Tolerance is the maximum permissible deviation of a part’s dimensions from the value specified in the drawing — and it is one of the first figures we ask for when placing an order for CNC machining. Not because we want to overwhelm the customer with technical details, but because the tolerance directly determines how much production costs, how long it takes, and whether the part will actually function as it should.

What tolerance means in practice

If the drawing specifies a diameter of 20 mm with a tolerance of ±0.01 mm, this means that the finished part must have a diameter somewhere between 19.99 mm and 20.01 mm — otherwise it will not pass quality control. The narrower this range, the more demanding the production process: slower cutting parameters, more intermediate checks, more expensive tools, and stricter conditions in the workshop.

Common tolerance values

In practice, we most commonly encounter three levels:

±0.1 mm — standard engineering accuracy, sufficient for most structural parts, covers and less critical components.
±0.01 mm — our standard for precision functional parts: bearings, pins, gear wheels, and components in assemblies requiring precise mating.
±0.005 mm and less — special applications where even a thousandth of a millimetre makes a difference: measuring equipment, parts for hydraulic and pneumatic systems, precision bearing housings.

When do you actually need higher precision?

Tighter tolerances are worthwhile where a part must fit precisely into an assembly, where a functional surface is subject to high wear, or where even a small deviation would cause leakage, vibration or premature wear. Typically, these include rotary bearings, sealing surfaces and gear components.

When is it worth accepting a less precise tolerance

Conversely, for mounting holes, covers, spacers or parts without direct functional contact, it is unnecessary to pay for tolerances that the part does not actually require. An experienced supplier will tell you themselves, when preparing a quotation, where tolerances can be relaxed without compromising functionality — and where they cannot.

Tolerances at Metal One

As standard, we achieve ±0.01 mm for both CNC milling and turning; for special applications, this can be as tight as ±0.005 mm. We carry out 3D measurements on every part before dispatch, so you know exactly what you’re getting — not just what’s written on the drawing. If you’re unsure what tolerance your part actually requires, send us the drawing and we’ll advise you.