"Two parts with identical geometry can differ in price by 300% based solely on how their tolerances are specified."
Tolerances are the invisible cost multiplier in CNC machining. Two parts with identical geometry can differ in price by 300% based solely on how their tolerances are specified. Understanding this relationship is essential for anyone ordering custom machined parts.
A tolerance defines the acceptable range of variation for a dimension. When you specify a hole diameter of 0.500" ±0.005", you're telling the machinist that any diameter between 0.495" and 0.505" is acceptable. Tightening that to ±0.001" means the acceptable range shrinks to 0.499" to 0.501" — a much harder target to hit consistently.
Why do tighter tolerances cost more? Three reasons: First, the machine must run at slower feed rates and take lighter cuts to avoid deflection and thermal distortion. Second, the setup must be more rigorous — better workholding, more precise tool measurement, and temperature-controlled conditions. Third, inspection takes longer because every tight-tolerance feature must be verified with calibrated instruments.
The most expensive tolerance mistake is applying a blanket tolerance to an entire drawing. When every dimension on your part is specified to ±0.001", you're paying premium prices for features that don't need that precision. A mounting hole that accepts a standard bolt doesn't need the same precision as a bearing bore.
Smart tolerance specification follows a simple principle: tight where it matters, relaxed where it doesn't. Bearing fits, sealing surfaces, and mating interfaces need tight tolerances. Cosmetic surfaces, clearance holes, and non-critical dimensions can often accept ±0.005" or even ±0.010" without any functional impact.
GD&T (Geometric Dimensioning and Tolerancing) provides a more sophisticated language for communicating your requirements. Instead of applying bilateral tolerances to every dimension, GD&T lets you specify exactly what matters — concentricity, perpendicularity, surface profile — and leave everything else at standard machining accuracy.
At 406 Industries, our DFM review always includes a tolerance analysis. James will identify dimensions where you're over-specifying (paying for precision you don't need) and dimensions where you might be under-specifying (risking assembly or performance issues). This analysis alone can reduce your per-part cost by 20-40% without changing the part's function.