Tolerances

Tolerances are the permitted deviations of a feature's size, form or position from its nominal (specified) value. Every manufactured part deviates from its ideal dimensions, and the tolerance defines how much deviation is still acceptable for the part to function correctly.

Engineers distinguish between dimensional tolerances (the accuracy of linear sizes), geometric tolerances (form, orientation, location and runout, covered by GD&T) and surface texture requirements (roughness). On drawings, tolerances are written as upper and lower limit deviations, as a symmetric deviation (for example 25 ±0.05 mm) or as an ISO tolerance class (for example 25 H7).

Setting tolerances is one of the key design decisions, with a direct impact on manufacturing cost. A tight tolerance (for example ±0.01 mm) requires a more precise machine, longer machining time and more inspection steps, which makes it significantly more expensive. Designers should specify a tolerance only as tight as the function actually requires.

For fits, where two components mate (a shaft in a bearing, a pin in a hole), the ISO 286 system defines standardized clearance, transition and interference fits. Choosing the right fit ensures that the assembly works and can be put together without special tools.

Every dimension on a production drawing needs a tolerance, either stated explicitly or covered by general tolerances (for example ISO 2768-m, or a tolerance block in the title block on drawings that follow US practice). Individually specified tolerances belong on functional dimensions: bearing and pin fits, mounting hole patterns, locating and sealing faces, and dimensions that interface with parts from other suppliers.

When designing an assembly, tolerances feed into a stack-up analysis (tolerance chains), which verifies that the assembly can be put together and will function even with the worst-case combination of limit dimensions.

The most common mistake is applying tight tolerances across the board "just to be safe". It raises the price, extends lead time and brings no functional benefit. The opposite mistake is a drawing without general tolerances, which leaves the manufacturer to guess the required accuracy.

Tolerances must match the manufacturing process. Values that are normal for grinding make no sense for 3D printing or an as-cast surface. A specified tolerance also has to be measurable. Ask the supplier what accuracy their process typically achieves and what tightening it would involve, and request an inspection report for critical dimensions.