PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 1
For example, if a hole with a 0.250 inch diameter with a
tolerance of 0.005 of an inch (€0.0025) is prescribed for a job, and the
shaft that is to be fitted into that hole is to have a clearance allowance
of 0.001 of an inch, the hole must first be finished within the limits and
the required size of the shaft determined exactly, before-the shaft can be
made. The hole is then termed the basic hole. If the hole is finished to
the upper limit of the basic dimension (0.2525 of an inch), the shaft would
be machined to 0.2515 of an inch or 0.001 of an inch smaller than the hole.
If the dimension of the shaft was given with the same tolerance as the hole,
there would be no control over the allowance between the parts. As much as
0.005 of an inch allowance (either clearance or interference) could result.
To provide a method of retaining the required allowance while
permitting some tolerance in the dimensions of the mating parts, the
tolerance is limited to one direction on each part. This single direction
(unilateral) tolerance stems from the basic hole system.
If a clearance
allowance is required between the mating parts, the hole may be larger but
not smaller than the basic dimension. The part that fits into the opening
may be smaller, but not larger than the basic dimension. Thus, shafts and
other parts that fit into a mating opening have a minus tolerance only,
while the openings have a plus tolerance only.
If an interference allowance between the mating parts is required, the
situation is reversed. The opening can be smaller but not larger than the
basic dimension, while the shaft can be larger but not smaller than the
Therefore, one can expect to see a tolerance such as
+.005, -0, or +0, -.005 inch, but with the required value not necessarily
.005 of an inch.
One way to get a better understanding of a clearance
allowance, or an interference allowance, is to make a rough sketch of the
piece and add dimensions to the sketch where they apply.
There are various workpieces made by the machinist that require certain
types of fits. These fits can be extremely close or they can be a loose or
Whatever type of fit, the engineer notes tolerances and
allowances on the blueprint.