Tolerances, Fits and GD&T
Tolerance specification is where most of a part’s cost is decided, and where most drawing disputes are won. This section covers the three layers a CNC drawing actually uses: geometric tolerancing to control form, orientation and location, general tolerances for everything not individually called out, and fits for the features that have to mate.
How to read this section.
GD&T frames control geometry and datums; ISO 2768 sets the default for untoleranced linear and angular dimensions; fits define the relationship between mating features. Most drawings use all three together, and an unreadable drawing is usually one where the author was not clear about which layer applies.
Geometric Tolerancing
- Geometric Dimensioning & Tolerancing (GD&T)All 14 geometric tolerance symbols with definitions, feature control frame examples, datum rules and MMC / LMC / RFS modifiers
- The 7 Most Expensive GD&T Datum MistakesThe datum selection errors behind most rejects and drawing disputes — primary datum, datum order, datum scheme — and how to fix them before production
General Tolerances & Fits
- ISO 2768 & IT GradesGeneral tolerance classes (f, m, c, v), linear and geometrical tolerance tables, IT grades IT01–IT18, and what is actually achievable
- Fit TypesClearance versus transition versus interference, the ISO hole basis, designations such as H7/g6 and H7/k6, and what shrink or interference does to assembly
Tolerance is relative to material and process. The same 0.01 mm feature is routine in aluminum and a serious problem in a hardened tool steel, and it may need grinding rather than milling. For the underlying depth, jump to Materials, Processes, or Applications.