True Position is a GD&T location tolerance restraint typically applied to holes or threads on a drawing. It is one of the most common GD&T callouts to help control positional alignment.
Table of contents
True Position Symbol
The above example demonstrates nearly every aspect of a GD&T callout. The far left denotes the true position GD&T symbol followed by the tolerance (.007″). After the tolerance is the tolerance modifier (M) and the primary, secondary and tertiary datum references.
How to Measure GD&T True Position
In essence, true position is actually a very straight forward callout. It requires that the overall location of the feature falls within a certain tolerance. Henceforth, true position is different from standard locational tolerances because it allows a feature to be reference from a different location than the drawing view.
A CMM (Coordinate Measuring Machine) is typically used to measure true position. At this point in time, it is the most accurate probing technology available. Reports can easily be put together referencing multiple datums with a variety of tolerance modifiers. Most CMM come with a robust software package and they can report what measurements fall in or out of tolerance.
Depending on the callout and what it is referencing, calipers may be used. However, this is not common nor is it advised since calipers are less accurate than a CMM.
Common True Position Callouts
- 1-3 reference datums
- .001”-.005” of tolerance per reference datum
- Common internal reference: blind, through or tapped holes
- Common external reference: OD of a part
- Reference datums are usually large planes or line segments