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Class-A Surfacing
While Class-A surfacing is in-and-of itself an art form, having the right tools makes a world of difference in the final product. For our surface modeling projects we use ICEMSurf software. ICEMSurf is regarded among automotive companies worldwide as a premier Class-A surfacing tool. What sets ICEM apart from other software is it’s surface analysis tools.
Surface sections, curvature sections, curvature plots and highlight plots are just a few of the quality feedback reports available to the modeler. The most powerful feature of ICEM is the fact that all of the plots and reports update in real time as the modeler makes changes to the surfaces.
One of the most often requested uses of laser digitized data is as a base to build surfaces from. This process can take two forms:
 
1) If surface data does not currently exist for a part, it can be built from the digitized data.
2) Surfaces may exist, but might not be current or up to date. In this case, the part could be digitized and compared with the original CAD data. Any changes would stand out in the comparison. The original CAD data could then be updated in the changed areas to reflect the parts current state.
What is Class-A Surfacing?
Class-A surfacing in the industry is commonly referred to as having 4 levels of continuity along surface boundaries, called G0, G1, G2, and G3.
Two more important aspects of the class A surface are the U and V parameters. These are values on the surface ranging from 0 to 1. They are used in the mathematical definition of the surface and for defining paths on the surface.
Note that UVs are not proportionally spaced along the surface. A curve of constant U or constant V is known as an isoperimetric curve, or U (V) line. In CAD systems, surfaces are often displayed with their poles of constant U or constant V values connected together by lines; these are known as control polygons.
The four levels of surface continuity are:
- G0--G0 continuity has a sharp edge between surfaces and a defined angle of deviation between surface normals.
- G1--G1 continuity is a tangent condition and will leave an intended highlight line visible along the boundary edge. This continuity value is absolute. It has been measured by the angle of the surface normal of both surfaces at the measuring point. The measurement of this angle has a direct angle tolerance.
- G2--G2 continuity is the first level that is commonly referred to as being curvature continuous. This level affects the third row of UVs on the surface at the edge where continuity was measured. G2 continuity is measured and given a tolerance. The fact that the curvature rate is still continuous at the third row gives the surface a lead in to the highlight portion of the surface, thus resulting in the desired condition of no highlight line along the surface boundary.
- G3--Recently, the next level of curvature continuity, G3, has become more prevalent in class A surfacing. G3 controls the fourth row of UVs. This level of curvature continuity plays a large role in what is generally accepted as the highest surface quality because curvature control to the 4th level must be obtained.
Although G3 is the highest surface quality, many times G0, G1 or G2 levels of continuity are desired, especially with todays emphasis on edge design.
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