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:
to
reduce friction between the rolling elements and the separator or cage, and
between the rolling elements and the races at any point where true rolling is
absent.
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.HARDWATER
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.
