I’m making mechatronic Dr Octopus arms (they’re gonna be pretty sweet), and I’m in the process of prototyping the segments. They’ll be roughly 2.5 cm in height, with a 2.5 cm gap from segment to segment, and they need to be attached securely, with smooth rotation on xyz to ≈ 10°. Planning on housing the servos in the base, and running cables down the arms to actuate them. Roughly 24 segments total, with separate control cables for the first 12 and second 12 (so the first half can bend in one direction, and the second half another), along with a drive cable down the center so it can rotate on the z axis. This also means I’ll need some empty space running down the center of the segments to run the drive cable and the control cables for the second half, so they don’t impede/aren’t impacted by the bending of the first half.

As far as I can figure, spherical bearings are my only real option here. As a test, I bought 4 steel spherical bearings from Amazon to connect the segments. And man, I love these things. I’d never handled them before, and they are smooth as butter. Unfortunately, they were like $7/per, and I need around 100 of them. So, slightly more than I’m hoping to spend haha. Aliexpress has them for ≈ $1.50/per, but the shipping is obscenely expensive, and would take like 2 months.

So I’m trying to figure out another option. I’m a member of a local maker’s space, so I have access to a bunch of tools (metal shop with lathe, CNC, FDM 3D printers), but still don’t think I have a great solution.

The tools for making these the right way are obscenely expensive. I’m thinking I could machine the race in two halves and weld/clamp them around the ball, which would be a ton of work, and wouldn’t be as smooth, but would probably be sufficient.

I could 3D print them, but I can’t figure out a process that would be as smooth as I want. They’d need to be filled/sanded, probably coated with a dry lubricant. But the filling/sanding process would introduce tolerance issues, and I can’t afford to have slop with how the cables will work.

Anyone have any insights here to help me out? Thanks!

  • HaphazardFinesse@sh.itjust.worksOPEnglish
    1·
    1 year ago

    I have been experimenting with the 3D printing, just not getting great results. Did a few test pieces over the weekend at 0.1 mm layers, the layer lines were still killing any kind of smooth motion I’d hoped for. Which is what got me on the fill/sand path, which is my next experiment. I’m pretty confident that with enough process experimentation, I’ll be able to get the action I’m looking for with fill->sand->possible resin coat->dry lube. Just gonna be a lot of work to get it right haha. Wish I still had access to an SLA printer; I printed out some kick-ass ball joints on a Formlab printer years ago.

    The print-in-place joints, in my experience, work just great, until you try to move them while they’re loaded; ≈ 50 N*m is a lot to ask of a non-finished plastic joint lol, and that’s probably around what I’ll be working with.

    print a tapered shaft that starts at -0.020in below your bearings internal diameter, and ends at +0.010in. Do the same for the external race but inverse

    Could you elaborate on that? Not sure I follow.

    Thanks for the help!

    • empireOfLoveEnglish
      2·
      1 year ago

      I’m saying make a shaft in your modelling program that is straight at the nominal dimension of your bearing race’s internal diameter such that you could slide the shaft inside the bearing inner race. But instead of being straight, taper the shaft from undersize on one end to much oversize on the other over about 3-4 inches. Profile like so. That way when you slide the race on and tap it, the taper wedges the race in place and you can quickly and easily swap parts in and out for lathe sanding. Do the inverse for the outer race- that is, make a hollow shaft where the inside tapers smaller instead of the outside tapering bigger. I do it all the time for holding handmade parts for outer sanding.