In addition to solid CAD models, the Transfer Die and Sheet Metal process groups support surface CAD models, which represent surfaces only, rather than solid geometry. Surface models typically model one side of stamped or soft tooled metal component.

When you use a surface-only model, you must enter the intended part thickness into the Blank Thickness field of the Cost Guide. The surface model together with the specified thickness allow aPriori to, in effect, extrapolate a solid model of the part, and then extract geometry and perform costing just as it would with a solid CAD model as input.

You can also specify blank thickness for surface models with the aPriori Bulk Costing interface: right-click on the desired parts (rows) and select Production Inputs.

The geometry and costing results for a surface-only model of a part are generally identical or extremely similar to the results for a solid model of the same part. But because information is missing from the surface-only model (even with the addition of a thickness value), some geometric processing might differ slightly.

One type of information that is missing from a surface model is the exact shape produced by the cuts that create the holes and edges of the blank. In these situations, aPriori assumes that the cuts are true, at 90 degrees to the surface.

For example, an elliptical opening in a model (either solid or surface) indicates a hole GCD, but the information in the surface model is not sufficient to determine whether the walls of the hole are cylindrical; so aPriori assumes that the hole is a complex hole (with non-cylindrical walls and an axis perpendicular to the part surface) rather than an angled simple hole (with cylindrical walls and an axis oblique to the part surface). The additional information provided by a thickness value still leaves this under-determined.

Similarly, a surface model lacks sufficient information to determine whether an edge is chamfered.

Note also that the GCD Area and Volume properties for a surface-only model of a part might differ slightly from the same properties for a solid model of the same part. This is because the information in the surface-only model under-determines the effects of bends and curves on volume and surface area calculations. So, in starting point VPEs, costs that depend heavily on surface areas and volumes (for example, costs for heat treatment and surface treatment) might differ slightly between surface-only and solid models.

Note that if you enter a value into the Blank Thickness field for a solid CAD model, aPriori ignores it, and issues a message indicating that thickness will be derived from the solid model rather than from the user-entered value. If your company does not use surface-only models, you can remove the Blank Thickness field from the Cost Guide if you wish—see the aPriori System Administration Guide for information on customizing the Cost Guide.

GCD extraction for surface models can be disabled with the aPriori property apriori.enable.surface.models.sheetmetal—see the aPriori System Administration Guide for more information.