Tolerancing

Tolerancing is the practice of specifying permissible variations in the size, shape, location, and orientation of features on manufactured parts. It defines how much a dimension or geometric characteristic may deviate from a nominal value and how those deviations affect fit, function, and interchangeability. Tolerancing encompasses both dimensional tolerances and geometric tolerances, the latter often expressed through geometric dimensioning and tolerancing (GD&T) standards.

Dimensional tolerances control the allowable size of features, using unilateral (one-sided) or bilateral (two-sided) tolerance limits. Geometric tolerances specify attributes such as form, orientation, and position, independent of size, using symbols and a feature control frame. Common geometric attributes include straightness, flatness, circularity, cylindricity (form); angularity, perpendicularity, and parallelism (orientation); and position, concentricity, and run-out (location). The concept of material condition, such as maximum material condition (MMC) or least material condition (LMC), is used in GD&T to relate tolerance zones to the size of a feature.

Tolerance analysis methods range from worst-case calculations to statistical approaches, influencing stack-up, manufacturability, and cost. Standards such as ASME Y14.5 and ISO 1101 provide frameworks for GD&T; ISO 286 covers general tolerances for holes and shafts. Inspection and metrology, including coordinate measuring machines and gaging, verify that parts meet specified tolerances.

Proper tolerancing balances function, quality, and manufacturability. Over-tolerancing can raise cost and reduce yield, while under-tolerancing risks improper assembly or failure. Effective tolerancing supports compatibility across parts, assemblies, and manufacturing processes.