Smith G.T. Cutting Tool Technology: Industrial Handbook

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•

Sulphur – to act as an ‘extreme pressure’ (EP) ad-

ditive to reduce frictional eects at the various cut-

ing interfaces.

NB S

ulphur was soon to prove unpopular as a sat-

isfactory EP additive, as it had a tendency to stain,

or erode certain decorative machined nishes for

specic metals and alloys.

8.2 Primary Functions

of a Cutting Fluid

In the previous section, it was recognised that two of

the primary functions of a cutting uid was to cool and

lubricate both the workpiece and cutting tool’s edge. In

addition, one could add the improvement of machined

surface quality and an increase in tool life (i.e. see Fig.

193b). Further, it has been shown that a reduction in

spindle power is an added bonus to many machining

processes, which oers considerable savings when this

reduction in electrical demand is accrued per annum.

If a problem occurs where work-hardened swarf dis

osal from the cutting vicinity presents an obstacle

to ecient cutting, then ushing this zone with ood

coolant, m

ay eliminate this diculty. Eective chip re-

moval by the application of ood coolant (Fig. 194a:

showing a twist drilling operation, 194b: milling with

the periphery using a ‘porcupine cutter’), can mini

ise an otherwise serious problem on machining cen-

res where large volumes

f densely-packed swarf can

impede the cutting process. Even on continuous cut-

ing operations such as when undertaking external/in-

a high contact angle (i.e. the angle of tangency that the spheri-

cal cap, or a bubble makes with the surface). When this angle

is considered on a ‘wetability scale’ it has a high contact angle

and as such, is not considered as ‘wet’!, due to its high curva-

ture (Fig. 208a), as indeed does a typical lubrication oil. Con-

versely, a liquid detergent does not have particularly a high

contact angle and as such, will chemically react with both the

oil and water and breaks-down this surface tension between

the concentrations of an unmixed oil and water. is loss of

surface tension between these two ‘products’ thereby produces

a basic mixture, or suspension and it then becomes somewhat

‘milky’ in appearance, thus it is oen termed a (basic) ‘emul-

sion’.

ternal turning processes, an insert’s chip-breaker will

break the swarf into convenient shapes and sizes, but

these chips may still necessitate ushing-away – being

deposited into a swarf conveyor and then onward into

an adjacent skip. Swarf removal has become of sig

icant importance as material removal rates have in-

reased with latest tooling advances and high-produc-

ion machine tools, where they may be continually fed

wrought material allowing them to operate untended

for 24 hours per day. Possibly

he most stringent test

for any cutting uid is in deep-hole drilling applica

ions (Fig. 58), where coolant is delivered under high-

pressure through suitable coolant holes and is forced

up to the cutting edge to not only cool the drill, but

provide lubrication and ush any swarf back and away

from the cutting vicinity. In fact, with extremely high-

pressure coolant delivery systems having pressures

>300 MPa, such as when using a through-the-nose

indexable insert short-hole drill as illustrated in Fig.

195, the advantages are: increased speeds; penetration

rates; more holes per insert edges are achievable – see

the following section for more

etails on this high-

pressure coolant delivery topic, with particular refer

nce to turning operations.

8.3 High-Pressure Jet-

Assisted Coolant Delivery

Probably the most important criteria in many metal

cutting operations is an acceptable chip control, with

respect to its: chip form; chip-ow; plus its chip-break-

ng ability. It has been mentioned earlier in Chapter 2:

Section 2.5, that good chip control will have an aect

on: tool life, machined surface texture; cutting forces;

reliability; etc. Productivity is strongly inuenced by

poor chip control, as the machine tool must be fre

ently stopped to manually remove the vast quanti-

ies of swarf present in the working area. is problem

becomes especially acute when turning smaller inter-

al diameters on products, since limited space soon

becomes lled and compacted with work-hardened

chips, that can damage the recently machined surfaces.

Reasonable chip control can oen be achieved by in

exable inserts with an appropriate cutting geometry

that is having chip-formers, these being developed to

meet the requirements for specic machining opera

ions.

Cutting Fluids 383

Figure 193. Heat dissipation during machining can be lessened by utilising appropriate cutting uids.

384 Chapter 8

Figure 194. ‘Standard-pressure’ (i.e. <40 bar) coolant supply for drilling and milling operations.

Cutting Fluids 385

Trends of late, have been toward either ‘dry-’ , or

‘near-dry’ m

achining strategies – more will be men-

tioned on these machining applications later in the

chapter, however many modern materials cannot be

machined dry, even with the latest coated cutting in

erts, because of the high temperatures generated in

the cutting vicinity. Typically, alloys such as: austenitic

stainless steel; high-temperature alloys; titanium, etc.;

demand the application of appropriate cutting uids.

In industrial machining applications, the availability

‘high-pressure cooling’ (

HPC) of cutting tools has

proven to be very eective when machining the met-

ls just mentioned, while at the same time increasing

production throughput. By

tilising a high-pressure

uid jet, it is possible to signicantly decrease cut

ing zone temperatures, while extending tool life – in

certain instances by >200%, operating with lower cut-

ing forces because of the improved frictional condi-

ions between tool/chip interface, with an attendant

reduction in machining-induced vibration levels. All

of these advantages will improve the machined surface

texture and oer better and more consistent dimen

ional accuracy, by a reduction in component process

variability.

Figure 195. High-pressure coolant supply for high penetration rate drilling. [Courtesy of Sumitomo Electric Hardmetal Ltd.].

386 Chapter 8

When utilising high-pressure jet-assisted machining

at cutting uid pressures of >110 MPa with a velocity

of >122 ms

–1

, some precautions need to be considered,

prior to applying this cutting uid strategy. Caution

should be made, when using certain types of cutting

tool geometries and grades, as they may not have been

designed for this increased level of coolant delivery,

which could if

inappropriately a

pplied, actually lower

productivity. e above mentioned eects do not only

depend on eective heat dissipation, but require the

contact length between the chip and the rake face to be

reduced. Since the application of a coolant by a high-

pressure jet, partially penetrates between the tool/chip

interface, via a ‘hydro-wedge’

which here is created

and, then provides hydrodynamic lubrication at this

position in the ‘friction zone’. Hence, the shorter the

contact length the lower the friction, causing a larger

shear angle, which in turn lowers the chip compres

ion factor (Fig. 196). is ‘hydraulic wedge’ – a

s a re-

sult of HPC, inuences the chip formation in several

ways, it aects both the ‘up- and side-curl’ , thus break-

ng them into manageable pieces as well as vectoring

the chips. By aiming the HPC cutting uid jet to either

the main, or secondary

utting edges this will inu

nce and aect chip-curling behaviour. is chip-curl-

ng action in turn, aects the resultant tool life, as it is

thought that a reason for this dierence in respective

tool life is due to the temperature distribution on the

rake’s face – as a result of the vectoring angle of the jet-

assisted coolant application.

8.4 Types of Cutting Fluid

Introduction

Modern cutting uids can be sub-divided into two

major classications: ‘Oil-’ , o

r ‘Aqueous-based’ , with

further sub-division into ‘Semi-synthetic’ , o

r ‘Synthetic’

uids (i.e. see Fig. 197, for a ‘family-tree’ and break-

2 ‘Hydrodynamic wedge’ , as its name implies, cannot actually

penetrate into the chip/tool interface, as the separation pres-

sures here – at the interface – are simply far too high. How-

ever, this hydrodynamic wedge acts as a sort of ‘lever’ (Fig.

196) on the emerging formation of the curling chip, changing

its contact length, which in turn, modies the shearing zone

and as a result, inuences the chip compression factor.

down of these cutting uid groupings). In most tech-

nological countries, relevant Standards for both the

chemical and technical requirements are published

concerning their: storage, usage and disposal, along

with their pertinent operator health needs. e ‘Aque

us-based’ cutting uids can be divided into either:

‘emulsiable’; o

r ‘water-soluble’ types (Fig. 197). e

former ‘Oil-based’ c

utting uids are supplied as ready-

to-use products, while the ‘Aqueous-based’ p

roducts

are normally oered in the form of a concentrate,

which must be admixed with water to the desired con-

entration, prior to use. Once these latter products

have been mixed with water, the ‘emulsiable’ v

ersions

form an ‘emulsion’

, whereas the ‘soluble’ type forms a

‘solution’

. In both cases, the resulting cutting uid is

termed ‘water-mixed’.

The ‘Ideal’ Cutting Fluid

Having accepted the fact that a cutting uid is a re-

quirement for the machining of many of today’s en-

ineering

aterials, be they either metallic, non-me-

allic in composition and are necessary for various

production processes. en one must ensure that the

selected ‘uid’ achieves its intended purpose, more

ver, that it does not create additional problems. ese

conditions imply that there are many and varied spe-

ic characteristics that an ‘ideal’ cutting uid should

possess, such as:

•

Optimum cooling and lubrication – clearly, the

‘ideal’ cutting uid would have the most favourable

cooling and lubricating properties, to ensure para-

ount cutting performance as measured by: pro-

uction rate; tool life; surface texture,

3 ‘Emulsions’ , are a disperse system (consisting of several

phases), which arises through mixing of two liquids which are

not soluble in each other. Hence, one liquid forms the inner,

or disperse phase, distributed in droplet form in the carrier

liquid (the outer, or continuous phase).

NB e emulsiable metalworking uids are what is known as

‘oil-in-water’ emulsions, that is the oil forms the inner phase,

conversely, its counterpart is formed by emulsifying metal-

working uids, which are ‘water-in-oil’ emulsions. (Source:

Cincinnati Milacron/Cimcool, 1991)

4 ‘Solutions’ , a

re a metalworking uid solution, these are water-

soluble uids mixed with water. (Source: Cincinnati Milacron/

Cimcool, 1991)

Cutting Fluids 387

Figure 196. The eect of rake angle on chip thickness – with and without coolant supply .

388 Chapter 8

Figure 197. The main types of cutting uids for machining operations.

Cutting Fluids 389

•

Acceptability to the operator (i.e. when machine

tool is manned) – as all operators have some de-

ree of exposure/contact with the cutting uid. Op-

rators will consider the lubricant’s overall perfor-

ance, but even when the uid is ‘perfect’ in every

other respect, complaints are likely if, for example,

the smell was unpleasant. e following features are

likely to be of particular interest to the operator:

–

Smell – ideally, the cutting uid should have

no p

erceivable odour, but if present, it certainly

should not be objectionable,

–

Colour and clarity – most operators prefer prod-

ucts which are perceived to be ‘clean and fresh’

throughout their life and, some operators prefer

dye-coloured translucent products for this rea

son,

–

Misting – high-speed cutting operations tend to

generate a mist. Occasionally these mists may

be associated with operator health problems:

dry-throats; stinging eyes; etc.; leading to com

laints. Although misting is largely dependent

on the: machine tool; its operation; atmospheric

ventilation; etc.; dierent uids have diverse

misting characteristics and, ‘ideally’ the uid

should be non-misting – more will be said on

the operator’s health issues later in the chapter,

–

Irritation to the skin and eyes – these operator

issues have been associated with physical con-

act from cutting uids, such as: skin

nd eye

irritation; itching; rashes, swellings; stinging;

etc. Once again, uid formulations that are ‘kind

and gentle’ are preferred. As mentioned above,

more will be mentioned on these health issues

shortly,

•

Long ‘sump-life’- with all machining uids having

a nite life, a

t some point, the machine’s cutting-

uid system must be completely emptied, cleaned,

ushed through and relled with new uid. ere

are numerous reasons why the cutting uid might

be regarded as ‘dead’ – these points will be raised

when investigating the ‘problems’ with cutting

uids later in the chapter. e uid’s life is an im

ortant economic consideration in terms of: uid

usage; labour costs; down-time; etc. Some leading-

manufacturer’s cutting uid formulations are capa

le of achieving signicantly better overall perfor-

ance and have an extended ‘sump-life’ over their

cheaper contemporaries. e increased ‘sump-life’

will enable better use of a company’s maintenance

department’s manpower resources, thereby en

bling it to be more ecient in their anticipated

‘planned maintenance scheduling’

over prescribed

shut-down periods, o

r ‘maintenance windows’ ,

•

Corrosion protection – all cutting uids are for-

mulated to provide corrosion protection to the

machine tool and the workpiece during and, for a

short time aer the cutting operation. In the main,

these uids should preserve their corrosive-protec

ion properties throughout their useful

ife, to avoid

the potentially expensive problems of rusting of the

machine and machined components alike, the lat

er being rejected by the customer. More informa-

ion will be given on corrosion protection later in

the chapter,

•

Low foaming properties – on some of today’s ma-

chine tools, they incorporate uid systems that

agitate the cutting uid to such an extent that foam

spills out of the coolant tank and onto the oor.

e ‘ideal’ uid will withstand: swarf-washing jets;

high-pressure uid delivery; centrifuges; etc.; even

when prepared with the soest quality water sup

ly. e subject of foaming will be addressed spe-

ically later in

he chapter,

•

Machine tool compatibility – no self-respecting

engineer wants to see their newly purchased, or

well-maintained older machine being attacked by

its cutting uid. e optimum uid should create

no detrimental eects on the machine tool’s: paint

nishes; seal materials; screens and guarding; etc.,

•

Workpiece compatibility – means that the widest

possible range of workpiece materials should be

achined with a particular, but versatile grade of

5 ‘Planned maintenance scheduling’ , many companies adopt

either a ‘Total Productive Maintenance’ (TPM), or ‘Reliabil-

ity-centred Maintenance’ (RCM) philosophical and practical

approach to their overall maintenance organisational needs.

NB TPM is dened as: ‘A system of maintenance which cov-

ered the entire life of every piece of equipment in every division

including planning, manufacturing and maintenance. (Source:

Japan Institute of Plant Maintenance – JIPM, 1971). RCM is

dened as: ‘A process used to determine the maintenance re-

quirements of any physical asset* in its operating context’.

(Source: Moubray, 1996) Oen, companies run both a TPM

and RCM strategy together, to achieve an overall high level of

maintenance planning discrimination, coupled to plant secu-

rity – in association with individual asset reliability.

* A physical asset is any piece of operating plant, or equip-

ment that requires a maintenance function to be undertaken

upon it at some prescribed time period, or requiring various

modications to it for its operating context.

390 Chapter 8

cutting uid, although certain non-ferrous metals

may have a susceptibility to staining, so here, it is

prudent to discuss the problem with the cutting

uid manufacturer,

•

Water-supply compatibility – a water-soluble cut-

ting uid should ‘ideally’ be capable of being diluted

with any w

ater supply. Geographical locations can

create variations in water supply and its condition,

this latter factor is especially true for water hardness

(i.e see Fig. 199b), where its hardness can vary quite

considerably. us, the ‘ideal’ cutting uid would

not cause the typical problems of:

foaming i

n so

waters; or forming insoluble soaps i

n hard waters,

•

Freedom from tacky, or gummy deposits – as water

soluble uids dry out on a machine, or component’s

surface, the water content evaporates to leave a resi-

ue which is basically the product concentrate. is

residue should ideally be light and wet, allowing

it to be easily wiped-o. However, any gummy, or

tacky deposits collect swarf and debris, necessitat

ng increased machine and component cleaning,

•

‘Tramp oil’ tolerance – is a lubricating, or hydraulic

oil which leaks from the machine tool and contam-

nates the cutting uid. Most modern machines are

equipped with ‘total-loss’

slideway lubricating sys-

tems which can contaminate the cutting uid with

up to a litre of oil per day – on a large machine tool.

e ‘ideal’ cutting uid would be capable of toler

ting this contamination without any detrimental

eects on its operating performance. Some cutting

uids are formulated to emulsify the ‘tramp-oil’ ,

while other uid formulations reject it, allowing

6 ‘Total-loss’ uid systems, are as their name implies in that they

purposely leak oil to the machine’s bearing surface, requiring

periodic tank replenishment. When this oil leaks-out of the

machine tool it is termed: ‘tramp-oil’ , therefore the oil will

eventually end up in the machine tool’s coolant tank, where it

is either tolerated by the coolant product, or is separated-out,

requiring periodic ‘tramp-oil skimming’.

NB ‘Tramp-oil’ losses are invariably not accounted for in

many production shops, which invariably means their ‘eco-

nomic model’ for such losses are habitually not considered,

or not even thought about by the company. It has been re-

ported that on a quite ‘large-sized’ horizontal machining cen-

tre, it can lose up to 365 litres of ‘tramp-oil’ per annum, which

is an on-going cost that needs to be addressed. Multiply this

individual machine tool loss by the number of machines in

the manufacturing facility and this will represent considerable

unaccounted for expenditure!

the residual ‘tramp-oil’ to oat to the surface for re-

moval by physical ‘skimming’ ,

•

Cost-eectiveness – but what does this term mean?

ere was a time when the cost-eectiveness was

simply judged in terms of the price per litre of the

product concentrate. Fortunately, there are only

few engineering companies who still take this view,

with most recognising that there are many inter

elated factors that contribute to cost-eciency.

Some of these factors might be the: dilution ratio;

sump-life; material versatility; tool life; machined

component quality; health and safety aspects; plus

many others.

Having identied the ‘ideal’ cutting uid features, one

must unfortunately face reality, as there is

no s

uch

product that encompasses all o

f these desirable charac-

teristics – at the optimum level i

n just one cutting uid

product. However, all c

utting uids are not equal and

even apparently similar products may well perform in

quite dierent ways! erefore, it is for the machine-

shop supervisors/managers – in

onjunction with

other interested parties: purchasing; health and safety;

unions; etc., to select a reputable supplier who is pre

ared to undertake the necessary survey and ‘trouble-

shooting’ exercise to recommend the best uid(s) for a

particular manufacturing environment.

Today, there are many dierent types of cutting

uids available they can be classied according to

widely varying criteria, although some unied system

of terminology exists in various countries guidelines

and Standards. is commonality of ‘language’ reects

both the chemical and technical requirements of the

users. On the basis of the various countries publicised

cutting uid literature, the following classication

is perhaps the most useful – from the user’s point of

view. Broadly speaking, it was previously shown in Fig.

197, that cutting uid groups are of two main types,

either

‘oil-’ , o

r ‘aqueous-based’. e ‘aqueous’ cutting

uids can be divided into ‘emulsiable’ a

nd ‘water-sol-

uble’ t

ypes. As has already been mentioned, the former

‘oil-based’ cutting uids are supplied as ready-for-use

products, while ‘aqueous’ types are normally found in

the form of a concentrate, which must be mixed with

water, prior to use. Once mixed with water, the

‘emulsi-

able’ c

utting uids form an emulsion, conversely, the

‘soluble’ v

ariety forms a solution. In both of these cases,

the

esultant cutting uid product is termed: ‘water-

mixed’. I

n the following section, the various types of

cutting uids currently available will be briey men-

tioned.

Cutting Fluids 391

8.4.1 Mineral Oil, Synthetic,

or Semi-Synthetic Lubricant?

Mineral Oil

In order to manufacture cutting uids the raw materi-

als are naturally occurring oils, such as: mineral oils;

animal and vegetable oils; or fats. Of these oils, the for-

er mineral oils are probably most commonly utilised

by the manufacturing industry. ese mineral oils, in

a similar fashion to naturally occurring oils, tend to

be complex mixtures of widely varying compounds.

Such

compounds c

onsist of carbon and hydrogen and as

such, are usually referred to a ‘hydrocarbons’. I

n addi-

tion, they will contain: sulphur; nitrogen; plus various

trace elements.

So that the

mineral oil c

an be separated out to form

a ‘stock oil’ – w

ith natural lubricating properties, ther-

mal processes a

re employed by the uid manufacturer.

ese partly-rened ‘stock-oils’ are still chemically

complex mixtures of hydrocarbons, with widely vary-

ng characteristics. By way of an example of the di-

erse nature of ‘crude oil’ , it is a mixture of more than

one thousand hydrocarbons, with dierent chemical

structures. Such widely varying characteristics make

it impossible to supply mineral oil to closely dened

specications, which limits its uses and performance

as a cutting uid. e complex structure of a cutting

uid made up entirely from naturally occurring oils, is

schematically illustrated in Fig. 198a.

Synthetic Lubricants

e use of Synthetic lubricants cannot be compared

with those lubricants that are extracted from natu-

ally occurring oils, since the properties of the latter

are always an aggregate of the properties of their many

dierent components, as such, cannot be exactly pre

icted. While the former synthetic lubricants are made

from two types of raw material:

Mineral oil – normally from: polyalpha olen and

alkali aromatics,

2. Polybutenes.

t present (i.e. from around the late 1980’s, until

now), synthetic hydrocarbons predominate, as they

are not derived from mineral oils, they have become

of increased importance. In particular, they include

derivatives from

‘fractioning’

of plant oils. e most

signicant classes of compounds a

re the esters and

polyglycols. ese synthetic lubricants being a solution

of chemicals, which usually contain: corrosion inhibi-

ors; biocides; dyes; in water. Moreover, they may con-

ain such additions as synthetic lubricity additives and

wetting agents. Synthetic lubricants f

orm transparent

solutions a

nd as a result, provide good visibility of the

cutting operation.

In use, synthetic uids require special attention in

their application, because they contain no mineral oil,

they tend not to leave a corrosion-protective oily lm

on machine surfaces. As a result, it is essential to lubri

ate exposed machine tool surfaces carefully. In addi-

ion to this lack of protection, there may be some eect

on certain paint nishes and even degradation of the

machine’s seals, as a result of this synthetic uid enter

ng the machine tool’s lubrication system. Normally,

these problems of practical usage, limit these synthetic

lubricants in the main, to grinding operations.

Semi-Synthetic Lubricants

Today, the use of Semi-synthetic lubricants, or ‘Micro-

emulsions’ – a

s they are sometimes known, has become

much more widespread, because of certain advantages

they have over mineral-soluble oils.

By increasing the ratio of: emulsier-to-oil in the

formulation, either by reducing the oil content, or

by increasing the level of emulsiers, the product

takes on dierent characteristics from those of min

ral-soluble oils. Due to this increased ‘ratio’ , the oil

particles formed, are signicantly smaller than those

found with the mineral-soluble oil types (i.e. see Fig.

201a). Hence, these

‘micro-emulsions’ , v

isually appear

to be translucent, o

r even transparent, owing to the fact

that the oil particles a

re smaller than the wavelength of

light (

i.e. <0.5 µm). is translucency is an obvious ad-

vantage where workpiece visibility is important to the

machine setter/operator. In addition, the high level of

emulsiers in the product leaves some ‘spare capacity’ ,

which enables the ‘micro-emulsion’ to emulsify any

oil-leakage from the machine. is emulsication of

7 ‘Fractionation’ , is the breakdown of crude oil into its constit-

uents (i.e. fractions), by distillation.

392 Chapter 8