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Contributor
Posts: 20
Registered: ‎11-12-2013
Accepted Solution

Problem with SHELL

163 Views, 6 Replies
11-26-2013 10:05 AM

Why can't I doo a SHELL form the body attached? The arc on the model doesn't get the 4 mm thickness.

Best regards.

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Distinguished Contributor
rishivadher
Posts: 127
Registered: ‎05-01-2009

Re: Problem with SHELL

11-26-2013 12:07 PM in reply to: gillesht

Hi

On your case you can just go to Sketch and make a Offset of single line

rishivadher
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Product Support
AndrewSears
Posts: 250
Registered: ‎08-21-2010

Re: Problem with SHELL

11-26-2013 02:08 PM in reply to: gillesht

 

A little more might be going on here.  We will need to investigate this and will get back to you.

 

Thanks,

Andy 

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Contributor
Posts: 20
Registered: ‎11-12-2013

Re: Problem with SHELL

11-26-2013 07:35 PM in reply to: gillesht

Hi,

I've replace the arc that was causing the problem by a spline and was able to create the revol. I'm now working to detail my model.

Best regards.

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Distinguished Contributor
rishivadher
Posts: 127
Registered: ‎05-01-2009

Re: Problem with SHELL

11-26-2013 08:15 PM in reply to: gillesht

ok, its solved :smileyhappy:

rishivadher
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Employee
Posts: 228
Registered: ‎12-14-2006

Re: Problem with SHELL

11-26-2013 09:03 PM in reply to: gillesht

Hi gillesht,

 

Glad to hear you got it solved! I did some analysis of this before you got this resolved, so here it is in case this is helpful:

 

The problem originated from the spline curve on the sketch. Near one end of this line, there seems to be some inconsistency with the points & tangency handles, causing the curve to have a slight overlap:

 

overlap.png

 

This was propagating into the revolved body, which was what caused problems for shell.

 

I was able to resolve this by redrawing the spline, simply clicking the existing spline points from the previous curve (attached f3d file). This avoided the overlap issue, and the body shelled successfully.

 

However, with this new spline, the shell only went as far as ~2.1mm. Beyond this thickness, the offset encounters another issue, which is a region of the offset becoming so sharp that two curves become 'separated' (the below diagram shows the offset just before it becomes separated): 

 

separation.png

 

This occurs because this region becomes sharper and sharper (i.e. the angle between the arc and straight line becomes smaller and smaller) as you offset by larger distances. You can improve the shell distance here by reducing the sharpness of this angle on the input body, or (even better) removing the sharp edge completely: i.e. if you add a small fillet here, you get a smooth offset, which can be shelled much further: 

 

filletbeforeshell.png

 

(If you don’t want to have a fillet here on the outside of the model model, one option is to do the above, then delete the fillet after shelling the model.)

 

Hopefully this helps in shelling to the required distance.

 

Something else (not directly related to the shelling failure): is it intentional to have so many points along the long sketch spline? The potential problem with this is that this introduces waviness into the surface (this would become apparent when manufacturing):

 

waves.png

 

Perhaps this was intentional? If not, in general, it’s best to use as few points as possible to define a spline curve - this allows the spline to flow more naturally, giving smoother results. Below shows a redrawn spline using only 4 points, giving a much smoother resultant revolved surface:

 

sketchcompare.png

 

bodycompare.png

 

Hope this is helpful; but if not, feel free to ignore this :smileyhappy:

 

Thanks!

Jake



Jake Fowler
Sr. Software QA Engineer
Autodesk, Inc.
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Distinguished Contributor
kingson138
Posts: 173
Registered: ‎06-26-2013

Re: Problem with SHELL

11-27-2013 03:35 PM in reply to: jakefowler
Hi Jake, This is a very detail analysis. Your last advice to use as fewer points as possible is very useful. I also noticed this when making "3D Rapid Prototyping". Unless a certain texture is needed it is better to remove the extra points and edges that are not needed. For metal works these are things we have to watch out! In fact one of the most important things that we have to check before final manufacturing. Thanks! A big Kudos for that!

Regards,
Kingson
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