Smith G.T. Cutting Tool Technology: Industrial Handbook

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nants. e outermost adsorbate layer is oen termed

the ‘

Beilby layer’

: ≈1 µm in thickness and consisting

of many complex factors. Notably, this ‘layer’ would

more than likely have hydrocarbons present and wa-

ter vapour, that originated in the coolant, or the at-

mospheric environment, respectively. Underneath

his metallic surface for work-hardening materials,

there is normally a plastically-strained region that has

usually been metallurgically altered. e depth of this

strain-hardened layer will vary somewhat, but it is in

he region of 10 µm, its actual thickness is dependent

upon the amount of plastic deformation induced by

the tool’s passage over the surface and is inuenced by

the metallic substrate’s composition. e plastic defor-

mation and work-hardening depths

, can penetrate

to fractions of a millimetre this is particular true, if a

‘wiper-insert‘, or roller burnishing tools is employed to

purposely create this localised hardened region to the

component’s surface.

Residual Stress Deformations

For any residual stresses acting within a body (i.e.

component), they will occur without any external

forces, or moments. Internal forces form a system that

is currecntly in a state of equilibrium and if portions

are removed – by machining, the equibrium status is

normally disturbed, resulting in potential component

deformation. is eect of machining distortion is

well-known to practising industrial engineers, when,

for example, machining just one side of a thin compo-

nent, this operation will cause a partial release of local

residual stresses causing it to bend and bow. If either a

casting, or forging has not been heat-treated for stress

relief and its needs asymmetrical machining (i.e. on

one side only), it is likely to deform aer unclamping

restraint from its work-holding device on the machine

tool. In an attempt to minimise this distortion created

by residual stress release, an experienced machinist will

release the clamping forces aer roughing cuts so that

82 ‘Beilby layer’ , on the machined surface is ‘practically amor-

phous’ – this condition being proposed by Sir George Beilby

around the beginning of the 20

century.

83 A

s an approximation, the depth of hardness penetration is ap-

proximately 50% to that produced by residual stress penetra-

tion, whereas the observational plastic deformation is about

50% greater than this penetration.

the stressed surfaces are equalised, prior to reclamp-

ing and taking a nish pass. If this unclamping and

then re-clamping activity is not possible, components

clamped in-situ on the machine tool are occasionally

vibrated at their natural frequency, to minimise these

induced residual stresses. Component deformation is

roughly proportional to the removed cross-section of

workpiece material. Any further nishing is usually

concerned with just a light cut to minimise any detri-

mental eects resulting from residual stresses by a pre-

vious production processing operation, or route.

e release of internal residual stresses must not be

confused with the input of such stresses by machin-

ing, as indicated in Fig. 186b. e machining process

generates residual stresses by plastic deformation (Fig.

187a), or from localised metallurgical transforma-

tions. In Fig. 186a, the residual stress eects inuence

a range of mechanical and physical properties of the

workpiece material, such as:

•

Deformation – this point has been alluded to above

and can create problem with small workpiece cross-

sections,

•

Static strength – is aected by the yeild point of the

workpiece material, which in turn, is inuenced by

the presence of residual stresses,

•

Dynamic strength – of the part in-service can oen

have its fatigue strength and life aected by the in-

uence of residual stresses present,

•

Chemical resistance – if certain metals are sub-

jected to induced residual stresses on exposure to

atmosphere over a period of time, then stress corro-

sion may occur,

•

Magnetism – residual stresses present, can aect a

component’s magnetic properties, creating distur-

bances of the crystalline structure.

Taper-Sectioning and Micro-Hardness Assessment

So that an improvement of metallographical inspection

of a sectioned machined surface can be made without

unduly aecting any form of surface distortion, ‘taper-

sectioning’ has oen been utilised. A tapered-section

(Fig. 187b), allows such sub-surface features as: phase

transformations; plastic ow zones; localised cracking;

bulk transportation and redeposit of material; to be in-

vestigated which would otherwise have been missed, if

only prolometry (i.e. surface topography assessment)

had been undertaken.

As its name implies, a taper-section overcomes the

limitation of perpendicular sectioning. By taking an

362 Chapter 7

angular planar slice through the components cross-

section, this modied cut angle enhances the substrate

magnication, without unduly distorting exposed sur-

face features – giving greater discretion when observ-

ing, or testing the surface topography. In Fig. 187b,

an 11° sectional cut improves surface discrimination

by increasing the vertical section magnication by

around ve times. e taper-section angle (TSA) will

thus be 79°, with the vertical magnication being ob-

tained from the following expression:

TSM = s

ecant (TSA)

Where:

TSM = t

aper-section magnication,

TSA = taper-section angle.

Oen, the exposed sub-surface feature of interest that

has been plastically deformed, or mechanically altered

s in the main quite small, somewhat less than 0.1 mm

in width. If a micro-hardness indentor such as either

Figure 186. The eects of residual stress and deformations of a workpiece by machining. [After:

Brinksmeier et al., 1982]

Machinability and Surface Integrity 363

the Vickers

, or the Knoop

is utilised (Fig. 187c) to

establish hardness readings in the vicinity of this re-

sidual stress zone, then more indentations are possible

using the Knoop, rather than the Vickers indentor, giv-

ing, more discrimination to the ‘foot-printing’ assess-

ment. A note of caution here when originally attempt-

ing to take the taper-section, is that it is quite possible

to metallurgical alter the sub-surface features, if when

taking the section too much heat is induced when cut-

ting it from the parent component. is comment is

also a valid statement for the subsequent grinding and

polishing of the removed taper-section, prior to metal-

lographical/hardness assessment.

Surface Condition – Being

Affected by Cutting Speed

Prior to discussing the surface and sub-surface modi-

cations to the machined part – shortly to follow, it

is worth taking a closer look at the series of photo-

micrograph images shown in Fig. 188. Here, a group

of identical metallurgical composition ferrous work-

pieces was machined, but at various cutting speeds.

It can be demonstrated that the role played in aect-

ing the machined surface condition, is signicantly

inuenced by the cutting speed, with its accompany-

ing amplication of induced temperature eects as

‘speeds’ are increased. Moreover, it can also be said,

84 ‘Vickers indentor’ , has a square-based dymond pyramid with

and indentor included angle of 136°. Its indentation is dened

as: ‘e load divided by the surface area of the indentation’. e

Vickers hardness [i.e. penetration] number (VPN), may be

determined from the following expression:

VPN = 2Psin(θ/2)/L

Where: P = applied load (kg), L = average length of diagonals

(mm), θ = angle between opposite faces of diamond (136°).

85 ‘Knoop indentor’ ,

has complex facets to its diamond indentor,

having angle of 130° (Short diagonal) and 172.5° (Long diago-

nal), respectively. is facet geometric indentor arrangement

(i.e. having a diagonal ratio of 7:1), leaves a signicantly nar-

rower and longer surface indentation, to that of the Vickers –

mentioned in Footnote 84. us, the Knoop hardness number

(KHN) has been dened by the National Bureau of Standards

(USA), as: ‘e applied load divided by the unrecovered pro-

jected area of the indentation’. e following expression relates

to the Knoop’s surface indentation:

KHN = P/A

= P/L

Where: P = applied load (kg), A

= unrecovered projected

area of indentation (mm

), L = length of long diagonal (mm),

C = constant – supplied by indentor manufacturer.

that a material’s properties are dependent on the strain

rate, with the type and magnitude of tool wear chang-

ing according to the cutting speeds, so simplistically

speaking:

•

Low cutting speeds – wear is normally character-

ised by attrition (i.e. mechanical removal of surface

layers),

•

High cutting speeds – here, attrition gives way to

diusion type wear and ‘Fick’s laws’ dominate the

cutting regime.

NB S

uch ‘broad classications’ of tool wear mech-

anisms occurring, aects the type of: surface pro-

duced; chip formation and strain behaviour.

In some interesting trials undertaken by Watson and

Murphy (1979) – which highlight the disguised nature

of the underlying factors in surface integrity investi-

gations. In this practically-based experimental work,

they used a cemented carbide insert on an alloy steel

(Fig. 188). It was found that the feedrate and D

have

only marginal eects on the sub-surface damage to a

machined workpiece, with the cutting speed being the

most inuential in this situation. is fact has been

established in Fig. 188, when a range of similar work-

piece specimens was machined with the only variable

being the cutting speed, as follows:

•

Photomicrograph a – the machined specimen was

machined at a very low cutting speed (2.6 m min

–1

)

e chip formation was discontinuous and the sur-

face shows an alternating eect of both chip forma-

tion and fracture, with some evidence of deposited

residual BUE. Here, the surface topography is the

result of complex interactions by various eects,

such as changes in shear angle in the contact area

between the tool and chip, plus ‘straining’ causing

increases in the chip thickness. ese phenomena

produce a variety of conditions, from strain-to-

cracking and visually introduces an irregular and

an alternating surface topography,

•

Photomicrographs a to d – cutting speeds in the

range from 11 to 59 m min

–1

, generate a continuous

chip formation. It is evident from these photomi-

crographs (b, c and d), that the surface texture was

gradually improving as the cutting speed increased,

lthough even at 59 m min

–1

, there was some indi-

cation of debris from re-deposited BUE here (i.e. in

‘d’),

•

Photomicrograph e – once the ‘optimum’ cutting

speed had been reached (112 m min

–1

– for this ce-

364 Chapter 7

Figure 187. The tribological action of machining and its aect on induced residual stresses and the micro-

hardness ‘foot-printing’ technique

Machinability and Surface Integrity 365

mented carbide insert grade), the surface texture

appears to be in the main, ‘good’ , with only isolated

areas of the topography exhibiting marginal work-

piece side-ow eects,

•

Photomicrograph f – when the cutting speed

was increased to 212 m min

–1

, then in these trials,

greater cutting insert wear-rate occurred and was

attributed to appreciable carbide edge breakdown,

although the surface topography indicated that an

excellent surface texture was present.

e machined surfaces produced at the lower range of

cutting speeds indicated in Figs. 188 a to d, shows evi-

dence of some re-deposited BUE material to greater-

or-lesser extent: having broken away from original

‘

BUE mass’ , then being re-deposited over several

adjacent machined feed cusps (i.e. see Fig. 28a, fully-

appreciate this eect). To obtain a better and deeper

understanding of these machined surface and sub-

surface eects at the extreme conditions of either very

low, or high cutting speeds: Figs. 188 a and f, respec-

Figure 188. Some photomi-

crographs of component surfaces

machined at dierent cutting speeds

– otherwise with identical cutting

data – illustrating the surface, but not

sub-surface steel’s condition. [Source:

Watson & Murphy, 1979]

366 Chapter 7

tively, the following comments can be made. When

longitudinal taper-sections were taken through these

specimens’ cross-sections, the ground, polished and

etched surfaces reveal their true substrate damage. In

the case of Fig. 188a, BUE was presents on the sur-

face, moreover, there was a cutting/fracture sequence

indicated with conrmation of work-hardening hav-

ing ‘layered scales’ of with cracks and crevices beneath

them. Conversely, the test specimen machined at high

cutting speed (Fig. 188f), there is some verication of

a ‘white-layer’ formation – which is a complex metal-

lurgical phenomena found in certain ‘abused’ ferrous

workpiece situations – more will be said on this condi-

tion shortly. In fact, the ‘good’ machined surface to-

pography disguises the fact that an underlying ‘white-

layer’ condition was present, having a local recorded

hardness of 860 H

VPN

. By way of comparison, if this

same alloy steel composition had received a ‘conven-

tional’ hardness heat-treatment process: heated and

water-quenched from 1200°C, then the bulk hardness

would only be approximately 700 H

VPN

– see Appendix

12 for Hardness Comparison Tables.

From these examples of cutting speed investigative

results and the previously mentioned discussion, it is

evident that the ‘optimum’ machined surface texture

is obtained when the cutting speed is closely aligned

to that of the tooling manufacturer’s recommenda-

ions, so here in this case it is ≈112 m min

–1

, with a

correspondingly ‘good’ surface topography/integ-

rity. If the cutting speeds had been employed at the

‘

higher’ cutting data (i.e. 212 m min

–1

), then one could

have been fooled into accepting this apparently ‘im-

proved’ surface topography. Nevertheless, underlying

this machined surface would be an unstable sub-sur-

face condition, which if used in a stressed and critical

in-service environment, it might potentially fail, by a

reduced fatigue-life – this is why the topic of surface

integrity is so important in today’s climate of potential

industrial litigation, when component failure occurs!

Surface Cracks and White-Layers

If any cracks are present at the free surface which ex-

tends into the material’s substrate, they are potential

sites for premature component failure – for highly

stressed in-service components. It has been reported

in the ndings of industrial enquiries into the UK

railway industry of late, that despite these railroad

tracks being precision machined and then occasion-

ally inspected by non-destructive (NDT)

techniques

– according to the maintenance schedule, instances

have occurred when these rails and particular on

high-speed banked corners – have delaminated. is

catastrophic rail delamination has caused several pas-

senger trains to lose contact with the rails and crash,

resulting in signicant loss of life. Hence, the method

of machining – ‘abusive’ – can contribute poor surface

integrity and to the susceptibility of these machined

surfaces to prematurely fail. In the case of milling op-

erations, it has been recognised for a number of years

that up-cut milling – alternatively termed ‘conventional

milling’ (Fig. 190a), can introduce a surface tensile re-

sidual stress into the surface layers of a milled work-

piece. If this machined component is then subjected

to both an arduous and potentially fatigue-inducing

environment, then the cyclical nature of continuous

stressing followed by its immediate stress release, can

initiate surface crack sites causing them to open-up,

which could result in premature part failure. Con-

versely, an identical machined component that has

been ‘down-cut’ – otherwise termed ‘climb-milling’

(Fig. 190b), will induce surface compressive residual

stresses. is surface layer with its residual stress com-

pression, has invariably been shown to remain closed

and thus, avoiding crack propagation and growth,

when machined under identical cutting data and en-

vironmental circumstances. Moreover, for many years,

it has been recommended that for CNC milling appli-

cations ‘climb-milling’ not only generates this favour-

able machined surface compressive stress eect, but is

a more ecient cutting process and as a result, draws

less spindle power. In Appendix 13a and b, two useful

‘nomographs, are given to determine either the cutting

data (Appendix 13a) this is related to the workpiece’s

diameter and, a diagram (Appendix 13b) to obtain the

spindle power from the anticipated chip area, respec-

tively.

In a machined surface, both craters and pits do

not pose too great a fatigue problem, as they cannot

achieve the ‘critical radius’ (i.e see Footnote 67) neces-

sary to instigate a site for crack initiation at a poten-

86 ‘Non-destructive testing’ (NDT), is a range of ‘non-invasive’

sub-surface inspection testing techniques, typically: Eddy-

current testing, Ultrasonics tests, X-ray investigation, etc., that

can, in many cases be automated for the detection of otherwise

hidden aws in the component(s).

Machinability and Surface Integrity 367

tial stress concentration location. Furthermore, cra-

ters and pits normally exhibit shallow depth-to-width

ratios and are normally only present a problem from

the ‘cosmetic appearance’. Cracks in the surface are

normally classied as either ‘micro-’ ,

or ‘macro-cracks’ ,

with these cracks having depth-to-width ratios of >4,

typically they can promote:

•

Reductions in: mechanical strength; fatigue life;

plus creep resistance

•

Increases in the susceptibility to stress-corrosion

•

Probability increase in a surface material break-out

and generation of debris,

•

Surface delamination and fatigue.

Cracks may be considered as either separations, or

narrow ruptures that interrupt the surface continuity

and normally include sharp edges, severe directional

changes, or both. Macro-cracks can usually be visu-

ally inspected with the naked eye, conversely micro-

cracks obviously require microscopic examination.

Oen these cracks are complex metallurgical interac-

tions which are exacerbated by an ‘abusive’ machine

regime, leading to an unacceptable surface condition.

A crack’s origin can be the result of several multifari-

ous phenomena, typically they can be an inter-granu-

lar attack that might be degraded by surface dissolu-

tion, via chemical processes. Whenever preferential

intergrannular attack takes place, it can additionally

promote a grain boundary network of micro-cracks

that can extend beneath the surface, tracing-out and

following the underlying grain boundaries. Even mi-

cro-cracks should not be ignored, as they can aect

the component’s functional performance, because they

act as a potential source for macoscopic crack fatigue.

Hence, once a crack has been generated it cannot be

successfully resealed, owing to subsequent contamina-

tion and continuous chemical reactions. In fact, the

process of fatigue failure (i.e. see Fig. 190 bottom-right

for photomicrographs of a cranksha’s fatigue failure

87 ‘Creep’ , is: ‘e time-dependent plastic deformation of materials

that occur under constant load at relatively high temperatures

and low stresses’.

88 ‘Stress-corrosion cracking’ (SCC), is: A combined mechanical

and chemical failure mechanism in which a non-cyclic tensile

stress [below the yield strength] leads to the initiation and prop-

agation of fracture in a relatively mild chemical environment’.

mechanism) can be characterised by three discrete

steps:

. Crack initiation – where a minute crack forms at a

particular site, such where a high stress concentra-

tion occurs,

. Crack propagation – during which time at which

the crack incrementally advances with each stress

cycle

3. Final failure – rapidly occurs, once the advanc-

ing crack has reached a critical size being close to

‘speed of sound’: Mach 1 – and is a catastrophic

failure mechanism.

White-Layers

e so-called ‘white-layers’

that can appear when

‘abusive machining’ certain ferrous work-hardening

materials, are a result of microstructural and metal-

lurgical alterations to the machined sub-surface layers

of a workpiece (Fig. 189c). is undesirable and un-

wanted ‘white-layer’ condition is visually apparent (i.e.

when a taper-section through the machined surface

has been taken), as it resists standard etchants and the

consequence is a visible ‘white-layer’ – when viewed

under an optical microscope.

89 ‘Striations’ , (also known as ‘Beach-’ , or ‘Clamshell-marks’

– see Fig. 190bottom-right), are concentric ridges that expand

away from the initial crack site(s), frequently appearing in a

circular, or as a semi-circular radial pattern.

NB is ‘striation eect’ is analogous to that of a stone be-

ing dropped into a still pond – with the stone entry being the

equivalent of the initial crack site, while the radial/circular

waves generated, are akin to the cumulating concentric stress

ridges – until they intersect with the pond’s bank [ie free-sur-

face].

‘White-layers’ ,

are a metallurgically unstable sub-surfaces

exhibiting a very hard localised state, with a supplementary

heat-aected zone (HAZ) beneath it, which is soer than the

overall bulk hardness of the workpiece’s matrix – hence, this

metallurgical instability. ‘White-layers, can be classied de-

pending upon whether it resulted from: mechanical; chemi-

cal; or thermal events, which also directly relates to machined

workpiece factors such as: strain; strain-rate; heating/cooling

rates; plus environmental conditions.

NB In the past, ‘white-layers’ were known by several terms,

such as: ‘white-phases’; ‘white-etchings’; ‘hard-etchings’; etc.

– depending upon the variety and type of ‘white-layering’ pro-

duction.

368 Chapter 7

Figure 189. The inuence of the cutting edge’s condition on the resultant machined surface integrity .

Machinability and Surface Integrity 369

In Fig. 189c, a ‘white-layer’ (i.e. for this ferrous

drilled part, being a localised untempered martensitic

phase of 63 H

) exists beneath the recast and rede-

posited layer, in this case produced by a ‘dull’ drill’s

cutting lips and margins. Due to the fact that the recast

layer (i.e. heat-aected zone – HAZ) has a similar met-

llurgy to that of the ‘white-layer’ , with the delineation

of these ‘white-layers’ regions and their accompany-

ing HAZ’s are not clearly dened. is latter HAZ is a

complex metallurgical condition, comprising of some:

untempered martensite (UTM); over-tempered mar-

tensite (OTM), while beneath these layers, the bulk

substrate material remains unaected. e thickness

of these ‘white-layer’ zones is strongly inuenced by

both the actual plastic deformation created here and,

to a lesser degree, by the thermal inuence of the pas-

sage of the tool’s edge over the machined surface as

heat penetrates into the locality of the component’s

surface. Probably the worst ‘abusive machining’ condi-

tions that can exist, are when drilling holes in work-

hardening materials having long length-to-diameter

ratios (i.e. L/D ratios of >12:1) with inadequate cool-

ant supply, creating high levels of friction, this condi-

tion being exacerbated by an ineciency produced by

a ‘dulled’ drill’s cutting lips.

Virtually all tooling even the most sharp – the no-

table exception here being monolithic faceted natu-

ral diamond cutting edges, have a nite tip radius of

≈

8 µm (i.e. see Fig. 184 – high-lighting the tool tip

‘rounding eect‘), this results in increased forces and

tool wear, which can transform the surface metallurgy

by thermo-mechanical generation. e case has al-

ready been made concerning the fact that machining

processes impart residual stresses into the surface lay-

ers, as indicated in the schematically-represented mill-

ing conditions shown in Fig. 190 and graphically, in

Fig. 191 for a series of milling operations where preset

‘wear lands’ were generated on the cutter’s teeth prior

to workpiece machining. is latter case (Fig. 191) of

articially-inducing a controlled ‘wear land’ onto the

face-milling cutter’s individual tooth (i.e. with the other

teeth removed, hence, acting as ‘Fly-cutter‘), then aer

91 By way of comparison of this untempered martensitic ‘white-

layer’ phase, a conventional high-speed steel (HSS) milling

cutter’s teeth would have had a maximum hardness aer heat-

treatment of 62 H

, which clearly signies the true local hard-

ness of these ‘white-layers’.

several milling passes plotting the residual stress levels

from the surface and into the 4340 steel workpiece’s

substrate under standardised cutting data (i.e the steel

specimens having previously been quenched and tem-

pered to a bulk hardness of 52 H

). Hence, the eect

of these dierent induced tool wear rates and their

inuence in terms of their respective magnitudes and

depths, can clearly be seen. Even when the cutting edge

as ‘sharp tooth’ , a certain degree of tensile residual

stress was apparent in the immediate surface region.

Here, directly under this tensile stress zone, the stress

concentration changed to one of compression (i.e. to

depth of ≈50 µm). As each milling cutter tooth ank

became steadily more worn, the substrate compression

layer also increased in magnitude, which could lead to

considerable workpiece distortion, once the clamping

forces had been released – particularly if only one-side

of the part was milled (i.e. see Fig. 186b).

If the forces involved in the machining process ex-

ceed the ow stress, plastic deformation occurs and

the structure is deformed. In the case o ductile materi-

als, the plastic ow can create a range of degenerative

surface topography characteristics, such as: burrs; laps;

BUE residue; plus other unwanted debris deposits. If

this deformation becomes severe as a result of exces-

sive plastic ow, any grains adjacent to the surface

may become fragmented to such an extent that little,

or no metallic structure can be metallographically re-

solved, therefore ‘white-layering’ will result. Normally,

a ‘white-layer’ region extends to quite a small depth

eneath the surface, in the region of 10 to 100 µm, de-

pending upon the severity of the ‘abusive regime’ of

surface generation. Considering Fig. 191 once again,

as can be seen, the residual stress is indicated along the

vertical axis, here instead, it is alternatively possible

to superimpose a micro-hardness axis – see Fig. 191

circular inset graph. A note of care is required when

changing the vertical axis from residual stress to that

of micro-hardness, as they are two distinct quantita-

tive values. As mentioned the hardness prole closely

approximates that of the residual stress curve, however

in the latter case, instead of tensile stress at the in the

surface region, the sub-surface layer could equally be

compressive in nature.

‘White-layers’ must be avoided under all occasions,

because of the unstable metallurgical condition, com-

pounded by the fact that the these regions act as po-

tential stress-raisers for any critically-engineered com-

ponent and can lead to premature failure, or at worse,

catastrophic failure in-service.

370 Chapter 7

Figure 190. Typical fatigue characteristics within the component’s surface region, being inuenced by the mode of

milling: up-cut or down-cut

Machinability and Surface Integrity 371