# Constraints in Assemblies

eric5935

Newbie

posts: 11

Registered: 2013-11-7

Message 1 of 6

Constraints in Assemblies
17-11-2013 05:02 . pm | View his/her posts only
I have an assembly with two parts in it: a plate with a hole in it and a bolt

If I try to add two constraints between the bolt and the plate (one coincident to the underside of the bolt and the top face of the plate and one concentric with the inside of the hold and the bolt), then I get an overconstrained system since the plate is a part, so I cannot select individual surfaces. I tried geometric constraints for this, but eventually ran into similar problems once I constrained the geometric references to the original part to allow for dragging the complete assembly.

Is there a better way to constraint a bolt to a plate in an assembly?

I'm new to 3D cad, so any suggestions on additional books/reading that I should do would be greatly appreciated.

Thanks,
Eric

davej

Assistant Engineer

posts: 73

Registered: 2013-1-16

Message 2 of 6

18-11-2013 02:05 . am | View his/her posts only
What your describing here is the very fundamental removal of some of your 6 degrees of freedom and it should and does work exactly as you tried.

If you look closely at your parts you somehow have one of them out of square. This would cause the error your getting.

By the way this technique you attempted is the only possible way to constrain a bolt to a plate. Bolt axis to hole axis or bolt od to hole id and a coincident to position the head is all you can do. Of course the bolt will still be free to rotate about it's axis but for now we won't worry about that.

I would suggest that you start over with new parts.

chris

Assistant Engineer

posts: 47

Registered: 2011-5-24

Message 3 of 6

18-11-2013 04:59 . am | View his/her posts only
if you have some special geometry on the plate or bolt such as if the faces on the bolt are not exactly 90 degrees to the shaft axis then constraining the bolt is impossible without using a few little tricks. I will put little surfaces on objects that for whatever reason do not act as I want. An Example is a simple O-ring, getting the o-ring to locate correctly into the groove just put a flat only a couple thousands wide on the o-ring. You can now locate it on the flat and because the flat is so small it does not change the appearance of the assembly. Another method is to place a datum at a desired angle or location and use that as the locator surface.

eric5935

Newbie

posts: 11

Registered: 2013-11-7

Message 4 of 6

18-11-2013 07:51 . am | View his/her posts only
Thanks for the tips for the o-ring alignment example -- that will help me out in the future. Right now, my case should be a lot simpler.

In reproducing a simple case for the 2nd time to share this, "Face/Datum" is showing in the context selection list when I click on Assembly -> Align and things are working -- before, this wasn't showing up -- I'll keep an eye on it to see why -- the only thing I did differently this time is I went through assemblies example (http://www.zwsoft.com/Service_Support/ser_zw3d/Show/en/200.html) before doing the latest test.

Now, for my more complex case, the same technique fails, so it must be a 90 degree issue as you mentioned, even though the parts *should* be at 90 degrees. I'll keep digging into it and report back with steps if I can reproduce the issue again.

Thanks,
Eric

chris

Assistant Engineer

posts: 47

Registered: 2011-5-24

Message 5 of 6

18-11-2013 07:38 . pm | View his/her posts only
if you post the two components maybe we could help, if necessary you can cut the plate to hide its true function if necessary.

eric5935

Newbie

posts: 11

Registered: 2013-11-7

Message 6 of 6

23-11-2013 02:51 . pm | View his/her posts only
For the simple case, everything started working after I did the assemblies example. What I'm assuming is that I had the wrong item in the pick-pull down and couldn't select the correct faces.

In my more complex design, I removed all of the constraints and started over and everything is fine now -- I think I had picked some wrong edges (instead of faces) since I didn't have the pick-pull down set to "All".

So, looks like beginner misfortune. Thank you very much for the posts to confirm that it *should* work which gave me enough energy to push forward some more.

Thanks,
Eric