Engineering Resource
Tolerance is the single biggest lever on the price of a machined part. Tighten it without reason and cost climbs fast; loosen it where it matters and the part fails in service. This guide is about finding the balance deliberately.
What precision CNC machining can hold
As a working baseline we hold ±0.005 mm on 3-axis work and ±0.01 mm as standard on simultaneous 5-axis, with surface finish down to Ra 0.2 µm achievable. These are the numbers we quote against. Critical features are verified in house rather than estimated.
Tolerances tighter than that are not automatically impossible, but they stop being a machining question and become a metrology and cost question.
What actually drives the cost
Tolerance is not free, and its cost is rarely linear — it steps up as you cross each threshold.
- Extra operations: a feature at ±0.1 mm may need one pass; at ±0.01 mm it needs a semi-finish plus finishing pass.
- Fixturing and set-ups: every additional set-up adds stack-up error and time.
- Temperature control: metals and especially plastics move with temperature, so tight work needs a stable environment.
- Measurement: verifying a tight tolerance takes CMM time, not a caliper check.
- Risk: the closer you cut to the limit, the higher the scrap risk priced into the quote.
How to specify tolerances sensibly
- Put a tight tolerance only on the dimensions that carry function, and leave the rest at general tolerance.
- Do not apply a tight default across the whole drawing — it multiplies cost for features nobody measures.
- Define datums explicitly so the inspector measures what the designer meant.
- Think in fits: a shaft and bore pair needs a controlled fit, not two independently perfect parts.
- Leave room to measure. A feature the CMM probe cannot reach cannot be verified.
Geometric tolerances carry intent better than size
Flatness, position, concentricity and perpendicularity often express a requirement far better than a tightened size tolerance. A position tolerance on a hole pattern, for instance, controls what actually matters — where the hole sits relative to its datums — instead of tightening an unrelated diameter.