Joints vs. Rigid Group

[Anonymous]

Ok, I think I'm starting to get the differences between the joint types.

The basic Joint allows putting two components together and allow different types of motion for one of the components.

The "As Built Joint" is used when two components are already aligned correctly, but need to be "connected". It still allows different types of motion for one of the components (less options though?).

A "Rigid Group" connects multiple components. No motion allowed, but all of them will be moved together.

Am I getting this right?

What I wonder now is if there are any performance differences between these? If I just need to connect a few aligned components, is it better to create a group or use multiple joints?

[scottmoyse]

Correct.... and there's not a lot of point in defining a joint unless you want movement or to use that joint in the simulation environment.

[TrippyLighting]

That depends.

With a rigid group, you basically lock the origins of components together. What happens when you edit the size/features of a component spends on how these origins are oriented.

With a "normal" joint you have to reference features, a face, edge etc to create a joint. That requires more clicks but cam be more stable in a parametric design.

[m.scottpeterson1]

There seems to be no difference in using rigid groups and rigid joints. They both lock the origins together even if the components are not in contact.

[m.scottpeterson1]

Rigid group is faster when selecting multiple components.

[nstevelmans]

Hi Made a little video off working with rigid groups and as built joints.

This is workflow when opening a stepfile that I downloaded from grabcad. And you want to make some movement in it.

I needed only 2 rigid groups joints and one as build joint.

Less joints is more performance

[Anonymous]

TrippyLighting has an interesting point I didn't think about. A "Joint" component will move if the component it is attached is changed in length for example.With an "As-Built Joint" or a "Rigid Group" it doesn't do that.

And the difference between "As-Built Joint" and "Rigid Group" is that the first allows to define "movement" between two objects, while the "Rigid Group" allows multiple objects in a fixed state. Unless I need that movement, both should behave similarly.

[Scoox]

This thread, along with a a fair share of experimentation, has helped me to finally understand the differences, advantages and disadvantages of the various assembly methods available. In this post I summarise my understanding, hoping that it will help other users.

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Fusion 360 provides FOUR methods of creating joints: regular joints, as-built joints, rigid groups, and origin joints. 

Regular joint:

• Always involves two components.
• Regular joints are used to join two components that are not yet aligned. In other words, regular joints do not require pre-alignment.
• 9 our of 10 times used to establish non-rigid joints.
• Usually references component features such as faces, edges, points, etc. The component origin is a special case of a feature.
• After applying the joint, the selected features of the two components become coplanar, colinear, concentric or coincident. In other words, regular joints usually result in the two components coming into contact.
• Users may then explicitly specify offsets to create separation between two components (to break the 'contact')
• Motion between components is possible, depending on joint type (rigid, revolute, slider, etc)

As-built joints:

• A special case of joint.
• Like regular joints, as-built joints also involve two components.
• Typically used when the two components have been previously aligned, or are loosely aligned (e.g. by hand) and the goal is to preserve their relative positions.
• Unlike regular joints, as-built joints reference component origins only, not faces, edges etc. Because of this, as-built joints can 'break' after re-dimensioning a parametric design.
• 9 out of 10 times used to establish rigid joints, although other joint types are possible.
• With regular joints distance is created by manually setting offsets; with as-built joints, the offsets are implicitly calculated to preserve the initial position of the current position of the components.

Rigid Groups:

• A special case of as-built joint. A rigid group is essentially a rigid joint between more two or more components.
• No kinematic relationships possible: rigid groups are always... rigid. Therefore rigid groups can be thought of as a subset of as-built joints.
• While rigid groups can be emulated identically using as-built joints, the advantage of rigid groups is that they are quicker to establish. As-built joints must be established two components at a time, if you want to join, say, ten components, a rigid group can deal with all ten components in one go, which considerably reduces the amount of work and timeline clutter. It's essentially a workflow shortcut.
• You can think of rigid groups as hair spray to fix freeze components relative positions.

Ground:

• Ground is a special case of rigid joint. It can be emulated using any of the three methods above. In this case the joint is between the selected component and the top-assembly origin. The exact same result can be achieved by explicitly creating a rigid joint between the component and the top-assembly origin or the top-assembly component.
• The main advantage of using 'Ground' is that it requires just one click. Again, it's a workflow shortcut.

There are many situations where any one of these methods can be used, but there's always one method that is quicker/easier, and that's the one you should be using. 

[Anonymous]

Scoox, that is brilliant. Thank you. I'm still trying to get my head around all this. So, my question is, how does combine (& align) fit into all this? Is combine a type of joint? Thanks. Pete

[Anonymous]

Your definition of the 'Ground' command seems to be a little off. My understanding is that 'Ground' fixes the position of an object in space. It is not therefor not relative to anything other than the origin point in your workspace.

It should be used to fix your base component. This allows you to have a datum object to reference your other components to (or build up from).

[TrippyLighting]

@Anonymous spot on! 

[Scoox]

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@Anonymous wrote:

Your definition of the 'Ground' command seems to be a little off. My understanding is that 'Ground' fixes the position of an object in space. It is not therefor not relative to anything other than the origin point in your workspace.

It should be used to fix your base component. This allows you to have a datum object to reference your other components to (or build up from).

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It's been a while since I wrote that. As far as I can tell, you can fix the position of an object relative to the workspace origin (in other words, the top-level assembly origin) using any one of Joint, As-built Joint and Rigid Group, as long as you select either the top In my opinion Ground is, or rather, used to be a very specific case of joint. The only advantage of using Ground was that the top-level  assembly was implicitly included in the selection so you basically saved one click, but, as you will have noticed, the current version of Fusion 360 no longer provides the Ground command, which agrees with my earlier post in that it was, for the most part, redundant (and confusing).

[davebYYPCU]

Really?

the current version of Fusion 360 no longer provides the Ground command,...

Ground is still a function.  Relates to a Component, is not found in the Assembly Menu, and is not a Joint, as by your definition it does not involve two components.  You need more than one Component in the file for it to appear.  A one Component file, does not need the Ground command, because that one is already grounded.  

Might help....

[Scoox]

You are right, thanks for the correction 😅 I didn't realise Ground was in the component context menu (other than the top-level component). From the fact that I had forgotten where it was you can tell I don't use Ground at all, and that's because joints and rigid groups get me the same effect. I personally find Ground redundant, unless I'm missing something in which case there is huge functional overlap between it and the other joint functions.

Technically the Ground function could have been put in the assembly menu and also the S-key menu and it would have made perfect sense.

[TrippyLighting]

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@Scoox wrote:

...I never use Ground, and that's because joints and rigid groups can be used to the same effect as grounding. I personally find Ground redundant, unless I'm missing something....

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"Groud" removes components for the joint solver computations, which has shown to have significant performance impact in assemblies with a lot of moving parts.

[Scoox]

@TrippyLighting I didn't know that, where can I find more info? I suspect that kind of detail might be too technical for most users—myself included—, but I'm interested.

[scottmoyse]

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@TrippyLighting wrote:

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"Groud" removes components for the joint solver computations, which has shown to have significant performance impact in assemblies with a lot of moving parts.

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That's an interesting distinction.

The other thing to consider about Ground, is that it only applies to the instance of the Component you set it on... if you Copy the Component (with or without child components contained within it), the Ground status gets cleared. Joints (both types) and Rigid Groups survive instancing because they are bound to the Component itself and not the instance. For those who may not know, if you have 1 component that's used X number of times to create X Qty of a component in your BOM, those are the instances.

[GRSnyder]

Another point to consider regarding Ground is that it's turned off when a design is imported or insert-derived into another design. So, generally best avoided in models that will be used elsewhere. You can achieve import-stable joints that mimic Ground using any of the approaches outlined above.