Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 198. The basic structure of an oil-based cutting uid and an ‘oil-in-water’ emulsifying molecule. [Courtesy of

Cimcool]

Cutting Fluids 393

Figure 199. The principle of polar and passivating corrosion protection and the minimum requirements for water

quality. [Courtesy of Cimcool]

394 Chapter 8

the oil

, keeps the machine tool cleaner and will delay

the formation of a layer of ‘tramp-oil’ on the surface –

which might otherwise encourage unwanted bacterial

growth. e

denition o

f Semi-synthetic cutting uids

can cause some diculty, but generally the oil content

is much l

ower than with the mineral-soluble oils, rang-

ing from approximately 10 to 40%.

Additives for: corrosion inhibition; bacterial con-

rol; lubricity

; EP; are employed in the same manner

as for mineral-soluble oils, also, there is oen an addi-

ion of a blue, or pink dye, as these translucent micro-

emulsions can appear to look somewhat ‘watery’ oth-

rwise. Although translucent micro-emulsions

initially formed, Semi-synthetics d

o not go cloudy in

use. ey contain excess emulsiers to ensure that ne

micro-emulsion of oil particles are formed in water. As

previously mentioned, these ‘spare’ emulsiers enable

the micro-emulsion to absorb tramp oil. Hence, as

these ‘spare’ emulsiers are consumed by suspending

the ‘tramp-oil’ , both the amount of oil in the emulsion

and the oil particle size increases. is

increase i

n oil

particle size causes m

ore incident light to be reected

and results in the visual ‘clouding eect’ w

ithin the lu-

bricant. In particular, this ‘cloudiness’ of the lubricant

is not necessarily an indication that there is anything

wrong with the uid,

t is merely an suggestion of the

oil absorbed by the cutting uid.

All cutting uids, whether ‘aqueous-’ , or ‘oil-based’ ,

may contain some: mineral oils; synthetic products; or

a combination of both. e choice of raw material and

composition depends on certain parameters and their

actual composition (i.e its formulation) will depend

8 ‘Emulsication of tramp-oil’ when using Semi-synthetic oils,

will only occur, until all of the ‘spare’ emulsiers are used up!

erefore, aer this time, the excess ‘tramp-oil’ will oat on

the cutting uid’s surface.

NB Some Semi-synthetic formulations will emulsify only

small quantities of ‘tramp-oil’ , while others can emulsify

much larger concentrations.

9 P

erhaps the easiest and best uid denition is this: ‘A semi-

synthetic cutting uid forms a translucent emulsion and con-

tains mineral oil’.

‘Lubricity’ , o

r ‘Oiliness’ as it is oen known, is dicult to de-

ne with any precision. One reasonable denition is that Lu-

bricity is: ‘[e signicant] dierences in friction greater than

can be accounted for on the basis of viscosity, when comparing

dierent lubricants under identical test conditions.’ [Source:

American Society of Automotive Engineers]

upon many factors, which is closely-guarded secret by

any lubricant

anufacturer.

8.4.2 Aqueous-Based Cutting Fluids

A large proportion of cutting uids used for machin-

ing operations are still of the aqueous-based types (Fig.

197), as they combine the excellent heat-absorbing ca-

acity of water, with the lubricating power of chemical

substances. Such cutting uids oer excellent cooling,

lubricating and wetting properties. Machine tools re

ire protection from the lubricant ingress and should

be compatible with lubricating and hydraulic systems

on the machine, making it possible to apply water-

mixed cutting uids to the manufacturing environ-

ment. e aqueous-based lubricants can be utilised

across quite a diverse range of workpiece materials,

ranging from steels, to non-ferrous metals.

An aqueous cutting uid can consist of naturally

occurring oils such as: mineral oil; synthetic mater-

ial; or a combination of both, but generally they are

present in the form of an emulsion, or solution – as

previously discussed. Other forms of cutting uids,

such as: suspensions; gels; pastes; are rarely used in

the production process. Hence, the commonest form

in which aqueous cutting uids are used is as an emul

ion. Much of this cutting uid terminology has al-

eady been discussed, but is worth restating, to ensure

that its signicance is suciently comprehended. An

emulsion is a disperse system formed by

ixing two

uids which are not soluble in each other. In the emul

ion, one of the uids forms the internal phase, which

is dispersed in the form of droplets suspended in the

external phase, or ‘medium’ – as its is oen known.

Such corresponding cutting uids are of two types:

‘emulsive’ , or ‘emulsiable’ – of which the former type

is normally the most commonly used. e ‘emulsive’

cutting uid consists of an oil-in-water emulsion, in

which the oil forms the internal phase. While its coun

erpart, the ‘emulsive’ type is the ‘emulsiable’ solu-

ion, consisting of a water-in-oil emulsion, but here,

the water is the internal phase – lately this cutting uid

has become less important.

An aqueous ‘emulsive’ cutting uid always contains

a stock oil, usually having a: mineral oil; synthetic

hydrocarbon; synthetic ester; or fatty oil, etc.; together

with certain additives to the formulation. e most

important additives tend to be: ‘emulsiers’; corro

ion inhibitors; stabilisers and solubilisers; anti-foam

Cutting Fluids 395

agents; micro biocides; as well as complex formers (i.e.

see Fig. 197). Consideration will now be given to each

of these ‘additives’ in turn:

‘Emulsifiers’

e ‘emulsiers’ are necessary to help form a stable

emulsion and as such, are very important for the tech-

ical characteristics of the cutting uid. ‘Emulsiers’

make it possible for the oil droplets to form and re-

ain suspended in water, preventing them from merg-

ng and oating upwards to form a surface layer in the

uid’s tank. ‘Emulsiers’ r

educe the surface tension and

form a lubricating lm a

t the boundary surface. ese

‘emulsier’ molecules are bipolar in characteristic and

as a result ‘line-up’ like the bristles on a brush, with

one end toward the oil and the other end facing the

water, as shown in Fig. 198b. In this way, the ‘emulsi



er’ forms a lm which is one molecule thick at the

boundary surface.

Corrosion Inhibitors

e main task of a corrosion inhibitor in any aqueous

cutting uid is to prevent the water in the uid from

corroding the exposed portions of the machine tool,

such as its: slideways; spindle nose; ballscrews; etc. e

mechanism by which dierent corrosion inhibitors

operate, will vary widely and one commonly used ver

ion of ‘inhibitor’ , consists of an additive

hich forms

a protective lm on the exposed metal’s surface

11 ‘Galvanic corrosion’ , for two metals in contact in the ‘electro-

chemical series’ the further apart they are in this ‘series’ , the

greater their electro-potential and the faster the rate of corro-

sion. For example, in this ‘series’ gold (i.e. being a ‘noble metal’)

is at one extreme, thus having a potential dierence of +1.70

v – being cathodic, while at the other end of the galvanic table,

calcium (i.e. being a ‘base metal’) has a potential dierence

of –2.87 v – being anodic. Hence, the anodic metal will cor-

rode, while the cathode remains unchanged, hence in gold’s

case, the term ‘noble’ metal is used.us, water-miscible u-

ids can penetrate between bolt threads, setscrews and xtures

and as water is an electrolyte – a liquid that can conduct an

electrical current, the presence of water produces a galvanic

electrical current ow between these mating parts. So, say on

a lightweight workpiece xture – perhaps made from alumin-

ium (–1.67 v) with this being located onto a machine tool’s

table – normally produced from cast iron (–0.44 v). us, the

potential dierence here being 1.23 v, which is not too acute,

as both these metals are in fact, relatively close-together in the

‘electro-chemical series’.

ese corrosion inhibitors consist of long, narrow

molecules which are negatively-charged and

s such,

are attached to the metal in contact (Fig. 199a – top

schematic diagram, shows:

rust protection b

y polari-

sation, w

hereas the lower schematic diagram depicts;

rust protection b

y a passifying lm). ese ‘lms’ that

are subsequently formed, are no thicker than just a

few molecules and as such, are invisible. Nevertheless,

such ‘lms’ can eectively prevent the electro-chemi

al process of corrosion, such as passivation by means

of nitride, but the latter type is now being eectively

phased-out.

Stabilisers and Solubilisers

Stabilisers considerably extend the life of the concen-

trate, w

hile solubilisers act to increase the oil’s solubil-

ity. V

arious alcohols and glycols can be used as stabi-

lisers, or solubilisers.

Anti-Foaming Agents

Anti-foaming agents are oen known by the alter-

native names of: ‘anti-froth-’; or ‘defrothing-agents’;

being utilised to prevent the formation of foam. Sili-

ones, while being subject to certain restrictions have

proved in the past to be very popular anti-foaming

agents. A typical restriction to that of using silicones

additives in machining operations, might be because

aerward it may prove dicult to either: paint; coat;

or adhesively-bond to the machined parts. In the

past when both the coolant pressures and ow rates

were low, foaming did not present too great a prob

em, but nowadays, the pressures and ow rates are

much greater and severe coolant agigtation can re-

ult, creating potential foaming conditions. Foaming

is at its most prevalent when a newly-charged clean

and fresh cutting uid is employed and as this coolant

is contaminated with: ‘tramp-oil’; metal nes; abra

ive grains; from the subsequent machining process,

these contaminants will tend to suppress foaming

tendencies.

NB Galvanic corrosion occurs between contact of dissimilar

metals – in the presence of an electrolyte. is electrolytic con-

tact might at the least cause either: surface staining; mild corro-

sion; or pitting, with its severity depending upon how long the

two metallic surfaces are in contact in the presence of water.

396 Chapter 8

Today, anti-foaming agents tend to be ‘branch-

chained’ higher alcohols – being insoluble in wa-

er, or as mentioned above, silicones. Both alcohols

and silicones evidently disrupt the foam-producing

surface lm with that of an alternative gas-perme

ble surface lm, causing the surface-active liquid

surrounding each bubble to drain away, causing the

foam layer to collapse. If severe foaming occurs, anti-

foaming agents are not the answer, as eventually these

‘anti-foams’ get carried away, or ltered-out of the

coolant on the resultant machined: chips and swarf;

workpieces; or on coolant lters. e problem to

foaming may

not b

e due to the lack of ‘anti-foams’ ,

ut may be the result of air leaks that are sucking air

into the coolant stream. ese air leaks oen arise

around the pipe unions, or at pipe-connectors to

either the valves and pumps in the coolant delivery

system.

Microbiocides

Microbiocides are oen added to the aqueous-based

cutting uid as they help prevent the dramatic and un-

ontrolled growth of microbes in the coolant. Micro-

biocides uses are normally limited, owing to the po-

ential skin-care consideration – more will be said

concerning this very important topic later in the chap-

er, when ‘health-issues’ will be discussed.

8.4.3 Water Quality

e main constituent of any aqueous-based cutting



uid is obviously water and by nature, it is impure. e

impurity depends on the source: rain-; river-; spring-;

ground-water; etc. e water may also contain: dust

particles; oxygen; nitrogen; calcium and magnesium

salts; oen with smaller quantities of: ammonia; bo

on; ourine; iron; nitrate; strontium; aluminium;

arsenic; barium; phosphate; copper and zinc. Addi-

ionally, the water has in its presence micro-organ-

sms, such as: algae; bacteria; fungi and viruses (i.e.

see Fig. 203); although in dierent orders of magni-

de. So, depending on its composition, water can

aect the aqueous-based cutting uid in many ways

and since the composition varies throughout the

year, these seasonal variations will have an eect on

its use. By far the greatest eect on the properties of

the cutting uid is caused by the hardness of the water.

Water’s hardness depends on the concentration of ele

ments

such as: calcium, magnesium and other heavy

metals like iron and manganese. Hard water may cause

a soapy deposit, which will eventually block lters, or

destabilise the emulsion and may have a detrimental

eect on the uid’s corrosion protection. Equally, so

water can be a problem, but for a dierent reason, in

this case it can promote foaming under ‘abusive’ ma

hining conditions.

e degree of alkalinity of the water can be ex-

ressed as a pH-value (

i.e. see the pH-scale shown in

Fig. 202b) and this is an important measurement, as

it aects its usage and can react to human skin

caus-

ing ‘serious complaints’ – more will be said concerning

these health issues later in the chapter. Alkalinity in

the main, aects the growth of microbes (i.e. see Fig.

203b) and the degree of corrosion protection aorded

12 ‘Water hardness levels’ , are calculated based upon the quantity

of ‘grains’ of hardness minerals the water contains. By way

of example, one grain of calcium carbonate, constitutes 17.1

parts million

–1

(ppm) per 3.785 litres (i.e. equivalent to a U.S.

gallon). ‘Salts’ such as sodium chloride and sodium sulphate

are found in hard water, where they contribute to corrosion, or

rust – if not ‘inhibited’. Moreover, the greater the cutting uid’s

solution salt content, the more coolant concentrate is required

to prevent subsequent corrosion. Further, coolant degradation

occurs with time and usage. For example, a new charge of

relatively so water admixed with coolant concentrate, will

initially have say, a 3-grain hardness, but aer one month’s use

its hardness will have increased to between 12–14 grains and,

aer two months this hardness will have increased still further,

to between 24–27 grains. is problem is exacerbated if the

water content evaporates, needing periodic cutting uid analy-

sis to maintain optimum coolant performance.One method of

signicantly reducing water of its hardness minerals, is to run

it through a water soener, which removes the calcium and

magnesium ions, replacing them with sodium ions, although

residue build-up will be signicantly reduced, corrosion may

now be a problem, so for this reason soened water is not

recommended when using water-miscible coolants. Other-

wise, boil the water – ensuring that no soener, or anti-cor-

rosion agents were present prior to using the condensed water

product (i.e from the boiling process). Deionized water is the

best source of pure water, as a deionizer removes all dissolved

minerals, creating distilled water.

13 ‘Human skin’ , varies from one body-region to another, but

generally, it has a pH-level slightly biased toward the acidic

region of the scale, at approximately 6.8 pH (e.g. a value of 7.0

pH is considered as ‘neutral’).

NB Skin also has a protective layer of natural oils, that act

to retard moisture evaporation, acting as a form of ‘defensive

shield’ against some forms of biological attack.

Cutting Fluids 397

by the emulsion. If alkaline levels increase this results

in improved protection, particularly when machining

ferrous workpieces. In view of the importance of water

composition for the eectiveness of a water-mixed

cutting uid, it is essential to know the quality of the

water source available and to take account of this fac

or when selecting a concentrate. Cutting uid manu-

acturers undertake water analysis, as do local water

companies. In Fig. 199b, the minimum requirements

for water quality for aqueous-based cutting uids is

shown.

8.5 Cutting Fluid

Classification – According

to Composition

Generally speaking, cutting uids are purchased under

the following classications, according to

heir com-

position:

•

Synthetic uids – are those cutting uids which

contain very little, or no natural oil. e various

components such as the actual cutting uid are

nely distributed in water, as such, they form a

watery transparent solution – shown in a schematic

representation in Fig. 200a. e applications of

synthetic cutting uids range from light-to-heavy

cutting, together with usage in grinding applica

ions. In order to ensure the necessary lubricating

power desirable for heavy cutting operations, some

of these products contain synthetic lubricants (Fig.

200b). e major properties of synthetic cutting

uids can be summarised as follows:

–

A very clean and transparent uid,

–

Excellent corrosion protection,

–

A long life cutting uid,

–

Outstanding cooling capabilities,

–

Easy to mix,

–

Does not burn, or smoke.

•

Semi-synthetic uids – can contain up to 41% oil

and when mixed with water they have a translucent

property (Fig. 200c). Extreme pressure (EP) addi-

ives and synthetic lubricant can be added, in order

to widen the range of potential workpiece materials

and applications. e properties of semi-synthetic

cutting uids can be summarised in the following

manner:

–

Very clean in appearance,

–

Excellent corrosion protection,

–

Long life of cutting uid,

–

Outstanding cooling capabilities,

–

Good wetting properties,

–

Easy to mix,

–

Does not burn, or smoke.

•

Emulsion uids – contain a high proportion of oil

and when the concentrate is mixed with water it

has a ‘milky appearance’ (Fig. 201a). Cutting uid

products intended for very heavy cutting operations

additionally contain EP additives (Fig. 201b). e

properties of an emulsion cutting uid, are sum

arised below:

–

Clean,

–

Oer good corrosion resistance,

–

Long life of emulsion,

–

Outstanding cooling capabilities,

–

Easy to mix,

–

Do not burn, or smoke.

Finally, for all of these various cutting uid types and

compositions, the dierences in the range of applica

ion of: synthetic; semi-synthetic; emulsion uids; de-

ends upon the respective machining requirements.

general, the heavier t

he cutting operation, the higher the

cutting forces p

roduced and the greater the oil content

required. is observation, means that synthetics a

normally used for lighter cutting o

perations, whereas,

emulsions a

re usually utilised for heavy-cutting appli-

cations, while the semi-synthetics t

end to be employed

as a general-purpose (

i.e. alternative) cutting uid.

8.6 Computer-Aided

Product Development

e latest cutting uids are very complex products and

a considerable amount of research and development (R

and D) is required to perfect them. e quantity of raw

materials that have diering characteristics and the

number of interactions between them, means that the

possible combinations are potentially enormous. Even

when most of the possible combinations are obviously

unnecessary and hence could be disregarded, this still

leaves the possibility of many thousands of coolant ad

itive permutations and their respective interactions

to investigate, which would be a ‘Herculean task’ to

398 Chapter 8

Figure 200. Schematic representation of synthetic variaties of cutting uids. [Courtesy of Cimcool].

Cutting Fluids 399

decipher and then to optimise! To press the point still

further, this situation of determining the optimum

combination is analogous to that of: ‘looking for a

needle in a haystack’ , where the conventional empiri

al methods become no better that in eect, searching

at random! Luckily a solution is at hand, by the evalu-

tion using computer technology, when utilised with

specially-developed programs. Computer-aided prod-

ct development will as a result, eciently provide a

solution backed-up by statistical techniques, enabling

many thousands of combinations to be assessed, re

ucing the nal choices to just a few cutting uid com-

inations. In this way it

s possible to rapidly and ac-

urately optimise the solution, as depicted in Fig. 204,

where a Computer-aided Design (CAD) application is

used to select – in this case – a corrosion inhibitor for

a potentially-new cutting uid. Such computer-based

techniques have brought about a means of develop

ng cutting uid products, using the CAD to not only

‘screen-out’ formulations which do not t the present

machining requirements, but can also uncover previ

usly unsuspected properties – resulting form syner-

Figure 201. Schematic representation of emulsion varieties of cutting uids. [Courtesy of Cimcool].

400 Chapter 8

Figure 202. A cutting uid emulsion’s diametral size (0.2 to 1.5 µm) in comparison with micro-organisms and ‘tramp oil’, together

with the ‘pH scale’. [Courtesy of Kuwait Petroleum International Lubricants]

Cutting Fluids 401

Figure 203. Bacterial contamination: aqueous cutting uid.

402 Chapter 8