Tangent Joint between Cylinder and Surface

[nikitanugent]

Does anyone have a better way of doing this?

I know that Fusion 360 doesn't have an equivalent to Inventor's tangent constraint, but something like that is necessary for the design I'm working on. I managed to make it work, but only by adding an extra part to the design:

This feels sloppy to me, as now I have a part that I need to keep hidden, and I'm worried that it will mess up my bill of materials, etc.

Other solutions that I have found so far tend to constrain the motion to a specific point on either the cylinder:

(here)  or the plane (here). Neither behavior works in this case, as the cylinder needs to be free to both rotate in two axes and slide in two axes with respect to the plane. Maybe a parametric sketch made in the parent component?

Joints are nice for some things, but by only giving us a limited combination of degrees of freedom to choose from, some things become way more complicated. I hope I never have to design a complex cam shape in Fusion 360. (I guess a motion link would be the only way to simulate that behavior.)

[jhackney1972]

The next time you have this situation, try the Joint Origin solution.  Joint Origins have the ability to redefine the Z and Z axis giving you the ability to place them on the surface of a cylinder and use them is situation where you need a tangent joint.  Take a look at the Screencast for the process.

[nikitanugent · Accepted solution]

Hi @jhackney1972,

That's identical to the first "solution" that I referenced in my original post. That doesn't work, because it isn't a true tangent joint: it is a planar joint that is tangent to a specific point on the cylinder surface, rather than tangent to any point on the surface. This doesn't allow the cylinder to rotate about its own axis with reference to the plane, which my part needs to be able to do. If that weren't the case, I'd just make the part square!

I have figured out a slightly nicer way to achieve the same result that I had before, but it still boils down to making an extra component. The only difference is that the extra component is now just two joint origins, rather than a solid body. I've attached a screencast of this solution here:

Tangent Cylindrical Constrain/Joint in Fusion 360 

(I can't embed the screencast because of a bug in the forums.)

This solves my problem for this component, but I'd still like to know if there's a better way to do this. My solution is only applicable to cylindrical (and, if that second joint origin isn't perpendicular, conical) surfaces. 

[jhackney1972]

Very nice!  I embedded your Screencast for you.

[nikitanugent]

@jhackney1972 

Thanks for embedding that! I hope someone finds it useful.

[davebYYPCU]

but I'd still like to know if there's a better way to do this,

that seems like hard work

It's a standard gimble set up, Ball Joint doesn't work for you? 

Grey disk revolves in the red cylinder?

Cylindrical vs Revolve - usually alignment errors,

Might help....

[nikitanugent]

Hi @davebYYPCU,

I don't think I'm understanding your comment.

It's a standard gimble set up, Ball Joint doesn't work for you? 

I don't see how a ball joint constraint would be applied to this? I need the cylinder to be able to rotate around the axis normal to the plane, and translate along the plane. If you're suggesting I design a ball joint instead of a gimbal, that also won't work for me in this design.

Grey disk revolves in the red cylinder?

If you're referring to the grey disc in the screencast, that revolves in the blue cylinder. That part works just fine using a revolute joint.

Cylindrical vs Revolve - usually alignment errors,

If you're referring to the section in the screencast where I mentioned that the yellow component is constrained using a cylindrical joint instead of a revolve joint, yes, I could use a revolve joint there, and it does work. But as I said in the video, that would over-constrain the model. Using a revolute joint there would make it more likely that the joints would begin to conflict as I start adjusting the sizes of the different components to make this manufacturable.

What would really solve my problem is if there were a "custom joint" option. One where the user can select what combination of translation and rotations they need free or locked. There are a number of other joints that are very difficult to model in Fusion 360 that would become trivial if that were an option.

[davebYYPCU]

The model you have is a gimble in my terminology, 2 axis of rotation, stationary green frame,

Set up the ball joint on the axis' of the red block, in your model with the green frame.  Lock the horizontal rotation.

The rest is mechanical.

Might help....

[nikitanugent]

Are you just describing how to get the gimbal portion to work? That already worked when I first asked the question. The red part is unnecessary if I only wanted gimbal operation, but that's not what I'm designing.

The issue is connecting the red part to the blue cylinder such that the blue cylinder remains within the rails of the red part. Unless I'm seriously misunderstanding something, I don't think what you're proposing does that. (I did try, just to be sure.)

I've attached a recent version of the part, which has the desired behavior, in case you wanted to try demonstrating the joint that you're proposing.

[davebYYPCU]

I have the Ball Joint and it works as required with a fudge, and it is this fudging, that prevents the rest of the model working, so I will leave it to you, with version that does work as you need it.

There is discussion about ball joints and their non intuitive nature within Fusion Support, that does not seem to crop up often.