A datum reference frame is three mutually perpendicular planes that a part is measured from, built from the datum features named in a feature control frame, in order. The primary datum stops three degrees of freedom, the secondary two more and the tertiary the last one, which fixes the part exactly as it will be held for inspection.
A datum feature is a real surface or feature on the part, marked with the datum feature symbol: a letter in a box attached to a triangle that touches the feature. The datum is the perfect plane, axis or point that a gauge (the datum feature simulator) creates from it.
The order of letters in a feature control frame matters. "A | B | C" means: seat the part on A first, with at least three points of contact. Then push it against B with two points. Then slide it up to C with one point. Change the order and you change the measurements, because a part is never perfectly square.
Pick datum features that match how the part is located in the assembly: the face it bolts down on, the bore it pilots on, the pin that stops it rotating. Datum features usually get their own form or orientation controls, such as flatness on A and perpendicularity of B to A.
A datum reference frame has no tolerance zone of its own. It is the coordinate system that the zones of position, profile, orientation and runout are placed in.
The figure shows a rectangular plate seated in a frame: three contact points on primary plane A, two on secondary plane B and one on tertiary plane C.
This page is about datums themselves. Each geometric control decides whether it needs them: form controls never do, orientation and runout always do, and position and profile normally do.
| Modifier | With datums and the datum reference frame |
|---|---|
| No modifier (RMB) | The datum simulator fits the actual datum feature. No shift is allowed. |
| Circled M after the letter (MMB) | The simulator is fixed at the maximum material boundary. If the datum feature is made away from it, the part may shift, giving datum shift. |
| Circled L after the letter (LMB) | The simulator is fixed at the least material boundary. |
| Common datum (A-B) | Two features used together as one datum, such as two bearing journals forming one axis. |
| Datum targets | Points, lines or small areas on rough surfaces such as castings, used instead of the whole surface. |
Example: reading the datum part of this frame on a 100 × 60 × 10 mm plate
Result: Every inspector who follows A, then B, then C gets the same answer. Swapping B and C gives a different result if the edges are not perfectly square.
Every GD&T symbol page: GD&T symbols chart
A datum feature is the real surface on the part. The datum is the perfect plane, axis or point that the gauge or fixture creates from it.
Real parts are not perfectly square. Seating on A first, then B, then C, fixes the part one way; a different order fixes it another way and gives different results.
The primary datum touches at least three points, the secondary two and the tertiary one. Together they stop all six degrees of freedom.
No. Form controls (straightness, flatness, circularity, cylindricity) never use datums. Orientation and runout always do. Position and profile usually do.
A common datum: two datum features used together to make one datum, such as the axis through two bearing journals.
Specific points, lines or areas on a datum feature, used on rough or large surfaces such as castings where the whole surface would not seat repeatably.
COPLAIN AI PLATFORM
Coplain turns datum setups (which face goes down, which edge goes to the fence) into photo-led inspection steps that every shift follows the same way.
Generate yours automatically with Coplain →Related Tools & Templates