CNC Machining Tolerances — Charts & Achievable Accuracy

CNC machining tolerances we work to

CNC machining tolerances decide whether a part fits on arrival, so we state ours plainly instead of promising one unachievable blanket number. Our default is ISO 2768-m for general dimensions not called out on the drawing, and ±0.01 mm on the precision features where the drawing asks for it. Surface roughness is confirmed against your drawing and the chosen finish, and any dimension we have not confirmed is quoted as “reviewed against your drawing.”

This page explains what each of those statements means, so you can mark a drawing once and quote it correctly the first time. For how we physically hold these numbers, see our equipment list and factory.

General tolerance: ISO 2768-m

ISO 2768-m is our stated general tolerance for dimensions the drawing does not specifically call out. It sets sensible defaults for linear sizes, radii and angles, so an un-toleranced dimension does not drift. If your drawing has its own general tolerance block, that block wins — we machine to your note. In practice, stating ISO 2768-m means the everyday dimensions on a bracket, flange or housing stay controlled even where the drawing is silent.

Stating the general tolerance up front also removes the most common quoting delay: a drawing with no tolerance note at all. Naming ISO 2768-m as the default means we can price and program without first asking how tight an unmarked dimension must be, so your quote stays within the 24-hour window.

ISO 2768-m linear tolerances (reference table)

Nominal length range	Permissible deviation (m class)
0.5 up to 3 mm	±0.1 mm
over 3 up to 6 mm	±0.1 mm
over 6 up to 30 mm	±0.2 mm
over 30 up to 120 mm	±0.3 mm
over 120 up to 400 mm	±0.5 mm
over 400 up to 1,000 mm	±0.8 mm
over 1,000 up to 2,000 mm	±1.2 mm
over 2,000 up to 4,000 mm	±2 mm

ISO 2768-m angular tolerances (reference table)

Nominal length of the shorter side	Permissible deviation (m class)
up to 10 mm	±1°
over 10 up to 50 mm	±0°30′
over 50 up to 120 mm	±0°20′
over 120 up to 400 mm	±0°10′
over 400 mm	±0°5′

These tables are the published m-class values; they are what an unmarked dimension on your drawing is held to. If your design needs something else, add your own general tolerance block and that takes precedence.

Precision tolerance: ±0.01 mm

Where a drawing calls out a tight feature, we hold ±0.01 mm. This is the number that matters on close tolerance bolts, shaft seats, bore fits and locating surfaces — features where a loose fit means rework or a scrapped assembly. We treat ±0.01 mm as a feature-by-feature commitment, not a blanket claim, because holding it everywhere would be slower and costlier than you need. If your drawing requires less than ±0.01 mm, we say so during DFM review rather than discover it after cutting.

The same ±0.01 mm applies whether the feature is a bore on an aluminium housing or a seat on a stainless shaft; what changes with material and geometry is the machining strategy, not the stated number. Tell us which features carry it and which do not, and we will price the two groups differently instead of charging tight-tolerance rates across the whole part.

Precision by process

Process group	Typical tight features	Tolerance we hold
CNC turning	Diameters, bores, faces, runout	±0.01 mm
Swiss-type turning	Small diameters, concentricity	±0.01 mm
Turning and milling composite	A milled detail relating to a turned diameter	±0.01 mm, one setup
Milling (3-axis)	Pockets, hole positions, locating faces (800 × 500 mm envelope)	±0.01 mm
Multi-axis (4/5-axis positioning)	Angled holes, multi-face relationships	±0.01 mm
Grinding / wire EDM	Fine fits, hardened material, sharp internal corners	Reviewed per drawing (partner network)
Threads	Pitch diameter, fit class	Per drawing and thread standard

For the equipment behind each row, see the equipment list.

Surface roughness and unstated dimensions

Surface roughness (Ra and similar callouts) is confirmed per drawing and per finish, because a turned surface, a milled face and an anodized surface all land differently. Tell us the roughness your assembly needs and we will state what the process and finish deliver. Where roughness affects a sealing face or a bearing seat, that callout is treated with the same care as a dimensional tolerance, because the two usually travel together. Likewise, any dimension, tolerance or material property we have not confirmed is written as “reviewed against your drawing” — a plain statement that the number will be checked and quoted from your file, not guessed.

How tolerance choices affect price

Tolerance is one of the few cost drivers you control from the drawing stage. Three rules cover most of it.

• Mark only what matters. A drawing where every dimension carries ±0.01 mm is priced as a tight part end to end. Marking the two or three features that actually locate or seal — and letting the rest run to ISO 2768-m — usually cuts cost more than any price negotiation.
• Tight tolerance costs an operation, not just time. Beyond the extra checking, tight features often force a second setup, a slower feed, a different tool, or a grinding step. On round parts, moving a tolerance onto a turned diameter instead of a milled face is frequently the cheaper route.
• Let the finish follow the function. A cosmetic surface and a sealing surface should not carry the same callout. Tell us which faces seal, slide or show, and we will state the finish each one needs.

The same rule covers cnc tolerances on materials and sizes we have not yet quoted: no number is invented to fill a gap. Send your drawing and we will return the tolerance reality — what we can hold, where, and at what cost — with your quote within 24 hours.