How can I mate contacts with 4 DOF in Onshape assemblies?

John_vd_Werff

I need to mate a hook to a rail in an assembly. The hook touches the rail with three line contacts and has no flat surfaces that are parallel to the rail. 
How can I mate the hook to the rail simulating the three line contacts?
  

Link to the assembly (in this simplified document the hook is constraint correctly to the rail in a sketch in the part studio, in real life the parts in the assembly are linked to different documents)

wayne_sauder

@John_vd_Werff
Would this work?
https://cad.onshape.com/documents/0cdba4db01184f17ae54bfa9/w/5cd90777edf69fa60da0d16f/e/4106905ce4371080ab48b4c5

John_vd_Werff

@wayne_souder
Good use of a mate connector in part studio!
That does work in my public demo document, in reality the parts are not in the same part studio but are imported from different documents.
I need a way to mate the two parts in such a way that the correct position of the hook is a result of the mates in the assembly.

john_mcclary

Fist we need to fix one part (i went with the blue one)

then I set a planar mate to the center of each part, so it's mated on center of the blue part

then add a tangent mate to each of the touching surfaces.
Use a vertex/surface when possible (my first mate in the gif)
then use line/surfaces as a secondary option
then use surface/surface as a last resort

this animation still took 3 recordings to show it working the first time... Tangent mates suck. They do in all 3D cad. But understanding they 'may break' is part of assemblies in any software. So, use the mate connector in the part studio first, but if you absolutely need to, then use the method I show as a second best.

even in the animation, you can see the second tangent mate did not work properly until I dragged the part closer to the edge. . .  I hate tangent...

John_vd_Werff

@john_macclary
Thanks for your clear instructions. I tried the tangent mates before posting on the forum, but didn't succeed. Moving parts close to the correct position does help a lot. I think your 3rd mate is not mechanical right. In reality the hook will touch the face of the bend, not the tangent edge of the bend. 
There is some interference now:


In this use case the error is neglegible and I can leave it at this.
We have this assembly also in Solidworks, and there I had the same issue, almost correct, but always a slight interference at that 3rd mate.

If however the bend on the rail were more then 90° the error would be too large to neglect.





I think that the tangent mate in Onshape might work a lot easier and be more robust if one could select specific faces to mate and not the entire circumference of the part.

john_mcclary

Thanks for pointing that out, i didn't notice the interference. 

So, i suppose we could trick with offset surfaces
Public file: hook&rail - Copy | dummy assy rail&hook (onshape.com)

go to your part studio, add a 0mm offset to the surfaces shown


insert and mate those surfaces to the parts
then use the surfaces as tangent selections

John_vd_Werff

Nice trick, thanks!
Works perfect:



I think this demonstrates that individual selectable surfaces would make the tangent mate much more usable.

john_mcclary

As an option at least, because the tanget surface selection is needed for cam follower mates

Maybe there should be a uihint to toggle tanget connected surface or single surface next to each tanget mate selection 

John_vd_Werff

I agree: add a toggle in the tangent mate UI to select single surfaces instead of connected surfaces

John_vd_Werff

@wayne_souder
Good use of a mate connector in part studio!
That does work in my public demo document, in reality the parts are not in the same part studio but are imported from different documents.
I need a way to mate the two parts in such a way that the correct position of the hook is a result of the mates in the assembly.

wayne_sauder

@John_vd_Werff
 Do you have enough symmetry or similarity between parts to do the math to place some mate connectors or can you use in-context? Maybe there are too many variables for that to work I'm not sure what you are dealing with, I have found that if you don't need motion then it is far faster and easier if you can just use a fasten mate. 

John_vd_Werff

@wayne_sauder
Calculating the correct position and orientation of the mate connector is possible, but requires quite some work.
I think that your first suggestion of placing the mate connector in the part studio is faster, I can sketch and extrude a simplified model with only the relevant geometry of the rail in the part studio of the hook. Tangent constraints in the sketch are much easier than in the assembly.

   

Using one single mate connector is faster and more robust then using 3 tangent mates on added surfaces. Both approaches are viable and have their own merrits.

wayne_sauder

 I agree both approaches have their own merits. I hope Onshape can improve the robustness of the tangent mate at some point then I could really get into using it. I tend to stay away from it as much as possible because of how fiddly it can be.