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Turbo Talk => Problems => Topic started by: IanC on June 10, 2019, 09:02:29 AM

Title: Can't unbend sheet
Post by: IanC on June 10, 2019, 09:02:29 AM
...and I can't see why  >:(

To be double sure I was parallel and had no un-connected nodes, I re-created from a polyline made of arcs and straight segments before using the offset tool, closing the ends, and making into a single polyline, then giving thickness as a Solid. I've also tried Simple Extrude rather than just assigning thickness.

So why can't I unbend? Neutral axis set to less that the sheet thickness.

Using 2018 ProPlat.
Title: Re: Can't unbend sheet
Post by: IanC on June 10, 2019, 09:39:47 AM
I think I've fixed it, well two problems actually so not sure which it was....

There was a bit of 'chaff' I discovered, and also I recreated by 2D filleting line segments before offsetting as before.

Title: Re: Can't unbend sheet
Post by: Jeffin90620 on June 10, 2019, 02:42:16 PM
Well, I'm confused.  Your 3D object is listed as a single Polyline under the Selection Info panel.

I traced the inside curve of the narrow face, then Offset it and added two short lines to connect the two pieces together followed by a Simple Extrude to duplicate your structure.  After I Exploded it to get an ACIS Solid, I tried Unbend Sheet, which only partially succeeded (the top facet on the outside curve, shown in blue, and the second flat section on the inside curve, starting from the other end, shown in red).


Jeff
Title: Re: Can't unbend sheet
Post by: Henry Hubich on June 10, 2019, 03:11:18 PM
I reset the 3D Thickness to zero; exploded the Polyline to lines and arcs; deleted all the arcs and all the lines except those on the inside; applied "Meet 2 Lines" to join adjacent line segments; applied Join Polyline to weld everything back together. Extruded to 400 mm; filleted joints to 40 mm radius; shelled the extrusion to 5 mm thickness. Unbent with no further difficulty. Modified drawing (v2018 format) is attached.

Henry H
Title: Re: Can't unbend sheet
Post by: IanC on June 10, 2019, 03:36:37 PM
Thanks :)

Jeff, yes it is a single 2D polyline, with thickness.

Henry, the fillet radii might appear very random, but they are what I received back (as a DXF/DWG) from a firm who are going to incrementally CNC-bend the part, which they assure me are correct. I don't really understand how they end up like that!

As I say, all resolved for now.
Title: Re: Can't unbend sheet
Post by: IanC on June 12, 2019, 08:32:16 AM
Hmmm, I've got another one!

This was created by first slicing and facet-editing a single ACIS solid to achieve the shape I needed. Quite a lot of times to achieve what I wanted, actually, but it looks fine.

Then I used Surface From Solid to 'hollow out', before again Facet Editing to delete unwanted faces. So now I have an ACIS solid zero-thickness entity, which I then Shelled to give it's 5mm thickness. So I know it's a consistent thickness.

So why won't it flatten?

The surface consists of three planes, which were then given a 55mm fillet along the two edges between planes.
Title: Re: Can't unbend sheet
Post by: IanC on June 12, 2019, 09:45:33 AM
OK, appears to be fixed. Slightly different workflow, although I tried the original way twice and still think it should work!

First I facet-edited the original ACIS-solid to remove the two fillet edges. The did the hollowing-out and deleted faces, before filleting the zero-thickness ACIS-solid. Then shelled to give the 5mm thickness.

Now it flattens! ;D
Title: Re: Can't unbend sheet
Post by: Henry Hubich on June 12, 2019, 10:47:42 AM
If you shell before filleting, you need to specify a larger radius on the outside. If you fillet first, the shelling operation takes care of this automatically.

Henry H
Title: Re: Can't unbend sheet
Post by: IanC on June 12, 2019, 11:42:12 AM
If you shell before filleting, you need to specify a larger radius on the outside. If you fillet first, the shelling operation takes care of this automatically.

Henry H

In both examples, I filleted prior to shelling. But in the second successful attempt, I applied the fillet to the zero-thickness ACIS-solid 'surface', as opposed to the ACIS-solid 'solid'.

Attached is the ACIS-solid 'solid':

Fillet > Surface-from-Solid > Delete Faces > Shell > Won't Unbend
Surface-from-Solid > Delete Faces > Fillet > Shell = Will Unbend

I don't really understand why one works and the other fails? Maybe I just need to accept the fact!  ;D