"Cavity" feature in assembly and global variables?

jan_rychter

I'm experimenting with OnShape for designs that use laser-cut plywood and there are two things I'm missing from SolidWorks.

1. A "cavity" feature. Plywood is often joined by mortise/tenon joints, so one part will have extrusions and holes need to be made in the other part to accomodate these extrusions. I'd much rather specify these only once. In SolidWorks, people (mis)use the cavity feature for this: in the assembly, I can specify that holes need to appear to conform to the extrusions that protrude into the part. From what I can see, OnShape has no assembly-level features, so I'm not sure if this is something that has been considered. I tried achieving the same effect using "derived parts" and boolean operations, and while one can get there, it's cumbersome: I have to position my parts (which I'll do again in the assembly anyway), and every part with holes needs to be derived from all parts with extrusions that will mate with it.
2. Global variables (SolidWorks calls them "assembly-level" and their usability is terrible): is there a way to specify variables like plywood depth on a project (document) level, rather than in each part separately?

andrew_troup

@jan_rychter


You are possibly misinterpreting the purpose of Onshape assemblies. 

They are essentially optimised for providing motion control; hence they are not meant to fulfill the same purpose as Solidworks assemblies.

I suggest you check out the videos on Multipart modelling, which is the intended way of building assemblies where there is no relative motion required. Here's a good one to start with:

https://www.onshape.com/videos/essentials-multipart

Part studios are optimised for static assembly building, and their in-depth implementation of boolean operations provide the in-context "cavity" capabilities I think you're seeking.

For many purposes, this turns out to be a much simpler way than we are used to in legacy MCAD, once the user gets familiar with exploiting all the possibilities they offer.

andrew_troup

@jan_rychter


You are possibly misinterpreting the purpose of Onshape assemblies. 

They are essentially optimised for providing motion control; hence they are not meant to fulfill the same purpose as Solidworks assemblies.

I suggest you check out the videos on Multipart modelling, which is the intended way of building assemblies where there is no relative motion required. Here's a good one to start with:

https://www.onshape.com/videos/essentials-multipart

Part studios are optimised for static assembly building, and their in-depth implementation of boolean operations provide the in-context "cavity" capabilities I think you're seeking.

For many purposes, this turns out to be a much simpler way than we are used to in legacy MCAD, once the user gets familiar with exploiting all the possibilities they offer.

jan_rychter

Just to clarify, this is the intended effect. I achieved it using derived parts and boolean operations, but that's quite a bit of work.

andrew_troup

@jan_rychter

Another user recently posted a model exploring mortise and tenon joints in a lasercut plywood box, which you will find if you search in the Public documents for "Laserkiste"

jan_rychter

Thanks! Indeed — coming from SolidWorks I thought assemblies were the way to, well, assemble parts together. I didn't go through tutorials, because I was surprised to find that the tool is so easy to use that I could simply start working and get the results I wanted 

I will go and watch the tutorials, especially as I can't see how I can insert additional parts into a Part Study (for example parts that I have imported). If Part Study is supposed to be the way to go for static assemblies, there has to be a way to also assemble things that were not created inside it.

andrew_troup

jan_rychter said:

......................
I will go and watch the tutorials, especially as I can't see how I can insert additional parts into a Part Study (for example parts that I have imported). If Part Study is supposed to be the way to go for static assemblies, there has to be a way to also assemble things that were not created inside it.

You've put your finger right on the area where this capability still needs the most development!
It is possible, but laborious, as parts need to be imported into their own tabs, then derived into the Part Studio, and the toolset for positioning derived parts is still somewhat klunky, in my opinion.

Onshape have tried very hard to avoid the pitfalls of the inter-document referencing which make legacy MCAD file structures and transfers (and collaborations) hard to manage, and this is an area where (I guess) some sort of compromise will be required to the purity of that vision.
I hope there will be input from smart users as to how we can have (as they say in Italy) "a full barrel AND a drunk wife"

brucebartlett

I'd suggest checking this discussion about a Laser Cut Box (laserkiste as Andrew mentioned above) , similar to your example.

Onshape Doc: https://cad.onshape.com/documents/343a8e986d3949eda17a2f1d/w/49e8acd910e749468db203a1

https://forum.onshape.com/discussion/2063/laserkiste-a-parametric-finger-joint-box-for-laser-cutting-stackable

andrew_troup

jan_rychter said:

......
I will go and watch the tutorials, especially as I can't see how I can insert additional parts into a Part Study (for example parts that I have imported). If Part Study is supposed to be the way to go for static assemblies, there has to be a way to also assemble things that were not created inside it.

Just came across this

https://www.onshape.com/videos/lets-import-a-part

which is a very good primer on importing non-native parts

jan_rychter

Thanks for your help. I did watch the tutorials, learned how to build multi-part "part studies". I think many people coming from SolidWorks will make a similar mistake with assemblies.

The other thing I was missing was that what I expected to be called "Import" or "Add" is called "Derive". Once I figured that out, the rest was straightforward.

As a side note, the current way of mating derived parts could be improved. It takes a lot of effort to properly position a part. I'm jealous, because assemblies get a better method — why can't we in the Part Study get the same method, too? I shouldn't have to explicitly create mating connectors if I can use the default ones?

Anyway, I moved my laser-related discussion to the "Laserkiste" thread.