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 assembled. 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, beforethe 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.
c.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.
d.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 basic dimension. 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 sliding fit. Whatever type of fit, the engineer notes tolerances and allowances on the blueprint.
The more familiar the machinist becomes with these terms, the easier his job will be. This task described the types of fits, tolerances, and allowances. Task 2 will describe the proper use and care of precision gages and measuring tools used in the machinist trades.