On a technical drawing a single dimension rarely appears on its own — more often a dimension with a tolerance, e.g. ø30 H7. It is the tolerance that decides whether the finished part will fit where it should, and whether mating parts will match. We explain what a tolerance is in CNC machining, how to read it and how to choose it so you do not overpay.
What a tolerance is
A tolerance is the permissible range of deviation of a dimension from its nominal value. No process makes a part perfect to the nanometre, so the designer specifies how much the actual dimension may deviate — e.g. 30 mm +0.02 / −0.00. The narrower the range, the higher the machining requirements (and cost).
IT accuracy grades and fits
In a notation like H7, g6, js9 the letter defines the position of the tolerance zone (hole — capital letter, shaft — lowercase), and the number is the IT accuracy grade — the lower it is, the tighter. By selecting a hole/shaft pair you design a fit:
- clearance — parts slide, easy assembly (e.g. H7/g6)
- transition — a compromise of guidance and assembly
- interference (press) — a press-fit joint, no relative movement
What tolerances we achieve
For typical turned and milled parts we work in the hundredths of a millimetre, and for precision components we go down to single micrometres. We check every critical dimension on a Mitutoyo CRYSTA-APEX coordinate measuring machine (accuracy 0.5 µm) — measurement and a report, not “by eye”.
How to choose a tolerance — and not overpay
The rule is simple: tighten the tolerance only where it is really needed. Every accuracy grade down means more machining time, inspection and scrap, i.e. a higher price. Leave non-mating dimensions safely in a general tolerance (e.g. ISO 2768-m). If you are not sure how to specify the tolerances — send a drawing or model, and we will advise during the quote.
Frequently asked questions (FAQ)
What does H7 mean on a drawing?
It is the tolerance zone of a hole in the 7th IT accuracy grade — typical for fits with a g6/h6 shaft.
What accuracy does CNC machining achieve?
For typical parts — hundredths of a millimetre; for precision parts — single micrometres, confirmed by measurement on a coordinate measuring machine.
Does a tighter tolerance raise the price?
Yes — every grade down means more machining and inspection. That is why it is worth tightening only the critical dimensions.
Have a drawing with tolerances? Send an enquiry — we will check feasibility and quote the part.
