Hi Brian,

> I think there is a "better to do this than that" set of "basic"
> rules item here but I dont know how to put in words myself?

I think some images will help!

So say you have 2 curves like this (here just one is selected):



You can select both of them and use Loft to make a surface there. But such a surface is collapsed at the points where the curves touched each other. To better explain it, imagine that the surface is a kind of gridwork of curves (that is basically what NURBS surfaces are actually). The grid of such a surface will look like this:



Notice how there is a collapsing down to a kind of "pole" area at the top and bottom where the curves touched.

These pole areas tend to cause problems with more advanced calculations such as offsets and intersections (both of which are used in the process of filleting). With freeform surface generation commands like Loft, it is easy for there to be little tiny ripples and bumps right in that pole area. That causes a lot of confusion about the precise direction of the surface normal in that area, and that's what causes these problems.

Some other kinds of surfaces like spheres or surfaces of revolution also have these poles, but they tend to be much more precisely formed at that area and without little wiggles or bumps. So they are somewhat more behaved despite having these poles, although still it can sometimes cause problems even then.


Now here is basically what happens with the alternate way that I was describing.

In this method things start with a central flat curve like this:



Then that gets extruded out to a surface:



Then that extruded surface gets trimmed to an outline:



That will create a very similar shaped surface. However, if you examine the structure of this surface, it has a lot more simple rectangular kind of layout to its grid, like this:



That's because when you trim an object, new trim curve are calculated with mark areas of the surface as being "holes", but the surface data is still there underneath the trim curves.

A trimmed surface like that has far more "stable" properties to it compared to the pinched one - even if it has some ripples or bumps in it, it doesn't have any spot where the surface might actually fold back over on top of itself, that's kind of what can happen sometimes with the pole situations unless things are very exact in that area.


So the thing to avoid is creating surfaces that squish down on themselves, unless you don't plan on filleting it.

If you plan on filleting, it is better to have things like extrusions which generate more rectangular grid type surfaces and then have trims that cut away parts.

I hope that might help?

- Michael