Dynamic length component based on joints?

[agmlauncher]

I am designing a truss tube Dobsonian telescope. Here's an idea of what a finished product looks like:

The way these telescopes work is there are two mirrors - one large one at the back/bottom near the ground, and a small one suspended in the small ring/tube at the top. These two mirrors must be placed at a very specific distance from one another, meaning the ring/tube at the top (called the upper tube assembly) must be a specific distance from the mirror box at the bottom.

Connecting the two is a truss assembly of 6 or 8 aluminum poles. The connection angles and length of these poles will depend on many different factors in the design of the telescope.

I'm wondering if there is a way to use Fusion to automagically adapt the length of the truss poles for me so that I don't need to use 3D trig to figure out how long they should be. Imagine the UTA and Mirror Box are both anchored in place at the right distance in a Fusion assembly, and then the truss poles are connected at each end via ball joints in Fusion. Is there a way to make Fusion automatically change/adapt/update the length of the pole in such a scenario?

[davebYYPCU]

Yes.

Connect the ends of the pipes to a sketch in each component.

Might help...

[Drewpan]

Hi,

In 2D your problem is similar to this:

If you assume that the two boxes at the top and bottom are the mirrors and the dashed construction line is the

distance required between the mirrors The two lines marked in red are fully constrained and will change length and

angle when you update the dimension marked in blue.

If you set the blue dimension as a parameter then the length of your trusses will be whatever the length of those lines

are less the mounting hardware.

To do what you want it may be useful to set this up as a 3D sketch and get all of the angles correct in 3d. They are then

quite easy to measure with the measure tool or through reading the values as driven parameters from the table.

Cheers

Andrew

[TrippyLighting · Accepted solution]

You don't need any joints for this!

3 skeleton sketches are all you need to lay the foundation for his design.

The third one is a 3D sketch that connects the pints in the top and bottom sketch.

The length of the connection trusses and the angles between them is  a result of the distance between the XY origin plane and the offset plane from it (2nd feature in the attached file).

With this set of skeleton sketches established, you can design components in place and don't need any ball joints!

[agmlauncher]

Thanks @TrippyLighting. I like the idea of sketching the skeleton in place, but I'm unsure how to design components in place around it. I'm used to designing components separately as their own assemblies and either importing them (if they are re-usable) or simply creating a new component off elsewhere in the workspace and then using joints to connect things.

Do you know of any tutorials/resources for this kind of workflow and how one might design components around a skeleton sketch?

[agmlauncher]

Alright, I believe I figured this out with your guys' help.

Attached is a proof of concept using a 3D skeleton and in-place sketching. The key for me was learning how to create sketch planes, particularly creating one along one of the truss sketch planes.

I also figured out a version of this using more complex component assemblies for the trusses and ball joints that works well, but this is a simple demonstration of how to achieve this using the advice given above.

You can adjust the sizes of the mirror or UTA sketches, or adjust the distance of the sketch plane of the UTA, and the truss poles will respond accordingly. From there you can get the measurements of the length of the truss pole, which is exactly what I would need to cut them to the right length.

Thanks again all!

[TrippyLighting]

Yep, that is what I tried to imply with "skeleton sketch".

One thing to be aware of when mirroring a component is that in a BOM this will be a separate component, so instead of one BOM entry if 8 Truss Poles, you'll have 4 Truss Poles and 4 Truss Pole (mirrored).

Also, if the poles don't change length dynamically you would not actually need a ball joint. I note that currently there are No joints in your design so components can be dragged around in the viewport. Not a problem (yet) but something to be aware of.