Cutting Fluids: Functions and Selection | Industries | Metallurgy

It has been seen that the amount of power lost due to generation of heat varies from 20 to 30 per cent and hence to reduce it, the use of cutting fluids came into existence in the field of production-technology. Cutting fluids can also be considerably reduced by using suitable cutting lubricants when machining metals at speeds below 0.7 m/sec.

Function of Cutting Fluid:

The functions of the cutting fluids are given below:

(а) To cool the tool and work piece and conduct the heat generated in cutting away from the cutting zone. It is essential to keep tool temperature below 250°C and 650°C for hardened and tempered carbon steel and HSS respectively because they lose their hardness above these temperatures.

(b) To decrease adhesion between chip and tool, provide lower friction and wear, and a smaller built up edge. In fact, the built-up edge can often be inhibited by the use of suitable cutting fluid and thus superior surface finish obtained.

(c) Cutting action is improved due to increased shear angle (which increases the chip thickness ratio). Lubrication also induces chip curl by reducing the rake contact length. Surface finish obtained by using cutting fluid while machining metals at slow speeds is superior than while machining dry.

(d) To wash away the chips and keep the cutting region free.

(e) It helps in keeping freshly machined surface bright by giving a protective coating against atmospheric oxygen and thus protects the finished surface from corrosion.

(f) To decrease wear and tear of the tool and hence to increase the tool life. At higher cutting speeds, fluids serve mainly cooling purpose.

(g) It improves machinability and reduces machining forces.

(h) To prevent the expansion of the work piece.

The first three of these functions are of much importance and the remaining are of secondary importance and are mainly concerned with the production of fine-surfaces.

The desirable properties of a cutting fluid are lubricating quality, high heat absorption capacity, high flash point stability, emit no fumes while in contact with hot surface, and harmless to workers and workpiece. It must be understood that at high cutting speeds heating of tool approaches adiabatic condition and heat flow by conduction has a smaller effect and thus coolant can’t be effective in cooling the tool. Lubricants or coolants with tungsten carbide or ceramic tools are seldom used as these have detrimental effect on tool life by causing thermal cracking.

Classification of Cutting Fluids:

Cutting fluids used may be in the form of solids, liquids or gases. Their purpose is to decrease the friction between chip and tool. The solid cutting fluids are in the form of free machining metals additives to the work material, surface treatment to tool faces or finely ground solid lubricants deposited on a tool from a liquid or a gaseous stream. Solid lubricants normally used are graphite, molybdenum disulphide and stick waxes etc.

Liquids and gases (used in cutting operations) are broadly referred to as cutting-fluids.

The liquid cutting fluids are of two general types:

(i) Materials having water bases, and

(ii) Material having mineral oil bases.

Many additives are used in conjunction with each of these types of fluids to accomplish a variety of specific objectives. The most common gaseous cutting fluids are the oxygen and the water vapour found in the ordinary atmosphere, while air is not generally considered a cutting fluid but in some machining operations it plays an important role.

As regards lubrication action, the best and most effective cutting fluids in mineral oil containing sulphur or chlorine additives. Because of the high cutting temperatures encountered, the cutting oil is broken down and it chemically reacts with the underside of the chip material to produce low shear strength chlorides or sulphides thereby reducing shear stress.

A. Solid Lubricants and Cutting Fluids:

Solid lubricants are employed in a finely divided state and are kept in suspension in the liquid vehicle by means of a dispersing agent. Stick waxes and bar soaps are sometimes used as a convenient means of applying lubricants to the cutting tools, but their applications are limited to tapping and sawing operations. In case of grinding operations, these solid lubricants are applied in the form of a paste which consists of finely divided talc in conjunction with fatty oil and sulphur.

B. Liquid Cutting Fluids:

For efficient cooling, a liquid possessing high specific heat and good machining metal wetting properties is necessary. It should also have good lubrication qualities.

Liquid cutting fluids are classified in two classes:

1. Water Solutions:

Water is an excellent cooling medium having the maximum specific heat, but has little lubricating properties and causes rust and corrosion. Another disadvantage of using water is that it possesses high surface tension due to which it balls upon the surface and does not spread over the surface. Water itself is seldom used but with some alkali or antiseptics (soda ash and tri-sodium phosphate or chlorine), it is quite often used.

2. Oil Base Cutting Fluids:

They are also known as straight oils. They may be mineral oils of any viscosity or fatty oils.

Some of the important oil-base cutting fluids used in machining are described under the following heads:

(a) Soluble Oil or Emulsions:

They are non-expensive type of coolants. In it, oil is mixed with water in certain proportions and a small amount of soap is added as an emulsified agent. This emulsion contains finite number of minute and invisible particles which give the mixture a creamy or milk white colour. The proportion of water for one part of oil varies from 1 to 50. If the mixture is weak, i.e. low in oil, it may cause corrosion.

In the mixture, the water used is of soft nature. When these types of soluble oils are used during machining metals, air circulation is required so as to remove the water by evaporation. The remaining oil forms a protecting layer over the machined work-piece and saves if from rust and corrosion. In case of grinding operations, the paste oils mixed with water are used as cutting fluids.

(b) Straight Fatty Cutting Oils:

The important classification of straight cutting oils are as given below:

(i) Mineral Soils:

The chief use of mineral oil for blending with base cutting oil is to obtain the properties required during different machining operations. These oils are normally used for light machining operations. Low viscosity oil is preferable when both cooling and lubricating effects are desired.

(ii) Straight Fatty Oil:

Now-a-days, these types of oils are not much used, firstly because of harmful effects particularly to the health of the operator and secondly because of their gummy characteristics during machining operation. The most important variety of this type of oil is lard-oil.

(iii) Mineral Lard Oil:

It is a mixture of mineral and straight fatty (lard-oil). This type of cutting fluid is used where greater lubrication and heavy machining operations are required. The proportion of lard oil varies from 10 to 40% and increases as the hardness of work-piece increases. It is frequently used in automatic screw cutting operation.

(c) Sulphurised or Chlorinated Cutting Oils:

Sulphurised or chlorinated cutting soil are used where there is a high-chip bearing pressure, which is generally found while machining metals tough alloy steels. These oils are mixed with mineral oils and produce metallic oxide or a metallic film lubrication instead of fluid film lubrication.

As a result of heat generated by cutting tool, sulphur may be cooked with fatty oils such as lard oil, thus obtaining a compound (sulphur base which is added to the mineral oil) or sulphur can be directly added to the mineral oil by the application of heat.

Similarly chlorine is added to the mineral oils like sulphur for obtaining cutting fluids suitable for high chip bearing pressure. When sulphur is mixed in lard oil it is called sulphurised cutting oil and when chlorine is mixed in mineral oil it is called chlorinated cutting oil. Sulphurised and chlorinated oils have both the lubrication and cooling qualities.

(d) Aqueous Solution:

When the function of the cutting fluid is only to provide cooling action to the work-piece and tool and to wash away the chips, the aqueous solution is used. In consists of water containing some amount of alkalies which may be any one of the following: sodium carbonate, potassium carbonate and borax etc. Sometimes lard oil and soft soap may be used to improve the properties.

Such types of aqueous solutions do not provide any lubrication effect but sometimes cause corrosion. A typical aqueous solution consists of one part each of sodium carbonate, lard oil, soft soap, and remaining water to make 10 to 12 gallons of aqueous solution. It is first mixed thoroughly and then boiled for one hour preferably by passing steam.

C. Lubricating Oil Purification:

Filtration system comprises of a paper band filter and a magnetic separator. It provides automatic filtration of ferrous and non- ferrous contaminants from circulating coolants and cutting oils. It is suitable for use on all types of grinding and CNC machines. The contaminated coolant is fed through an inlet on to a rotating magnetic drum.

Magnetic drum separator is used for coolant filtration on all types of machines. Rotating Magnetic drum arrests all ferrous particles and these are then cleared-off into a swarf bin. The ferrous impurities adhere to the drum and are removed by the scraper.

The coolant is then made to flow on to the filter paper, which in turn filters the minute ferrous and non-ferrous particles. The paper, when choked moves forward automatically due to an electronic sensor device. The filtered coolant collects in the tank below and is re-circulated to the machine through a suitable coolant tanks.

D. Gaseous Type of Cutting Fluids:

Among the various types of gaseous cutting fluids, air (still or compressed), carbon dioxide and argon are the important examples of this class. Ordinarily air is seldom used because of its poor cooling, lubricating and anti-seizing properties. Its application is mostly limited to grinding and metal cutting processes where the main function of the cutting fluids is cooling.

It is found to be an effective coolant if it is used in sub-zero cooled state. Carbon dioxide is another most commonly used gaseous cutting fluid. It possesses an excellent heat extracting property, but its application is limited to low machinable and high strength thermal resistant alloys because of its higher cost.

E. Chemical Coolants:

Chemical coolants are mixture of a number of chemical components dissolved in water. All of them are used as coolants, but some of them very well exhibit lubricating properties also. Basically they are water modifiers and are recommended particularly for machining operations in which the main function of the cutting fluid is to act as a coolant.

They are classified in three groups:

(i) Pure Coolants:

They are made up mostly of water softeners and rust inhabiters and have very little lubricating properties.

(ii) Coolants and Lubricants:

In addition to pure coolant, such chemical coolants have extra property of wetting action, i.e. lubricating the surfaces.

(iii) Lubricating Coolants:

Coolants of such types are water softeners, rust inhabiters, wetting agents and also have chemical lubricating property. Chlorine, sulphur and phosphorus etc. are the examples of this class.

The following are most commonly used chemical constituents:

(i) Phosphates and Borates-for water softening.

(ii) Amines and Nitrites-for rust prevention.

(iii) Soap and Wetting agents-for lubrication and reduced surface tension.

(iv) Glycoles-as a mixing agent.

(v) Germicides-for controlling bacterial growth.

F. Minimal Lubrication Machining:

Considering ecological and environmental problems and stringent rules, trend is nowadays for minimal lubrication machining. This technology makes used of polycrystalline diamond pads and correctly dimensioned coolant channels. This technique results in same or better surfaces as with wet machining, good operation parameters (like speed), and high dimensional accuracy.

Selection of Cutting Fluid:

Depending upon the nature of work, the cutting fluids are selected on the following basis:

1. Viscosity:

It plays an important role in the selection of cutting fluid. If the viscosity is too low (mineral oils), the oil may be too volute and will also break in smoke when applied to the cutting zone. On the other hand, if viscosity is too high, the consumption of oil will be higher due to adhesiveness. Also heavy oil will keep dirt and chips in suspension for a longer time causing the reduction of the soil life.

2. Cost of the Cutting Fluid:

Cost is, of course, an important consideration in all engineering problems. In case of cutting fluid, it is not easy to ascertain the cost-wise value of one product over the other. It should be realised that the actual cost per kg of two cutting fluids considered may have little to do with their actual value.

The life of the fluid, loss during operation, allowable increase in production, type of finish and operation acceptance are also some of the other important factors which must be evaluated and weighed in assessing the relative desirability of any cutting fluid.

3. Chemical Action:

Fluids with higher carbon chains give low forces. Fluids with odd number of carbon atoms give less force while those with even number give high force.

Not only the chain length but also a minimum quality of surface product will be required to produce smooth finish and low cutting force. The result obtained by experiments performed with mercaptanes, disulphides, alcohols and chlorides are given in Fig. 26.5.

The lower coefficient of friction was achieved by using n-lauryl-mercaptane. A general decrease in coefficient of friction with increase in chain length may be noted. A cutting fluid with optimum concentration can reduce the cutting force and temperature but the area of contact also decreases and thus the quantity of chemical product is insufficient for new conditions so that cutting force and the temperature once more increases and fluctuations occur with alternate finished and rough surfaces.

This was observed in an experiment with 15° rake, tool cut length 40 cm, fluid used was n-decanol with benzene. Chip thickness ratio and force was observed to vary for an interval of six seconds which is very high to account for chatter or other mechanical vibrations.

No cutting fluid is best suited for cutting of ductile metals. Benzene is poor with almost all except copper. Turpentine is good with copper but poor with stainless steel, dianrial sulphide and CC1₄ are good in almost all cases cut CCl₄ is bad with lead. Acetic acid is very poor with Al.

Basic Actions of Cutting Fluids:

The basic actions of cutting fluids, which assist in cutting processes are:

(1) Cooling

(2) Friction reduction (lubrication)

(3) Reduction of the shear-strength of the work-material.

Cutting fluids initially used to act primarily as coolants. By flowing over a tool, chip and the work-piece, a cutting fluid can remove heat and thus reduce temperature in the cutting-zone. This reduction in temperature results in decrease in tool wear rate and an increase in tool life.

This occurs because, first, the tool material is harder and so more resistant to abrasion and wear at low temperature and secondly the diffusion rate constituents in the tool material are less at lower temperatures. Opposing these two effects, a reduction in temperature of the work-piece will increase its shear flow stress, so the cutting forces and power consumption may increase to some extent.

In addition to influencing tool-life, the cooling effect is of importance in reducing thermal expansion and distortion of work-piece. The cooling action also brings about some small improvements in surface finish, which is due to the production of thermal gradients in the chip, increases chip curl and reduces built- up edge formation.

The fluid that finds its way between the asperities between the chip and tool in the vicinity of the tool point is subjected to the following unique combination of conditions:

(1) High local temperatures, approaching the melting point of the chip metal on the high points of the surface asperities.

(2) High local pressure, approaching the hardness of the metal cut.

(3) Clean freshly produced surface.

(4) Highly stressed metal.

Under these conditions the chip may be made to react with the fluid to form a low shear strength solid lubricant. The thin layer of the solid thus formed prevents the formation of the weld between the chip and the tool hence reduces the coefficient of friction between the chip and tool. In this way, the cutting fluid is used in reducing tool friction.

When the coefficient of friction is decreased in metal cutting, not only a decrease in the friction will result, but also a decrease in shear work, due to the resulting increase in shear angle. An increase in shear angle corresponds to a decrease in shear strain which gives rise to a smaller shear stress, and hence the net result is a decrease of shear energy per unit volume when cutting with an increased shear-angle.

Application of Cutting Fluid during Machining Operation:

In order that a cutting fluid is most effective in lowering tool friction, it must penetrate to the point of the tool. The nature of chip-tool interface is shown in Fig. 26.1, where the hills and valleys that form a labyrinth of fine capillaries, are shown to a greatly increased scale.

Surface tension forces in the fluid and the action of a pressure difference of one atmosphere due to the tendency of forming a vacuous cavity by the tool as it penetrates the work-piece will cause the fluid to flow between the capillaries and reach the tool point against the adverse motion of the chip. The cutting fluid as it penetrates the very fine labyrinth of capillaries, is not in the form of liquid, but in the form of vapour.

The cutting fluid is carried to a point close of the tool-tip in the liquid state by capillary forces and is then converted to vapour upon absorption of some of the heat generated by cutting process. The vapour could then penetrate the capillaries, physically absorbing on the freshly cut nascent metal.

There are two possibilities from where the fluid enters, one at A and the other at B as shown in Fig. 26.1.

The motion of the chip across the tool face will tend to prevent fluid from reaching the tool tip from A and similarly the motion of the work-piece across the clearance surface of the tool will tend to prevent fluid from entering at B.

The relative velocity of the surfaces at B will be greater than that at A. The distance of intimate contact to be traversed at B will usually be much less than at A. The net result is that there will usually be a greater tendency for penetration along the clearance surface than along the tool face.

The effectiveness of the cutting fluids at higher speeds becomes ineffective particularly when the speed of fluids is very small. In order that a cutting fluid works effectively at higher speeds, high velocity streams are used.

This can be obtained from a small nozzle of 0.3 to 0.375 mm diameter through which fluid is forced under pressure of 20 kg/cm². This high velocity jet is thought to be useful in causing the liquid to reach more quickly at a point close to the tool tip. The two most important applications of the cutting fluids are shown in Fig. 26.2.

The process shown in Fig. 26.2 (a) is the conventional method and is readily adopted. By adopting the process shown in Fig. 26.2 (b) a considerable improvement in the tool life and surface-finish has been obtained.

Effect of Cutting Fluid on Tool-life:

By using cutting fluid effectively during the machining operation, the tool-life increases considerably. Carbon steel tools that have less heat resistance have maximum increase in tool-life (nearly 50%). In case of high speed steel tools, the increase in tool-life is nearly 25%.

Carbide tools may show slight improvement in tool life, but may also be damaged by thermal chipping. The effect on tool-life for different tool materials is represented on a log-log graph (Fig. 26.3) between cutting speed and time for a desirable amount of tool wear.

Disadvantages of Using Cutting Fluids:

It cannot be stressed too strongly that the use of cutting fluid must be economically justified. It costs money to apply a cutting fluid (the fluid itself, pumping system, collection and filtering system etc.) These costs must be covered by gains resulting from overall production, cost reductions or by better financial returns on improved products. In addition, cutting fluids may introduce undesirable side effects which can either preclude their use or restrict it to a limited range of operations.

Some of the side effects are as follows:

1. Physiological Effects on the Operator:

In the extreme cases some fluids, which are beneficial to the cutting process may produce toxic vapours which are harmful to health. An example of this class is an inorganic compound, carbon tetrachloride.

2. Effects on Work Material:

Work materials may be strained by certain fluids. For example copper alloys tend to be strained by oils containing a high proportion of the sulphur; titanium alloys get cracked when chlorinated cutting fluids are used, and rust on ferrous alloys when water is used.

3. Effects on Machine Tools:

The cutting fluid should not rust machine tool components, and sideways etc. Also in some cases they attack copper base bearings of the machine. These effects limit the use of cutting fluids to some extent.

4. Other Effects:

In some cases, the fluid may separate into various constituents when left for long period undisturbed, or it may form a thick gummy layer when it dries.