Sheet metal unfold elliptical arc?

gregory_holmberg

When converting to sheet metal, why can't I unfold a surface that's in an elliptical arc?

I create an elliptical arc in a sketch, then I extrude this as a new surface, up to the face of a non-flat part.



Next I create a sheet metal model, converting this surface.  I get an error message saying "Sheet metal model cannot be unfolded."

Why?  I have successfully done this with extrusions of lines and circular arcs.  Why not an extrusion of an elliptical arc?

Here's my project.

Greg

gregory_holmberg

@glen_dewsbury Looking at your success case, I think a work-around is to do what you did.

Instead of extruding the elliptical arc up to a face, extrude blind farther than needed, and then split the resulting band-like surface with the arching surfaces.  In your document, the sheet metal model was able to deal with the resulting surface.

I also discovered another work-around.  Instead of extruding the elliptical arc up to a face, extrude it up to part, using the surfaces of the arches.  At least this worked in my guitar document.

@lana With this work-around, I am happy.  I still think OnShape should either fix it or document the limitation of elliptical arcs compared to circular arcs.

Greg

lana

As surprised as you are. I'm bugging it to investigate further.

gregory_holmberg

lana said:

As surprised as you are. I'm bugging it to investigate further.

lana, were you able to recreate the behavior?  Have you learned anything about it?

lana

Have not had a chance to dig into this yet - will update when I have a new information.

glen_dewsbury

@gregory_holmberg
Out of curiosity I tried what you described and had no problem with SM tool to produce a flat wrap of the guitar body or just an ellipse with a copy of your document. Hope this sample helps.
https://cad.onshape.com/documents/8cd137c3946287f51bc78b02/w/f68b317ec042fd55f2f076b6/e/8ad36712cf15c80b33b090e5

gregory_holmberg

Thank you, @glen_dewsbury That was very helpful.

I analyzed the difference between the two cases, and discovered that the problem occurs when the elliptical arc is extruded up to a face.  In your example, you extruded blind to a given depth, and then the elliptical arc could be unfolded.

In my case, I extruded the elliptical arc up to a face, and then when creating the sheet metal model, I get the message "contains faces that cannot be thickened".

Circular arcs do not have this problem.

@lanaI created a minimal test case to demonstrate the issue.  In this document, there is both an elliptical arc and a circular arc.  Each is extruded up to and down to faces of some arched surfaces.  The circular band is converted to sheet metal successfully. When I try to convert the elliptical band to sheet metal, I get the message "contains faces that cannot be thickened".

I can see no reason for the difference.  If there is a good reason, then the limitation should be documented.  I didn't see any limitations mentioned in the docs about converting elliptical arcs to sheet metal.

I really, really would like this fixed, if possible.

Thanks,

Greg

gregory_holmberg

@glen_dewsbury Looking at your success case, I think a work-around is to do what you did.

Instead of extruding the elliptical arc up to a face, extrude blind farther than needed, and then split the resulting band-like surface with the arching surfaces.  In your document, the sheet metal model was able to deal with the resulting surface.

I also discovered another work-around.  Instead of extruding the elliptical arc up to a face, extrude it up to part, using the surfaces of the arches.  At least this worked in my guitar document.

@lana With this work-around, I am happy.  I still think OnShape should either fix it or document the limitation of elliptical arcs compared to circular arcs.

Greg

glen_dewsbury

Out of curiosity I went back to your original post and reviewed the SM feature of the sides. What I found was kinda odd. The convert has selected a single part that is a closed loop and not cast an error. It should have. (Maybe something for OS techs to look at) I could not select faces to exclude. The corner mods used had no affect on the closed loop. Instead it was accepted but produced no flat view.
While in theory it might be possible to form this from a closed loop but certainly not practical. OS follows typical sheet metal practices that would not allow this.