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Specifying Tolerances on Custom Metal Parts Without Overpaying
By Bondao engineering · 12 November 2026
Almost every RFQ we open carries the same line: ±0.01 mm on all dimensions. Sometimes it was copied from an older drawing. Usually the part does not need it.
Tight tolerances are not free. They change how a part is machined, how it is measured, and how many parts get scrapped. The fix is simple: decide which dimensions actually deserve a callout, then let a general tolerance standard cover the rest.
Start with what the dimension does
Ask what happens if a dimension is 0.05 mm off. If the answer is "nothing", it belongs under a general tolerance note.
A bore that takes a bearing is different. There the fit matters, so call out the fit class (H7/g6, for example) and the surface finish rather than a bare ± number. On a hole pattern that has to line up with a mating plate, a positional tolerance does more good than six separate ± values, because it gives the assembler clearance exactly where clearance is needed. A cover or a bracket with no mating face is fine at ±0.2 mm.
We redraw sketches this way fairly often. The quote drops, the part is identical, and inspection takes half the time.
Put a general tolerance note on the drawing
ISO 2768 is the usual choice for machined parts. A note such as ISO 2768-m covers every linear and angular dimension that has no individual tolerance. Legacy prints sometimes say DIN 7168; that standard was withdrawn, but plenty of old drawings still carry it and the intent is the same.
Once the note is on the drawing, only the handful of functional dimensions need a number. The print gets easier to read, and the shop can pick a sensible process instead of planning for the worst case on every feature.
Editor: optionally place the tolerance-band SVG diagram (from the HTML page) here, with its caption.
What different processes can hold
Indicative ranges for a production run. Material, geometry and batch size all move these numbers.
Process | Comfortable | Tighter work | Note |
±0.05 mm | ±0.01–0.02 mm | Diameters are easier to hold than lengths | |
±0.05 mm | ±0.02 mm | Positional accuracy depends on the setup | |
±0.2 mm | ±0.1 mm | Burr direction often matters more than ± | |
±0.3 mm as-cast | ±0.1 mm machined | State whether the dimension is as-cast or after machining | |
±0.1 mm | ±0.05 mm | Best value on repeat runs |
Where the extra cost comes from
Below about ±0.02 mm on a turned or milled feature, we slow the feed, add a finish pass, and measure every part rather than sampling. Thermal drift starts to matter on long runs. Scrap goes up.
None of that is a problem when the feature needs it. It is a problem when forty dimensions on the print all sit in the same tight band because writing one note was quicker than thinking about forty features.
The rule we use in our own shop: if a dimension has no functional reason, do not give it a tight number.
Before you send the drawing
Mark the dimensions that mate, seal or align.
Give those a fit class or a positional tolerance, not just a ± value.
Add a general tolerance note. ISO 2768-m is a safe default.
State surface finish only on sealing or bearing faces.
Say what the part is made from, and whether a material certificate is required.
That last one catches more delays than people expect. A quote built on 6061 aluminium and a quote built on 304 stainless are different jobs.
Common questions
What tolerance should I put on a non-critical machined dimension?
None. Add a general tolerance note such as ISO 2768-m to the drawing and leave that dimension untoleranced. The standard covers it, and the drawing stays readable.
Is ±0.01 mm achievable on CNC turned parts?
Yes, on short runs with a controlled setup and stable temperature. It needs a finish pass and full inspection, so reserve it for functional diameters and sealing faces rather than the whole part.
Do I need GD&T on a simple bracket?
Usually not. A general tolerance note plus one or two fit callouts is enough. Add positional tolerance when a hole pattern has to line up with a mating part.
