Why do Mates and Unrelated Assemblies Become Inconsistent After Adding New Mate?

PilotGuy

Hi there,

I'm seeing an odd problem when working in an assembly. I'm assembling an outdoor deck and currently working on the railings. I've added the railing posts and several sections of the infill (balisters and top and bottom railings) and everything is going fine. However, when I add one more section of infill, many previous mates become broken. Other assemblies within the parent assembly also get marked as broken. In addition, the infill assembly itself stops acting like an assembly. The piece of the assembly to which I am mating moves into the correct postion, however the rest of the assembly components don't follow it. Any advice?

Deck assembly that shows the problem is below, find 'Deck for sharing'. The very last mate feature, Fastened 44, is causing the problem. The 52 Inch Infill assembly is the one that falls apart after the mate feature.
https://cad.onshape.com/documents/33637adbd655fd61347d777f/w/c3bfbf403b268284abaa623e/e/d2bf2231b6809fa22388ae6f

Thanks for any advice or help from Onshape support.

alnis

For large assemblies, this sort of weird/inconsistent behavior can happen due to Onshape's "flexible-by-default" paradigm for assemblies. But never fear, there is an easy solution!

But first, some background/explanation: all subassemblies are flexible in Onshape by default. That means that the relationships for all subassemblies are calculated all of the time unless otherwise specified (specifically, when a subassembly is inserted into a larger assembly). For example, if you insert a gearbox into an assembly, all of the gears will spin immediately. On the other hand, in other CAD packages like Autodesk Inventor, subassemblies are not flexible by default. All components are "locked" relative to each other in a subassembly when that subassembly is inserted into another assembly. If you want a gearbox to spin, you must right click it and set it as flexible. This means that by default, Inventor and other programs may appear to be able to handle larger/more complex assemblies, but if you start making subassemblies flexible in other CAD packages (as Onshape does by default), the model will quickly disintegrate and slow down significantly.

The way you can fix this is by telling Onshape "hey, this subassembly doesn't move" by making a group with the subassembly in it. This will mark it as not flexible, and the model will work properly. Here's an example:
https://cad.onshape.com/documents/2c06d734f3a5472629549e45/w/14b2336bd80fbd1942ae1e53/e/530467ddf31fcb3f78ee99be

The one red/failing mate that's left has a missing mate connector, not related to the rest of the assembly turning red.

I hope this helped!