites will take place

in the absence of an absorbed film, and the pair of asperites

will be temperature-welded together to form a junction.

Continued movement of the chip will cause failure of the junction at its weakest point and a particle of the chip is transferred to the tool.

(2)"Diffusion will now

particle of the chip and the

Conditions of high temperature

occur between the

adjacent particle

transferred

of the tool.

and large diffusion gradient at

the junction are favorable to diffusion and allow migration to

the chip at the expense of the lo~alized region of the tocl, so

that the tool material adjacent to the transferred chip particle

becomes weakened.

(3}"The particle, which may have accumulated additional chip

material, will now be 'wiped off' the tool face by the oncoming

chip. Since the weakest section of the particle, on account of

diffusion, now lies within the surfaces of the tool, when the

particle leaves the tool face it will carry with i t a

microparticle of the tool."(1:129-132)

Drozda and Wick ref ere to the resisLance of any of these

wear mechanisms, so that the cutting tool will have a long life

and i t will keep its shape and cutting effeci~ncy, as a

requirement of the tool(2:3.1). Kaczmarek defines the tool life

as the "working time of the tool point, or cutting time under

constant cutting conditions until conventional dulling of the

tool point [the loss of the tool's capacity to machine workpieces

under given conditions] occurs." He also explains that as the

cutting speed is varied within a wide range, the tool life may be considerably increased or decreased(5:82).

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a.

3

John L. Feirer and Earl E. Tatro describe the three kinds of

chip that may be formed during the cutting process. These kinds

of chip are the discontinuous chip, the continuous chip, and the

continuous chip with a built-up edge. The discontinuous chip, or

broken chip (Fig 3.), is a small chip produced when the workpiece

is compressed in front of the cutting tool until a piece of the

workpiece is broken away. The discontinuous chips are each

individually separate. WORKPIECE d b c e WORKPIECE WORKPIECE

f \ ~ ~ . Howa discontinuous chip is formed. The edge of the cutting tool in a and b is compressing the metal. The chip in c and d is breaking away from the metal, and the chip in e is now completely separated.

WORKPIECE

The continuous chip (Fig 4.) is produced the same way as the

discontinuous chip, except the piece of workpiece that is broken

off is continuous, ie. not separate pieces(3:8-10).

WORKPIECE

f\~ 4. A continuous chip is H ideal way of machining metal.

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4

According to Black, because of little time for the chip to

aquire a film of oxide from the atmosphere, the newly formed chip

will scrub the surface of the cutting tool so that "asperites of

the tool and chip will be in contact at high temperature and

pressure", and as a result, particles from the chip may adhere to

the surface of the tool. Other particles roay then combine with

those from the chip that have adhered to the tool, creating a

built-up edge (Fig 5.).

WORKPIECE

'F \ G. ~. A continuous chip with built-up edge results in a roughe surface.

Because the particles of the chip have more time to adhere

to the tool at low cutting speeds, the built-up edge forms more

quickly. In the machining at high cutting speeds, friction at

the tool-chip interface is lower, resulting in slower formation

of the built-up edge. High cutting speeds are therefore favored

because of low wear and because of a good finish of the surface.

The built-up edge damages the workpiece in two ways:

(1) "By the change in shape of the active cutting edge due to the pressence of the built-up edge

(2)"By the presence of fragments dislodged from the built-up edge as they pass into the tool-workpiece interface."(1:ll7)

To show the relationship between Wear of a cutting tool and

time, five tool ~aterials witt their corresponding diffusion

coefficients and predicted wear rates, given by P.O. Hart~ng and

B.M. Kramer, were used(4:77). The equations for the time of

diffusion and wear length are the following:

Time of Diffusion t:

1-t

=

x

(7)

5

where x is the given distance,

coefficient.

and D is the diff~si~n

Wear Length L: L

=

1

R

( 2 )

where R is the predicted wear rate.

A graph of length vs. time was plotted to compare the

results (Fig 6.). A second graph was also plotted using LOG

values of the first graph as another form of co~parison (Fig 7.).

As can be seen in Graph 1., a linear relation can be obtained

from the five materials, excluding WC. Likewise in Graph 2., a

linear r2lation between ZrC, TiC, NbC, and VC can bE obtained

with WC resul~ing in an unlike manner.

Values of in Table 1.

the results of Eq~ations (1) and (2) can be seen

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LIST OF REFERENCES

1. Black, Paul H. Theory of Metal Cutting, McGraw-Hill, IJ.Y ..

1961.

2. Drozda, Thomas J. and Charles Wick. Tool and Manufacturing

Engineers Handbook. McGraw-Hill, Michigan, 1983.

3. Feirer, John L. and Earl E. Tatro, Machine Tool Metal

Working, McGraw-Hill, N.Y .. 1961.

4. Hartung, P. H. and B. M. Kramer. Tool Wear in Titaniu~

Machining.

5. Kaczmarek, J. Principles of Machining by Cutting, Abrasion,

The machining process : cutting