Tolerances, fits, and stack-ups

Nominal is not actual

A nominal diameter describes a target. Every physical part varies with machine capability, tool wear, material behavior, measurement error, and environmental conditions.

An interface must be designed using allowable ranges, not just nominal values. The distinction is particularly important when parts come from different suppliers.

Three useful fit concepts

• Clearance fit: the assembled interface retains space between its mating surfaces.
• Transition fit: dimensional variation permits either a small clearance or a small interference.
• Interference fit: assembly requires elastic deformation, thermal assistance, or a defined process.

Tolerance stack-ups

A worst-case stack adds extreme permissible variations in the direction that affects function. A statistical stack may be useful for process planning, but it relies on justified distribution and independence assumptions.

Do not turn an average measurement into a guaranteed fit. Characterize the complete tolerance range and include surface finish and temperature where relevant.