subassembly joints

[tinglett]

(Warning...I am very new to Fusion 360...)

I have been very successful with 360 so far, having previously been using sketchup for some time for woodworking designs.  Here I have started on the base for a fold-up workbench that will have a folding side with a little pivoting lock.  I've got the real drawing much further along, but this simple example shows a problem I ran into:

http://a360.co/2lkpJ79

There are two joints in this partial model.  The first is a little lock catch in the side assembly and that one works fine.  I animate it, and it does exactly what I want.  However, I have a second revolute joint between the side assembly and the back that sort-of works, but not really.  By "sort-of" I mean that when I animate the joint, the little flag dutifully revolves in the right direction, but the entire side assembly that is supposed to revolve with it will gray out and (apparently) not participate in the joint.

Perhaps this is something that is simple as "you can't join sub-assemblies like that."

Todd

[Anonymous · Accepted solution]

@tinglett

Try to add an as-built joint (rigid) between "Side" and "Side assy".

Manfred

[tinglett]

Manfred,

Sorry for the delay (day job getting in the way... 😞 ), but I never thought about the joint relationship between parent/child like this.

I also realized in my haste to make a demo, I didn't ground the back.  By doing that and the rigid as-built joint it sort-of works.   By "sort-of" I mean that I can now manually swing the side back and forth on the revolute joint, but if I animate it the whole assembly goes gray while the little joint flag (only) goes round and round.  But that's ok.

I updated the test model in case anyone reads this in the future, so to see the original problem you'd need to delete the Rigid2 joint under Side Assy.

Todd

BTW, one real oddity I've noticed is that if I drag the side by the little pivoting lock, the movement is totally bizarre.   There's something weird going on there.  I was expecting to be able to wiggle that lock back/forth as it has a revolute joint, too.  Dragging the side from points directly on the side itself behaves perfectly fine.

[Anonymous]

@tinglett

"...but if I animate it the whole assembly goes gray while the little joint flag (only) goes round and round..."

I try to explain this (hope I´m right - otherwise somebody in the community will correct my statements....).

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For some reasons, you created the "side joint" between "Back" and "Side Assy" - please keep that in mind.

Now try two things:

1) right click on "side joint" and choose "animate joint"

2) right click on "side joint" and choose "animate model"

"animate joint" should do what you described and "animate model" should also rotate "side" and "lock" components.

Why?

My interpretation of 1): Fusion just animates the two components that are connected by the joint. Not more. So it rotates "Side Assy".

Ok, you dont´t believe me because only the the joint flag goes round and round. Now try this: make the origin of "Side Assy" visible and try again "animate joint". What you should see now is that the origin of "Side Assy" is also going round and round. So "Side Assy" does rotate!

My interpretation of 2): Fusion calculates all the joints. So it finds out: ohh, there is an as-built joint (rigid2) between "Side Assy" and "Side"! So I have to rotate "Side" together with "Side Assy". And so on......

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I don´t know the reason why you created the joint between "Back" and "Side Assy". An alternative would have been to create the joint directly between "Back" and "Side" (the component, not the assembly). In this case, you would not need the rigid joint between "Side" and "Side Assy" (solution for the original question).

Regarding "drag component" and "bizarre movements". Because of this I avoid dragging if there are several joints.Or you have to temporarily lock joints, then it might be more predictable.

For a better understanding of the assembly/joints stuff I strongly recommend the excellent video of Kevin Schneider (Autodesk University Presentation):

http://au.autodesk.com/au-online/classes-on-demand/class-catalog/classes/year-2017/fusion-360/lo-dm0...

Regarding your example, listen to what Kevin says about:

- parametric and kinematic positions

- assemblies are kinematically flat

Manfred

[tinglett]

Manfred,

Awesome!   And I do want to thank you for trying to shed light on this.  Your explanation makes perfect sense to me!   Perhaps for future readers of this post I should add that the reason the side assy goes "gray" while the revolute joint goes round and round does make perfect sense if "animate joint" only animates the two components that are jointed.  To be more clear, the Side Assy component goes gray simply because by itself (i.e., not including child components) it has no body elements to it.  So there's nothing to show but a dimmed extent region of the sub-assembly.  But if I "animate model" everything is included and it does what I expected.  Wow...I think I have that in my head now.  A huge thanks!

I also believe the "bizarre movement" comes out of the fact that if I were to grab the lock in real life, it would behave a bit odd as well in that some motions would also pull the side back and forth.  That, in combination with mouse movement in 2d trying to emulate 3d action could result in "bizarre movement" :).  I'll watch that video and hopefully will learn more about how to think about this.

As to why I chose to revolute the Back + Side Assy vs. Back + Side, this is purely from my lack of experience with Fusion 360, I think.  What's going through my head is that the Side Assy as a whole is the thing that's hinged to the back, rather than a single part of the Side Assy (i.e., the Side).  In the future I'll probably just join with the Side in a case like this simply because it's easier to select the side as I can do that trivially in the model rather than by selecting it in the browser, and at the moment I'm not seeing any downside.

Todd