Smith G.T. Cutting Tool Technology: Industrial Handbook

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valve-driven hydraulically-servo controlled ‘computer’

called ‘Whirlwind’.

In the late 1970’s, with the advent of microproces-

sor technology, these later NC machine tools were

converted to Computer Numerical Control (CNC),

oering a signicant stride forward in operator-us-

ability, via on-board editing – without the costly and

timely re-punching of NC paper tapes each time a

minor modication occurred to the NC program.

Today, CNC machine tools have fast multiple-proces-

sor controls, with on-line computer graphics, enabling

new programs to be written and ‘prove-out’ while the

machine tool cuts other components, or the programs

can be automatically down-loaded by a Direct Nu-

merical Control (DNC) data-link from the CAD/CAM

workstation, or via remote satellite-linkage from other

sites either locally, or internationally. e design and

development of some of today’s and the future machine

tools, utilise ultra-fast CNC microprocessors, coupled

to orthogonal multi-axes linear-induction motor-

driven slideways, that can be precisely monitored via

laser-controlled positional encoders, with ultra-fast

co-axial spindles. Moreover, non-orthogonal-axes

controlled machine tools are under development, us-

ing simultaneous mulitple-axes slideway control, with

hybrids having tool spindles that incorporate multiple

angular orientation together with their linear slideways

for truly sculptured free-form surface machining capa-

bilities. Even today, operations carried out by several

machine tools are now being incorporated into one

hybid machine tool, with such as: turning, milling and

grinding at one set-up. In the near future, the machine

tools will have slideway acceleration/decelerations of

faster >5g’s, with these machines having the ability to:

rough-turn, mill, heat-treat, grind critical features, all

remotely-controlled via satellite from the CAD/CAM

designer, signicantly speeding-up the product devel-

opment process time-to-market.

2.2.2 Pioneering Work in Metal

Cutting – a Brief Resumé

Basic research into metal cutting did not commence

until approximately 70 years aer the rst machine tool

was introduced. In 1851, early research by Cocquilhat

was into the work required to machine a given volume

of material by drilling. By 1870, the terms ‘chip’ and

‘swarf’ were introduced by the Russian engineer Time,

where he attempted to explain how chips were formed.

In 1873, Hartig tabulated research into metal cutting

in a book, which was the rst authoritative work on

the subject. A more practical metal cutting description

was given by Tresca (1878), ustilising visio-plasticity

models

. In 1881, a presentation at the Royal Society

of London by Lord Rayleigh of Mallock’s metal cut-

ting research ndings was given. Mallock’s scientic

study of carefully etched specimens of the workpiece

and attached chip for both ferrous and non-ferrous

metals, where he observed them using a microscope

(magnication: x5). Mallock correctly surmised from

his investigation of his ‘models’ that the cutting pro-

cess was basically one involving shearing and, that

friction occurred in forming the chip, emphasizing

the importance of this friction along the cutting tool’s

face – between the chip and the tool. e sharpness

of the cutting edge was also mentioned and the rea-

sons for instability of the cutting process, leading to

unwanted vibrations, or ‘chatter’. Moreover, Mallock

employed basic lubricants in this work, noting that

the application of lubrication reduced chip/tool inter-

face friction. ese general observations by Mallock

mentined above, oer a surprisingly close approxi-

mation to today’s theories on the ‘mechanics of metal

utting’ , although his equations for the work done in

internal shearing and chip and tool friction were in-

correct, surprisingly, he was unaware of the ‘plasticity

models’ by Tresca and his theory of ‘plastic heating’.

To compound the metal cutting problems still further,

in 1900, an unfortunate ‘step backward’ in the under-

standing of the metal cutting process was taken by

Reuleaux. He suggested that a crack occurred ahead of

the tool’s point and likened the cutting action to that

of splitting wood, regrettably having popular support

for some years.

In 1907, a seminal paper by the now-famous Amer-

ican researcher Taylor, who published his 26 years of

15 Tresca’s visio-plasticity models, involved scoring a grid of

accurate closely-spaced lines onto the edge of a specimen

of metal to be machined, then partially cutting it at a preset

depth of cut and leaving the chip attached. He then investi-

gated the plastic deformation that had taken place as these

grids were distorted and buckled by the action of machining.

Both lighter and deeper cut depths were investigated in this

manner, across a range of metal specimens. Tresca noted that

ner depths of cut introduced greater plastic deformation than

larger cut depths, stating that stier and more powerful ma-

chine tools were needed to benet from these recommended

greater depths of cut (i.e. undeformed chip thickness).

Turning and Chip-breaking Technology 51

practical experience into investigation and research

ndings in metal cutting. Taylor, was fascinated by

the application of time-and-motion studies that could

be applied within the machine shop and in particular,

‘piece-work systems’

. In order to enable the progres-

sion through optimisation of these time-and-motion

studies, new cutting tool materials were employed,

in particular high-speed steels (HSS). Taylor investi-

gated the eect that tool materials and in particular,

cutting conditions had, on tool life during roughing

operations, in order to assist in the application of these

time-and-motion studies. His principal objective was

to establish empirical laws that would enable optimum

16 Piece-work systems are where a set time allowance is given for

a particular job, or a batch and, a bonus is agreed if the worker

performs this task within the allotted time.

cutting conditions to be attained. By establishing op-

timum cutting data for metal cutting operations and

employing ‘piece-work systems’ at the company, Taylor

was able to increase the Bethlehem Steel Company’s

output by 500%. Of particular note, was the fact that

the empirical law governing the cutting tool and its

anticipated tool life

is still used today, in the study

of machining economics – more will be said on this

topic later in Chapter 7 (Machinability and Surface

Integrity).

Notable in the years prior to World War Two, were

the contributions made into the generation of data on

cutting forces and tool life, initially by Boston (1926)

17 Taylor’s machinability work produced a fundamental dis-

covery, namely, that the interface temperature existing at the

tool’s cutting edge controlled the tool-wear rate.

Figure 24. The formation of a continuous chip, based upon the ‘deck of cards’ principle. [After: Piispanen, 1937].

52 Chapter 2

and later, by Herbert (1928). Around this time, the cut-

ting speeds were steadily improving with the arrival of

new cutting tool materials, such as cemented carbide.

In 1937, Piispanen introduced his so-called ‘Deck of

Cards’ principle as an explanation of the cutting pro-

cess (see Fig. 24 for Piispanen’s idealised model, with

Fig. 25 depicting sheared chips at a range of cutting

speeds). Here, Piispanen’s model depicts the workpiece

material being cut in a somewhat similar manner to

that of a pack of cards sliding over one another, with

the free surface an angle, which corresponded to the

shear angle (ϕ). So, as the tool’s rake face moves rela-

tive to that of the workpiece, it ‘engages’ one card at

a time, causing it to slide over its adjacent neighbour,

this process then repeats itself ‘ad nitum’ – during

the remainder of the cutting process. Some important

Figure 25. Variations in chip morphological surfaces at dierent cutting speeds, giving an indication

of the various shearing mechanisms. [Source: Watson & Murphy, 1979]

Turning and Chip-breaking Technology 53

limitations are present with Piispanen’s model, namely

that it:

•

exaggerates strain in homogeneity,

•

shows tool face friction as elastic rather than plastic

in nature,

•

considers shearing takes place on a completely at

plane,

•

assumes that BUE does not occur,

•

takes an subjectively assumed shear angle,

•

takes no account of either chip curling, or predic-

tion of chip/tool length.

NB P

iispanen’s model is easily understood and does

contain the major concepts in the chip-forming

process – admittedly for simple shear in the main.

By way of further information concerning chip mor-

phology: the micrographs of chip surfaces illustrated

in Fig. 26 show in these cases, that the morphology

indicates a semi-continuous chip form. ese chip

forms point towards the fact that noticeable periodic

variations have occurred, perhaps as the result of the

stress becoming unstable, rather than resulting from

any vibrational eects produced by the machine tool.

Any such instability, has the eect of causing minute

oscillations (i.e. backward and forward motion) in

the shear zone, while the machining takes place. e

dierences in segment shapes shown and their fre-

quency occurring at diering cutting data in these

micrographs, are thought to be dependent upon the

frequency of the shear plane’s oscillation relative to the

cutting speed.

A considerable volume of fundamental work on

machining research has been undertaken over the

last few years, but during World War Two (i.e. from

a European perspective), Ernst and Merchant (1941)

produced another signicant paper dealing with the

mechanics of the machining process – some of these

research ndings will be briey dealt with in the chap-

ter on Machinability and Surface Integrity, along with

other contributions to this subject.

2.3 Chip-Development

Most metallic materials can be considered as rela-

tively hard to machine and this is evident from all of

the reported literature on the subject of metal cutting,

indicating that shearing occurs in a concentrated re-

gion between the chip and tool, this eect being de-

picted schematically in Fig. 26. e overall machining

process is well concealed behind a amalgamation of:

workpiece material, high speeds and feeds, elevated

temperatures and enormous pressures

. e actual

cutting dynamics in contemporary machining opera-

tions, utilises just a few millimetres of physical contact

between the tool and the chip of a precisely-shaped

cutting edge geometry in an exotic mixture of tool ma-

terial to eciently machine the workpiece – this being

an impressive occurrence worthy of note.

In the early work on machining, it was thought that

the chip was formed by deformation along a shear

plane, elastically in the rst instance, then plastically

as the evolving chip passed through a stress concentra-

tion. e Piispanen model (i.e. Fig. 24) illustrates this

point, where workpiece material is being cut by pro-

gressive slip relative to the tool point, an angle which

corresponded to that of the shear plane. Here (i.e. Fig.

24), it shows how each chip segment forms a small, but

very thin parallelogram, with slippage occurring along

its shear plane.

In an orthogonal cutting process

, as the workpiece

material approaches this ‘shear plane’ it will not be-

gin to deform until it reaches the ‘shear plane’. Here,

it is transformed from that of simple shear, as it moves

across a thin shear zone, with the minute amount of

secondary shear being virtually ignored, as is the case

for tertiary shear – this being the equivalent of a slid-

ing friction but having a constant coecient of fric-

tion. Chip deformation in reality, is produced over a

zone of nite width, usually termed the ‘primary shear

zone’ (see Fig. 26). As the chip evolves, the back of the

chip tends to be roughened, due to the plastic strain

being inhomogeneous in nature (see Fig. 25). is

shearing action creates a particular chip morphology

as a result of the either, stress concentrations, or by

presence of points of weakness in the workpiece be-

18 Interface pressures between the chip and the tool are nor-

mally exceedingly high, typically of the order of 1,000 to 2,000

N mm

–1

, with temperatures in certain instances at the tool’s

face reaching approximately 1100°C.

19 Orthogonal machining, is when the cutting tool’s edge (i.e. rake

face – see Fig. 19b) is presented ‘normal’ to the evolving chip and

thus, to the workpiece, at 90° to the relative cutting motion. at

is, little if any, side shearing action occurs, while the chip is be-

ing formed as it progresses up the tool’s rake face – eectively

created by two distinct cutting forces: tangential and axial.

54 Chapter 2

Figure 26. Schematic representation of a sing-point stock removal process, during the continuous cutting of ductile metals.

Turning and Chip-breaking Technology 55

ing machined

. Once the chip deformation begins, it

will continue within this ‘zone’ , as though here in this

vicinity, the workpiece material is exhibiting a form of

negative strain-hardening.

e oblique cutting process

presents a dierent

and much more complex analytical problem, which has

been the subject of a lot of academic interest over the

years. Even here, the whole cutting dynamics change,

when the tool’s top rake surface is not at, which is the

normal status today, with the complex contoured chip-

breaker geometries nowadays employed (typically il-

lustrated in Figs. 4, 10 and 27a).

Actual chips are normally severely work-hardened,

in particular with any strain-hardening materials (for

example: high-strength exotic alloys employed for

heat-resistance/aerospace applications) as they evolve,

by the combined action of: elevated interface tempera-

tures, great pressures and high frictional eects. Such

machined action of the combined eects of mechanical

and physical work, produce a ‘compressive chip thick-

ess’

, which is on average, dimensionally wider than

the original undeformed chip thickness (see Fig. 26).

e rake angle depicted in Fig. 26 is shown as posi-

tive, but its geometry can tend to the neutral, right

through to the negative in its inclination. As the rake

angle changes, so will the complete dynamic cutting

behaviour also change, modifying the mechanical and

20 As the shear plane passes through a particular stress concen-

tration point, it will deform more readily and at a lower stress

value, than when one of these ‘points’ is not present.

21 Oblique machining, is when the rake face has a compound an-

gle, that is it is inclined in two planes relative to the workpiece,

having both a top and side rake to the face, creating a three-

force model (see Fig. 19a), where the cutting force mathemati-

cal dynamics are extremely complex and are oen produced

by either highly involved equations, or by cutting simulations.

is latter simulated treatment is only briey mentioned later

and is outside the remit of this current book. However, this

information on dynamic oblique cutting behaviour can be

gleaned, from some of the more academic treatment given in

some of the selected books and papers listed at the end of this

chapter.

22 Compressive chip thickness is sometimes known as the: chip

thickness ratio (r)* – being the dierence between the unde-

formed chip thickness (h

)

and the width/chip thickness of the

chip (h

*Chip thickness ratio (r) = h

** (i.e. illustrated in Fig. 26).

** h

= W/ρwl

Where: W = weight of chip, ρ = density of (original) work-

piece material – prior to machining, w = chip width (i.e D

l = length of chip specimen.

physical properties within the chip/tool region, as the

various deformation zones are distinctly altered. In ef-

fect, due to rake angle modication (i.e. changing the

rake’s inclination), this can have a profound aect on

the: cutting forces, frictional eects, power require-

ments and machined surface texture/integrity.

e chips formed during machining operations can

vary enormously in their size and shape (see Fig. 35a).

Chip formation involves workpiece material shearing,

from the vicinity of the shear zone extending from the

tool point across the ‘shear plane’ to the ‘free surface’

at the angle (ϕ) – see Fig. 26. In this region a consider-

able amount of strain occurs in a very short time in-

terval, with some materials being unable to withstand

this strain without fracture. For example, grey cast

iron being somewhat brittle, produces machined chips

that are fragmented (i.e. termed ‘discontinuous’), con-

versely, more ductile workpiece materials and alloys

such as steels and aluminium grades, tend to produce

hips that do not fracture along the ‘shear plane’ , as

a result they are continuous. A continuous chip form

may adopt many shapes, either: straight, tangled, or

with dierent types of curvature (i.e. helices – see Fig.

35a). As such, continuous chips have been signicantly

worked, they now have considerable mechanical

strength, therefore eciently controlling and dealing

with these chips is a problem that must be overcome

(see the section on Chip-breaking Technology). Chip

formation can be classied in a number of distinct

ways

, these chip froms will now be briey reviewed:

•

Continuous chips – are normally the result of high

cutting speeds and/or, large rake angles (see Figs.

26 and 27b). e deformation of workpiece mate-

rial occurs along a relatively narrow primary shear

zone, with the probability that these chips may de-

velop a secondary shear zone at the tool/chip inter-

face, caused in the main, by frictional eects. is

secondary zone is likely to deepen, as the tool/chip

friction increases in magnitude. Deformation can

also occur across a wide primary shear zone with

23 One of the major cutting tool manufacturer classies chips in

seven basic types of material-related chip formations, these

are: Continuous, long-chipping – mostly steel derivatives, La-

mellar chipping – typically most stainless steels, Short-chip-

ping – such as many cast irons, Varying, high-force chipping

– many super alloys, So, low-force chipping – such as alu-

minium grades, High pressure/temperture chipping – typied

by hardened materials, Segmental chipping – mostly titanium

and titanium-based alloys.

56 Chapter 2

Figure 27. Chip-breaking inserts and chip control whilst turning – in action. [Courtesy of Iscar Tools].

Turning and Chip-breaking Technology 57

curved boundaries, with the lower boundary being

below the machined surface (Fig. 26), which may

distort a soer workpiece’s machined surface – par-

ticularly with small rake angles and at low speeds.

Strain-hardening of this type of chip, results in it

becoming harder than the bulk hardness of the

original workpiece material (see Fig. 28c, where the

bulk workpiece hardness is 230 H

and the work-

hardened chip is ≈350 H

). is increase in the

chip’s strength and hardness will depend upon the

shear strain (see Table 4 for details of the Rheololog-

ical

status, related to the inclination of the tool’s

rake angle). erefore as the rake angle decreases,

the shear strain will increase, causing this con-

tinuous chip to become both harder and stronger

– behaving in a similar manner to that of a rigid,

perfectly plastic body. In order to satisfactorily deal

with long continuous work-hardened chips, that

could either wrap around the machined workpiece,

potentially spoiling the surface texture, or become

ensnarled around tooling, or even, reduce ecient

coolant delivery to the cutting edge, with integrated

tool chip-breakers having been designed and devel-

oped – see Fig. 27b.

24 Rheology is a branch of science dealing with both the ow

and deformation of materials, with the shear strain rate, oen

termed just the shear rate. (i.e usually quoted in Pascals-sec-

onds ‘Pa-s’).

•

Continuous chips with a built-up edge (BUE) – when

machining ductile workpiece materials, a built-up

edge (BUE) can form on the tool’s tip. is BUE con-

sists of gradually deposited material layers from the

workpiece, hence the term ‘built-up’ (see Fig. 28).

As cutting continues, the BUE becomes larger and

more unstable, eventually partially breaking away,

with some fragments being removed by the under-

side of the chip, while the remainder is randomly

deposited on the workpiece’s surface (Fig. 28a).

is process of BUE formation, shortly followed by

its destruction, is continuously repeated during the

whole cutting operation. e BUE deposited on the

workpiece will adversely aect the machined sur-

face texture. e BUE modies the cutting geom-

etry, creating a large cutting tip radius (Fig. 28a and

b). Due to the BUE being severely work-hardened

by the action of successive deposits of workpiece

material, the BUE’s hardness signicantly increases

by around 300% over the bulk component hardness

(Fig. 28c). At this severely work-hardened level, the

BUE becomes in eect a modied cutting tool. Nor-

mally, an unstable BUE is undesirable, conversely, a

thin stable BUE is as a rule, regarded as desirable,

as it protects the top rake surface. e formation

mechanism for the BUE is thought to be one of ad-

hesion of workpiece material to the tool’s rake face,

with the bond strength being a function of the af-

nity of the workpiece to that of the tool material.

is adhesion, is followed by the successive build-

up of adhered layers forming the BUE. Yet another

factor that contributes to the formation of a BUE,

Table 4. Strength and hardness of chips when turning mild steel.

Rake angle (γ°): 45 35 27 10 10 10

Feed (f - mm rev

–1

) 0.30 0.30 0.20 0.20 0.20 0.20

Cutting speed ( V

- m min

–1

) 50 50 168 168 168 76

Cutting uid: Soluble oil None None Soluble oil None None

ensile strength (UTS – kg mm

–2

) 75 84 91 92 93 95

Vickers hardness number (Hv) 272 289 302 320 314 325

/UTS 3.6 3.4 3.3 3.5 3.4 3.4

Shear strain (Pa-s) 1.1 1.7 2.1 2.9 3.1 4.0

[Source: Nakayama and Kalpakjian 1997]

58 Chapter 2

Figure 28. The development of a continuous chip with Built-Up Edge (BUE), its typical hardness distribution and its aect

on the machined surface

Turning and Chip-breaking Technology 59

is the strain-hardening tendency of the workpiece

material. erefore, the greater the strain-harden-

ing exponent, the higher will be its BUE formation.

From experiments conducted on BUE formation, it

would seem that the higher the cutting speed, the

less is the tendency for BUE to form. Whether this

lack of BUE formation at higher cutting data is the

consequence of increased strain rate, or the result

of higher interface temperatures is somewhat open

to debate. However, it would seem that a paradox

exists, because as the speed increases, the tempera-

ture will also increase, but the BUE decreases. e

propensity for BUE formation can be lessened by:

Changing the geometry of the cutting edge –

by either increasing the tool’s rake angle, or de-

creasing the D

, or both,

II.

Utilising a smaller cutting tip radius,

III.

Using an eective cutting uid, or

IV.

Any combination of these factors.

•

Discontinuous chips – consist of adjacent work-

piece chip segments that are usually either loosely

attached to each other, or totally fragment as they

are cut (Fig. 29). e formation of discontinuous

chips usually occur under the following machining

conditions:

. Brittle workpiece materials – these materials

do not have the machining capability to un-

dergo the high shear strains,

I. Hard particles and impurities – materials with

these in their matrix, will act as ‘stress-raisers’

and actively encourage chip breakage,

II. Very high, or low cutting speeds – chip veloc-

ity at both ends of the cutting spectrum, will

result in lack of adherence/fragmentation of

the chip segments,

V. Low rake angles/large D

’s – either small top

rakes and heavy D

’s will decrease the adher-

ence of the adjacent chip segments,

. Ineective cutting uid – poor lubricity, com-

bined with a meagre wetting ability, will en-

courage discontinuity of chip segments,

I. Inadequate machine tool stiness – creating

vibrational tendencies and cutting instability,

leading to disruption of the machining dynam-

ics and loosening of chip segments.

As mentioned in ‘Roman II’ above, the hard particles

and impurities tend to act as crack nucleation sites,

therefore creating discontinuous chips. Large D

’s

increase the probability that such defects occur in the

cutting zone, thereby aiding discontinuous chip for-

mation. While, faster cutting speeds result in higher

localised temperatures, causing greater ductility in the

chip, lessening the tendency for the formation of dis-

continuous chips. If the magnitude of the compressive

stresses in the both the primary and secondary shear

zones signicantly increase, the applied forces aid in

discontinuous chip formation, this is because of the

fact that the maximum shear strain will increase, due

to the presence of an increased compressive stress.

B Due to the nature of discontinuous chip forma-

tion, if the workpiece-tool-machine loop is not su-

ciently sti, this will generate vibrational and chatter

tendencies, which can result in an excessive tool wear

regime, or machined component surface damage.

•

Segmented chips – are sometimes termed: in-, or

non-homogeneous chips, or serrated chips. is

chip form has the characteristic saw-toothed pro-

le which is noted by zones of low and high shear

strain (Fig. 30). ese workpiece materials possess

low thermal conductivity, as such, when machined

their mechanical strength will drastically decrease

with higher temperatures. is continuous thermal

cycle of both fracture and rewelding in a very nar-

row region, creates the saw-toothed prole, being

particular relevant for titanium and its alloys and

certain stainless steel grades. For example, to ex-

plain what happens in realistic machining situation,

the specimen Fig. 30a is displayed, for an austenitic

stainless steel quick-stop micrograph. is micro-

graph being the result of a less than continuous ma-

chining process (1), utilising a 5° top rake-angled

turning insert. Here, variations in the cutting pro-

cess have created uctuations in the cutting forces,

resulting in waviness of the machined surface (2).

Prior to the material yielding, then the shearing

process occurring, the workpiece material has de-

formed against the cutting edge (3). To explain how

changing the top rake angle inuences the resul-

tant chip formation for an identical stainless steel

workpiece material, Fig. 30b is shown. Machining

has now been undertaken with a 15° top rake, pro-

moting a more continuous machining process than

was apparent with the 5° tool (i.e. illustrated in Fig.

30a). is more ecient cutting process, results in

smaller variations in the cutting forces (1 and 2).

e chip is seen to ow over the rake face in a more

60 Chapter 2