Smith G.T. Cutting Tool Technology: Industrial Handbook

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tests’ , conversely ‘absolute’ tests are known as ‘long

tests’. By their very nature, the ‘short tests’ merely in-

dicate the relative machinabilities of two, or more dif-

ferent combinations of tool and workpiece. Whereas,

the ‘long tests’ can produce a more complete depiction

of the anticipated conditions for various combinations

of tool and workpiece, but as their name suggests,

they are more time-consuming and costly to develop

and perform. Some of these test regimes are briey

reviewed below, but more information can be obtained

from the listed references at the end of this chapter.

‘Ranking’ Machining Tests

A series of these ‘ranking’ tests for fast assessment of

actual production conditions has been devised over

the years and some will be mentioned below, but this is

by no means an exhaustive account of all such testing

programmes, they merely indicate the relatively well-

tried-and-tested techniques, such as:

•

‘Rapid facing test’ – this consists of a turning op-

eration, requiring facing-o a workpiece, preferably

having a large diameter, using an H

SS tool

. e

machinability is assessed by the distance the tool

will travel radially-outward, from the bar’s centre,

prior to its catastrophic tool failure. is ‘end-

point’ as it is known, is compared with a similar

trial, where the distance for tool failure by using a

reference material

was previously determined,

NB A

lthough the ‘Rapid facing test’ quickly assesses

one particular test criterion that a machinability

rating can be based upon, it suers from a number

of limitations. Firstly, the material’s diameter may

be smaller than that which one would ideally prefer

to use for the test. Secondly, if the workpiece mate-

rial’s structure is not homogeneous

, then this test

only indicates properties over the diameter-range

2 ‘HSS tool material’ is utilised, because under these extreme

machining conditions, it will rapidly promote catastrophic

tool failure as the forces steadily increase together with esca-

lating tool interface temperature, as the tool’s edge is fed radi-

ally-outward during the subsequent facing operation.

3 ‘Reference materials’ ,

are normally those workpiece materials

that are considered to be ‘easy-to-machine’ , as their name sug-

gests they, at the very least, give a ‘base-line’ , or datum, for

some form of machinability comparison.

4 ‘Homogeneity of material’ ,

refers to a uniformity of its micro-

structure and having isotropic properties.

used. is latter problem of lack of homogeneity of

the workpiece material, can be somewhat lessened

by boring-out the material at the workpiece’s cen-

tre, prior to commencing the test.

•

‘Constant-pressure test’ – this is quite a popular

testing technique and can be undertaken by a va-

riety of methods of machining assessment. For

example, in turning, machinability is measured by

utilising predetermined geometry in association

with a constant feed force. e technique has been

used to some eect on the machining of free-cut-

ting steels. is test is essentially a measure of the

friction between the chip and tool, which is re-

lated to the specic cutting temperatures generated

whilst machining, together with its eects on the

tool’s wear-rate,

NB N

ormally a turning centre has a constant feed

force, in order to obtain relevant data. An engine-/

centre-lathe can also be employed to acquire iden-

tical data, but a tool-force dynamometer is used

to measure this feed force, then plotting a graph

of this feed force with its associated frictional ef-

fects, but this requires more eort and takes longer.

Similar constant pressure tests can be employed for

drilling processes.

•

‘Degraded tool test’ – consists of workpiece ma-

chining with a soened (i.e. degraded) cutting tool.

e test’s ‘end-point’ is determined either: when a

specied amount of tool ank/crater wear has been

reached, or at catastrophic tool failure,

NB I

f machinability testing is carried out on soer

and more easy-to-machine materials – typically on

various alloys of brass, then just a small variation in

soening the tool steel prior to cutting, has a dras-

tic eect on the results obtained, but for harder-to-

machine materials this eect is signicantly less-

ened.

•

‘Accelerated cutting-tool wear test’ – as an alterna-

tive to deliberately soening the tool (i.e. Degrad-

ing tool test), in order to speed-up the machinabil-

ity process the cutting speeds are increased. If the

cutting speeds are signicantly increased, the tool

will not behave according to the predictable tool life

272 Chapter 7

equation

– due to the articially-elevated cutting

temperature generated.

NB I

t is not prudent practice to extrapolate tool-

life data beyond that actually obtained during test-

ing in order to obtain quantitative information

about other ranges and conditions, with diering

operations and parameters. As a result, this test is

usually classied as a ‘ranking test’.

‘Ranking’ – Non-Machining Tests

Whenever there seems to be a need to experiment with

material cutting using perhaps one of the techniques

just mentioned, it is important to establish whether

any savings gained will be recouped in the actual pro-

duction operation. If a company is unsure of the likely

cost benets of such testing, then a strong case can be

made not to test the material at all! Fortunately, non-

machining tests exist that can be utilised in these doubt-

ful situations, rather than ‘working blindly’ – with no

relevant cutting data, to base the applied cutting con-

ditions upon. Several of these ‘ranking’ non-machining

tests can be employed, such as:

•

Chemical composition test – a variety of tests have

been developed by which workpiece materials are

‘ranked’ according to their primary constituents. It

is obvious that the results from such tests are only

relevant when materials of similar type, having

identical processing conditions/thermal history

are to be machined.

5 Taylor’s tool life equation(s), has been utilised for many years,

to determine the ‘end-point’ of a cutting insert’s useful life,

under steady-state cutting conditions. e basis of the general

formula: V

= C, has been modied and expanded to obtain

an equation for the ‘economical cutting-edge life’ for a speci-

ed feed, as follows:

= (1/α – 1)(C´

/C´

+ t

)

Where: T

= economical tool life, α = slope of the VT-curve (i.e.

measured from a plotted graph), C´

= cutting-tool cost per

cutting edge (i.e. see ‘Machining costs’ – later in the chapter),

C´

= machine charge per minute (i.e normally established by

the machine shop management), t

= tool-changing time for

the cutting operation – this will vary according to whether the

tooling is of the conventional, or quick- change type.

6 ‘ermal history’ ,

refers to the heat treatment thermal cycle

that the component in question was processed, describing the

time at temperature, with any modications to the tempera-

ture-induced regime on the heat-treated part.

NB Given the above limitations, these tests have

proved to be quite valid and successful for screen-

ing a workpiece material prior to actual machining.

Typical examples of this test type, rank materials

using a V

scale – giving cutting speeds in m min

–1

and the machinability index of 100 (i.e. utilised by

the ‘Volvo test’ – not shown). A close correlation be-

tween the chemical composition test and ‘absolute

tests’ has been obtained with accuracies claimed

to within 8%. For example, the relationship be-

tween chemical composition and cutting speed is:

Cutting speed (V

) = 161.5 – 141.4 × %C – 42 – 4 ×

%Si – 39.2 × %Mn – 179.4 × %P + 121.4 × %S.

•

Microstructure tests – are principally concerned

with the type of microstructure present in say, a

steel workpiece, specically: inclusion type, shape

and dispersion. e test method gives a good in-

dication of the likely machinability, but requires

highly-specialised laboratory equipment for such a

metallographical investigation although materials

can only be ranked, as either: good, bad, or indier-

ent.

NB E

arly work here, primarily investigated low-

to-medium carbon steel microstructures, notably

considering the spacing between pearlite laminae

achieved by heat treatment. e pearlite-to-ferrite

proportions clearly inuenced the materials hard-

ness value (e.g. Brinell). When a cutting speed was

selected (e.g. V

), a machinability rating could be

obtained for either life at: a constant speed (min-

utes), or relative speed for a constant tool life

(m min

–1

). It has been observed that when >50%

pearlite was present, combined with a relatively high

bulk hardness

, then good machining characteristics

occurred. In recent years, commercially-available

steels have trace elements added to aid machinabil-

ity, the so-called free-machining steels. Typically,

sulphur and manganese additions, create manga-

nese sulphide, with their shape, size and distribu-

tion within the steel’s matrix, playing a major role

in aiding machinability factors.

7 ‘Bulk hardness’ , is a term that is used to state the overall hard-

ness of the test specimen, not its micro-hardness – which only

establishes localised hardness levels.

Machinability and Surface Integrity 273

•

Physical properties test – requires specialist equip-

ment in order to perform this test. e physical

properties of the workpiece material are utilised in

order to determine its machinability ranking.

NB R

esearchers, have produced a general machin-

ability equation using a dimensional analysis tech-

nique and, by utilising conventional test methods

to establish and measure its: thermal conductivity,

harness (Brinell), percentage reduction in area,

together with the test sample’s length. is ‘Physi-

cal properties test’ ,

gives close agreement with the

cutting speed for a range of ferrous alloys, al-

though when brittle materials are assessed, the lack

of a yield-point

and the much smaller reductions

in area – aer tensile testing – may cause potential

ranking problems.

‘Absolute’ Machining Tests

As their name implies, the ‘absolute tests’ are utilised

in order to obtain a comprehensive data-gathering

machining-based activity, on particular types of work-

piece and cutting tool combinations. Many of these

‘absolute testing’ techniques have been devised, with

several of them listed below, including the:

•

Taper-turning test – being undertaken by turning a

tapered workpiece. As a result of turning along the

taper, the cutting speed will proportionally increase

with increasing taper diameter – this also being in

proportion to the cutting time. By originally estab-

lishing the cutting speed, the changing-rate of the

8 ‘Yield-point’ , refers to the strain* at which deformation be-

comes permanent, when the material is subjected to some

form of mechanical-working. e yield-point strain for fer-

rous and many ductile materials is well-dened, illustrating a

‘sharp’ transition from elastic-to-plastic deformation – where

a permanent ‘set’ occurs. However, this is not the case for

many brittle materials, here when say, a tensile test is con-

ducted, an articial ‘proof-stress’ value is used to intersect the

stress/strain curve plotted, to establish its safe-working level

of operation – see the relevant References for more in-depth

details.

*‘Strain’ , is a measure of the change in the size, or shape

of a body – referring to its original size, or shape. For ex-

ample, linear strain is the change per unit length of a linear

dimension – aer some form of mechanical working. For a

tensile test specimen that has been subjected to a tensile test, it

refers to its linear dimensional change from its original gauge

length.

cutting speed in conjunction with the amount of

tool ank wear – for two separate tests – allows the

values of the constants (i.e.‘α’ and ‘C’) in Taylor’s

equation for tool wear – see Footnote 5 – to be de-

rived and, the tool life established for a range of fu-

ture cutting tests. As the D

must be consistently

maintained throughout the test, either a CNC pro-

gram must be written – using one of the standard

‘canned-routines’ available, or a taper-turning at-

tachment is necessary on an engine-/centre-lathe,

NB S

ome major advantages accrue from this com-

prehensive testing technique, not least of which is

that results are valid for a range of pre-selected cut-

ting speeds and, the test is of relatively short du-

ration, but closely agree with many thorough and

longer test methods. Although, the results obtained

may not be representative of actual cutting condi-

tions, owing to the fact that the cutting tool, ma-

chines at diering diameters throughout the taper

turning test.

•

Variable-rate machining test – achieves similar

results to the previously described ‘Taper-turn-

ing test’. In this case, the increase in cutting speed

is obtained by turning a parallel testpiece axially,

whilst simultaneously increasing the cutting speed

as the tool traverses longitudinally along the work-

piece. Once again, the constants are derived for the

‘Taylor equation’ aer a minimum of two tests have

been completed,

NB 

e main advantages of this method over the

‘Taper-turning test’ , are that a standard testpiece

can be used and the results probably reect truer

actual turning conditions – in that consistent diam-

eters are being turned, although this argument is

somewhat debased, if the turning of complex free-

from component geometry is demanded for the

production part.

•

Step-turning test – was developed to overcome

some of the problems associated with the two pre-

viously described testing techniques. In the ‘Step-

turning test’ method, a range of discrete diameters

and speeds are utilised to determine the ‘Taylor’s

constants’. is test, shows close agreement with re-

sults obtained from the two previously-mentioned

‘absolute test’ methods,

•

HSS tool wear-rate test – this test assesses machin-

ability by measurement of the tool’s ank wear, pro-

274 Chapter 7

duced when machining free-cutting steels, with the

major parameters being the elemental additions to

the metallurgical composition of these steel grades.

NB 

ese tests are undertaken in a similar manner

to the: ISO 3685:1977 Standard, for a long ‘absolute

est’ , but it was withdrawn in mid-1984.

All of the above ‘absolute testing’ programmes, relate

to turning operations, principally due to the fact that

the tool is engaged in the workpiece test sample for a

reasonably lengthy period of time. is tool/workpiece

engagement, allows for ‘steady-state’ conditions to be

developed, having the additional benet of producing

relatively consistent ‘Taylor constants’. From a more

practical viewpoint, the author has developed some

other testpieces, which have proved somewhat useful

in actual industrial machining applications, where a

more representative machinability situation was de-

manded. Just some of these testpieces, along with a

discussion of their relative merits, will now proceed.

Practical Testpieces – for CNC Applications

e premise behind the development of the testpiece

depicted in Fig. 145, was to attempt to ‘mirror’ the ac-

tual production operations and to a lesser extent, the

physical geometry of a particular component part.

Here, the component geometry was devised to be ma-

chined on either a machining centre, or a turning cen-

tre with the facility of driven tooling and at the very

least, having an indexing workholding spindle/chuck.

With this testpiece, the part is preferably a thick-walled

tube that can be bored out, OD turned, circular inter-

polated (i.e. milled), drilled and tapped – as the drill-

ing size, is also an M6x1 tapping size. is allows the

component’s geometric features to be inspected ‘On-

machine’ – using metrological inspection routines in

association with touch-trigger probes and, ‘O-ma-

chine’ employing a CNC Co-ordinate Measuring Ma-

chine (CMM). ese identical parts were from a series

of exhaustive tests undertaken on both ferrous metals

and aerospace-grade aluminium stock. Of particular

note, was that when a milled circular interpolated fea-

ture – the boss, was assessed on the machining centre,

it gave more accurate readings than its equivalent in-

spection routine on the CMM. is perceived dier-

ence in accuracy and precision, was the result of part

changes caused by both relaxation of the clamping

forces – upon release – and the greater temperature

dierential between these workpieces when inspected

on the CMM. However of note, was the fact that in

general for the inspection of part features, the CMM

showed a four times improvement in repeatability, to

that of the touch-trigger probing undertaken on the

machine tool, as the following Table 9 indicates:

e above type of practical ‘testing regimes’ are gen-

erally termed: ‘Production Performance Tests’ (PPT).

Typically, these PPT’s can be utilised to determine the

maximum production rate – in parts per hour. Al-

though it must be said, that with shis normally con-

sisting of between 6 to 8 hours duration of potential

‘

in-cut time’ , this to a certain extent, limit’s the achiev-

able machined surface nish requirement, particularly

if a ‘Sister tooling strategy’ is not operated. One of the

main problems connected with PPT’s, is that invari-

ably free-cutting metals are usually selected for long-

term testing, meaning that any wear-related data takes

awhile to accrue. Despite this slight reservation, actual

cutting data can be employed, which represents almost

optimum machining conditions, leading the way to

Table 9. A comparison of the machined component tes-

tpiece accuracies by either: ‘On-’ , or ‘O-machine’ inspection

procedures

PARAMETERS: MACHINES*:

- equipped with Renishaw touch-

trigger probes:

Machining Centre

(Vertical)

MM (LK CNC

Micro4)

Scope Full range of: X-, Y- and Z-axes

Direction of test Uni-directional

Positional Accuracy ±

13 µm X-axis ±8 µm

Y-axis ±5 µm

Z-axis ±6 µm

Repeatability ±

10 µm ±2.5 µm

* Machine tools here, are part of a fully-industrial Flexible Manufac-

turing Cell (FMC), comprising of Cincinnati Milacron equipment:

200/15 Turning Centre, 5VC Vertical Machining Centre, T

776 Ro-

bot- equipped with twin back-to-back grippers – for component

loading/unloading, LK Micro4-CMM, DeVlieg Tool Presetting Ma-

chine, Component workstation, Cell Controller, all equipped with

Sandvik Coromant quick-change tooling (Block Tools and Varilock

Tooling), plus DNC-link to a CAD/CAM workstation – being desi-

gned and developed by Cincinnati Milacron and the Author, when

acting as an Industrial Engineering Professor at the Southampton

Solent University.

Machinability and Surface Integrity 275

Figure 145. General machinability test piece for CNC machine tools.

NB Holes marked ‘A, B and C’ are machined at dierent cutting speeds, as are the turned, bored and milled dimensions.

276 Chapter 7

‘full’ production operational machining, meaning that

with some degree of condence, manufacturing dic-

tates and objectives will be met.

In Fig. 146, a commercial (PPT) testpiece has been

developed showing typical machining data employed,

based upon the secondary machining operations de-

manded by many companies on Powder Metallurgy

(P/M) components – where light nishing cuts, or ac-

curate and precise screwthreads are demanded. Here,

the cutting insert can turn three dierent diameters

– usually in some form of arithmetic progression

, so

that feedrate longitudinally can be metrologically as-

sessed. Moreover, the insert’s passage over the surface

can be metallographically-inspected and a micro-

hardness ‘footprint’ across a tapered section can be

undertaken, to see if any surface/sub-surface modica-

tions have occurred. More will be said on this subject

later in the chapter, when discussing the eects of ‘ma-

chined surface integrity’. is design of using a thick-

walled tube (Fig. 146), that can be produced from ei-

ther wrought stock, or P/M compact processing – the

latter, giving a controlled ‘density’

across and along

the part, makes it particularly ‘ideal’ for any secondary

machining machinability trials. Boring operations can

also be conducted on such a testpiece geometry, al-

lowing roundness parameters and its associated ‘har-

monic prole’ to be metrologically assessed, in conjuc-

tion with any ‘eccentricity’ with respect to the OD and

9 ‘Arithmetic progressions’ , are normally utilised for many ap-

plied machining (PPT) trials as they give a ‘base-line’ for the

research work and increase at a controlled amount. For ex-

ample, a feedrate, could begin and increase as follows: 0.1, 0.4,

0.7, 1.0, 1.3, … mm rev

–1

– with the ‘common dierence’ being

3. As a mathematical expression, this simple arithmetic pro-

gression, can be written as follows:

a, a+d, a+2d, a+3d, a+4d, a+5d, … where the ‘common dier-

ence’ is ‘d’ , giving the:

n´

term as: a+(n–1)d.

‘P/M Density’ , refers to either the uncompacted, or free-par-

ticulates and is termed its ‘Apparent density’ (AD). is term

AD, is used to refer to the loose material particulates prior to

PM compacting, to describe the density of a powder mass ex-

pressed in grammes per cubic centimetre of a standard volume

of powder. is AD diers from that of its ‘compacted density’

– which will vary depending upon the consolidation (i.e. com-

pacting) technique utilised. For example, double-compaction

– pressing the powder in the dieset from both ends, or us-

ing ‘oating diesets’ – to simulate double compaction, in this

latter case, pressing from one end only, will produce a more

uniform bulk density throughout the ‘green compact’ as it is

known – prior to its subsequent sintering process.

ID – these machined surfaces both being produced in

a ‘one-hit machining’ operation – then inspected by a

suitable roundness testing machine.

e main advantage of using industrial-based

(PPT) testpieces similar to that shown in Fig. 146, is

that ‘canned-cycles’

, can be used to produce the un-

dercuts, turning passes, or screwcutting operations

on each part. Moreover, optional ‘programmed-stops’

can be written, allowing the research-worker/operator,

to have the facility to stop machining at a convenient

point as desired, at the press of a button – giving a

measure of control to the automated CNC machining

processes. If a series of testpieces are to be machined, it

is important that all of the parts machining sequences

are known and that they are laid-out in a consequtive

logical fashion. is allows one to measure the dete-

rioration with machining time for the sequence of tes-

tpieces produced. To this end, not only should some

unique and logical part numbering system be used,

but in the case of P/M testpieces, the top and bottom

for each compact should be established. As when each

one was initially compacted, its local density have var-

ied and, for consistency for all machining undertaken

with each test piece, it needs to be held in the same

orientation.

Oen it is possible to amalgamate two previous

ranking machining test regimes into one, this is the

case with ‘Accelerated Wear Test’ (AWT) illustrated

in Fig. 147, this test being a combination of both the:

‘Rapid Facing’ and ‘Degraded Tool’ tests – previously

described. In the case of the AWT technique, this hy-

brid test’s aim is to assess the relative machinability of

either wrought, or secondary machined P/M compacts

11 ‘Canned-cycles’ , this is a preset sequence of events that is ex-

ecuted by issuing a single command, which may remain active

throughout the program, or in this case will not, for a par-

ticular ‘canned-cycle’ *. For example, once the preset values/

dimensions together with the required tool osets have been

established, then a preparatory function entitled a ‘G-code’

can be used, such as a G81 code, which would initiate a sim-

ple drilling cycle, in association with the following G84 code

which would then specify a tapping cycle on this drilled hole,

or alternatively, a G32 code commences a threading cycle and

so on. – which considerably reduces both the complesaty and

overall length of a CNC program.

*G-codes fall into two categories, they are either ‘modal’ , or

‘non-modal’. A ‘modal’ G-code, remains ‘active’ for all subse-

quent programmed blocks, unless replaced by another ‘modal’

G-code. Conversely, a ‘non-modal’ G-code will only aect the

programmed block in which it appears.

Machinability and Surface Integrity 277

Figure 146. A turning and boring surface texture test piece.

278 Chapter 7

Figure 147. Machinability testing utilising an ‘accelerated testing procedure’ – a combination of the rapid facing and degraded

tool tests

Machinability and Surface Integrity 279

on a moderately short timescale. Normally in many

previous testing programs, an uncoated cemented car-

bide P20, or P10 grade would have been used, since

these grades withstand both higher speeds and have

better tool wear resistance to that of previously utilised

cutting tool materials. However in this case, an P25

grade was chosen, which is a degradation from the

optimum P20 grade, but it should still perform satis-

factorily. Furthermore, the cutting speed was raised by

2.5 times the optimum of 200 m min

–1

, with all fac-

ing operations being conducted at a ‘constant surface

speed’

of 550 m min

–1

Typical tool-life curves produce by the AWT tech-

nique are illustrated in Fig. 148, showing the expected

three stages of ank wear. is ank wear being a func-

tion of: the initial edge breakdown, steady-state wear –

as the insert’s ank progressively degenerates and -

nally, catastrophic insert edge breakdown – as the edge

completely fails. Detailed metallurgical analysis can be

made as to the reasons why some P/M compacts per-

formed better than others, by reference to the litera-

ture on the metallurgical interactions between the tool

and the compact – this subject being outside the scope

of the present discussion. e facing-o secondary

machining operation meant that aer 10 facing passes,

a pre-programmed ‘optional stop’ can then be applied,

to allow both tool ank wear and compact surface tex-

ture to be established. e faced-o surface texture re-

sults can then be superimposed onto the same graph –

for a direct comparison of ank wear and for that of

the machined surface texture parameter. Without go-

ing into too much detail of the specic aspects of the

processing and metallurgical interactions present here

on the composite graph, some compacts abraded the

cutting insert more than others, while the ‘faced’ sur-

face texture, generally seemed to get worse, then im-

prove and nally worsen again. However, this is a

complex problem which goes to the ‘heart’ of the vi-

12 ‘Constant surface speed’ , this can be achieved by employing

the appropriate ‘canned-cycle’ G-code accessed from the CNC

controller, which allows the testpiece’s rotational speed to in-

crease as the faced diameter decreases*.

* Normally there is a restriction on the rotational speed limit

– created by the maximum available speed for this machine

tool, which would normally be reached well before the cutting

insert has coincided with that of components centre line, but

because in this instance, the compacted testpiece is hollow, the

rotational restriction does not present a problem.

sual aspect of machined surfaces – wherein the real

situation is that surface texture continuously degen-

erates, and it is only the burnishing (i.e.‘ironing’) of

the surface that ‘masks’ the temporary improvement

in machined surface – more on this topic will be made

in the surface integrity section. What is apparent from

using the AWT technique is that on a very short tim-

escale, considerable data can be generated and applied

research assessments can be conducted both speedily

and eciently. is topic of exploiting the minimum

machining time and data-gathering activities to gain

the maximum information, will be the strategic mes-

sage for the following dialogue.

Machinability Strategies: Minimising Machining

Time, Maximising Data-Gathering

Prior to commencing any form of machinability tri-

als, parameters for cutting data need to be ascertained

in order to minimise any likelihood of repetition of

results, while reducing the amount of testpieces to be

machined to the minimum. Data obtained from such

trials must be valid and to ensure that the cutting pa-

rameters selected are both realistic and signicant a

disciplined experimental strategy based upon the ‘De-

sign of Experiments’ (DoE) approach is necessary – see

Fig. 149. Here, a ow-chart highlights the step-by-

step approach for a well-proven industrial technique,

to maximise the labour-intensive and costly exercise

of obtaining a satisfactory conclusion to an unbiased

and ranked series of machinability results. ere are

a range of techniques that can be utilised to assess

whether the cutting data inputs, namely: feeds, speeds,

’s, etc., will result in the correct inputs to obtain

an extended tool life, or an improvement in the ma-

chined surface texture from the testing program. One

such method is termed the ‘Latin square’ – which as-

sesses the signicance of the test data and its interac-

280 Chapter 7

Figure 148. Graphical results obtained from the accelerated machinability test, illustrating how ank wear and

surface texture degrades, with the number of facing-o passes

Machinability and Surface Integrity 281