Grounded components (yet again ;-)

[johan.rutgeerts]

I have an issue with a grounded component. Found some posts but I still don't understand:

This is the issue:

• I have a motor assy (MS17HD2P4100) containing a body and an axle component:
  
   
  • The motor body component is grounded,
  • The motor axle component has a revolute joint wrt the motor body component,
  • The top component contains a Joint Origin, to easily constrain it when assembling it in some other assy.
    
    
• In this motor assy component, all works as expected:
  • You cannot drag the motor body,
  • You can drag the axle and it will rotate wrt the body.
    
    
• However, when you assemble the motor assy component in some other component, the ground icon disappears, and the motor body component floats wrt the motor assy component (see file in attachment).  --> Why? 
  
  

  Using the Joint Origin of the motor assy component obviously does not work to position the motor body (as it now floats wrt the motor assy component).
  Referencing the motor body origin as joint reference does work as expected, as well as the revolute joint between axle and body.

• I know instead of grounding I could have used a fixed constraint, a rigid group or an as-built joint. So I'm not looking for a solution for this; I want to understand why grounding a component does not work as I expect.

I found several forum threads: Grounded Component Constantly Un-Grounding Itself!, Any opinion on Ground vs As-Build-Joint from the last 360 LIVE stream? and Why does the grounded component move?

I also searched the documentation, but I did not find a comprehensive description of the grounding functionality (within reasonable searching time).

It is still not clear, e.g.:
@TrippyLighting writes "Grounding locks the origin (not the geometry) of the grounded component to the top level origin in that design. If an assembly with one, or more grounded parts is inserted into another assembly, now the previously founded parts in that assembly are floating.

That is where many people get confused."

Still confused 🙂 

"Grounding locks the origin (not the geometry)"

--> I don't get that. I grounded the body component and cannot drag any geometry. So why would grounding 'not lock the geometry'?

"of the grounded component to the top level origin in that design."

--> I think key in this sentence is "in that design"? I see indeed that the 'grounded icon' disappears upon assembly, however what is the concept behind this? I am very interested in a clear use case for which this functionality has a practical use ( @jeff_strater ?).

EDIT: Also: where is this behavior (i.e. that grounding is disabled upon assembly) described in the documentation? I could not find it, though maybe I don't know where or how to properly look?

Thank you,

Johan

[jeff_strater]

"However, when you assemble the motor assy component in some other component, the ground icon disappears, and the motor body component floats wrt the motor assy component (see file in attachment).  --> Why? "

This was just a design decision made in Fusion development.  Ground does not come across when inserting components.  If it did come across, I suspect that there would be way more complaints than with the current behavior.  Ground is often applied in a sub-assembly so you can test its mechanism out in that sub-assembly.  But, if ground did persist across Insert, then that component would be grounded in the larger assembly, which is not desirable in most cases.  Take your motor, for example,  If that motor was then inserted in another design, and constrained to a moveable arm, you would not be able to move the arm, because the motor was grounded.

[TrippyLighting]

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@johan.rutgeerts wrote:

...

"Grounding locks the origin (not the geometry)"

--> I don't get that. I grounded the body component and cannot drag any geometry. So why would grounding 'not lock the geometry'?

I can see that my statement is confusion. When you create a new component in Fusion 360 it contains only an origin. You can already ground that component. 

[johan.rutgeerts]

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@jeff_strater wrote:

This was just a design decision made in Fusion development.  Ground does not come across when inserting components. 

Ok.

If it did come across, I suspect that there would be way more complaints than with the current behavior. 

There's no complaint, except that I don't find this behavior explained anywhere in the official documentation.

Can you point out where to find it, or is this lacking?

Ground is often applied in a sub-assembly so you can test its mechanism out in that sub-assembly. 

If the mechanism is not constrained wrt the subassy that contains it, then the subassy will float wrt the assy it is assembled in (even if joints are defined between the mechanism components and other subcomponents or geometry of the new top assy).

So I still lack to see a good use case, except maybe if the subassy contains both subcomponents (floating) as well as bodies (fixed). But that seems a rather exotic use case.

Of course it is personal preference whether you find it acceptable or not to have floating components in your design.

But, if ground did persist across Insert, then that component would be grounded in the larger assembly, which is not desirable in most cases.  Take your motor, for example,  If that motor was then inserted in another design, and constrained to a moveable arm, you would not be able to move the arm, because the motor was grounded.

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That depends if the grounding relation is defined static, relative to the active top level component at the time of definition, or dynamic relative to the most toplevel component of the current context . I had assumed the former, but given your example I conclude it's the latter. There's no possible way to know this as an end user.

Hence the need for clear documentation.

Regards, 

Johan

[TrippyLighting]

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@johan.rutgeerts wrote:

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That depends if the grounding relation is defined static, relative to the active top level component at the time of definition, or dynamic relative to the most toplevel component of the current context . I had assumed the former, but given your example I conclude it's the latter. There's no possible way to know this as an end user.

Hence the need for clear documentation.

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It is indeed the latter. I cannot find a description of how grounding works in the documentation either.