undertolerancing

Undertolerancing is a tolerancing practice in engineering where the allowable variation for one or more features is set too tightly relative to what can be reasonably produced, measured, or assembled. It occurs when design tolerances do not adequately reflect process capability, measurement error, or assembly clearance, effectively constraining parts beyond what the manufacturing system can reliably achieve. The result is reduced manufacturability and potentially higher cost due to increased scrap, rework, and inspection.

Causes include overly optimistic assumptions about process capability, insufficient tolerance analysis, and a misalignment between design intent and production realities. In geometric dimensioning and tolerancing (GD&T) terms, undertolerancing often appears as tight tolerances without considering achievable variation of datums and mating features, or as tolerances that leave little or no clearance in assemblies.

Effects include lower yield, higher inspection effort, frequent fit problems during assembly, and increased design changes or supply chain delays. It can also mask underlying process issues by forcing parts to meet stringent specs rather than addressing root causes.

Mitigation involves early tolerance analysis and design for manufacturability, using capability studies (Cp, Cpk) to set realistic limits, and allocating tolerances to critical features. Employing GD&T properly, allowances, and robust design strategies reduces the risk of undertolerancing by aligning tolerances with actual production and inspection capabilities. Example: specifying a hole at 10.00 mm with ±0.01 mm tolerance when typical drilling processes can only hold ±0.05 mm, which may cause assembly or interference problems.