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18 Surface finishing processes
Grinding
Grinding is an operation performed by a rotating abrasive wheel, which removes
metal iSrom the surface of an object. It is usually 2i finishing operation which gives
a good surf ace finish and high dimensional accuracy to workpieces which have
already been machined by other methods. Very little metal is removed in this
operation. Grinding is also used to machine materials which are too hard to be
machined by other methods.The different types of grinding processes are:
(1) External cylindrical grinding, including centreless grinding (2) Internal
cylindrical grinding (3) Surface grinding (4) Form grinding.
External cylindrical grinding is used to produce a cylindrical or tapered surface
on the outside of a workpiece. The workpiece is rotated about its own axis as it
moves lengthwise while in contact with a revolving grinding wheel.
Internal cylindrical grinding is used to produce cylindrical holes or internal
tapers on a workpiece. The workpieces are rotated about their own axis, while the
grinding wheel rotates against the direction of rotation of the workpiece.
Surface grinding is used to produce flat surfaces. The work may be ground by
using either ihQ periphery or the end face of a grinding wheel. Form grinding uses
specially shaped grinding wheels to accurately finish surfaces previously machined
to a special shape like threads, gear teeth and spline shafts. Complex forms like
punches and dies can be ground by using special machines calledyVg grinders .
Centreless grinding is a method of grinding in which the workpieces are not held
between centres (Fig 18.1). The work lies on a work rest between the wheels. Both
grinding and regulating wheels rotate in the same direction.The work and the
regulating wheel are fed forward, forcing the work against the grinding wheel. The
axial movement of the work is obtained by tilting the regulating wheel.
Abrasive wheels
The abrasives used in grinding wheels are small particles of silicon carbide or
aluminium oxide. Grinding wheels are made by using a suitable material to bond
116 18 Surface finishing processes
the abrasive particles together. Different particle sizes and different kinds of
bonding materials are used to make a whole range of grinding wheels, each
suitable for a different type of work and finish. Silicon carbide wheels are used to
grind low tensile strength materials, such as the tips of cutting tools, ceramics, cast
iron, brass, etc. Aluminium oxide wheels are better suited for materials of higher
tensile strength, such as most types of steel, wrought iron, tough bronzes, etc..
Lapping
Lapping is an abrading process that is used to produce geometrically true
surfaces, achieve high dimensional accuracy, secure a fine surface finish, and
obtain a close fit between two contact surfaces. In the lapping process, fine
particles of an abrasive material are mixed with oil to form a paste. Lapping is
done by rubbing the surface of the work with an object called a lap in an ever
changing path. The lap is made of a relatively soft porous material like cast iron
or copper.The paste is rubbed into the lap, an action which causes the abrasive
particles to become imbedded in it. Laps are operated by machine or by hand.
Honing
Honing is usually used to finish internal or external cylindrical surfaces which
have been previously machined or ground. The abrasive is in the form of a flat
stone or stick called a hone. A few of these stones are mounted round a metal
cylinder to form a honing tool which is reciprocated axially, while being in contact
with the rotating work piece.
Superfinishing
Superfinishing is a process which uses large bonded abrasive stones to produce a
surface finish of extremely high quality. A reciprocating motion is given to a large
stone which presses lightly on the work piece. The work piece itself is rotated or
reciprocated depending on its shape.
Polisliing and buffing
Polishing is used to remove scratches and small defects from a surface. This is
done by the cutting action of fine abrasive particles applied to wheels made of
cloth, leather, felt, etc. The wheel is rotated while the workpiece is held against it.
