Most geometric tolerances reference one or more datums. A datum reference consists of a datum identifier that refers to one or more datum features. A datum identifier is usually a single datum letter such as A or D, but it can be a sequence of up to three letters. In PC-DMIS, a datum identifier can refer to any of these:
A single feature, such as a plane, line, point, cylinder, circle, cone, sphere, or width—this is often called a single datum.
A pattern of like features of size that all have the same nominal size and size tolerance; this case is restricted to cylinders, circles, spheres, and widths—this is often called a datum pattern.
PC-DMIS only allows you to use a single 1D width as a datum. You cannot use patterns of 1D widths as a datum.
A geometric tolerance may reference two or more datum identifiers at once by joining them with a hyphen, such as C-D; this is often called a common datum. PC-DMIS supports many combinations of features as common datums. For more information see “Common Datums”.
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