Using Offset Objects
-
The data should be pre-aligned to the part using other non- GD&T techniques before attempting GD&T analysis, such as Quick Align to CAD or relationship fitting.
-
It is necessary to specify an SA Offset Object within the datum an-
notation which will be used to determine the degrees of freedom to constrain for that datum during alignment.
This offset geometry is what will “maintain contact” during alignment to subsequent datums and/or the main tolerance feature and so it needs to be chosen in such a way that it represents the control the datum is intended to exert over the solution. For example, a slightly curved plate designated as a datum can be treated as either a planar surface or a cylindrical surface. The offset object selection clarifies this.
GD&T evaluation begins with an initial alignment step performed internally and also use an iterative solution. When surface datums are involved both are disabled. This was necessary to ensure stable results with surface datums and since the data is required to be close before starting they aren’t needed anyway.