Engineering Resource

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. Every critical dimension is verified on a Zeiss CMM 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.