How to Specify Tolerance on a CNC Aluminum Part

A drawing with ±0.02 mm on every dimension looks rigorous and costs twice as much as it needs to. The useful question is which dimensions the assembly actually depends on.

Start from the assembly

List the features that touch another part: mounting faces, bores, pilot diameters, hole patterns. Those carry the tight tolerance. Everything else — outer profiles, clearance holes, non-functional edges — can usually sit at general tolerance.

Always state the datum

A tolerance without a datum is ambiguous, and the shop will pick one for you. Mark the primary mounting face and the two edges that locate the part, and reference hole patterns from that datum rather than from an arbitrary corner.

Watch these three interactions

• Flatness plus thickness: a tight flatness call on a thin base is expensive; consider a corrected face instead of an overall thickness call.
• Surface finish plus anodizing: coating grows the part. Leave allowance where fits matter.
• Thin walls: symmetric stock removal drives distortion more than any single tolerance value.

What we need for quoting

3D model, 2D drawing with datums, alloy and temper, surface finish, quantity, and anything the part must mate with. With those we can confirm feasibility and propose a sampling plan.