Cutting Fluids Used in Lathe Machine

A large amount of heat is generated during the process of metal cutting. The major part of this heat is carried away by the chip and the rest is taken by the job and the tool. This heat is harmful for the cutting edge of the tool. It has been observed that as the cutting speed increases the heat also increases. The formation of the built-up edge is also promoted due to this heat. To reduce this action, the heat can be effectively removed by using proper cutting fluids or coolants. The use of cutting fluids improves machinability, tool life and surface finish and also provides considerable reduction in machining forces   and chip interference.

A cutting fluid has the following functions:

(a) It carries away the heat and thereby prevents a dangerous rise in temperature.

(b) It lubricates the chip, work piece and tool and thus reduces the heat of friction.

(c) It improves finish on the job and saves it from rusting.

(d) It avoids welding between chip and tool by providing a film, which helps in reducing friction.

(e) It increases the life of tool.

(f) It flushes out the chips and keeps the cutting point clear.

(g) Sometimes a cutting fluid prevents expansion of work also.

(h) It improves machinability and reduces machining forces.

Main Properties of a Cutting Fluid

Basically cutting fluids are used for cooling and providing lubrication to the job. It should posses following main properties:

(a) It should fulfill all the requirements mentioned against its functions.

(b) It should not cause discoloration to the work.

(c) It should not give smoke or create fog when used.

(d) It should not give bad smell.

(e) It should also be ensured that the cutting fluid gives no harm to the operator.

Classification of Cutting Fluid

The cutting fluids are available in three main classes: -

(a) Cutting Oils - They include mineral cutting oils. These are further divided into two classes: -

(i) Active - Active cutting oils are used in most severe operations on the toughest steels. The triple combinations of fatty oil, chlorine and sulphur additives impart wettability, oiliness and anti weld properties. It enables the cutting oil to be effective over a wide range and extreme range of pressure.

(ii) Inactive - In Inactive cutting oils the extreme pressure additives are tightly bonded within the hydrocarbon structure of the oil and hence they are not released so easily for reaction. For machining bronze or other alloys the inactive oils are used.

(b) Soluble Oils or Water Emulsions  - Soluble oil is a blend of mineral oil, emulsifying agents and coupling agents. Additives like rust inhibitors, anti foam agents, water softening agents and bactericides are also commonly added to soluble oils. These are used in machining both ferrous and non-ferrous metals. Emulsion is formed by mixing soluble oil with water in the ratio of 1:10 to 1:40 for general machining and up to 1:80 for grinding. If the mixture is too weak, it may cause corrosion to the job and machine both.

(c) Gaseous Fluids - Gaseous fluids are rarely used because of their high cost. Mist is the most commonly used gaseous fluid. Modern mist coolant system use compressed air to atomise the coolant and carry it to the point of cutting in the form of mist. Gases like carbon dioxide, freon and helium are used for special applications. Gaseous fluids perform lubricating, cooling and flushing action.

Selection of Cutting Fluid for Different Materials

The following recommendations are followed in normal practice for machining different materials.

(a) Low Carbon Steel - Soluble oils are used for turning, drilling and milling operations. Soluble oil with one part of oil and 10 to 20 parts of water is most satisfactory. For tapping, mineral oils are used. For grinding, soluble oils (with 1: 80) are best. For broaching in steels, a heavy mineral oil or soluble oil emulsion is used. For Thread cutting, soluble oil with 1: 15 is used.

(b) Alloy Steel and High Carbon Steel - For Turning, mineral lard oil or 70% mineral oil and 30% Sulphur base oil is used. For Drilling, generally soluble oil with 1: 15 or mineral oil is used.

(c) Cast Iron - It is generally worked dry or compressed air may be used as coolant.

(d) Brass - It is machined dry when it is a cast brass. For brass rod straight mineral oil or mixture of mineral oil with 10% lard oil is used.

(e) Aluminium  - Some  of the Aluminium alloys have been successfully machined dry. But to attain best results kerosene oil mixed with 10% mineral oil is used.

Carbon dioxide finds its application in machining low machinable and high strength thermal resistant alloys. It extracts heat very rapidly from the chip tool interference. The application of carbon dioxide is limited because it is comparatively costlier coolant. It is, therefore, used for cutting at high speeds or machining special types of alloys.