The mechanism of chip formation with hard turning steel

These concentrated shear bands curve downward and gradually become parallel to tool face as the chip moves up the tool face Fig... These photomicrographs reveal the following: A complete

The Mechanism of Chip Formation with Hard Turning Steel

M.C Shaw (11, A.Vyas Arizona State University, Tempe, Arizona, USA

Received on January 6,1998

A b s t r a c t

Steels having a hardness of HRC 60 or greater are presently being finished by

t r m n g instead of grinding This is usually done using a polycrystalline cubic boron nitride insert having a rather large nose radius on a very rigid machine at a relatively high cutting speed In order t o understand this process it is important that the sequence of events occurring in the formation of the unique type of chip involved be correctly identified Experimental evidence t o this end is presented and discussed in fundamental terms

Keywords: Cutting, Chip formation, Hard turning

I n t r o d u c t i o n

In a previous paper (Shaw and Vyas,l993)

the mechanics of chip formation involved when

hard (brittle) materials are cut was considered

Since that time a great deal more experimental

work has been performed The basic picture

presented there still holds, the only exception

being the nature of the non-etching white layer

observed when a hard high carbon steel is turned

a t a relatively high speed This white layer has

now been identified as a mixture of untempered

martensite and .; iron by electron diffraction using

a transmission electron microscope on a carefully

thinned specimen The 1993 paper should therefore

be considered an introduction t o this one

N o m e n c l a t u r e

In (Shaw and Vyas ,1993) the various chip

types that have been identified were itemized as

f 01 lows:

Steady state tvpxs: Concentrated shear

zone, Pie shaped shear zone, Extended shear zone

due t o a blunt tool-tip

_Cy_fiLtypes: Discontinuous, Wavy, Saw-

tooth, Built up edge (BUE)

In addition, noncyclic changes in chip

thickness are sometimes obtained, particularly

when pure materials are cut a t very low speed

In the literature the term segmental chip is

often used t o describe all of the cyclic types,

particularly the wavy and saw-tooth types This

is unfortunate since these two types of chips are

distinctly different For example, the cycle

frequency for a wavy chip is typically about 100

Hz while that for a saw-tooth chip is 2 t o 4 orders

of magnitude greater Also, wavy chips do not

have sharp points while saw-tooth chips do (Fig.1)

Cycle Frequency The mean cycle frequency for either of these chip types may be readily determined by dividing the speed of the chip (vchip) by the mean spacing

of points of maximum chip thickness (p,) :

= (vchip/pc (1 1

Origin O f The Saw Tooth Chip The saw tooth chip was first identified about the same time as the wavy chip (wavy chip :Bickel 1954; saw tooth chip, Shaw et al, 1954) and there is a considerable body of literature pertaining to the mechanism of formation and the characteristics of each The saw tooth chip was found while studying the machining characteristics of a new structural material (titanium) having unusually low values of thermal conductivity and volume specific heat (Fig.2) Also, the concept of

adiabatic shear introduced by Zener (1948) in connection with the mechanics of ballistic impact was a relatively new popular concept Unfortunately, it was suggested (Shaw et al 1954) that the saw-tooth chip observed when turning a titanium alloy might be due to periodic adiabatic shear This misconception has persisted to the present

Low Speed Turning of a Very Brittle Material

Nakayama (1972) found that saw-tooth chips were produced when highly cold worked (brittle) 40160 brass was cut under orthogonal conditions at very low speed

He observed shear cracks forming periodically at the free surface which ran down the shear plane toward Ihe tool tip This divided the chip into blocks that slid past each other as the chip moved up the face of the tool (Fig.3) Quick-Stop Tests

In the case of hard steel turned at a practical speed, chips are found to show the block-like structure of Fig 3 near the free surface plus bands of concentrated shear extending downward from the cracks defining the edges of the blocks These concentrated shear bands curve downward and gradually become parallel to tool face as the chip moves up the tool face (Fig 4) In order

to obtain a betler idea of the sequence of events

responsible for chips like Fig 4, a series of quick stop

tests was performed on Ti-6AI-4V cut under orthogonal

conditions Figure 5 gives two representative results

Figure 5a shows the situation at the beginning of a cycle

while Fig 5b is about half way through a saw tooth cycle

These photomicrographs reveal the following:

A complete crack (.con:inuous across the width

of the chip) extending about half way down a

straight shear plane toward the tool tip, followed

by a region that does not appear to be completely

cracked but weakened by microcracks (Fig 5a)

A band of concentrated shear going all the way

to the tool face in a straight line (Fig.Sa), followed

by bands that begin to curve toward the tool face

more and more as the chip moves up the tool face

(Fig 5b)

Movement of blocks of material that gradually

proceed outward due to sliding along the fully

cracked surfaces together with extension

of bands of concentrated shear in the micro-

cracked region (Figs 5a and b)

Thinning of the microcracked region as the chip

moves up the face of the tool (compare distance

D1 T with D2T in Fig 5b)

A gradual approach to the final shape of the chip

as it moves up the tool face requiring several

cycles before the chip leaves contact with the tool

No evidence of adiabatic shear is found along the

fully cracked surfaces such as C2 D, in Fig 5b

Fig.1 Chips commonly referred a) waW chip b) saw-tooth chip

Fig 2 Ti -140A chips a) Cutting

fDm (45.7 m/min.); Feed (f) = mm/r); Rake angle a = +so

D i s c u s s i o n

In the discussion that follows the completely

cracked region where a continuous crack extends across

the chip width is designated GC (gross cracked region)

while that corresponding to the region where cracks are

discontinuous across the chip width is designated MC

(microcracked region)

The significance of the thinning of the MC region

as the chip moves up the tool face is that this gives rise

to a cutting ratio (r) greater than one This is usually the

case when hard steel is turned with a negative rake tool

Important consequences of r>l is that the speed of the

chip (VC) will be greater than the cutting speed (V) and

the shear angle will be greater than 45O

The significance of the gradual approach to the

final chip shape involving several cycles of chip-tool

contact is that any slight variation in the cracking pitch

(p,) will not be reflected in a fluctuation of the shear

angle This causes the pitch of the “teeth” of the chip to

be remarkably constant removing the effect of any slight

variation in stress concentration in the original surface

The fact there is no evidence of adiabatic shear on

the GC surfaces (such as C2D1 in Fig 5b) suggests that

the root cause of saw-tooth chip formation is periodic

cracking and not adiabatic shear The only adiabatic

shear involved in fig 5b is in the MC region which begins

to develop only after a GC region forms Therefore, any

attempt to predict the onset of saw-tooth chip formation

due to an increase in cutting speed or feed (Fig 2) will

involve fracture mechanics and not heat transfer

t o as segmental

speed (V) = 1 5 0 0.0104 ipr (0.26

b) V = l O O fpm (30.5m/min; f = 0.0052 ipr (0.1 3

mm/r); cr - +So (after Shaw e t at, 1954)

Fig 3 Saw-tooth chip when turning highly cold worked brass a t very low speed with negative rake tool (after Nakayama, 1972)

Hard Turning of Steel

Steel in the hardened state is being finished today under conditions that produce surface finishes comparable with those in fine grinding (Ra=0.2 to 0.4 pm) This is possible due to the availability of ceramic and cubic boron nitride tools of improved quality and machine tools of greater rigidity To produce surfaces of the

desired finish at a reasonably high feed rate it is

necessary to use tools having a relatively large nose

radius Figure 6 shows a typical turning arrangement

where a nose radius (I) of 3 mm (0.1 18 in.) is making a cut

at a feed rate (f) of 125 pm/r (0.005 in/r) , The depth of

Cut (d) will be much less than r, so that all cutting is on

the nose radius The scallop left behind on the finished

surface will give a theoretical peak- to- valley roughness

(Rt) of f2Br (independent of the depth of cut) T o a good

approximation the theoretical arithmatic average

roughness (Ra) wit1 be f2 /32 r)

b r the example of Fig 6a:

Ra= (125~lO-~)~/1(32)(0.003)] = 0 163pm (6.52 1 in)

For dry turning with a sharp tool and a rigid system

the actual surface roughness including vibration and

other non- geometrical effects will be within a factor of

two of the above value

Figure 7a) shorn the chip of Fig 4 oriented

along the negative rake tool face as a free body,

and just below, the tool is shown in the process of

making a cut This is a snapshot of a saw-tooth an

instant after crack formation, where the element

just formed has slid outward a small distance DC,

According t o Nakayama ( 1 974) the equal and

oppositely directed forces R and R' should be

parallel t o CD Forces R and R' are shown

resolved parallel and perpendicular t o the shear

plane (Fs and NS respectively) In this instance

the tool face friction force (F) is very small while

there is a very significant zone showing bands of

concentrated shear

The gross cracked region of the chip (GC)

extends from C, t o D, while the microcracked

region (MC) extends fom D, t o T The hodograph

for the GC region is given in Figure 7b)

The cutting ratio (r) for this chip may be

found by dividing the undeformed chip thickness (t)

by the mean chip thickness (Tc) However the

composite surface CID1 + C2D2 + %D3 + etc

was found t o corrrespond t o the equivalent length

of uncut surface on the work This was

demonstrated by coating the original surface of

the work with soot and then producing a replica of

the surface of the chip by pressing a soft plastic

material into the back of the chip Valleys on the

chip become peaks on the replica with slopes CD

coated with soot Microscopic meeasurments on

the replica revealed that mean length CD

corresponds t o the mean distance between cracks

on the work (p) Therefore a convenient method

of finding r is t o divide the mean tooth pitch (pc)

by the mean value of C2D2 (=p) Thus,

r = pc/p = Vc/V (2)

There is a small tooth-to-tooth variation of

pc and p for the chip of fig 4 When all

combinations of pc and p are cosidered the mean

value of pc/p is found t o be:

pc/p = 1.59 .t 0 15 ( 1 59.t 10%) = VC/V

Fb.4 Case carburized steel chip (HRC=62) V=338fpm (103 m/min); f=0.01 l i p r (0.28 m m h ; d=O.Ol l i n (0.28 mm); nose radius=0.125 in (3.1 8

mm); (x=-7O

Fig 5 Quick-stop photomicrographs of Ti-6AI-4V

chips a) shortly after formation of gross crack

(GC) at free surface showing extent of GC and MC

b) about halfway between cyclic cracks V=l72fpm ( 5 Z m h i n ) ; t=O.O07in (0.01 2 mm), b= 0.100 in (2.54mm); rx=-7'; tool material, WC

I \ : I

I

‘ I

r

I

I

Fig 6 Cutting geometry for hard turning with tool

having relatively large nose radius

The cutting ratio for the saw-tooth chip of

Fig 4 was obtained by the conventional method

involving measurement of chip length and weight

(see for exmple, Shaw 1984) and found t o agree

with the above value For the chip of Fig 4

the’mean value of the cracking frequency was

found t o be 18 kHz bv use of eq.1 This approahes

the upper limit of the audio frequncy range and has

been verified by dynamic masurement during saw-

tooth chip formatiion

The very inhomogeneous strain in the MC

region of Fig.4 will give high temperatures in the

bands of concentrated shear along the

microcracked extensions of the cJross cracks, for

high cutting speeds It is only these bands that

involve adiabatic shear For ferrous alloys the

temperature in these concentrated shear bands

may exceed the transformation temperature where

ferrite (fi iron) changes t o austenite ( y iron)

Evidence of this transformation is found in the non-

etching white bands in Fig 4 and other similar

photomicrographs of saw-tooth chips of ferrous

alloys machined a t high speed As previously

mentioned these white areas have been identified

as untempered martensite + y iron However,

before rapid cooling and during chip foimation,

these bands will be y iron a t high temperature

which is a relatively soft material that offers

little resistance t o plastic deformation, as would

be the case for a molten metal It is the presence

of high temperature y iron along the tool face that

gives such low values of tool face coefficient of

friction (about 0.05 in Fig 7)

The distance one segment slides relative t o

i t s neighbor during one cycle ( 0 C2 in Fig 7a) will

depend upon the distance between cracks on the

When p p p (r>l), this is a result of the

compressive stress on the material in the MC zone

work (PI

being sufficient to cause elongation of the MC region of the chip Material in the GC region is carried along with the MC material, resulting in r for the entire chip being greater than one

The drawing below the photomicrograph of Fig.7a) is consistent with the quick-stop photomicrographs of Fig, 5

The hodograph in Fig.7b only holds for the GC region since the direction of the shear bands in the

MC region are continuously changing direction as the chip moves up the tool face

The shear angle + may be obtained from Fig 5b where:

( 3 )

r = V,/V = sin +/cos (C ++)

Knowing r and a, the value of I.$ that satisfies

Eq 3 may be found

An energy balance for the specific energy for the chip of Fig 4 may be readily perfomed but space limitations do not permit this t o be included here

The Adiabatic Shear Theory

According t o the adiabatic shear theory the root cause of saw-tooth chip formation is a catastrophic thermoplastic instability where the decrease in flow stress due t o thermal softening associated with an increase in strain more than offsets the associated strain hardening A

number of papers suggest adiabatic shear as the origin of saw-tooth chip formation A few of these are Davies et al, 1996 and 1997; Komanduri

et al, 1982; Koenig e t al, 1984; Recht, 1964 and 1985; Sheikh-Ahmed and Bailey, 1997; and Zhen- Bin and Komanduri, 1997 The most recently proposed model based on the adiabatic shear theory is given in Flg 8

Comparison of t h e Two Theories

The quick stop photomicrographs of Fig 5 are useful in comparing the two theories First of all, a thermally initiated process should be expected t o have its origin where the temperature

is a maximum which is a t the tool tip This is in agreement with the model of Fig 8 but not with reality (Fig 5) The crack in Fig 5a clearly runs from the surface toward the tool tip, initially along a relatively straight shear plane A shear crack should be expected t o initiate near a point of maximum shear stess where the compressive stress is a minimum (i.e.at the free surface) Although it is normally assumed that stress along the tool face and shear plane are constant,

this is not so The normal stress on the shear

plane rises exponentially from zero a t the free surface t o a maximum a t the tool tip Evidence for

this and for a similar variation of stress on the

tool face is given in Sarnpath _and Shaw 1983

AS higher normal stresses are encountered

as a shear crack progresses downward from the free surface toward the tool tip, a continuous gross crack will be gradually converted into a

Fig 7 Chip of Fig.4 oriented t o tool face a) Free body of chip b) Hodograph of GC region

discontinuous microcrack This is seen t o be the

case in Fig.5a It should be noted that a titanium

alloy was employed in obtaining Fig.5 which should

favor the adiabatic shear theory due to its low

values of thermal conductivity and specific heat

Figure 8 shows a recent model (Zhen-Bin and

Komanduri, 1995) employed to explain saw-tooth

chip formation in terms of an onset of adiabatic

shear In this model an adiabatic shear band runs

from the tool tip A in Fig.8a along a straight line t o

the free surface As the chip moves forward the

concentrated shear band just formed rolls down

onto the tool face as block (1 ) glides outward along

two adjacent shear bands While this model

explains the sharp point it is not in agreement with

Fig 5a in that the adiabatic shear band shown from

‘C t o D in Fig 8c is not found experimentally in

.Figs 4 or 5

Even when surfaces CB in Fig.8~ are carefully protected with a hard material t o prevent alteration or loss during polishing, no evidence of adiabatic shear has been found on such surfaces with the electron microscope a t very high magnification While there is no apparent reason the concentrated shear bands bend down and approach the tool face in the model of Fig.8, the reason for this is evident in the shear crack theory

The fact that work-piece hardness (brittleness) is so important relative t o the onset

of saw-tooth chip formation further supports the crack initiation theory

(b)

Fig 8 Model used by Zhen-Bin and Komanduri for

thermal analysis based on adiabatic shear theory

(after Zhen- Bin and Komanduri, 1995)

of saw-tooth chip formation further supports the

crack initiation theory

Concluding Remarks

Considerable experimental evidence sup-

ports the concept that the root source for saw-

tooth chips is cyclic cracks that initiate a t the

free surface of the work and proceed downward

along a shear plane toward the tool tip and not

adiabatic shear If the material is sufficiently

brittle these cracks may be continuous across the

width of the chip (called gross cracks, GC)

essentially all the way t o the tool tip (Fig 3) For

less brittle materials higher cutting speeds are

required for saw-tooth chips t o form and

continuous cracks will become bands of dis-

continuous microcracks as high crack arresting

normal stresses are encountered close t o the tool

tip There are then two regions as the chip

proceeds up the tool face - the material between

gross cracks sliding outward, and deformation in

the MC region confined primarily t o concentrated

shear bands that gradually bend downward and run

along the tool face (Figs 5 and 7)

If the chip speed is high enough when hard

turning steel, the temperature may reach a value

high enough t o cause a transformation to austenite

This will offer little resistance t o deformation

acting as though it were molten metal After rapid

cooling the transformed shear band material

becomes a very hard non-etching white layer

consisting of untempered martensite and retained

austenite Tool face friction is found t o be

unusually low when a white layer exists along the

tool face side of a polished and etched chip

The point spacing in a saw-tooth chip is

remarkably constant but does vary slightly due t o

impel fections in the original surface However, this small variation is not reflected as a change in rake angle, since several cycles are involved in chip-tool contact as the final geometry of the chip evolves (Fig 5)

The need for nomenclature t o distinguish between the several types of cyclic chips cannot

be overemphasized because the basic mechanisms involved for each is entirely different

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