Parts in subassembly move when inserted in a master assembly

ben_druce254

Hi All

I'm new to Onshape (Solidworks user for 10yrs) and don't fully understand subassemblies within master assemblies. Here's a quick video where I explain what I'm trying to understand: https://www.loom.com/share/b65d18ff1a514fe09cf1cf3496481d1b

T crux of it is this: I need to understand how I can build subassemblies that are accurately represented in a master assembly where: 

1 - Parts in the subassembly don't move independently in the master assembly

2 - Updates to the subassembly are represented in the master. 

Thanks. 

eric_pesty

Sub-assemblies are always "flexible" in Onshape so any degrees of freedom are always propagated "upwards" in the assembly structure.
I guess there are a couple of ways you could handle this.

One option would be to create "named positions" (you could crate one in your sub assembly and "re-use" it in your higher level). The downside is that it's more of a "bookmark" to get you to the position but things are still free to move.

You could also add mates in the higher level to further constrain things (but it won't update based on what you do in the lower level).

Or you add a configuration in your sub-assembly where there is no more "degrees of freedom". You could use a "group" feature but it doesn't update automatically so maybe not what you want, or just adding/configurating your mate limits... 

Maybe a more important question in this case is why you are "under constraining" the items in the sub-assembly as they are not meant to be physically moving once packaged in an upper level... Maybe if you can explain what you are trying to achieve we might be able to provide better guidance but it looks to me like you would be better served by having things fastened in the sub-assembly and using exploded views (if you need to show them spaced apart).

Technoid

Subassemblies are always "flexible" in master assemblies? Oh no, that's... not good. In my case, I have a sub-assembly which is its own complex mechanism. Imagine a robotic arm (master assembly) with a complicated gripper on the end (subassembly). I want the subassembly to have unconstrained degrees of freedom when working on it by itself, but I don't want it to flop around as I manipulate its position in the master assembly. This is something I've used frequently in Solidworks, and I am very shocked to learn that no such option exists in Onshape. 

NeilCooke

henry_fiorentini said:

Subassemblies are always "flexible" in master assemblies? Oh no, that's... not good. In my case, I have a sub-assembly which is its own complex mechanism. Imagine a robotic arm (master assembly) with a complicated gripper on the end (subassembly). I want the subassembly to have unconstrained degrees of freedom when working on it by itself, but I don't want it to flop around as I manipulate its position in the master assembly. This is something I've used frequently in Solidworks, and I am very shocked to learn that no such option exists in Onshape. 

A quick fix (pardon the pun) is to add the subassembly to a Group at the top level. It will be marked as an error, but it will fix any subassembly mates temporarily.

nick_papageorge073

henry_fiorentini said:

Subassemblies are always "flexible" in master assemblies? Oh no, that's... not good. In my case, I have a sub-assembly which is its own complex mechanism. Imagine a robotic arm (master assembly) with a complicated gripper on the end (subassembly). I want the subassembly to have unconstrained degrees of freedom when working on it by itself, but I don't want it to flop around as I manipulate its position in the master assembly. This is something I've used frequently in Solidworks, and I am very shocked to learn that no such option exists in Onshape. 

Consider the converse. In real life, the gripper would flop around. OS makes this super easy to do. In Creo, (IDK about SW), that would be a pain to do. Eric's and Neil's posts above have a few solutions if you don't want it to flop around.