topic Re: Crease in surface in Fusion Design, Validate & Document Forum

Crease in surface

tanwinghoe1983 — Sat, 23 Oct 2021 05:15:05 GMT

I am looking for some opinion on how I can remove the crease in this surface. I have attached the file here. I am aware that the sketches are not defined. At the moment, I am just experimenting with different workflows to see what will give me the best surface.

I also have a question regarding the "connected" continuity option for lofts. Do this option make the surface normal to the sketch plane? In this particular design, I actually made a "helper" face for the loft to align to. But this resulted in an error. If the connected option ensures that the surface is normal to the sketch plane, then I can just leave it as connected.

Re: Crease in surface

TrippyLighting — Sat, 23 Oct 2021 10:52:36 GMT

The crease is the result of using arcs with tangency to straight lines.

The best surface results can be achieved with splines and G2 (curvature continuity) constraints.

Also the two top lofts are triangular, meaning they are lofted into singularity. That will almost never result in a good surface and very often is he root cause for not being able to shell, thicken or surface-offset geometry.

Aslo, the connected condition only means connected. it does NOT guarantee normal direction. Yo'd have to enable "direction". That's not what you want either 😉

I'll look at this a

Re: Crease in surface

tanwinghoe1983 — Sat, 23 Oct 2021 11:56:28 GMT

Hi Peter, thanks for the advice. I will look into using splines. However, I find it hard to see how I can proceed with a non-triangular loft. Noted on your comments regarding "connected". Also, if you don't mind, could you also roughly explain how the "free edges" continuity option works?

Re: Crease in surface

wmhazzard — Sun, 24 Oct 2021 02:01:12 GMT

This is a great video that might help you out.

https://www.youtube.com/watch?v=BFHaQGhxURs&t=479s

Re: Crease in surface

TrippyLighting — Sun, 24 Oct 2021 09:06:41 GMT

I'd call the video good, but not great. It would be great if he provided links to some sources that explain the concepts he omitted to explain in the video.

If I recall correctly he never explains why you need three aligned control points to achieve G3 curvature continuity.

He also does not explain why G3 in Fusion 360 can only be achieved to flat surfaces.

When you loft between rails defined by sketches and the sketches have G1/G2/G3 constraints applied to define the transition between the sketch curves, that only guarantees that the transition of the resulting lofted surface is G1/G2/G3 in exactly the areas defined by the rails. It does not guarantee that transition across the entire surface. IN order to guarantee that we need to set the proper condition in the loft dialogue for the loft profile.

We only have up to G2, however. For straight surfaces, the rails provide all the definition needed, but for arbitrarily curved surfaces we'd need a G3 condition in the loft dialogue.

The documents I refer mostly to when users are interested in surfacing are the Autodesk Alias Theory Builders.

Written ages ago by the creators of the software, it's still some of the best documentation out in the wild!

Then I usually recommend the Handlebar 3D youtube channel. The guy works professonally in vehicle design and uses Alias for surfacing. A lot of the concepts he explains apply generally to surfacing but it also becomes clear where Fiosn 360 still has a lot of room to grow.

So the video only helps with the bottom part. While that is substantially more work than just creating circular fillets it's a necessary step in creating this seemingly simple object. The really tricky area is the top, however. I have yet to find a good approach that actually returns a good surface.

Re: Crease in surface

TrippyLighting — Sun, 24 Oct 2021 09:50:00 GMT

So this is what I've got so far. the top is a "simple" patch. Until a most recent update that increased the surface quality of the patch tool I would have never considered using a patch to create this surface but it works OK in this case.

There are still 2 areas that need some work (I'll leave those for you to discover) but so far it looks OK. The problem is that the patch tool does not offer a lot of control over particular areas of a surface. You can add internal rails, but if you check the resulting surface with the curvature map tool, you'll find that you might have gotten the shape in that particular area, but the overall surface quality went down.

Re: Crease in surface

tanwinghoe1983 — Sun, 24 Oct 2021 10:43:58 GMT

Thank you for the effort, Peter. Wow, the zebra lines look really clean. I would not have thought the patch can work that well over such a big surface. In my second attempt, I tried to use four sided lofts as much as possible, with the exception of 2 small portions using patch. It did get rid of the crease but the surface quality does not match that of your effort here. Attached the file here.

Also, I have noticed that you used splines instead of a regular slot sketch for the base. Does that improve surface quality?

Re: Crease in surface

TheCADWhisperer — Sun, 24 Oct 2021 12:15:28 GMT

@TrippyLighting wrote:
The crease is the result of using arcs with tangency to straight lines.
The best surface results can be achieved with splines and G2 (curvature continuity) constraints.

@tanwinghoe1983 wrote:
Also, I have noticed that you used splines instead of a regular slot sketch for the base. Does that improve surface quality?

Re: Crease in surface

TrippyLighting — Sun, 24 Oct 2021 12:40:19 GMT

4-side lofts are only part of the "recipe". Do you know why?

Before continuing to model I would encourage you to thoroughly read through the Autodesk Alias Theory builders I linked to in my last post. Then let's talk ...

Re: Crease in surface

TrippyLighting — Sun, 24 Oct 2021 12:43:14 GMT

@tanwinghoe1983 wrote:

... Wow, the zebra lines look really clean ...

Actually, they don't, but you'll need to open and rotate the model I had attached to see where they don't look clean 😉 There are three areas that need work in this model.

Re: Crease in surface

TrippyLighting — Mon, 25 Oct 2021 22:59:32 GMT

Now they look clean:

Re: Crease in surface

tanwinghoe1983 — Tue, 26 Oct 2021 12:07:17 GMT

Looking at the split lines, it looks like you have built the surfaces in a different way. Would you be able to share the file? Thank you.

Re: Crease in surface

TrippyLighting — Tue, 26 Oct 2021 14:11:03 GMT

Model is attached. The key here is to trim surfaces in such a way that you can create a fairly rectangular shaped 4-sided loft.

When you use G2 conditions the trimming often needs to be quite generous, otherwise there will be a lot of tension in the surface.

After checking with Zebra stripes I always check surfaces in the render environment with a dark, high gloss material.