Monday, 5 January 2015

Sheetmetal cone and Unfold/Refold bug

Recently I stumbled onto a very annoying bug and regardless of many methods to get around it I couldn't. It's simple enough to replicate. It just requires me to create a sheetmetal part and a cone with an included angle of more than 60 degrees. Then use the Unfold command and refold (using the same stationary reference) and the refolded part is truncated and offset from the original cone model and UCS. But there is a simple enough solution (workaround) that's not immediately obvious.



To further explain the problem see this Screencast video. It it you can see that I have used the plate end with the fillet as the Stationary Reference, the fillet is just an indicator that the same end is being used for the Unfold and the Refold. It's presence has no bearing on the problem being shown, nor does the Cut feature.



The 3 methods I've used to try and solve this problem are :-
  1. Lofted flange, rip, unfold
  2. Revolved surface, thicken, rip, unfold
  3. Revolved surface part derived into a Sheetmetal part, rip, unfold
All of these fail with the same 2 problems, a truncated model when the Refold is made and the resultant cone is offset from the original modeled features and the UCS.

My initial search in the forums found little of any use so I posted this looking for any wise insight. Not a lot turned up so I continued digging and eventually found this forum entry from 2009 of the same problem (which means that it has been around since at least then) with a potential solution at the bottom.

The solution is to use a Revolved Surface and Thicken approach, but the key is to NOT use a Full type Revolve. Change the selection to Angle and type in something like 359.9 degrees. Then thicken the Surface and the Unfold/Refold functions now work properly.



Below is another Screencast video of this successful technique.




Kudo's for this solution goes to forum poster sumit_waghmare.

The bug seems to be with a cone that has an included (developed) angle of more than 60 degrees. When flattened this creates a swept angle (measured between the flat cut ends of the plate) of more than 180 degrees. In the example shown in the 2 videos the included angle is around 76 degrees and the flat part swept angle is around 222 degrees. If I reduced the included angle to 60 degrees then the Unfold & Refold operations would work as expected.


While a solution (read workaround) has been found it's disappointing that users need to go to this extent to find a valid workflow. All of the preceeding 3 workflows are valid so this leaves it to Autodesk to lift their game and fix this bug. I have raised a support case but given that this seems to have been firstly identified in 2009 I'm not holding my breath waiting for a proper fix. My VAR has confirmed that this problem still exists in Inventor 2015. #BogusAutodesk

Update 23-01-2015 :- Autodesk Customer Support have raised an ADN Incident ID which can be seen here.