Geometric Operations - Things to Consider, Part 1

Source:
Dr. Richard Lowrey, Mechanical CAD Consultant

Geometric operations in solid modeling are volumes created using cross-sectional planer profiles and dragging functions, such as blends, extrudes, sweeps, revolves, fillets, corner rounds, and local surface deviations. It should be noted here that in a solid modeling system these geometric operations result in a single closed solid entity. Similar operations are available in surface modeling that result in one or more individual surface entities.

Some "hybrid" modelers can support both at the same time.
These volumes may have constant or variable cross sections and linear, multi-linear, curvilinear, or axisymmetric paths. The common naming conventions used for such entities are: extrude, sweep, blend or transition, and revolve.

Here are some things to consider when construction geometric operations.


Blending Concerns

• Additional profiles to control cross sections may be difficult to position accurately if the trajectory curve varies rapidly.
• Due to their approximate nature, blends can be created, sectioned where refinement is desired, new improved profiles constructed at the cross sections, and a new blend created.
• As additional cross sections are added, check for surface continuity between profiles as waviness can be introduced.
• The number of segments per profile are required to be equal as each segments is blended in consecutive profiles. Arcs and lines of profiles may need to be subdivided.
• Blends are sometimes restricted to profiles on parallel planes.

Sweep Concerns

Depending on the application, the sweep volume may not self intersect along the sweep trajectory as it would if a figure-8 trajectory were used. Arc or spline segments of the trajectory that are small as compared to the profile can cause self-intersection.


Extrusion Concerns

Two or more non-intersecting outside planer profiles on the same plane can be extruded simultaneously to create multiple unconnected parallel solid primitives. Unconnected objects can define a valid solid, but applications such as automatic finite element meshing and automatic NC tool path generation can not use a solid of multiple volumes.