CNC Machining Tolerances: What Is Achievable

By CHX Metal Engineering Team · Published 2026-10-09

A practical guide to CNC machining tolerances - standard and precision bands, how tolerance drives cost, GD&T datums, and what to put on a drawing. Parts are machined from aluminium, stainless steel, brass, titanium and engineering plastics.

Tolerance is the single line on a drawing that most often decides part cost. Put a tight tolerance where it controls function and the part stays economic; put it everywhere and you pay for inspection you did not need. Here is how tolerance works in a CNC shop, in practice.

Standard and precision bands

For most turned and milled metal parts, a standard tolerance of +/-0.005 mm (about +/-0.0004 in) on a dimension under 100 mm is routine and does not require special process control. That is the band we work to by default at CHX Metal.

When a feature genuinely needs it, we work to +/-0.005 mm (+/-0.0002 in) on critical features, verified by CMM. That is a different regime: it usually means leaving stock for a finishing pass, controlling temperature, and planning the inspection method before the first part is cut. Tight tolerances are also harder to hold on thin walls and long slender sections, because the part moves under cutting force.

Why tolerance drives cost

A tight tolerance does three things at once: it slows the cut (lighter passes, slower feed), it adds process steps (rough and finish operations, sometimes a stress-relief or stabilising step), and it multiplies inspection time (a CMM check on one feature versus a full layout).

The largest cost is often inspection, not machining. A part that calls out +/-0.005 mm on ten dimensions needs all ten verified, on every batch, with a record. A part that calls it out on one functional datum needs only that one verified. Same machine, very different price.

Rule of thumb: reserve the tight band for the features that touch something else - bearing bores, gear seats, pilots, and fits. Let everything cosmetic or stand-off run at standard tolerance.

Datums and GD&T

A tolerance is only meaningful relative to a datum. Choose datums that reflect how the part is held in the machine and how it is measured on the CMM, so the tolerance chain matches the manufacturing chain.

Concentricity and runout callouts are the common trap. If runout is referenced to a datum the fixture cannot access, or to a surface that is itself machined in a second operation, the shop either adds an operation or the parts argue at inspection. A better approach is to reference runout to the same bore that the part is turned from, and to keep the number of datum references small.

Feature and material effects

Achievable tolerance depends on the feature, not only the number. A ground outside diameter on a Swiss-type part is easy to hold. A bore with a high depth-to-diameter ratio is much harder, because the boring bar deflects. A cross-hole's position tolerance depends on fixturing, not on the lathe's accuracy.

Material matters too. Free-cutting brass and easy aluminium hold tight tolerances readily; 316 stainless and titanium spring back, generate heat and work-harden, so the same tolerance needs more care. Plastics move with temperature and moisture, so a tight tolerance on a plastic part should be qualified with the shop first.

What to put on the drawing

• State a general tolerance block (for example, ISO 2768-m) and then call out only the exceptions.
• Reference critical fits with the mating part or an explicit fit class, not just a number.
• Use GD&T for position, runout and profile where the function needs it - and pick datums that are machinable and measurable.
• Do not blanket a tolerance across a drawing; it signals that the designer has not prioritised, and the shop quotes to the worst case.
• Send the 3D model as well as the 2D drawing, and state the revision that governs tolerances.

How tolerance is verified

Inspection should match the tolerance. Standard tolerances are checked with calibrated hand tools and spot checks; critical features are verified on a coordinate measuring machine (CMM) with a first-article inspection report on the first part of a batch. At CHX Metal, critical dimensions are measured on an in-house CMM, and a first-article report and batch traceability can ship with the order.

If your quality plan needs a specific inspection report format - PPAP-style, ISIR, or a cup of the measurement data - say so at quotation, because the format is part of the scope.

Frequently asked questions

What is the tightest tolerance you hold on CNC parts?

Standard tolerance is +/-0.005 mm, with +/-0.005 mm on critical features such as bearing bores and pilots, verified by CMM in-house. Tolerances tighter than that are possible on specific features, but they should be discussed with the shop so the process and inspection can be planned.

Does a tighter tolerance always cost more?

On the feature that needs it, the extra cost is real but contained. The expensive mistake is applying a tight tolerance across a whole drawing, because it multiplies inspection time without improving function.

What is the difference between +/-0.005 mm and +/-0.005 mm in inches?

+/-0.005 mm is about +/-0.0004 in and +/-0.005 mm is about +/-0.0002 in. Both are within the range of good CNC practice; the deciding factor is the feature and how it is measured.

Can you work to a tolerance I have not fully specified?

Yes. Send the drawing and we will propose a tolerance scheme as part of the DFM feedback, flagging any callout that will be hard to hold or measure.

Shenzhen Changhexin Precision Hardware Co., Ltd. (CHX Metal) · Shajing Street, Bao'an District, Shenzhen, Guangdong, China · ISO 9001:2015 (NOA198428) · ISO 14001:2015 (NOA2100184)