Well, here’s the I.D.milano answer. A class “A” surface is one without physical imperfection. “A” surfaces are the ones seen immediately by the customer and can be contrasted with the less visible and therefore less important class “B” surfaces that are less critical. These highly aesthetic “A” surfaces are closely associated with the automotive industry. When you wander around a new car in the showroom, all of the large panels you see will need to be of this quality. Anything painted with a metallic basecoat and a transparent clearcoat and then polished until it is gleaming will show up the slightest defect. When you have a hundred spotlights glaring down on each panel, the slightest unintended deformation of the reflection is unacceptable. This is the reason for this designation.

By the way, you will find the “B” surfaces on non-glossy interiors, stiffening stampings for the underside of the bonnet and boot and on partially hidden areas that you might have to go out of your way to see. Smaller consumer products are no different. Something like a mobile phone or an Ipod that will be closely examined daily effectively is all “A”. The only place for defects is in the battery compartment.

That’s the easy part. Now, the hard part. How to get the “A” bit into your surfaces.

At this point the discussion turns immediately to continuity. C1? C2? C3? What is required? Well, the consensus is that you need at least second degree, C2 or G2 to achieve class “A”.

"Now I’m screwed", you might be saying. I can’t create variable section sweeps with G2. I can’t create sketched curves with curvature continuity between new or existing entities. What am I to do?

(By the by, if you are using Pro/Engineer, you can talk about the “G” version. Please don’t ask me to remember the mathematical difference between C and G. Its really not that significant.)

Without the ISDX module, PTC force you to create curves between 2 points in Pro/Engineer. This allows you to force G2 and then create boundary surfaces that are also continuous. However, it is a truly horrible option. Curve between 2 points is not a strong feature in Pro/Engineer and very difficult to tweak. An even worse option is to create composite curves, either by using overlayed splines within the sketcher or by creating a seperate approximate curve feature. This will also enable you to achieve G2. You’ll just have a miserable looking transition area.

You may now realize that even if you can achieve G2, this alone does not guarantee class “A”. In fact, forcing surface continuity at every opportunity does not necessarily make your surfaces better. You may discover that after having spent considerable time achieving G2, the quality of your surfaces is worse than before!

There is another reason why this may occur. In addition to the lack of appropriate surfacing tools, there is another problem. Curves and surfaces inside Pro are only built to the third order (ie. the polynomial after quadratic). This means that it often has difficulties adapting to particularly dynamic changes in curvature. Compare this with ISDX and Alias that use much more flexible curves (up to order 8 or 9 if I remember right). By the way, if you are from PTC and reading this, please correct me if any of my statements are incorrect.

The catch with quality is that there is no quick and easy solution. Checkboxes can give you a measure of success but they don’t necessarily provide the whole picture. G2 curvature can often be a good thing, but it’s not a complete solution to quality surfacing.

The image above is designed to portray how well-built surfaces can be constructed with G1 (tangency) continuity and how rubbish surfaces can be made with G2 continuity. In each case there are 2 curves created in Pro/Engineer.

A) This is G2 continuity. Where the two curves meet the length of the porcupines is identical. The inflexion in the curvature plot will show up as a distorted highlight on the surface. Each curve is decellerating up to the point where it meets the adjacent curve at which point there is a significant acceleration.

B) This is G1 continuity. The second curve was sketched with only a Tangent connection. While the continuity is not perfect, it is better than (A) because the change in acceleration at the transition point is more subtle. Compared with (A) this curve has been crafted rather than thrown together.

C) This is G1 continuity. It is the worst of the bunch. Not only is there a significant difference in curvature value at the transition point but there is the same decelleration followed by accelleration that was evident in (A).

D) This is G1 continuity. This is a very common case in Pro/E. There is a step change in curvature but the first curve flows into the second without any obvious inflexion. While there will probably be a visible seam at the transition, the overal quality will be reasonably good and probably better than (A). If this surface has a slight texture, the defect will probably not be visible. If it is a polished surface, better rebuild.

Note that these examples are all 2D curves yet I am talking about surfaces. Whether you are creating extrusions, sweeps, or boundary surfaces, you will be sketching or using existing curves. The quality of the final surface is entirely dependent on the quality of the base curves. This is especially true in the case of boundary surfaces. If you have four input curves, the quality of the final surface will be at least as bad as that of the worst curve.