When Modifying Surfaces  Don LaCourse, eDocHelp
November 18, 2004
reprinted by permission from
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.
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.
About the Author Don LaCourse is principal partner of eDocHelp, which provides e-documentation, online help, technical writing and 3D modeling services for the CAD/CAM, manufacturing and other service-oriented industries. eDocHelp recently launched 3DCADTips, a 3D CAD resource site. Don has over 25 years of experience in design and documentation. He gained much of his experience during his 10 years as a tier-one automotive designer with Textron, Inc. He also helped NASA and the European Space Agency (ESA) design and document their joint SpaceLab 1 project that flew on board several of the early space shuttle missions. Don also brings product and injection mold design and documentation experience to eDocHelp. Don had his first book published in 1995 serving as editor-in-chief for the McGraw-Hill publication "Handbook of Solid Modeling," where he contributed, edited and managed over 25 industry-leading authors. He is currently a contributing editor for Cadalyst magazine.
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