Downloaded from https://www.everyspec.com APPENDIX C MIL-STD-31000A Figure 5. Image showing the control points corresponding to the Greville points in the figure above. Surface Greville points that are outside the surface trimmed area should be excluded from the point sample unless all Greville points end up outside the surface trimmed area (an uncommon occurrence. In such cases at least one sample point must be generated within the trimmed surface area using another sampling technique. It is not necessary to sample points on analytic surfaces such as planes, cylinders, ellipses, and conic sections.
Where analytic surfaces intersect with NURBS surfaces, it sufficient to verify the position of the edge curves. When two analytic surfaces intersect, the actual position of the surfaces may be validated by comparing the parameters of the surfaces. For example, a plane surface may be described at a minimum by six real numbers: the origin (3 coordinates) and the unit normal vector at the origin (3 coordinates). C.6.1.2.2. Allowable deviations. The allowable deviation between sample points and the curves or surfaces on which they should lie is expressed as a tolerance + 0.XXX units of measure.
The tolerance used should be less than the manufacturing tolerances employed to make the part, but greater than the allowable edge-surface deviation of the target CAD system. The tolerance applicable to sand castings, sheet metal, or rubber parts is generally greater than the tolerance for precision milled parts. In the absence of other information, contractors that employ the metric system may employ a tolerance of + 0.01 mm. In English units, a tolerance of + 0.0004 inches is common for precision machinery. C.6.1.2.3. Reporting of deviations.
When sample points are used to confirm the geometric accuracy of surfaces following translation to another format, the number of points required to be examined is generally too large to allow this task to be performed interactively. Consequently, the evaluation of differences between the points and their corresponding surfaces should be performed by automated validation software. The validation software should produce a report showing the distance between sample points and their boundary curves or surfaces. Points that fall outside of the allowable tolerances should be noted in the report.
The form of the report may be graphic or tabular and shall be approved by the contracting officer prior to the submission of 3D models. 74