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FARO® Knowledge Base

Calculating GD&T True Position Tolerance With Measure

The American Society of Mechanical Engineers (ASME), ANSI Y 14.5M, is very specific in its description of the meaning and usage of true position tolerance. FARO® has incorporated true position into CAM2®, Measure, Q, and Gage with the intent of matching this specification as closely as possible based upon the amount and type of data that is made available as normal measurements.

True position is defined as a cylindrical tolerance zone about the nominal centerline through the thickness of the material into which a hole is drilled or the height of a stud.

In order to apply this concept to measurements, FARO software engineers made these assumptions.

  • The datum features specified on the engineering drawing are used to establish the alignment.
  • There are small deviations between the nominal primary datum and the measured primary datum.
  • Material thickness is much less than the diameter of the feature.
  • Feature centerlines are perpendicular to the primary datum.

As it concerns RFS (regardless of features size) and MMC (maximum material condition), you can apply these conditions to features only and not to datums. If your engineering data requires the application of these conditions to datums, you will need to calculate these manually.

How True Position is Calculated

For CAM2, Measure, Q, and Gage software, the true position of any feature must sit parallel to the XY plane for the calculation to work correctly. If this requirement is not met, try rotating the coordinate system or making a new coordinate system for that specific true position call out.

For Measured Circle Features

The true position is calculated as twice the distance between the measured and the nominal as projected to the plane of the measured circle.

  • X = Distance between measured and nominal in plane of measured circle, True Position = 2 * X

A more exact definition would be twice the distance between the measured and the nominal as projected to the plane of the nominal circle. But, because the plane of the nominal circle may not be provided, use the plane of the measured circle.

For Other Measured Point Reducible Features

The true position is calculated as twice the distance between the measured and the nominal as projected to the XY Plane.

  • X = Distance between measured and nominal in plane of measured circle, True Position = 2 * X

In this case, MMC can not be applied, as there is no measured diameter from which to calculate a bonus tolerance. If the features in question do not lie in the XY plane, they should then be measured as circles. If the features can not be measured as circles, and do not lie in the XY plane, then manual calculations will need to be performed in order to determine the true position.

For Measured Slot Features

True position for slots controls the position of the measured slot axis compared to the nominal slot axis.