When modifying surfaces

Source:
Don LaCourse, Principal Partner, eDocHelp
Editor, 3DCADTips.com - 200,000 CAD Resources!!

There are many techniques available to modify surface geometry both locally and globally. Our discussion here is limited to any concerns these modification techniques raise for the ultimate development of trouble-free solid geometry. Each tip below is also listed as separate posts in this forum.

Trimming Surfaces

Surfaces may be trimmed or divided by curves and or other surface boundaries. This is a very convenient and powerful technique to seemingly discard unwanted surface areas. However, trimmed surfaces do pose concerns for the development of solid models and for data exchange through neutral formats such as IGES and STEP.

Many trimmed surfaces in a single model will increase database size and complexity since more information is required for storage (original surface, trim boundaries, and resulting trimmed surface). Also, the resulting surface edge is dependent on the accuracy and dependability of your 3D CAD system's surface trimming functions. Air tight surface enclosures are required for solids so always check trim boundaries that are to mate with other surfaces.

Extending Surfaces

Surfaces may be extended in either or both the U and V directions. Extending surfaces is very helpful during the development of surface geometry. For example, to establish an additional section curve further away from a surfaces current boundary, or to allow the continued tangency of subsequent surface functions. As a rule, surface extensions are used as a means to an end rather as an end to themselves. The development of new surface boundaries is generally the result of surface extensions.

Surface Tangency

Some surface operations such as fillets and corners maintain surface tangency during their construction. Such surface areas should not be reconstructed from their boundaries as recommended here for other surface areas. Even though their boundaries would be the same, the surface patch operation does not take tangency into account and the resulting surface may differ from the one it is intended to replace. This is especially true for the corner surface operation.

Local Surface Deviations

Surfaces may be modified locally. Any deviation that maintains a surface's current boundaries and thus the solid's current topology is acceptable unless the surface deviation internally or externally pierces another solid face. Local surface deviations take several forms. Here are a few:

• Radius Dome - A smooth radius surface deviation computed to fit a selected surface.
• Section Dome - Replaces an entire planar surface with a blended, extruded, or swept sculptured surface.
• Local Push - Smoothly deforms a circular or rectangular surface region.