Question about how components move in joints

[Catherine__P]

Hello, 

I am still getting to grips with the basics of Fusion 360. One thing I am often confused about is how Fusion determines which component moves when you make a joint. I read somewhere that the component you click on first should be the one that moves, but I am not sure whether this means: 

a) that the 1st component moves position physically to meet the position of the 2nd component in the design OR

b) that 'movement' here means that the 1st component will be the one to move around the 2nd component when the joint is animated (ie provided it is not a rigid joint)

I *think*, having done some experimentation, that it is b. Which makes me ask: how do you move a component to join the existing position of another component. It often happens that I can't achieve this, and the *wrong* component moves to the position of the other one. Even when I ground the one I want to stay in place I sometimes still can't make it work. For example, (I have attached the file I am working on), here I wish to join 'side eyeball joint: 1' (via the rod on the internal 'ball (4): 1' component (I know I have messed up a bit already with multiple components inside other components, but hopefully doesn't matter too much for this question)) to the hole on the  right hand side of 'eyeball: 1'. I try creating a join by clicking on the rod on 'ball (4): 1' first, and then the inner cylinder shape of the hole on the eyeball. The ball joint moves over to the eyeball as I want (looks like the eyeball will stay in position), but as soon as I click 'OK', the eyeball itself has moved over to the initial position of the ball joint! How do I get the eyeball to stay in position? Note that this is even with the 'central ball joint' (which is currently holding the eyeball in position) grounded. I thought that if a component is grounded, it can't move? The ball joint I am trying to move is not grounded, so I really don't understand why its position is being prioritised.

Any help understanding this much appreciated! Many thanks. 

[Catherine__P · Accepted solution]

I think I have sorted this - I click on the component I want to move first, and then after clicking 'OK' on the joint, I revert position on the top right (rather than leave it capturing that position). This seems to bring both components back to the original position of the first component. Posting here in case it's of help to anyone else.

[jhackney1972 · Accepted solution]

You are piling up useless sub-assemblies in your Browser and also duplicating Joints.  In the video I will go over this.  Also, you need to drill down into your sub-assembly to select the grounded component.  In some cases, if your sub-assembly is modeled to the origin, you can ground it but you are not doing this. 

As far as selecting components when placing Joints, I would suggest you select the component your wish to have move to another jointed or grounded component first.

Model is attached.

[Catherine__P]

This is super helpful! Thank you so much! I didn't realise that you can remove a sub-component and put it back into the top level - I will be cleaning up a lot of files now. Thanks for your help! The video was great

[jhackney1972 · Accepted solution]

If you try to organize sub-assemblies, as I did, keep your Undo button handy!  I did a lot of testing, on your assembly, before I did it making sure of you dependencies.  This should not be necessary as you should not create sub-assemblies unless they are absolutely needed.  Activate the sub-assembly you are going to work on, so that all components, bodies, and sketches will fall under the activated sub-assembly.

You are working on a far too complex assembly to learn on.  Drop back to something simpler, get you understanding of Joints and sub-assemblies down pat before attempting such a project. 

[Catherine__P]

Yes it is giving me a bit of a headache - it is getting a bit complicated for what I understand! The sub-assemblies were piling up as I've been creating the separate components in other files and then inserting them in - and each time I do that it has to create another sub-assembly. Anyway it is a great tip to know how to remove them now. Many thanks!

It is probably that I need to give up on this model for now while I go back and focus on learning some basics, but I was wondering if you have any insight on what is happening in this updated model attached. 

So far the eyeball movement is working OK when I drive the servo horns. The top servo, moving eyeball up and down, seems to work completely fine. However, for the right-hand servo, controlling side-to-side movement, the sockets on both ball joints immediately rotate 90 degrees clockwise as soon as the servo horn is driven (such that the open end of the ball socket attached to the servo is facing left, rather than down). I keep trying to assemble it so that in a resting position, every joint is set to 0 degrees, but this keeps happening. If you have any idea what I can do to fix it, that would be amazing! 

Many thanks!

[jhackney1972 · Accepted solution]

I opened you model, labeled the two Servo Motor rotation Joints, so I could find them, and it seems to work OK.  Added a Motion Link between them and then you see some motion cross-over in the eye movement which is to be expected because of the main ball joint which has unlimited motion. 

I am going to send you a Private Message, please read it.  You will find it by clicking on the envelope up in the right hand corner of a Forum page.  You must be logged in to see it!  You may also get an email notification depending on your Forum settings.  It will take a few minutes to compose, but it will arrive.

[Catherine__P]

Thank you very much. The motion link is a good thing to know about, I will try using that feature more. The twisting ball joint seems to have stopped for me now so that seems to have sorted itself out anyway. 

Many thanks for your help and insights into ball joints!