CLASS A SURFACING

A Class surfacing and its importance: A class surfaces are those aesthetic/ free form surfaces, which are visible to us (interior/exterior), having an optimal aesthetic shape and high surface quality.

Mathematically class A surface are those surfaces which are curvature continuous while providing the simplest mathematical representation needed for the desired shape/form and does not have any undesirable waviness.

Curvature continuity: It is the continuity between the surfaces sharing the same boundary. Curvature continuity means that at each point of each surface along the common boundary has the same radius of curvature.

Why Class A is needed:

We all understand that today products are not only designed considering the functionality but special consideration are given to its form/aesthetics which can bring a desire in ones mind to own that product. Which is only possible with high-class finish and good forms. This is the reason why in design industries Class A surface are given more importance.

Understanding Class A surfaces:

1. The fillets - Generally for Class A, the requirement is curvature continuous and Uniform flow of flow lines from fillet to parent surface value of 0.005 or better (Position 0.001mm and tangency to about 0.016 degrees).

2. The flow of the highlight lines - The lines should form a uniform family of lines. Gradually widening or narrowing but in general never pinching in and out.

3. The control points should form a very ordered structure - again varying in Angle from one Row to the next in a gradual manner (this will yield the good Highlights required).

4. For a Class A model the fillet boundary should be edited and moved to form a Gentle line - and then re-matched into the base surface.

5. Matched iso-params in U & V direction are also a good representation of class A.

6. The degree (order) of the Bezier fillets should generally be about 6 (also for arc Radius direction) sometimes you may have to go higher.

7. Also you have to take care of Draft angle, symmetry, gaps and matching of surfaces Created with parent or reference surfaces.

8. Curvature cross-section needles across the part - we make sure the rate of Change of curvature (or the flow of the capping line across the top of the part) is Very gentle and well behaved.

The physical meaning: 

Class A refers to those surfaces, which are CURVATURE continuous to each other at their respective boundaries. Curvature continuity means that at each "point" of each surface along the common boundary has the same radius of curvature.

This is different to surfaces having;

Tangent continuity - which is directional continuity without radius continuity - like fillets.

Point continuity - only touching without directional (tangent) or curvature equivalence.

In fact, tangent and point continuity is the entire basis most industries (aerospace, shipbuilding, BIW etc ). For these applications, there is generally no need for curvature.

By definition:

Class A surface refers to those surfaces which are VISIBLE and abide to the physical meaning, in a product. This classification is primarily used in the automotive and increasingly in consumer goods (toothbrushes, PalmPC's, mobile phones, washing machines, toilet lids etc). It is a requirement where aesthetics has a significantcontribution. For this reason the exterior of automobiles are deemed Class-A. BIW is NOT Class-A. The exterior of you sexy toothbrush is Class-A, the interior with ribs and inserts etc is NOT Class-A.