CAM2 True Position Explanation and Interpretation
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 with the intent of matching this specification as closely as possible based upon the amount and type of data that is made available during a normal measurement session.
True Position is defined as a cylindrical tolerance zone about the nominal centerline through the thickness of the material into which the hole was drilled (or the height of the stud).
In order to apply this concept to CAM2 measurements, one must make some simplifying assumptions:
1. The datum features specified on the engineering drawing were used to establish the CAM2 alignment.
2. There are small deviations between the nominal primary datum and the measured primary datum.
3. Material thickness is much less than the diameter of the feature.
4. Feature centerlines are perpendicular to the primary datum.
In regards to MMC and RFS, CAM2 can apply these conditions to features only, and not to the datums. Should engineering data require the application of these conditions to datums, manual calculations will need to be performed to determine the true position.
CAM2 calculates the true position of features as follows:
For Measured Circles, the true position is calculated as twice the distance between the measured and the nominal as projected to the plane of the measured circle.
True Position of Measured Circles:
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, since the plane of the nominal circle is not generally provided, it is practical to use the plane of the measured if the aforementioned, simplifying assumptions are valid.
For all 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.
True Position for Non-Circle Point Reducible Features, 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.
Keywords: true position, mmc, rfs
