Fairly new here, need help with mates and overdefined assemblies

riccardo_ingrosso

Hi there. This is my first post so I hope I'm not breaking any rules and I apologise if I am asking a stupid question.
I am trying to design a mechanism with two identical arms on either side of an assembly. There are several mates of different types and everything works fine until I come to the final two mates. I need two revolute mates in exactly the same spot, one on the top of each arm. With one suppressed, all is good but with both active the assembly is over defined.
Can someone help me to understand why this is and how to fix it?

NeilCooke

Change one of the revolute mates to a cylindrical mate - it's a tolerance thing where the translation part of the two revolute mates are fighting each other. Not obvious I know - sorry.

riccardo_ingrosso

Hi Neil, thanks for the reply. I found this suggestion on another thread and tried it but I still couldn't get it to work. I think I may be over-complicating things anyway, so I might just go "back to the drawing board".

NeilCooke

Share your doc URL if you want some help.

tim_hess427

@riccardo_ingrosso - coming from other CAD programs, I found that I had to retrain myself a bit with mates in Onshape. I find it helpful to think in terms mating parts rather than mating geometry.

That means, if I'm mating a bolt to hole, I used to think about mating the bolt and hole to be concentric, then mating the head of the bolt to be planar with with the top of the hole. With Onshape, all off that can be specified with a single "fixed" mate and the appropriate mate connectors. So, now I'm thinking about how I want the bolt part to be fixed relative to the part with the hole. Then, the selection of my mate connectors define the geometric relationship. 

Apologies if you figured all of this out already, but I wanted to pass along something that I didn't realize right away. Perhaps it may help ensure things aren't over-constrained if you're going back to the drawing board with your assembly. 

riccardo_ingrosso

Thanks for the replies. I haven’t really used any other CAD programs so I’m completely fresh when it comes to all the concepts involved here. I’ve completed the courses in the learning centre and found them incredibly useful and easy to follow, so I’m ok with the basics. This is just a hobby for me, I don’t need it for work, although I am a fabrication engineer, so time isn’t an issue. Also, I’m a bit of a stubborn old $&@:! and I like to figure things out for myself if I can. Having said that, it’s good to know there are people on the forum willing to help when I get too frustrated.
Thanks again.  

riccardo_ingrosso

Hi Neil

To share the Doc URL, is it a simple copy and paste of the address bar? Like this?

https://cad.onshape.com/documents/30deccb5aaa93639cd708721/w/844cce8d07a1b51ac503c020/e/421d5ede1ddb0a1ab47ecd58<br>
This is the next version. Is there a quick and easy way to duplicate the scissor mechanism from where it is to the opposite side of the hatch?

Thanks in advance.

NeilCooke

Your "mistake" was to rely on the Tangent mate (requires a lot of calculation and therefore can sometimes be unpredictable) - I changed that to a Pin/Slot mate and changed one revolute to cylindrical.

https://cad.onshape.com/documents/cd7bf61e2d5385230c72ee7c/w/3bf68953f7ce394fbac5c6d1/e/9418aeaedad65a596156eca2

No mirror as yet, you'll have to replicate on the other side - you can however copy and paste the 2 linkages and they will at least retain the revolute between them.