What mates should I use for a Reuleaux triangle rotation?

john_hackington

Hi,

What mates should I use to get the Reuleaux triangle to rotate within the square in this assembly?
https://cad.onshape.com/documents/c7076af1b7a1f5c3cad9aa33/w/328860a966179beb4ec367d4/e/723a794413f54436d42a9000

An illustration of how the rotation should look like:
https://commons.wikimedia.org/wiki/File:Reuleaux_triangle_Animation.gif

Thank you!

philip_thomas

This is very similar to a Wankel engine - very easy to do.
You will need;
an eccentric cam
2 revolute mates
1 mate relation

john_hackington

Hi Philip,

Thanks for the cue! I'm able to successfully get the rotation with what you've suggested 
https://cad.onshape.com/documents/c7076af1b7a1f5c3cad9aa33/w/328860a966179beb4ec367d4/e/723a794413f54436d42a9000

Wondering though, do you think it's possible to get the same rotation without adding a physical external cam?

@philip_thomas

MBartlett21

@john_hackington
@philip_thomas

I have made an example showing it using two tangent mates:

https://cad.onshape.com/documents/6837f48dc8d88fe44c98ed6c/w/edf8e3d61121b32e22b73ef6/e/94fe1bf6544bf4b399de5df3

Just make sure you rotate the triangle slowly otherwise it goes wonky.

NeilCooke

@john_hackington if you don't want to use physical cams, use a sketch! https://cad.onshape.com/documents/825620768ae3a5beefc53dc9/w/e8a5d084cdc07c1cee0e63fc/e/f60a9dbb830285433efcaa50

philip_thomas

This is very similar to a Wankel engine - very easy to do.
You will need;
an eccentric cam
2 revolute mates
1 mate relation

john_hackington

Hi Philip,

Thanks for the cue! I'm able to successfully get the rotation with what you've suggested 
https://cad.onshape.com/documents/c7076af1b7a1f5c3cad9aa33/w/328860a966179beb4ec367d4/e/723a794413f54436d42a9000

Wondering though, do you think it's possible to get the same rotation without adding a physical external cam?

@philip_thomas

MBartlett21

@john_hackington
@philip_thomas

I have made an example showing it using two tangent mates:

https://cad.onshape.com/documents/6837f48dc8d88fe44c98ed6c/w/edf8e3d61121b32e22b73ef6/e/94fe1bf6544bf4b399de5df3

Just make sure you rotate the triangle slowly otherwise it goes wonky.

philip_thomas

@john_hackington - nicely done!

@mbartlett21 - tangent mates are awesome, but they are also very finicky (in any cad system) - wherever there is more than one solution, or a singularity (eg tool in a corner touching two faces its supposed to be tangent to), then they tend to become unpredictable or 'stuck'.
Johns solution is far more robust and mimics the driving mechanics of the mechanism.
Be like John

john_hackington

@philip_thomas - Thanks for the encouragement! I believe @mbartlett21 was trying to answer my contemplation of whether it could be done without an external cam with his example. It's great that we get to use this opportunity to learn from you the pitfall behind tangent mates and get a better understanding of the cad system

On further thought of eliminating the physical external cam, I realize that the cam being "physical" is all a matter of my own perception. I could easily just hide the external cam and it would just be an invisible implicit rule of how my actual physical part should move.

@mbartlett21 - Thanks for looking into the problem and trying to devise an alternative solution! I did consider using tangent mates too, but besides the wonkiness of the resultant mechanism, the Reuleaux triangle would also no longer be an object of constant width if we fillet the edges.

MBartlett21

john_hackington said:

On further thought of eliminating the physical external cam, I realize that the cam being "physical" is all a matter of my own perception. I could easily just hide the external cam and it would just be an invisible implicit rule of how my actual physical part should move.

@john_hackington

BTW, even if you hide the part in the assembly, it will still show up in any drawing that you do of the assembly.

john_hackington

mbartlett21 said:

BTW, even if you hide the part in the assembly, it will still show up in any drawing that you do of the assembly.

@mbartlett21

You're right indeed! I did a quick search and it seems to be an improvement item that they're working on. Hopefully there'll be a function in the near future for it.

https://forum.onshape.com/discussion/6956/part-visibility-in-assembly-drawings-for-illustrating-assembly-steps

philip_thomas

Gents - not only is there an enhancement request, there is a technical paper written by our top guy that describes how to do exactly what you're looking for!

https://learn.onshape.com/learn/article/branching-to-control-part-visibility

MBartlett21

philip_thomas said:

Gents - not only is there an enhancement request, there is a technical paper written by our top guy that describes how to do exactly what you're looking for!

https://learn.onshape.com/learn/article/branching-to-control-part-visibility

@philip_thomas

But suppressing the parts will make the mates not work.

mahir

It would be just for assembly drawing purposes. Your main assembly would still have the cams unsupressed.

john_hackington

Thanks everyone for the inputs! It's a treasure trove of information out there and it's great to learn something new everyday. Kudos to the Onshape team too for the extensive learning materials and the continual improvement on the product

NeilCooke

@john_hackington if you don't want to use physical cams, use a sketch! https://cad.onshape.com/documents/825620768ae3a5beefc53dc9/w/e8a5d084cdc07c1cee0e63fc/e/f60a9dbb830285433efcaa50

philip_thomas

@mbartlett21 - You asked how to control the visibility of parts in a drawing, that's exactly what the technical paper explained how to do
What are you trying to do?

john_hackington

@NeilCooke

Wow! Thanks Neil, this is so elegant, it's very much just what I was looking for! I'm utterly delighted in all the capabilities that I continually uncover from Onshape

MBartlett21

philip_thomas said:

@mbartlett21 - You asked how to control the visibility of parts in a drawing, that's exactly what the technical paper explained how to do
What are you trying to do?

I wanted it to be able to rotate properly and not have the parts in a drawing.
Don't worry, @NeilCooke answered it in his post above (https://forum.onshape.com/discussion/comment/37486/#Comment_37486)

MBartlett21

Just noticed,
The reuleaux triangle intersects the outside at some points.