GDT:Datums Overview
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| Symbol | Datums Overview |
|---|---|
| Type | Datums |
| Category | Datums |
| Standard | ASME Y14.5 |
| Author | Admin |
| Datums | Defines a datum reference frame from datum features |
| Modifiers | Material condition modifiers and datum feature simulator rules |
| Tolerance zone | Datum-related tolerance zones and simulators |
| Image | GDT-Datums-Overview.svg |
| Common mistakes | Unstable datum selection; missing datum order; mixing unrelated datum features |
| Related | GDT:Position |
Summary
Datums establish the reference frame used to locate and orient toleranced features.
Definition
A datum is a theoretically exact reference derived from a datum feature. Datum features are real surfaces or features on the part that are used to establish the datum reference frame.
When to use it
Use datums whenever a feature needs to be located or oriented relative to other features or functional interfaces.
Examples
Example: Primary, secondary, tertiary datums
A flat surface as datum A, a perpendicular face as datum B, and a hole axis as datum C.
Common mistakes
- Choosing unstable or non-functional datum features.
- Omitting datum order or mixing the order between segments.
- Using datum features that do not reflect assembly or inspection constraints.