Introduction to Basic Manufacturing Processes and ... - always yours

Inspection and Quality Control 467
of size. It is equal to the algebraic difference between the upper and lower deviations and has
an absolute value without sign. Its value is a function of the basic size and is designated by
a number symbol, called the grade. There are two basic ways of specifying the working
tolerance: (1) bilateral and (2) unilateral, tolerances. Bilateral tolerances are used where the
parts vary in both directions from the desired or nominal size. Unilateral tolerances are used
where it is important for the dimension to vary in only one direction. Components produced
will fall close to the desired dimension but can vary in only one direction. An example is the
drilled hole, as the drill is made close to the normal hole size, it is seldom possible to drill
a hole undersize. All drilled holes therefore should carry only a plus tolerance. Since the
tolerance is allowed on one side of the nominal size, the system is said to be unilateral. Closer
the tolerance higher is the cost of product.
26.3 INTERCHANGEABILITY
The dimensions of mating parts are generally controlled to have a proper fitting of
matching parts for its optimal functional requirement. Providing dimensions on components
or parts is the job of a product designer. Interchangeability of the parts is, therefore, a major
pre-requisite for economic production, operation and maintenance of machinery mechanism
and instruments. It is therefore very much possible to interchange spare parts in various
machines, tractors, motor cars, machines tools, airplanes and many, others so that they can
be dismantled for replacement of parts in service conditions in the field, and also in many
local workshops with least possible loss of time. In order to produce interchangeable or
identical parts, the components of interchangeable system and the various terms related with
inter-changeability of the mating parts should be understood clearly by the product designing,
manufacturing and product inspecting staff working in industries.
26.3.1 Size
It expresses the numerical value of a length in a particular unit on the part. The basic
size of a part is its nominal dimension from where all variations are generally made. It is
determined by the part designer from its functional requirements to meet the specified
objective. The other term used with respect to a part is the nominal size. The nominal size
is the size of the part specified in the drawing as a matter of convenience. It is used primarily
for the purpose of identification of a component and is never used in the precision measurement
of parts. A rigid attitude towards the maintenance of a basic size of the part may increase the
manufacturing cost and a little variation in dimension is accepted resulting in a size, which
is different from the basic size of the part. This is called the actual size. The actual size of
a dimension or part is its measured size. An actual size of a ready part will, therefore, always
deviate from one specified in the drawing, i.e. from the nominal or basic size of the part.
Whereas the difference between the basic size and actual size must not exceed from a certain
limit otherwise it will interfere with the interchangeability of mating components during
assembly or sub assembly of parts.
26.3.2 Limits of Size
Limits of size are two extreme permissible sizes between which the actual size exists.
The maximum limit for a dimension is the largest permissible size, while the minimum limit
for a dimension is the smallest permissible size. The maximum or minimum sizes represented
by tolerances are called the limits. The basic sizes deviation and tolerances for both shaft and
hole are illustrated in Fig.26.1.