How to specify CNC machining tolerances without paying for accuracy you do not need

A drawing where every dimension carries ±0.01 mm does not produce a better part. It produces a more expensive one, because the shop now has to inspect every feature, slow the process down and scrap parts that were never functionally wrong.

Start from function

Ask what each dimension does. A bearing bore, a mating pilot diameter or a sealing face has a real reason to be tight. An overall length, a clearance hole pitch or a cover outline usually does not.

The features that genuinely need control usually fall into four groups:

• Fits — anything that mates with a purchased component: bearings, dowels, O-rings, shafts.
• Interfaces — bolt patterns and mating faces that have to line up with another of your parts.
• Function — clearances where too much or too little movement changes how the product behaves.
• Appearance — gaps that a user will see, which are often tighter than the mechanical need.

Everything else can normally live under a general tolerance. ISO 2768-m is a reasonable default for machined metal parts, and calling it out explicitly saves the argument later.

Why tighter costs more

Three things change when a tolerance tightens:

1. The process — a feature that could be milled in one pass may need a finishing pass, a different cutter or a second setup.
2. The inspection — tight features get measured, and measurement takes time on a CMM rather than a caliper.
3. The yield — a process running at its limit produces more scrap, and that cost lands in the price.

Moving a thermal or clearance feature from ±0.1 to ±0.02 mm can easily add 20–40% to the cost of the part without changing whether it works.

GD&T, used sparingly

Geometric tolerancing is the right tool for position, perpendicularity, flatness and profile, because it says what actually matters instead of boxing in a coordinate. Use it where the function is geometric — a bolt pattern, a sealing face, a bearing seat — and keep the datum structure simple.

A datum scheme that references three features on one drawing view and two on another is a common source of disagreement; a drawing that defines a primary, secondary and tertiary datum once is not.

What to tell your supplier

If you are unsure, say so. A short note — “these two bores are bearing fits, the rest is cosmetic” — is more useful than another decimal place, because it lets the shop plan the process around the features that matter and stop over-controlling the rest.