Profile of a Surface
Profile checks are quite versatile. They provide the ability to evaluate size, form, orientation and location and the tolerance boundary can either be symmetrical or asymmetrical as needed, using any number of datum references. Measurements are fit to the features to minimize the point deviations as allowed by the any degrees of freedom not locked down by the datums and the max deviations are then com- pared to the tolerance boundary to evaluate the check.
Evaluation Process within SA:
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Feature measurements are allowed to rotate and shift, depending on the datum constraints to minimize the point deviations relative to the specified surface.
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Evaluation is made based on the greatest point deviation with respect to nominal surface and tolerance boundary.
Reporting:
Surface profile, by default, establishes a tolerance zone which is equally disposed inside and outside the surface. The check is a measurement of the thickness required to enclose the data while still being equally disposed. Which is why its double the largest error.
Measured Deviation is calculated by first finding the point furthest away from the boundary wall (the boundary being the extents of of the tolerance zone which could be asymmetric with respect to the surface), then computing how far to the boundary as a positive number [A].
When a check passes, the Measured Deviation is reported as:
[Total Tolerance Value]-[A*2].
When a check Fails, the Measured Deviation is reported as:
[Total Tolerance Value]+[A*2].
Distance Out of Tolerance is reported as: [A*2]
In order to specify an asymmetric tolerance zone for a profile check, open the annotation properties and edit the two tolerance fields.
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Tolerance. Defines the total tolerance band used by the check from the inner extent to the outer.
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Outer Tolerance. Defines the location of the outer tolerance on the positive or outer surface. This value controls the relative placement of the full tolerance with respect to the surface. For a symmetric tolerance this should be exactly 1/2 the tolerance value, and for a tolerance exclusively on the outside of the sur- face this value should equal the total tolerance.
Composite Surface Profile
Composite Surface Profile Checks provide a means to evaluate the overall position of a set of features and combine that with a tighter evaluation of the relative position of those features with respect to a less restricted set of datum constraints all within a single check.
Evaluation Process within SA:
What makes Composite Surface Profile checks unique from two separate position checks is that composite checks provide a greater degree of freedom in the lower segment check than would be allowed through a secondary check. Take the following check for example:
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The part is first fit to Datum A, B, and C, which fully locks down all degrees of rotation and translation and the upper segment of the control frame is evaluated.
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The pattern is then completely released to translate freely and rotate within the degrees of freedom not locked down by datum A to fit the nominal pattern. They key here is that translation is completely unconstrained.
This is very different from two independent checks. In this case the upper segment is evaluated, and then in the second check is evaluated separately. In this case the difference is slight but the part is limited in its ability to move toward and away from datum A, where in a composite check that would not be the case.
The difference is more apparent with two datums in the lower seg- ment. The part is both constrained in position to datum A and B. This restriction in translation and only allows the part to only slide in a straight line perpendicular to the datums to fit the data, while a composite check would only hold rotation for the lower tier evaluation.