i think class-A surfaces are basically what u see from outside. (we can say the visible surface)

that can be either digital one or a physical model

It does not have to be an exterior part, interiors also have class A surfaces.

There is alot more to being able to produce class A surfaces than just continous curvature.

A clay (physical model) will normally be scanned in to give a series of sections. This will then be worked on by the surface guys to get the math model A surface. They will probably use that to re-mill another clay. This may happen several times as they tweak it and make changes.

Class is industry wide, if a customer can see it (exterior and interior) it will be class A.

CATIA / ICEM SURF and other packages can be used to produce class A surfaces. You need a package that allows you to really control your surfaces.

Class A is something visible: Like the hood of your car.
Class B is visible but not essential: Like the door aperture.
Class C is not visible, like the seat mounting under the carpet.

This article has a few numbers to define a class A surface.

http://www.design-engine.com/stories/classa.htm

to quote them

"Our Understanding for 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. We also 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. "


It was written by

For further information contact:
Pankaj jha
Tata Autocomp Systems Ltd.
106 A Muttha Chambers, 3rd Floor,
S. B. Road, Pune- 411 016, INDIA
Tel.- +91-20-5660690
Fax- +91-20-5660699
pankajjha@tacogroup.com
http://www.tacogroup.com

class a -surface was around before the computer therefore a definition of class-a surface has a little earlier history.  basically any surface that warranted the attention of the styling studio was a studio product  (inferring ownership) and thus would require the best surface production available at the time.  The curvature continuity ability in certain softwares makes the technicalities of quality surface easier , but does not define class-a surface.

class a -surface was around before the computer therefore a definition of class-a surface has a little earlier history.  basically any surface that warranted the attention of the styling studio was a studio product  (inferring ownership) and thus would require the best surface production available at the time.  The curvature continuity ability in certain softwares makes the technicalities of quality surface easier , but does not define class-a surface.

   I agree with what HDS said.But this article is related to the use of ICEM Surf,perhaps not for the users who use imageware to perform Class A.

   According to the imageware training,we can find that typical values that Class-A designers look for are: 0.001mm positional continuity and 0.05 degrees tangential continuity.  Curvature continuity differs depending on the application.  Each customer can vary, but these values should stand-up in almost any surface development situation.  These figures produce surfaces edges that are indistinguishable to human sight or touch.