Smith G.T. Cutting Tool Technology: Industrial Handbook

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tion of the mouth and throat; by inhalation, will ir-

ritate the lungs; by ingestion, of the stomach – it may

promote nausea; and aect other internal organs.

If exposed to toxic mists over a long period of time,

this could cause lasting damage to both external and

internal bodily-parts, with at the extreme condition,

promoting the growth of malignant tumours. In order

to restrict misting and minimise operator health risks,

then special-purpose ltering systems have been de

eloped, which will be briey reviewed below.

e conventional mist-collection technology,

such as: lters; rotating drums; or cyclones; will col

ect particles of >1 µm in diameter,

ut cannot cope

with smaller sub-µm particles. Further, it has been re-

orted that brous lters once they are wet, lose ef-



ciency over time – see Fig. 212. erefore, the opti-

um manner of removing sub-µm mists are by tting

one of the following: High-eciency Particulate Air l-

ers (HEPA); Electrostatic Precipitators (

ESP’s); or Fi-

bre-bed s

ystems. Probably the two best systems for re-

moval of sub-µm mist particles are the HEPA and ESP

systems. Each one has its disadvantages, with HEPA l-

ers being expensive and become clogged, thereby los-

ng eciency. So, when disposable lter replacements

are needed this hidden replacements cost, will result

in both costly maintenance and disposal. While,

SP’s

need frequent maintenance and cleaning, thus rep

esenting a continuous on-going cost burden. Mean-

hile, Fibre-bed systems oer high eciency in mist

collection, but with ease of maintenance, although

they are larger requiring more electrical power to op

rate them.

Vegetable Oil-Based MWF’s

Driven by the health and safety concerns of both

workers and manufacturers alike, vegetable oil-based

MWF’s have been developed, to substitute for the same

machining operations as either the mineral-, or petro

eum-based uids, currently undertake. It has been

reported that compared with mineral oil-based cut-

ing uids, the alternative vegetable-based MWF’s, en-

ance cutting performance by extending tool life while

improving machined surface texture, with the addi-

ional benet of being an environmentally-friendly

MWF. In particular, Soybean oils h

ave shown con-

siderable promise as a practical alternative to ‘tradi-

ional’ MWF’s, where they have improved component

surface texture and reduced tool chatter. One of the

principle reasons for these surface texture and ma

hining improvements, is that the vegetable oil-based

MWF’s have enhanced lubricity, coupled with a slight

‘polar-charge’ – which acts to attract the vegetable

oil molecules to the metallic surface being tenacious

enough to resist any subsequent wipe-o. e oppo

ite is true for a mineral-based oil, where there is no

molecular charge, so oers little improvement in lu-

ricity.

Mineral-based MWF’s are just straight hydrocar-

on, while their vegetable oil counterparts contain

oxygen, which is tenaciously-attracted to the sterile

elevated temperature of the recently-machined work

iece’s metallic surface, thus it bonds more strongly

– acting as a result as a better lubricant. Yet another

performance benet of utilising vegetable-based oils

over their mineral-based equivalents, is that they have

a higher ‘ash-point’

, which reduces both the ten-

dency for smoke formation and re-risk. Yet another

reason for selecting a vegetable-based MWF over its

mineral-based counterpart, is that it has a high natural

viscosity

. Hence, as the machining temperature

increases, t

he viscosity of the vegetable oil drops more

slowly t

han for that of a mineral oil. Conversely, as the

temperature falls, t

he vegetable oil remains more uid

than i

ts counterpart mineral oil. us, facilitating more

ecient and quicker drainage from both the swarf and

workpiece. e high viscosity index

of vegetable oil

ensures that it provides more lubricity-stability, across

the operating temperature range being found during a

range of machining operations. High viscosity allows

vegetable oils to be used as a slideway lubricant and

for gear lubrication in gearboxes, acting as a so-called:

‘multi-functional uid’ (

i.e. see Section 8.9).

Along with the above stated benets, there is also

a down-side to vegetable-based uid applications, the

limitations are that they lack sucient oxidative sta

34 ‘Flash-point’ of oils, is the instantaneous ignition of the oil at

a specic temperature, without the aid of a ame. So, in the

case of a Soybean oil it has a ash-point of 232°C, while a typi-

cal mineral oil has a ash-point of just 113°C.

35 ‘Viscosity’ , c

an be dened* as: ‘e resistance of a uid to shear

force.’ erefore, the shear force per unit area is a constant

times the velocity gradient, with the constant being the coef-

cient of viscosity. SI units are: Newton-seconds per square

metre (Ns m

–2

), denoted by the Greek symbol: ‘µ’. [Source:

Carvill, 1999]

*While another denition for a uid’s viscosity is: ‘e bulk

property of a uid, semi-uid, or semi-solid substance that

causes it to resist ow.’ [Kalpakjian, 1984]

36 ‘V

iscosity index’ , can be dened as: ‘A measure of a uid’s

change of viscosity with temperature: the higher the index, the

smaller the relative change in viscosity.’ [Kalpakjian, 1984]

Cutting Fluids 423

bility for many machining applications. us, a low

oxidative stability means that the oil will oxidise quite

quickly during use, becoming thick as it polymerises

to a plastic-like consistency. Once the oil has become

too thick, or even too thin for that matter, the cutting

tool’s edge(s) will quickly wear-out. Vegetable oils be

ome oxidised and as a result, will chemically change,

along with their viscosity and lubricating abilities.

ere is some concern among users of vegetable-based

cutting uids, that this oil reacts with the environment

(i.e. oxygen and metals), thus breaking-down, which

is not the case for petroleum-based products. Both of

these uid products oxidise with heat, but vegetable

oils are more susceptible to oxidation. While another

Figure 212. At the lter some droplets and volatiles are re-

moved from the atmosphere, but the remainder pass through

and are re-entrained. Other particulates are ‘indenitely’ re-

tained, but with time reduce lter eciency. Optimum lters

. maximise droplet removal, while minimising evaporation and

re-entrainment – at a reasonable pressure-drop. [Source: Raynor

P. & Leith, D. – Univ. of North Carolina, 2003]

424 Chapter 8

drawback to utilising vegetable-based oils, are its lack

of hydrolytic stability

. Typically, when making an

emulsion; obviously oil and water are present; so if ox-

gen and some form of alkaline component is at hand,

it may cause certain ester linkages within the vegetable

oil to break down. ese broken-down components

act in a dierent manner to that of the original vegeta

le oil, thereby aecting its cutting uid performance.

Conversely, mineral-based cutting oils are resistant to

hydrolytic reactions. Vegetable oils can support micro

ial growth more readily than the equivalent mineral-

based cutting uids. Although this vegetable oil’s bio-

egradability is ideal for subsequent

aste treatment,

conversely, when this product is ‘festering’ in a ma-

hine’s sump, it becomes both smelly and sour, via its

bactericide and fungicide reactions. Finally, for many

companies, probably the biggest limitation in changing

over to vegetable-based products in machining opera

ions is its purchase cost. For example, canola oil, can

cost up to 300% more than its equivalent petroleum-

based product and to compound the nancial problem

still further, costly ingredients are necessary to control

oxidation and enhance its biological stability – consid

rably adding to the nished product’s cost.

8.12 Fluid Machining

Strategies: Dry;

Near-Dry; or Wet

So far, this chapter has been principally concerned

with all aspects of ood/wet coolant applications to the

overall machining process. Several other complemen-

ary cutting strategies can be adopted, these include:

dry; near-dry; together with wet machining; thus, in

the following sections a discussion of these important

issues and concerns will be briey mentioned.

37 ‘Hydrolytic stability’: ester molecules consist jointly of con-

densed fatty acids and alcohols; so the vegetable oils will

naturally exist as esters – oen termed ‘triglycerides’ , these

being a condensation of fatty acid, plus glycerine. Under the

right conditions, the triglyceride can split and revert back to a

fatty acid and glycerine, which acts dierently from that of the

original ester. In the case of mineral-based oils, they do not

contain these ester linkages and as such, will not break down,

nor ‘hydrolise’.

8.12.1 Wet- and Dry-Machining –

the Issues and Concerns

In the past twenty-ve years the cost of cutting u-

ids has risen from just 3% of the overall

ost of the

machining process, to that of >15% of a production

shop’s cost. Cutting uids and especially ones that are

oil-based products have become something of a liabil

ty of late, this is due in the main, to many countries

‘Environmental Protection Agencies’ , strictly regulat-

ng their ensuing disposal – at the end of their natural

life. In many countries ‘spent’ cutting uids have been

re-classied as either ‘toxic-’ , or ‘hazardous-waste’ ,

moreover, if they have been found to have machined

certain alloyed and exotic material workpieces, they

are under even harsher disposal regulations. Due to

the increasing popularity

oday of high-speed machin

ng (HSM) – more will be said on this subject in the

following chapter – coupled to increased cutting data

and the application of coolants via high-pressure sys

ems, these factors have signicantly contributed to

the creation of air-borne mists within the workshop

environment (i.e. see Fig. 212). Such coolant mists

now have even stricter permissible exposure levels

(PEL’s) imposed in the working environment, to regu

ate and control these air-borne particulates, thereby

minimising workers health risks. us, the cost of:

uid maintenance; record-keeping; with strict compli

nce to current and proposed regulations, have rapidly

increased the overall price of cutting uids. In many

manufacturing companies involved in a signicant

amount of machining operations, they are consider

ng the strategy of cutting dry, to overcome the cutting

uid-based costs and disposal concerns during and

aer their subsequent usage.

For many companies involved in signicant work

iece machining operations, they are unsure if they

could cut all their components ‘dry’. Furthermore, they

are under the impression that to achieve higher cutting

data and ‘hard-part’ machining, then cutting uids

are essential in achieving these objectives. Moreover,

many companies also believe that the cost of chang

ng from a ‘wet-’ to ‘dry-machining’ operations would

be prohibitively high. Neither of these impressions are

true. So, by machining ‘dry’ it can be considered as a

standard operational procedure for most metal-cut

ing operations, including: turning, drilling and mill-

ng operations (i.e see Figs. 39, 49 and 168a, respec-

ively). Moreover, it is not only possible to ‘hard-part’:

turn (Fig. 15) and bore (Fig. 65b); or mill (Fig. 172);

etc.; but these can now be classied as highly-prot

ble ‘dry-machining’ activities.

Cutting Fluids 425

Probably the chief obstacle to dry-machining ac-

ceptance, is that conventional wisdom dictates that

MWF’s are vital in attaining acceptable machined n-

shes and will considerably extend the tooling’s life. In

many circumstances these are valid points, but with

some of the advanced cemented carbide grades and

high-technology coatings, such tooling can be oper

ted at higher cutting data than was previously the

case and, cope with their elevated machining tempera-

res. In fact, if interrupted cutting o

ccurs, the hotter

the cutting zone b

ecomes, the more unsuitable will be

the application o

f a cutting uid – as the thermal shock

becomes greater with a ‘wet-machining’ strategy.

Present tool coating technologies are vital to dry-

machining applications, as they both control the tem

erature uctuations, while restricting heat transfer

from the cutting vicinity to the insert, or tool. Mul-

iple coatings act as a heat barrier because they of-

er a lower thermal conductivity to that of the tool’s

substrate and the workpiece material. us, coated

inserts/tooling absorb less heat and as a result, can tol

rate higher cutting temperatures, allowing more ag-

ressive cutting data, whilst not debilitating the tool’s

life. Coating thickness is also important, as the thin-

er the overall coatings, the better they can withstand

temperature uctuations, that might otherwise arise,

if thicker coatings

ere utilised. e main reason for

this improved thermal shock performance of thinner,

rather than thicker tool coatings, is that a thinner coat

is less likely to incur the same stresses, hence, they are

less susceptible to cracking as a result. So, by running

thin coatings in ‘dry-machining’ operations, normally

extends tool edge life by up to 40%, over thicker coat

ings

38 ‘ermal shock/fatigue’ , the cyclical nature of both rapid

heating followed by immediate cooling – in for example face-

milling (i.e see Fig. 213 – top), or when interrupted turning

(e.g. when eccentric turning, or OD/ID machining with either

splines and keyways present), promotes potential tool edge

fracturing – resulting from the cyclic thermal stresses and in-

creased temperature gradients, being exacerbated by the ap-

plication of a cutting uid.

39 ‘

in coats-v-thick coats’ , the former coating oers longer life

than the latter coating process. Today, it is normal to utilise the

coating process of: physical vapour deposition (PVD) as this

type of coating is thinner and will adhere/bond more strongly,

than the alternative chemical vapour deposition process. For

example, a TiAlN PVD coated insert/tool can have a hardness

of 3,500 Hv, withstanding cutting temperatures up to 800°C.

‘Dry-machining’ – some Factors for Consideration

•

Adopting a ‘dry machining’ strategy will only make

sense, if all t

he cutting processes in the part’s manu-

facture can be performed without coolant,

•

Only by utilising specialised cutting tool geome-

tries, can ‘dry-machining’ be possible and eective,

•

Tooling typically having special hard multi-layered,

or diamond-like coatings, e

tc., to isolate heat and

create minimal thermal conduction a

cross the tool/

chip interface,

•

Employing cutting tool materials producing sharp

edge geometries – t

o reduce heat,

•

For drilling operations, utilise ‘so-glide’ coatings –

for lubrication, w

ith the necessary and appropriate

ecient chip transportation

eometries,

•

Speedy and ecient removal of both chips and as-

sociated steam – b

y suction – are important factors

in ‘dry-machining’ ,

•

Utilise new machining concepts, plus the latest

fully-enclosed m

achine tools – whenever possible,

•

Employ faster, rather than slower cutting data, to al-

low t

he majority of heat to be conned to the evacu-

ated chips.

8.12.2 Near-Dry Machining

e strategy of ‘near-dry’ machining is not a new con-

cept, it has been in existence for more than 50 years.

However, this machining and lubricating approach

is still not a universal practice, which is surprising

when one considers the real benets that accrue from

the practice over its ‘wet-machining’ counterpart. As

its name implies, in ‘near-dry’ machining little lubri

ant is used – normally vegetable-based oils, meaning

that both cutting uid treatment and its disposal are

eliminated. Further, instigating a ‘near-dry’ machining

strategy means that there are fewer worker health risks

from resultant mists, which might otherwise create: re

NB From a metallurgical/materials science viewpoint, the:

TiAlN – PVD tool coating can attribute its superior mechani-

cal/physical properties to an amorphous aluminium-oxide

lm that forms at the tool/chip interface, as some of the alu-

minium of the coating surface oxidises at these elevated ma-

chining temperatures. While, even more exotic multiple-type

diamond-like coatings can be applied and their like, which of-

fer even greater cutting performance – in certain machining

circumstances, when applied to the tool’s cutting edge(s).

426 Chapter 8

spiratory problems: skin irritations; etc. e ‘near-dry’

cutting approach can be exploited across a wide range

of either ferrous, or non-ferrous workpiece metals.

Most machine tools are equipped with the capabi-

lity of supplying ood coolant to the cutting process,

together with ‘through-coolant’ tooling systems, mean

ng that the cost to recongure for that of a ‘near-dry’

technology is not prohibitive. Assuming the worse-

case scenario of requiring a through-coolant tool

ng system, then probably just over $5,000 at today’s

prices should prove sucient capital to complete the

task. Some re-tooling to complement the ‘near-dry’

machining production techniques may be necessary,

allowing the

recise application of lubricant to the cut

ing edge(s). Further, the user must consider a method

for ecient chip removal from the cutting area. Usu-

lly, with external ‘near-dry’ cutting operations, the

lubricant is transported within the media of a com-

ressed air application, via the correct-sized aperture

nozzle – pointed toward the cutting zone. Control of

the volume of lubricant delivery to the tool and work

iece area is critical, with the common misconception

being that more l

ubricant is better! e optimum ar-

rangement for ‘near-dry’ lubricant application, is when

the minimum o

f over-spray and resultant misting does

not occur.

With external ‘near-dry’ operations, dispensing

systems usually consists of reservoir metering pumps

and valves, being mounted on the machine tool’s exte

ior – at some convenient location. While the nozzles

are strategically-mounted so that they can easily be

directly aimed at the tool’s cutting edge(s). Normally,

the nozzles are a manufactured from either copper,

or plastic and ‘snap-together’ – being much smaller

in size than their ‘wet-machining’ counterparts. For

internal machining operations, having tooling with

‘through-the-nose’ delivery, the lubricant is mixed

with compressed air prior to delivery to the cutting

zone. e admixing of compressed air and lubricant

keeps the lubricant in suspension, with

hese oil par

icles being broken-down into minute particles prior

to being fed into the compressed air jet stream – on

their way to the tooling.

For ‘conventional’ ood coolant delivery the sys

ems, the coolant channels are lled with cutting uid,

which inevitably nds its way to the cutting zone. If

however, in a ‘near-dry’ machining conguration, a

heavy mist of lubricating oil oats through the com

ressed air, attempting to negotiate all of the twists

and turns on its way to the cutting zone, this may pres-

nt a potential lubrication clogging/starvation prob-

em. Hence, for a successful ‘near-dry’ delivery system,

the lubricant channels

eed to be smooth and even,

with direct ows from the coolant pump to the cutting

zone.

A basic misapprehension by some machine tool

designers and manufacturers, is that copious volumes

of ood coolant are necessary to remove large quanti

ies of swarf. In fact, just the opposite can occur, as

wet chips will not only pack tightly together but have

a surface tension property to them, tending to make

them adhere to machine tool surfaces (i.e. see Foot

ote 29, ‘Lang’s chip-packing ratio’ i

n Chapter 2). is

is not generally the case for ‘near-dry’ lubrication, as

the chips here, have a thin layer of non-oxidising lu-

ricant surrounding them and with their evacuation

velocity – aer being machined, coupled to gravita-

ional eects, means that they will fall to the bottom

of the swarf tray, or into the chip conveyor. It is good

working practice to use the external air-only supply’s

blow-o nozzles to clear away chips form the cutting

area

, however, it is not recommended to use the oil/

mist to achieve chip clearance, as it will simply blow

the lubricant straight past the cutting edge(s), while

probably creating an unwanted oil-misting problem.

It is possible to incorporate both ‘wet-’ and ‘near-dry’

lubrication systems onto

he same machine tool. It

has been reported that for external/internal work the

change-over from one system say, from ‘wet-’ , to the

other – ‘near-dry’ , takes about 3 minutes to complete.

For ‘near-dry’ machining to be successful, it de

ends upon various factors, including: workpiece ma-

erial to be machined; tool geometry and its coating(s);

speeds and feeds selected; plus other important fac-

ors. If applied correctly, ‘near-dry’ machining has sig-

icant direct and indirect benets to the machining

process as a whole.

Economics of: ‘Dry-’; ‘Near-Dry’;

and ‘ Wet-Machining’.

For any tool and workpiece lubrication strategy to

operate eectively, a range of cost factors need to be

considered, regardless of the method of machining

40 ‘Chilled compressed air’ , has been successfully utilised in the

past for not only removing chips from the cutting vicinity,

but on certain materials, the continuous application of chilled

compressed air acts simply as a form of ‘basic lubricant’ for

the cutting process in hand.

Cutting Fluids 427

undertaken. In Fig. 213, a table has been constructed

to show the relative merits of the three machining

strategies previously discussed, namely: ‘dry-’; ‘near-

dry’; or ‘wet-machining’. e cost component for each

of these lubrication strategies has been broken down

into its relevant parts, with some of them not being ap

licable to every lubrication application. If one ignores

the individual cumulative factor in the overall cost

and simply looks at the ‘bottom-line’ , namely, the total

relative costs for each process, then a clear message is

being given here! Explicitly, that ‘wet-machining’- in

certain cases, when compared to ‘dry-machining’ is

Figure 213. Indicates the comparative costs for utilising either: ‘dry-’, ‘near-dry-’ or ‘wet-machining’ strategies.

428 Chapter 8

>330% more expensive overall, this being a good rea-

son to look carefully at employing ‘dry-machining’

techniques – when applicable!

In Appendix 14, a MWF ‘trouble-shooting guide’

has been included, to help establish the relative causes

and remedies for certain uid-related problems – as

they arise.

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430 Chapter 8

Machining and

Monitoring Strategies

‘Inque brevi spatio mutander saecla animantum

Et quasi cursores vitai lampada tradunt.’

TRANSLATION:

‘e generations of living things pass in a short time

and like runners hand on the torch of life.’

Titus LUCRETIUS Carus

(94 – 55 BC)

[In: On the Nature of the Universe, II]

9.1 High Speed Machining

(HSM)

Background to HSM

Possibly the rst work of note on HSM was that of Sa-

lomon who ran a series of applied experiments from

the period 1924 to 1931, when a German Patent was

granted for this work. e Patent was founded upon a

series of cutting speed curves plotted against machin-

ing temperatures for a range of materials (Fig. 214). In

these tests Salomon achieved peripheral cutter speeds

f 16,500 m min

–1

, utilising either y-cutters – for chip

morphology data, or helical milling cutters – notably

when cutting aluminium. Salomon contended that

the cutting temperature ‘peaked’ at a specic cutting

speed, which he termed the ‘Supercritical speed’

, fur-

ther, when the speed was increased still faster the tem-

perature he noted, decreased. Moreover, either side of

this ‘supercritical speed’ zone, it was suggested that

‘unworkable regimes’ occurred where the HSS cut-

ters could not withstand the severe forces and tem-

peratures generated. As mentioned, when the cutting

speed increased beyond this ‘zone’ the temperature

reduced to those expected by ‘normal data’ cutting

conditions, permitting practical cutting operations to

be carried out. e problem being with this early HSM

work is that any theoretical rationale is not available

and the experimental procedures are somewhat un-

clear, but Salomon can be considered the founder of

ultra-high speed machining (UHSM) – taking cutting

1 ‘Supercritical speed’ mentioned by Salomon when HSM, has

never been truly substantiated. e practical data was based

upon chip morphology* experiments with ‘y-cut‘ climb-mill-

ing of: non-ferrous alloys; ‘so’ aluminium; red cast brass; the

latter being ‘un-machinable’ with HSS cutters between 60 to

330 m min

–1

(i.e. see Fig. 214). As the machining parameters

and experimental details only exist as a partial fragment of the

original German work. is fragmented machining informa-

tion, is because during the Second World War, unfortunately,

the vital details were lost, moreover, none of the participants

in this work also surviving to explain how this applied re-

search data was collated.

*Chip morphology was achieved by situating a heavily wax-

coated board, this being strategically positioned to allow the

y-cut chips to stick to the board – during the peripheral

climb-milling experimental operation, ready for future analy-

ses.

data beyond that considered in the so-called ‘Taylor-

equations’.

Eectively it can be established that there were four

distinct periods of advancement in the eld of UHSM,

with the rst one being from the early 1920’s to the late

950’s, with each period during this time and there-

aer, being separated by a signicant event. Obviously

during the rst period, the work instigated by Salomon

(i.e. in the 1920’s – alluded to above), was followed by

the originally-commissioned United States Air Force

(USAF) major research contract, being from 1958

to 1961. Previous to this contract award, little in the

way of UHSM work in the preceding decades had oc-

curred, apart from that in the States by Vaughn (1958).

Vaughn, shortly became aware of the Salomon Patent,

acquiring limited information on this work through

the United States Consul in Berlin. Vaughn’s (Lock-

heed) group were also familiar with the many tech-

nical references concerning the ‘art’ of oil well tube

perforation utilising perforated cutters, these ‘cutting

actions’ being employed to perforate oil well casings

at explosive speeds. Such background work, now meant

that Vaughn had ‘set the scene’ for the second period

of UHSM development.

e USAF Materials Laboratory commissioned

study – mentioned above, which was awarded to Lock-

heed (i.e. Vaughn’s group), with the objective of evalu-

ating the ‘machining response’ for a selected range of

high-strength materials to that of (surface) cutting

peeds of up to 152,400 m min

–1

. e ‘principal aim’

of this ‘USAF-commissioned work’ was to increase

producibility, while improving both the quality and

ciency of the fabrication of aircra/missile com-

ponents. Vaughn’s experimental apparatus included

the use of modern-day ex-military cannons, which

were positioned on ‘railed-sleds’ to minimise subse-

quent recoil upon ring, while obtaining the desired

ballistic exit velocity of the ‘material-slug’ for the cut-

ting speeds

. Unfortunately, these ‘machining results’

2 ‘Ballistic cutting speeds’ , were obtained as the cannon red

the projectile (i.e. a specic material-slug) at ultra-high speed

out of the cannon. At the projectile’s exit from the cannon’s

barrel, a very robust cutting tool arrangement was situated

and held in an accurate position to take a linear cut along the

exiting slug’s periphery. e evidence of this ballistic cutting

action was then recorded by very high-speed photographic

equipment – this being both electronically-timed and stra-

tegically positioned, for visual dynamic recording and future

reference and analysis.

432 Chapter 9