What is the Difference Between Tolerance and Allowance?

The difference between tolerance and allowance lies in their definitions and applications in manufacturing and engineering:

• Tolerance: Tolerance is the amount of random deviation or variation permitted for a given dimension. It puts a limit on machining inaccuracy and is used to specify the amount of error permitted in making a part. Tolerance can be unilateral or bilateral, with unilateral tolerance defining a variation on just one side of the basic size, and bilateral tolerance varying on both sides of the basic size.
• Allowance: Allowance is a planned difference between a nominal or reference value and an exact value. It is a design feature that defines the planned deviation in dimensions of mating components. Allowance can be positive (clearance) or negative (interference). If both mating parts are not machined according to the defined allowance, the engineering fit will not function properly.

In summary:

• Tolerance is used to specify the amount of error permitted in making a part, putting a limit on machining inaccuracy.
• Allowance is a planned deviation in the dimensions of mating parts to achieve the correct mate, necessary for proper engineering fit.

It is important for design engineers and machining experts to understand the distinction between tolerance and allowance, as they are linked but different.

Comparative Table: Tolerance vs Allowance

The difference between tolerance and allowance can be summarized in the following table:

In summary, tolerance is about accepting and respecting differences, while allowance is about permitting or granting a specific amount or variation in dimensions. Tolerance is related to the limits of a given dimension, while allowance is an intentional difference between the dimensions of mating parts.