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Edge to plane parallel mate removes 2 DOFs?

Question asked by Dara Kong on Jul 22, 2020
Latest reply on Aug 11, 2020 by Dara Kong


I'm trying to mate two plates having two holes each. I need the holes to match/align, and the plates to touch each other. But I have two constraints : I need the fewer possible mates, an most importantly I need my assembly to have zero redundant mate (for complex motion analysis). The final assembly should look like that :

The final assembly should look like this


So I used a coincident mate between these two circles so there's only one degree of freedom (DOF) left :

The circles being coincident mated


After that, I usually use a plane-to-plane perpendicular mate to eliminate the remaining DOF being the rotation around the Z axis. But because my plates have round corners, I can't use this technique. Instead, I used an edge-to-plane parallel mate which is theoretically supposed to eliminate only one rotational DOF :

The parallel mate



But it seems that SolidWorks thinks that an edge being parallel to a face removes two rotational DOFs instead of one! Here's a proof of that using motion analysis :

Proof that SolidWorks doesn't understand edge-to-plane mate


It's super annoying since I know for a fact that the edge can still rotate around itself, AND the edge can also rotate normal to the plane/face, so there should be 2 free rotations (around edge and normal to plane). And because a parallel constraint never removes any translation, my assembly should be fully defined without having any redundancy in my mates. But of course, here's what SolidWorks tells me when I try to add my coincident mate :

Redundant mates due to weird behavior of parallel mate


Finally, here are my questions about edge-to-plane parallel mates in SolidWorks :

  1. Is this a bug or I'm missing something?
  2. Should this kind of mate really remove 2 DOFs or just one like I think?
  3. Is there a workaround for this?
  4. Is there another way to achieve what I want?
  5. Is it okay to have redundant mates for motion analysis?


The files are available for you to download if you need them...


Thank you for your time! Your help would be really appreciated!