PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 1
d.
If a dimension is given as 3.000 inches, the accuracy expected is
0.0005 of an inch; or if the dimension given is 3.00 inches, the accuracy
expected is €0.005 of an inch. The €0.0005 is called in shop terms, "plus
or minus five ten-thousandths of an inch." The €0.005 is called "plus or
minus five thousandths of an inch."
FIGURE 1.
BASIC DIMENSION AND TOLERANCE.
4.
Allowance
a. Allowance is an intentional or prescribed difference in dimensions of
mating parts to provide a certain class of fits or a desired fit.
(1) Clearance Allowance.
This allowance permits movement between
mating parts when assembled.
For example, when a hole with a 0.250 inch
diameter is fitted with a shaft that has a 0.245 inch diameter, the
clearance allowance is 0.005 of an inch.
(2) Interference Allowance. This allowance is just the opposite of a
clearance allowance. The difference in dimensions in this case provides a
tight fit.
Force is required when assembling parts that have an
interference allowance.
If a shaft with a 0.251 inch diameter is fitted
into the hole identified in the preceding example, the difference between
the dimensions will give an interference allowance of 0.001 inch.
As the
shaft is larger than the hole, force is necessary to assemble the parts.
b.
What is the relationship between tolerance and allowance?
In the
manufacture of mating parts, the tolerance of each part must be controlled
so that the parts will have the proper allowance when
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