Every checking fixture starts from the part's datum reference frame. Datums are the surfaces, axes or points a drawing declares as the origin for measurement, and the fixture must recreate them exactly.
The primary datum constrains the largest degrees of freedom — typically a plane resting on three points. The secondary datum removes rotation and translation in a second direction, and the tertiary datum locks the final degree of freedom, fully seating the part.
Getting this 3-2-1 constraint right means the part sits the same way every time, so a go / no-go pin or profile gauge reads true. Get it wrong and the fixture measures its own error, not the part's.
When we quote a gauge, the first thing our engineers study is your datum scheme. A clear GD&T drawing lets us build a fixture that mirrors how the part will be measured on a CMM — and how it functions in the assembly.
