Manufacturing & engineering
Tolerance
Definition
A tolerance is the allowed variation from a nominal dimension, such as 10.00 ± 0.05 mm. Tolerances drive fit, cost, and inspection, and they have to be specified deliberately because a CAD model on its own stores only nominal sizes.
Also known as: dimensional tolerance, tolerancing
Updated
Types of tolerances#
Size tolerances can be written as a plus-minus value, as limits, or as a standard fit such as a hole and shaft pairing. A title block often sets general tolerances for any dimension without its own callout. Geometric tolerances, defined in the US by ASME Y14.5, control form, orientation, and location relative to datums.
Tighter tolerances cost more to machine and inspect. A good drawing puts tight values only where the function needs them. AI tools generate nominal geometry, so tolerances on generated parts still have to be chosen by someone who knows how the part is used.
Sources
Related terms
GD&T
GD&T is a symbolic language for specifying the allowed variation in a part's form, orientation, location, and runout relative to datum features. In the US it is defined by ASME Y14.5, and it tells the manufacturer and inspector which geometry actually matters for fit and function.
PMI
PMI is the dimensions, tolerances, GD&T, materials, surface finishes, and notes attached to a 3D CAD model instead of, or in addition to, a 2D drawing. It is the core content of model-based definition.
Drawing
An engineering drawing is a 2D document generated from a 3D model, with views, dimensions, tolerances, and notes. Many shops still quote and inspect from drawings even when they also receive a STEP file.
DFM
DFM is the practice of shaping a part so it can be made reliably and economically with the intended process. It covers choices such as wall thickness, draft, tool access, bend radii, hole sizes, and tolerances the supplier can actually hold.