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russ
Posts: 11
Registered: ‎06-15-2011
Message 1 of 6 (985 Views)
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Interference analysis claims self-intersecting surfaces on identical parts

985 Views, 5 Replies
06-15-2011 12:50 PM

Hi all,

 

I'm rather new to Inventor, still learning.  I've designed a part from scratch that is rotationally symmetric, that is, it mates to itself when rotated 180 degrees, thus creating a shelled solid from a single part.  When I put two instances into an assembly and constrain them face to face plus constraining vertices at the top and bottom and run interference analysis, it tells me "The attempted operation would create self-intersecting surfaces...."  Can someone explain why this happens and how I might solve it?  I've done this with a nearly identical part and the worst I got was some tiny interference area when I didn't do the assembly right.

 

I do struggle with figuring out how to specify the constraints on this time of same-part assembly.  Am I doing it right?  I've tried different constraints, but the results are always the same.

 

Thanks for any help at all.  Part and assembly attached.

 

-Russ

-Russ
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rdyson
Posts: 880
Registered: ‎04-11-2005
Message 2 of 6 (977 Views)

Re: Interference analysis claims self-intersecting surfaces on identical parts

06-15-2011 01:47 PM in reply to: russ

Not quite sure why the self intersection error, but if you reconstrain using origin planes (to avoid constraint conflicts) and move the parts away from each other by a very small amount, you can get interference results. The snaps at the bottom show a bit of interference. Don't know if that's what's causing the self intersection error or not.

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russ
Posts: 11
Registered: ‎06-15-2011
Message 3 of 6 (967 Views)

Re: Interference analysis claims self-intersecting surfaces on identical parts

06-15-2011 04:39 PM in reply to: rdyson

rdyson, thank you for your quick response.  There will be interference with the snaps if the part faces are not fully mated.   But how were you able to tell there was interference if Inventor will not actually do an interference analysis due to to the self-intersecting surfaces issue?

 

My biggest concern is that I need to know whether it's safe to ignore this self-intersecting surfaces issues because it's just an Inventor anomaly, or whether there's a real issue before I spend several hundred dollars to have a prototype milled.

 

-Russ

-Russ
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JDMather
Posts: 26,289
Registered: ‎04-20-2006
Message 4 of 6 (963 Views)

Re: Interference analysis claims self-intersecting surfaces on identical parts

06-15-2011 07:02 PM in reply to: russ

russ wrote:

  There will be interference with the snaps if the part faces are not fully mated.    

-Russ


Not sure exactly what this means.
I did fully constrain using origin planes as suggested.
Only reason I can think of why you get the error is because it is two instances of same part (which you incorrectly changed name of) mated to each other.

Put in an offset distance of the xy plane of .00001 (far far far less than the possible manufacturing tolerance) and did get the interference calculation (negligable) (which again, does not take into account manufacturing tolerance - assumes perfect parts).

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Contributor
russ
Posts: 11
Registered: ‎06-15-2011
Message 5 of 6 (944 Views)

Re: Interference analysis claims self-intersecting surfaces on identical parts

06-16-2011 09:23 AM in reply to: JDMather

Now I get it!  I didn't quite understand the explanation.  Let me summarize what I understand as the answers to my two questions:

 

  1. It isn't clear why Inventor claimed there was a self-intersecting surface, but there's a workaround.
  2. The best way to constrain snap-together parts is via origin planes.

I didn't know how to constrain via origin planes, so for others who don't, here's what I did:

 

  1. Open the Origin folder for the non-grounded part and click on the XY Plane.
  2. An Assemble button appears on the plane.  Click on it.
  3. Open the Origin folder for the grounded part and click on the XY Plane.
  4. If needed (as it was for me), enter a small negative offset number (like -0.00001).

Ultimately, for reasons I don't understand, as the numbers should have worked out, I had to reduce the snap trap extrusion's depth from 0.06 to 0.057 and increase its offset from the front of the part from 0.12 to 0.122.  I didn't need to change the width at all.  I don't know if, in the world of plastics milling and injection molding, these tiny changes are relevant, as I'm very new to this, but if Inventor's happy, I'm happy.

 

Many thanks to rdyson and JDMather for taking the time to help me.

 

-Russ

-Russ
Distinguished Mentor
rdyson
Posts: 880
Registered: ‎04-11-2005
Message 6 of 6 (940 Views)

Re: Interference analysis claims self-intersecting surfaces on identical parts

06-16-2011 10:28 AM in reply to: russ

Your welcome.

And since you are new to plastic design, there is a philosophy called "steel safe design". It states that it always easier to add plastic than take it away since adding means removing steel from the tool and removing plastic means adding steel to the tool. So tolerance your parts such that if they don't fit perfectly the first time, it's easy to make adjustments.

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