It is difficult to manufacture any product or component to its exact size. Tolerance on
the parts is therefore the amount of variation in size tolerated to cover reasonable imperfections in workmanship and it varies with different grades of work.
Tolerance on a dimension can also be specified as the difference between the maximum limit of size and the minimum limit 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.
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