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Tolerances and allowances.
Ideal interchangeable manufacturing.
Factors affecting ideal interchangeable manufacturing.
Basic requirements-Basic dimensions.
Why tolerances are specified…???
Unilateral and Bilateral Tolerances.
Tolerance is the total amount that a specific dimension is
permitted to vary;
It is the difference between the maximum and the minimum
limits for the dimension.
For Example a dimension given as 1.625 ± .002 means that
the manufactured part may be 1.627” or 1.623”, or anywhere
between these limit dimensions.
It is the difference between the permitted minimum and
maximum sizes of a part.
Tolerances and Allowances
Modern industry has developed on the basis of
The interchangeable manufacturing means the production of
parts to such a degree of accuracy as is necessary to permit
the assembly and proper functioning of the parts without
further machining or fitting, although the individual parts
may have been made at different times or in different
In other words, the parts are theoretically, at least,
A basic dimension is the theoretical size, which, for practical reasons,
is only approximated; or, it is the dimension that would be obtained if
perfection were possible and did not result in increased costs.
However, since perfection is impossible and also unnecessary, so far
as the dimensions of machine parts are concerned, it is general
practice to give a base or basic dimension and then indicate by
supplementary “tolerance” dimensions just how much the actual
dimension can vary from the basic without causing trouble, how much
inaccuracy is allowable allow able without causing a part to fit or
During inspection a surface scratch on a component cabinet
is assessed or judged to be unacceptable to customer. The
inspection criterion calls for “no scratches” and the
inspection call is a pass/fail or go/no go option.
However such scratches can also be held to a range or
“tolerance” of acceptance.
Why tolerances are
The tolerances are applied to the dimension of all
A tolerance indicates how much a part may deviate from its
ideal or basic dimension and still function properly. For
example a bearing may require a 1-inch diameter bore to
accommodate a 1-inch diameter journal. To make the
bearing exactly 1- inch would be impossible , and even if this
could be done, the bearing would wear in actual use to a
somewhat larger dimension.
Unilateral and Bilateral
The term unilateral tolerance means that the total tolerance, as related
to a basic dimensions, is in one direction only.
For example, if the basic dimension were 1-inch and a tolerance of
0.002-inch were expressed as 1.00 - 0.002, or as 1.00 + 0.002, these
would be unilateral tolerances, since the total tolerance in each case is
in one direction.
If the tolerance were divided so as to be partly plus and partly
minus, it would be classed as bilateral tolerance.
Thus, 1.00(+0.001 & -0.001) is an example of bilateral tolerance,
because total tolerance of 0.002 is given in two directions– plus