Cutting fluids, of which cutting oil are by far the most important type, are liquids applied to the tool and material at their point of contact during machining. They facilitate the machining operation in various ways. In the first place they keep the tool and work piece cool. This reduces the rate of wear of the tool, and by preventing excessive expansion it simplifies the task of machining the work piece to the exact final dimensions required. Secondly, cutting fluids reduce power consumption by their lubricating properties. Thirdly, they prevent welding of the chip to the tool, an effect which is liable to occur when tough materials are machined. They also have various auxiliary effects such as improving surface finish, washing away chips and acting as protective against corrosion.

Cutting fluids are divided into three main classes, 'soluble' oils, 'neat/straight' oils and 'water base' fluids. Soluble oils are emulsifiable with water and are normally used in the form of aqueous slurries. The main types of straight cutting oils are extreme pressure (EP) oils, which are designed for neat use. Water base fluids are true solutions of salts and other materials in water. 

The properties required in a cutting fluid depend on the operation. For rapid, light machining such as turning and grinding, cooling power is usually more important than the lubricating power but the reverse is true of slow, heavy operations such as broaching. Soluble oil slurries and water base fluids have greater cooling power but less lubricating power than straight oils, consequently there is a tendency to use them for light operations and straight oils for heavy work. This general rule is however, considerably modified by various other factors, such as the type of metal being cut and whether the operation is roughing or finishing.

SOLUBLE OILS

These oils form stable emulsions with water. (Strictly speaking the term 'soluble ' is a misnomer, because emulsions are true solutions). These are opaque and clear types of soluble oil. The opaque type forms a milky-white emulsion, which does not permit a clear view of the job allowed by the clear-type, the translucency of which is of special value in precision work.

The following points concerning the storage and use of soluble oils are important:

A. STORAGE

Exposure of packages to moisture or to excessive heat or cold should be avoided.

B. MIXING

To make the emulsion, add the necessary quantity of oil to water (never the water to the oil), meanwhile stir constantly. Do not use hot water or steam. Use absolutely clean vessels.

C. HARD WATER

The use of excessively hard water to make the emulsion may cause separation of emulsion, possibly followed by the rusting of the work piece or machine tool. This can be avoided by softening the water (upto 40 ppm CaCO3 is adequate) before making up the emulsion. 

D. MAINTENANCE OF EMULSION

During use the concentration of oil in the emulsion becomes lower because more oil than water adheres to the metal shaving and finished parts. It is therefore, necessary to make the additions to maintain the strength of emulsion. This is done by adding the concentrated emulsion prepared separately for the purpose. To know how much of this to add, the strength of the improvised emulsion should be measured. A way of doing this is, to add 10% by volume of strong hydrochloric acid to a quantity of the emulsion in a measuring glass. The acid breaks up the emulsion, the oil rises to the top and the amount can be read off. From this the required percentage of oil to water in the concentrated emulsion can be estimated.

E. APPLICATION OF SLURRY

In applying the emulsion it must be directed on to the work in an adequate stream. It must not form a spray, nor contain air bubbles formed in the circulating system.