Datum

Related terms

GD&T

GD&T is geometric dimensioning and tolerancing, the symbol system defined in ASME Y14.5 and ISO 1101 for controlling the form, orientation, location, and runout of part features relative to datums. It states functional intent more precisely than plus-minus dimensions and often allows larger tolerances where they do not matter.

True position

True position is a GD&T location control that defines a tolerance zone, usually cylindrical, around the theoretically exact location of a feature such as a hole, relative to datums. A diameter-zone position callout gives about 57% more tolerance area than the equivalent plus-minus square zone.

Fixture

A fixture is a custom workholding device that locates and clamps a part in a repeatable position for machining, welding, assembly, or inspection. Good fixtures locate from the drawing's datums. Fixture cost is usually a one-time charge on the quote, separate from piece price.