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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