
There is a great deal of debate and confusion regarding quality surfacing and what may or may not represent a class “A” surface. Many Design studios state that they offer this kind of service. Some firms like ICEM (PTC) claim to have Class “A” software. This leads to many users wondering what they are using. Is it only a B modeller? What’s the deal? Should you be doing Class “A”?
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.