Position checks provide the ability to evaluate size, orientation and location. The number of datums specified, the number of measurements and the type of feature determines the way in which the evaluation is conducted. Here we will just consider a basic hole position check.
The degrees of freedom controlled by the datums determine if it checks a point location or an axis. In evaluating the True Position of a feature relative to a single datum axis you are essentially checking the coaxiality of that feature relative to that datum.
Note that when specifying a MMC or LMC position tolerance a second annotation must also be included on the same feature to specify the nominal dimension. If you do not an error “No size tolerances defined for feature” will be seen. This second annotation will be automatically recognized and included in the evaluation.
Evaluation Process within SA:
The feature measurements are used to establish the center point or axis of the measured feature.
The maximum point or axial deviation is minimized after aligning to the datums and using any degrees of freedom not locked down by the datums.
The maximum deviation is doubled to define the measured deviation, any bonus tolerances are applied, and compared to the tolerance zone for evaluation.
Reporting
Measured Deviation is calculated by first finding the largest center or axis point deviation from the nominal feature and doubling it to define the tolerance band required for the part to pass.
Distance Out of Tolerance is reported as the radial distance from the maxi- mum deviation to the tolerance boundary. Or the distance a drill bit would have to move for the part to pass when checking the placement of a hole.
Tolerance Definitions Zones
Several Tolerance Zones options can be set for position evaluations on different features.
None. Which is a default setting that will apply either a cylindrical, planar or spherical tolerance zone based on the selected geometry type.
Cylindrical. Tolerance zone is a cylinder sized by the tolerance value centered around the nominal cylinder axis. The axis of the actual cylinder must fall inside this tolerance zone.
Planar. Two parallel planes spaced apart by the tolerance value centered on the nominal cylinder axis are used to define the tolerance zone. This allows the actual to slide back and forth between the planes. Note that this setting requires a fully con- strained condition or it will rotate to a perfect result.
Spherical. Currently only supported for spheres.
The geometry type has a direct influence on how a position check is evaluated. Cylinders can be evaluated as lines, circles, or full cylinders based on a user select. The extent of the evaluation is established by the extent of the measurements unless otherwise specified.
The offsets on points used in a position check on a line are ignored. The expectation as that measurements are on the axis.
Composite Position Checks provide a means to evaluate the position of a pattern of features and combine that with a tighter evaluation of the position of each feature relative to each other feature within a single check. So for example, this allows you to check if a bolt hole pattern matches the correct location on the part at a lower tolerance while also verifying that the pattern is correct at a higher tolerance.
Evaluation Process within SA:
The upper tier is evaluated following the position check guidelines above.
The feature is then released to translate freely and rotate within the degrees of freedom not locked down by datums in the second tier and evaluated again.
The check is passed only if both evaluations pass.