Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 252. In-process gauging, used for up-dating the cutting process voiding ‘tool-drifting’ during

a production run. [Courtesy of Mahr/Feinpruf]

Machining and Monitoring Strategies 513

as that depicted in Fig. 251b. Aer rough-milling,

any semi-nishing and nishing operations can be

undertaken in the hardened condition,

2. H

ard-part milling – this is mainly where small-di-

mensional parts, or components requiring the pro-

duction of shallow-cut features that can be readily

milled (e.g. threads – Fig. 251a; gears and hobs –

Fig. 251c) – in the hardened state,

3. E

lectrical discharge machining (EDM) – is usually

utilised when the part incorporates thin features,

requiring deep cuts, thus the EDM process may be

the only practical solution to this problem.

Hard-Milling – Tool Selection and Replacement

For most die and mould operations, selecting the ap-

propriate cutter geometry is important, with many op-

rators choosing ball-nosed end mills (Fig. 249 – bot-

tom right) for such hard-part milling work. Such

eometry is chosen for rouging and nishing oper-

ations, because the tool’s large radius dissipates both

the heat, while ‘spreading’ the cutting forces across its

longer cutting edges. Additionally, the ball-nose end

mill enables the user to cut closer to the net shape of

the part’s geometry at high speeds and feeds. When a

part incorporates wide and at areas across its base

– needing to be milled, a corner-radiused tool should

ideally be utilised aer the surface has been roughed-

out with the ball-nosed tooling. e logic behind em-

ploying the corner-radiused tool for nishing, is that

with its smaller radius it cannot dissipate the heat and

orces as readily as the ‘ball-nose’ , this is why it is usu-

ally used for semi-nishing/nishing operations when

hard-part milling. If a square shouldered part feature

is needed, then a ‘square-ended tool’ is only used aer

the ‘ball-nose’ has roughed-out the component’s fea-

ture – leaving the minimum of stock to be removed.

is ‘square-ended’ cutter – due to its sharp corner, has

a tendency to chip/fracture, since it acts as a ‘stress-

raising source’ for the heat and cutting forces.

Tool rigidity is important, with the tool’s shank

being much larger in diameter to that of the cutting

diameter. With ball-nosed cutters, a small dra an-

gle of about ½° is employed for additional strength,

while the tool’s neck is usually slightly-relieved when

HSM milling straight walls. In both of these cases, the

tool’s projection from its holder should be kept to a

minimum – to improve its intrinsic rigidity. Returning

briey to the former case of the cutter having a dra

angle. Another reason for the ½° dra angle when ma-

chining dies and moulds, is that if for example, when

the hardened die has a dra angle of 5°, the cutters

modied relief should be ½° clearance, producing an

included angle of cutter body relief of 4½°.

uring machining a die cavity (Figs 246 – bot-

tom right and 249b), the excessive heat that is gener-

ated modies the part’s surface topography, which in

turn, reduces component accuracy. One technique to

inimise such heat generation and retention while

milling, is by controlling the radial step-over (i.e. pick-

feed) distance for adjacent tool paths – when taking

‘

parallel cuts’

(Fig. 84c). is radial step-over is the

distance between the centrelines of successive paral-

lel cuts ‘a

e (p)

’ – shown in Fig. 247b. erefore, for ball-

nosed roughing-out operations, this radial step-over

should ideally be between 25 to 40% of the cutter’s

diameter. Conversely, for nish-milling – for a given

cusp height on a at surface, the radial step-over can

be calculated, as follows (Fig. 247b):

adial step-over (mm) = √4(a

× D

) – 4 (a

)

Where: cusp height is the chordal deviation – nish

tolerance, thus, a

= cusp height (mm), D

= tool diam-

eter @ a set D

(mm).

Cusp height (mm) = D

2 – √ (D

– a

e (p)

)/4

Where: a

e (p)

= radial step-over (mm).

(Sources: adapted from Sandvik Coromant, 1994; Ma-

carthur, 2001)

Since, the radial step-over determines the length of

time each cutting edge spends in the actual cut, in

conjunction with the amount of time it has to cool,

prior to re-entering the following cutting-pass. is

in eect, simplistically determines the quantity of

heat that will accumulate in both the tool and the ma-

chined workpiece. So, when the radial step-over is too

wide, the heat builds-up in the cutter’s edge, due to the

fact that there is insucient time for it to conduct the

heat away, before it re-enters the following cut. While,

smaller step-overs can facilitate ‘almost’ a continuous

cooling action, which limit’s the heat generation and its

59 ‘Parallel cuts’ – when milling, are sometimes referred to as

‘lace cuts’. If they are not parallel – such as when pocket-mill-

ing a triangular feature, where parallel cuts would be ineec-

tive, then the technique here is to utilise variable step-over

cuts, termed: ‘non-lace cuts’.(Source: Smith et al., 1993)

514 Chapter 9

retention, allowing a slightly higher cutter rotational

speed to be programmed. A suitable coating selec-

tion for example, on a cemented carbide cutter, will

also enable higher speeds to be realised. In hard-part

milling operations, coatings, such as: titanium car-

bonitride (TiCN) can withstand temperatures up to

400°C, comparing this to titanium aluminium nitride

(TiAlN), which can withstand cutting temperatures of

up to 800°C, indicates for many hardened alloy steel

dies and moulds, the latter coating makes for a wiser

choice in these production circumstances.

e selection of speeds and feeds will also aid in

controlling heat build-up. With large chip thicknesses

helping to remove heat build-up in the tool and work-

piece. When chip loads are too light, the heat quickly

builds-up – as edges tend to rub the surface being

milled, which in turn, aects tool life and may create

‘white-layering’ in the sub-surface in steel-based prod-

ucts. So using the largest possible chip loads improves

through-put of parts. By way of illustration, if the chip

oad per tooth should be 0.2 mm, but instead it is only

0.05 mm, then a workpiece that normally takes around

18 minutes to machine, will now actually take 72 min-

utes. is increased time means the tool’s edge will

now spend 400% longer in-cut.

Flood coolant should not be used when adopting an

HSM milling strategy for hardened metals (>40 HR

In the USA some industrial trials were conducted into

milling such hardened workpieces and, it was reported

that by not using ood coolant then tool life was in-

creased by 500% – on average. ese trials including

various methods of coolant delivery, via: through-

the-tool coolant holes; coolant grooves; coolant hoses;

and for normal- and high-pressure coolant applica-

tions; in all cases the tool life was reduced. e main

problem with the various forms of coolant delivery

it would seem, is the result of the cemented carbide

tooling suering from ‘thermal-shock’ ,

creating by

the high tool/chip interface temperatures and the im-

mediate ‘quenching-eect’ of the coolant application

– this ‘thermalcycling behaviour’ occurring at very

fast rates. Nonetheless, work-hardened chips in the cut-

ting vicinity must still be evacuated from deep recesses

and pockets to avoid the ‘recutting eect’. By using an

air-and-mist application – close to the tool’s edge, this

will provide a means of swarf removal, while produc-

ing some ‘token’ cutting edge lubrication – assuming

that ‘coolant-eect’ permissible exposure levels (PEL’s)

can be safely dealt, thereby with minimising potential

health hazards.

Any decisions concerning tool replacement will de-

pend on the users machining needs, with the tool fail-

ure generally being apparent by the naked eye, or un-

der low optical magnication – simply observing the

cutting edges to determine the ‘wear-patterns’. In-cut,

a worn tool’s edges will tend to emit a dull ‘red glow’

this indicates that excessive forces and heat are being

generated in the cutting zone, shortly leading toward a

apid and catastrophic tool failure condition. is ‘vis-

ual glowing eect’ is initially usually conned to the

60 ‘Tooling – glowing red’ , this temperature-induced machining condition has been widely reported. Trent, 1984, stated: ‘Under very ex-

ceptional conditions, when cutting fully hardened steel, or certain nickel alloys at high speed, chips have been seen to leave the tool red

hot* – i.e. a temperature of over 650°C’.

*is term ‘red hot’ – relating to temperature is somewhat vague, as shown in the chart, for: Variation of colours with temperatures

– tempering, stress relief and hardening:

Colour: °Fahrenheit: °Celsius: Colour: °Fahrenheit: °Celsius: Colour: °Fahrenheit: °Celsius:

Straw yellow 430 220 Light blue 590 310 Faint red 950 510

Light brown 465 240 Grey 615 325 Dark red 1150 620

Brown 520 270 Grey-purple 660 350 Dark cherry 1175 635

Purple 545 285 Grey-blue 705 375 Cherry red 1300 705

Dark blue 565 295 Dull Grey 750 400 Bright cherry 1470 800

NB Temperatures above are either slightly rounded-up, or -down. Conversion: °Celsius = 5/9 (°F - 32)

(Sources: Avner, 1974; Bofors, 1981)

Machining and Monitoring Strategies 515

cutting edge corners – where high stresses and tem-

peratures are generated, which can be precisely tem-

perature-monitored by thermographic equipment

, or

simply rather crudely, by naked eye observation – with

the machine tool’s lights turned out!

So, by applying the correct tooling in a consistent

and repeatable manner, becomes a vital factor for any

form of predictability with all hard-metal machining

applications. is is also particularly true for any form

of hard-part: drilling; reaming; and tapping opera-

tions; where these production processes oer serious

challenges to the cutting edges, as the bulk hardness of

the workpieces increase to >40 HR

9.10 Ultra-Precision

Machining

Introduction

In the last few years, there has been a momentous drive

toward producing components and indeed assemblies,

signicantly more minute than was previously the case.



e demand might be to locate and align mechanical

parts together in much closer proximity, or perhaps,

oering improved functionality and providing en-

hancement of power-to-weight ratios necessary for

electronic micro-circuitry. In fact, the adjacent circuit

dimensions for nanometric electronic devices can have

proximity to each other of: 0.000006 mm (i.e 6 nm ≡

6 × 1

–9

m).

61 ‘ermography’ , utilises the infra-red radiation emitted by

a temperature-induced body. ese thermographic cameras,

oer considerable benets to any form of actual temperature-

monitoring applications. ermal gradients, hot-spots and

heat losses can be observed – by ‘line-of-sight’ measurements

– during actual machining operations. ey can also be used

to assess temperatures in electrical cabinets, servo-motors,

ballscrews, etc., for actual condition monitoring of the ma-

chine tools. (Source: Smith et al., 1996)

NB Typical temperature ranges for thermographic cameras

are: –40°C to >2,000°C with a thermal sensitivity of 0.08°C,

making then ideal for some forms of tool temperature moni-

toring – assuming that the chip-stream is away from the cam-

era’s lens. (Source: Flir Systems™)

e challenges for the whole of the ultra-precision

manufacturing industries, are to be able to make and

supply miniscule devices that will meet these latest de-

sign objectives. Before, discussing the tooling require-

ments and machining techniques necessary to pro-

duce these diminutive parts, oen with a high volume

demand. It is worth trying to comprehend the ‘true

dimensional size and scale’ of these miniature compo-

nents and assemblies.

Previously, the term ‘hair’s breadth’ was oen

quoted as a very minute dimensional size, but if

one looks at Fig. 253a, here, the large circle is sup-

posed to represent the diameter of an actual hair

– for comparison. Although even here, a hair is not

of uniform diameter. In some very simple comparison

tests undertaken about 6 years ago (Smith, 2002). He

plucked one of his own hairs from his head – that he

could not really aord to miss!, plus four more from

several other people in the vicinity! en, he located

this group of hairs within an scanning electron mi-

croscope (SEM) chamber and simply measured them.

e surprise was that they varied quite considerably,

ranging from the smallest hair: at φ3

0 µm to that of the

largest hair at: φ1

00 µm. So, the diagram (Fig. 253a) in-

dicating that the hair’s size was ≈φ8

9 µm is somewhat

misleading as a form of measurement criteria, as we

will begin to appreciate, that the dierence of a few

‘microns’ can be excessively out-of-tolerance in some

ultra-precision components and assemblies. Even the

previous high-accuracy value of a ‘micrometre’ – oen

simply termed the ‘micron’ this dimensionally-being

–6

m (i.e. illustrated against the hair for comparison

in Fig. 253a), which is not considered and exceptional

dimensional size to ‘hold’ in today’s ultra-precision

machining world. In fact, the technical challenge now

and into the future, is not one of the actual manufac-

ture of these parts (Fig. 253b), but measuring them,

as the old statement, that: ‘We make it, then measure

it, at ten times this accuracy’ , does not hold true any-

more.

We are ‘almost routinely’ of late, making ultra-pre-

cision components at the ‘atomic levels of resolution’ ,

so how can one measure sub-atomic sized components

at the ‘absolute limits’ of today’s metrological instru-

mentation? is form of ‘innitesimal measurement’ ,

is where the term ‘uncertainty of measurement’ really

does become and important factor. ere are so many

variables in the actual manufacturing process that can

inuence the overall dimensional sizes of critical fea-

tures with these miniscule components.

516 Chapter 9

Figure 253. Micro- and nano-machining of parts is a big challenge for both today and tomorrow.

Machining and Monitoring Strategies 517

e term ‘ultra-precision machining’ – that is its

accuracy and precision, really does need to be more

clearly stated

. Many people wrongly surmise that ‘ul-

tra-precision manufacture’ only refers to very minute

components, but one can also relate to a large-scale di-

mensional workpiece, that has certain features for ex-

ample, its machined diameter, or face (Fig. 257b), to-

gether with its overall manufactured width, or length

(

Fig. 254 – top), to these ultra-precision dimensions.

In recent years, the distinct challenges with respect

to ‘micro-machining’ manufacture have been reason-

ably successfully overcome, but ‘nano-machining’

(10

–9

m) and indeed, ‘pico-machining’ (i.e. 10

–12

m), is

now distinctly on the horizon (i.e see Fig. 254, for an

indication of the relative ‘sizes and scales’ demanded –

of late. ese latter machining operational strategies,

oering simply massive challenges in terms of the:

•

Production environments – controlling and moni-

toring the temperature, humidity atmospheric pres-

sure, cleanliness and dust ingress, together with any

oor- and air-borne vibrational eects,

62 ‘Accuracy’ , here and to make it more readily understood, will

be stated in distinct two ways, thus:

‘Accuracy’ , can be established by the dierence between the

actual position of the machine’s slide and the position de-

manded by the CNC, or:

‘Accuracy’ , is the conformity of an indicated value to a true

value (i.e. an actual, or accepted standard value – this being

a qualitative term only).So, the accuracy of a control system

can be expressed as the: Deviation, or dierence between the

ultimately controlled variable and its ideal value – usually in

the steady-state, or sampled instants.

‘Precision’ , on the other hand, is: e degree of discrimina-

tion with which a quantity is stated – e.g. a three digit nu-

merical, discriminates among 1000 possibilities.

NB Precision* is oen contrasted with accuracy. For exam-

ple, a quantity expressed with 10 decimal digits of precision,

may only have one digit of accuracy. (Source: Smith, 1993)

*Bell (1999) states that: ‘Precision’ , is a term meaning ‘neness

of discrimination’ , but is oen misused to mean ‘accuracy’ , or

‘uncertainty’. Its use should be avoided if possible – accord-

ing to Bell. So if this latter statement is the actual situation,

then some singular confusion reigns, when we use the word

‘precision’. In eect, we should always say: ‘accuracy and pre-

cision’ , as uniquely and simply metaphorically-depicted in the

well-known Archery: ‘Target analogy’ – for arrows hitting a

target, thus:

Wide arrow scattering, but centred (on average) on the

‘gold’ – accuracy,

Arrows o target centre (i.e. from the ‘gold’), but closely

grouped – precision,

Close arrow grouping on the ‘gold’ – accuracy and precision.

(Source: Oakland, 1986, Smith et al., 1993)

–

•

Work-holding security, part restraint; manufac-

turing with its potential for component distortion

– perhaps achieved by some form of ‘cryogenic ma-

chining’ , together with monitoring tool wear the

component,

•

Retrieval of parts once manufactured – this collec-

tion operation of minuscule machined parts aer

manufacture is not without problems, as at best,

they will be almost invisible to the naked eye (i.e

see some of these ‘larger parts’ manufactured in

Fig. 253b),

•

‘True’ dimensional measurements – of such ma-

chined part’s actual features, need to be measured

perhaps when in-situ as it is being manufactured,

or later, when any distortions through subsequent

handling and temperature eects have been nulli-

ed.

ese major manufacturing and measurement prob-

lems will have to be addressed in the relatively near-

future, if these latter ‘invisible to the naked eye’ parts

some of which being beyond the visible spectrum for

our sight are to be dealt with outside the ‘research

environment’ and into actual ultra-precision produc-

tion.

9.10.1 Micro-Tooling

Introduction

e question oen posed when considering mi-

cro-tooling, is: ‘What constitutes a micro-tool?’ For

exa

mple, some automotive engineering companies

might consider micro-tooling to be <φ0

.95 mm, while

others in say the aerospace industries, would set an

arbitrary level at <φ0

.55 mm, conversely, in the medi-

cal and optical industries they would be more in-

clined toward diametral values of <φ0

.06 mm. So in

eect, it all depends upon a company’s ‘working-di-

mensional criteria’ ,

rather than an actual dimensional

size. Micro-tooling – such as endmills, are currently

commercially available in the USA, that have been

manufactured to φ0

.006 mm (i.e ≡ 6 µm), sitting quite

nicely within the ‘micro-machining dimensional tol-

erance zones’. Contrast this to other company’s, who

would even consider ‘nano-machining’ to operate with

orkpiece dimensional features set at <0.025 mm (i.e

≡ 2

5 µm)! So, there is even some confusion as to what

constitutes either: ‘micro-’ or, ‘nano-tooling’ and the re-

spective features that they might adequately machine.

So, as a ‘start-point’ ,

if we average these (above) three

518 Chapter 9

Figure 254. Relative dimensional sizes and scales, for:

• machining of accurate and presicion parts or

• equated to their respective measurement.

[Source: Smith, et al., 2002]

Machining and Monitoring Strategies 519

industrial-versions of what constitutes a ‘micro-tool’ ,

we obtain the following dimension for ‘our tooling’:

<φ0

.95 + 0.55 + 0.06/3 (mm) = ≈<φ0.52 mm, or by sim-

ply and conveniently rounding-down, let us consider

in our discussion, that any form of ‘micro-tooling’ is to

be set at, or below: φ0

.5 mm.

e geometric features of any ‘micro-tools’ cannot

simply be considered as minute version of ‘macro-tool-

ing’. More specically, we simply do not just scale-

down say, a φ1

2 mm drill, then manufacture its geom-

etry as a micro-tool of, for example: φ0

.25 mm drill

(Fig. 255a). Micro-tools have to be ‘engineered’ to pro-

vide eective chip evacuation, this is particularly rel-

evant when hole-making (Fig. 255b), yet remain rigid

enough to withstand the cutting forces generated and

not fracture under these conditions. In the following

sections concerning this review on micro-tooling, the

production processes of: drilling; milling; and boring

tools; will be briey mentioned.

Micro-Drills and Drilling

Probably the most signicant dierence when about

to utilise these minuscule tools, compared to their

macro-drilling counterparts, is that an operator can-

not even see what size they are, without suitable visual

magnication! is means that a micro-drill’s careful

handling – of these fragile tools, plus their safe stor-

age are vital. Micro-drills require correct containment

and appropriate labelling, allowing them to be readily

identied. Due to their minute diameters, micro-tool-

ing require rotational speeds of ≈1

00,000 rev min

–1

order to obtain the correct peripheral speeds, thereby

minimising the cutting forces acting on the micro-

eometry of the cutting edges. So that the minute

geometric features of the cutting edges are maintained

(Fig. 255b), it is desirable to have a very ne carbide

grain structure

– to strengthen the tool’s edges. If just

the smallest amount of uncontrolled lateral force oc-

curs, it can cause either tool edge fracture, or instigate

complete breakage. us, if a micro-tool’s edge is just

slightly chipped then this in itself may not adversely

63 ‘Micro-tooling materials’ , cemented carbide’s increased ri-

gidity over other tooling materials, makes is susceptible to

fracture. A good substitute micro-tool material is M-35 co-

balt steel, it is a compromise between carbide drills and those

manufactured of M-2 and M-7 HSS. Heat generated drilling

holes, will ‘roll’ a drill’s edge, thus it becomes: dull; ploughs;

and breaks. Cobalt improves drill ‘red-hardness’.

aect cutting, but a 5 µm edge-chipping on a φ100 µm

tool will radically modify the tool’s geometry and seri-

ously impair its cutting performance. is miniscule

cutting edge modication can cause the tool to break,

or damage the part’s features – requiring very careful

handling of such micro-tooling, in order to obtain the

optimum cutting performance.

A drill’s feature that needs to be modied from that

of its comparable ‘macro-drilling’ equivalent, is the

drill’s web ( i.e. see Fig. 47- bottom), this being the cen-

tral portion of the tool that extends axially along the

ute – gradually thickening as the distance increases

from the tool’s point. So for a micro-drill, the web is

proportionally thicker, because there has to be some

‘core-strength’ to a micro-tool. By way of illustration,

n a micro-drill a web of say just 25 µm is simply not

robust enough, as such, it would not work. To reduce

a micro-drill’s stress and prevent it from binding in

the hole, a back-taper

is purposely ground – playing

a major role in drilling eciency. While, another mi-

cro-geometric consideration for these minute drills is

its cutting edge sharpness, becoming of critical impor-

tance as the relative tool diameter gets smaller – this

being a limitation to eective micro-machining. For

example, on a macro-drill, if the cutting edge has a

5 µm cutting edge radius, it is considered somewhat

sharp, but this would hardly be the case for a micro

drill. Moreover, on a micro-drill if the cutting edge

adius is 10 µm and it is taking a 2 µm chip load, its

not just considered as ‘dull’ , but it is highly-negative

raked! e PVD-coating process on micro-drills can be

successfully accomplished, if the drill is manufactured

with a reduced width of edge preparation, thus main-

taining its sharpness (i.e by way of illustration, Fig 18

shows typical ‘edge-preps’ for macro-cutting inserts).

In any form of micro-drilling operation, very high

spindle speeds are necessary, for example, if a φ0

.2 mm

micro-drill is utilised, then the spindle speed should

be ≈8

0,000 rev min

–1

, in order to prevent the creation

of high drill thrust and torque forces, which might

64 ‘Micro-drill back-taper’ , this is where a slight decrease in

the drill’s diameter is being specically peripherally-ground,

decreasing in size from the drill point up to and toward the

shank.

NB Back-taper on a micro-drill is generally relieved by be-

tween: 5 µm and 13 µm; because the ute’s lengths are usually

<25 mm. Conversely, on a ‘macro-drill’ this back-taper lies be-

tween: 13 µm and 25 µm per 25 mm of length.

520 Chapter 9

Figure 255. Some typical micro-tooling: drills and boring tools.

Machining and Monitoring Strategies 521

otherwise lead to premature tool breakage. While an-

other note of concern when micro-drilling is its pene-

tration rate. If too high a drill feedrate is programmed,

then the micro-drill will immediately fracture. Some

micro-drilling manufacturing companies either rec-

ommend a single-ute assymetric drill geometry – al-

lowing high chip loads, coupled with an ecient chip

evacuation process, conversely, another approach is to

increase the number of drill utes, but this may cause

chip evacuation problems with ‘sticky’ workpiece

materials. At present, micro-drills can normally drill

holes with L/D ratios of 5:1, but it is anticipated that

these L/D ratios will soon be up to 10:1. In any micro-

drilling operation, the rst few revolutions of the drill

are crucial (Figs. 49 and 50), as the drill’s point expe-

riences eccentric forces as it enters the cut, with any

workpiece irregularities causing the drill to ‘walk’ – re-

sulting in its bending, breakage, or at the very least

some ‘helical wandering’ (i.e axially – see Fig. 70) as

the drill penetrates into the part. In order to mini-

mise the eccentric forces as a micro-drill enters a hole,

many micro-drilling manufacturers recommend that a

pilot hole (Fig. 50b) of between 1-to-2 drill diameters

deep is produced, utilising a short and rigid pilot drill.

A pilot drill’s point angle (

i.e. see Fig. 46 – top) should

have an included angle that is either identical, or

greater than that micro-drill producing the nal hole.

If smaller included angles were selected, as the drill en-

ters the pilot drilled hole, this causes the micro-drill’s

cutting edge to chip. is tool wear-eect is because,

as the micro-drill’s more shallow point angle initially

contacts the previous pilot-drilled hole – with its more

acute angled geometry, as the micro-drill enters work-

piece, this contact will take place at the outer edges of

the lips before the drill point touches the hole’s surface.

In lieu of a pilot-drilled hole, then begin the feedrate

at somewhat less than the nishing feed, or perhaps,

utilise a ‘pecking-drilling action’ – drill to a predeter-

mined depth, partially withdraw the drill, then drill

deeper into the workpiece, once more partially with-

draw the drill, then repeat this sequence. ‘Pecking’ has

the further benet of avoiding dwell at the bottom of a

‘

blind hole’ , this being an important surface integrity

feature with work-hardening materials.

ole tolerances that have been satisfactorily micro-

drilled in a range of workpiece materials are of the

rder: ± 5 µm, with tolerance-in-roundness (TIR) of

<2.5 µm. By utilising coolant delivery at high pressure,

either through-the-drill, with the ‘larger drill sizes’;

or alternatively ood coolant for minute micro-drills;

usually allows a 30% increase in cutting speeds coupled

to extended drill life. Although care should be made

when utilising through-coolant drills

, as their small

coolant hole diameters will simply clog unless the cool-

ant has been passed through s

ome form of micro-ltra-

tion unit, to remove ‘nes’ and other types of potential

clogging debris.

Micro-Mills and Milling

Over the last few years, with the advances in cutting

tool materials in combination with that of cutting tool

technology, has led to signicantly smaller milling

cutter diameters with more complex geometries be-

ing produced (Fig. 255-bottom le). In fact, several

important technologies have developed during the last

decade to assist the cutting tool manufacturers to cater

for the micro-machining industries. Probably the most

important of these new technology applications is the

design and development of very high accuracy six-axis

CNC tool grinding machines, having temperature con-

trol and coolant condition monitoring – these being

key elements in the cutter-grinding process. Grinding

tolerances held by these machines on say, an φ0

.12 mm

ball-ended end mill, must be within <2.5 µm (TIR).

Complementary to these multi-axes CNC grinders,

has been the improvements in diamond grinding

wheel technology, in conjunction with appropriate

grinding wheel metrological inspection techniques,

that have contributed to the signicant advancement

in micro-tool manufacturing quality and productivity.

Whilst continuing our discussion on the cutting tool

material front, micro-milling cutters are now being

produced from extra-ne grain cemented carbides, al-

lowing sharp cutting edges in conjunction with good

milled surface nishes. For example, one Japanese mi-

cro-tooling manufacturer oers a standard range of

‘

micro-mills’ , from: φ5 µm to φ5

5 µm; in incremental

65 ‘Ethanol’ , is an alternative applied lubrication strategy, to that

of either the usual water-based ood coolant method, or by

through-the-tool delivery. Here, ethanol is a form of alcohol,

occurring naturally in the sugar fermentation process, its

benets are that it has less-than-water viscosity, enabling it to

penetrate into the tool/chip interface in a superior fashion to

that of other coolants. Ethanol is usually delivered to the cut-

ting zone in the form of a spray-mist. While another bonus

of the application of ethanol is its low evaporation point, giv-

ing it eciency in cooling and as a lubricating agent for tool

spindle speeds (i.e. of up to and including) >60,000 rev min

–1

Moreover, ethanol simply evaporates and this eect negates

any disposal costs, while it provides a slight ‘chilling-eect’ on

the part, minimising thermal growth problems on miniature-

sized components.

522 Chapter 9