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I have a PLA 3D-printed object made of several rings with a diameter of 150 mm and a thickness of 6 mm. The height of each ring is about 30-35 mm but it does not matter so much.

I'm looking for a way to keep the rings together (one on top of another). I cannot place anything inside the ring (like a thicker area to host a nut). Most of the rings may have a closed top or bottom, so I cannot access the inner part.

I'm using three tabs (at different angles to force the correct alignment) that fit inside matching slots on the counterpart:

enter image description here

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They are a bit stronger than pins and holes due to the greater surface of the base layer, but of course are useful only for alignment.

Due to the very small thickness of the ring I cannot place a screw inside them to keep in position, the inner wall cannot hold a screw!

I was thinking about L-shaped tabs and slots to fix the rings rotating them a bit, but:

  1. it would not easy to clean the horizontal leg of the slots
  2. nothing would prevent to rotate the rings in the opposite way (even accidentally) so the disconnect

For my use-case scenario, I don't need anything too strong. Just avoid to disconnect the rings when taking the object in hands. I tried with magnets but:

  1. to make an acceptable resistance I needed 7-8 per ring (due to their small size)
  2. the electronics inside uses Hall sensors so I cannot use magnets!

Any idea?

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  • $\begingroup$ Do you need to be able to disassemble this at random times? $\endgroup$
    – Zeiss Ikon
    Oct 4, 2022 at 13:06
  • $\begingroup$ @ZeissIkon yes, it may happen. Otherwise I could just glue them $\endgroup$
    – Mark
    Oct 4, 2022 at 13:33
  • $\begingroup$ Double sided tape? Not the foam tape type, but the tape that is used to glue kite fabric together before stitching, is this something that you can use? $\endgroup$
    – 0scar
    Oct 4, 2022 at 14:57
  • $\begingroup$ @0scar, not sure which type are you talking about. Do you have any link? I would avoid tapes and glues, but I may definitely give it a try $\endgroup$
    – Mark
    Oct 5, 2022 at 7:27
  • $\begingroup$ @Mark E.g. 3M GPT-020F is only 0.2 mm thick, but I think I used Nitto double sided tape for my kite ages ago... $\endgroup$
    – 0scar
    Oct 6, 2022 at 13:04

2 Answers 2

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One possibility, if the wall thickness is sufficient, might be to put screw threads in the actual tube wall.

The wonderful thing with CAD and 3D printing is you can absolutely control the thread start points and shoulder position, so the clock orientation of the parts when they're screwed together is perfectly repeatable from one part to the next.

The potential down side is that threads require some minimum wall thickness to avoid introducing a line of weakness where the root diameter is too close to the inside or outside diameter of the main part. One way to help this is to use finer threads, or square or Acme thread profile, which can grip strongly without a great deal of engagement depth.

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  • $\begingroup$ Actually, this is a great idea. I have to play a bit with the CAD to understand if I have enough knowledge to design the threads in that way. $\endgroup$
    – Mark
    Oct 5, 2022 at 7:29
  • $\begingroup$ The only downside I can see (but it can be acceptable) is I have some slots that run across all the rings to put in cables. With your method it's impossible to inspect a ring after assembly. $\endgroup$
    – Mark
    Oct 5, 2022 at 7:33
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You could use velcro. I find it very easy to work with because you can cut it precisely with scissors or craft knife, and it's great for just holding things together but won't come undone by accident.

It's cheap and there are lots of different thicknesses at sewing shops. The ones I use come in sheets and have an adhesive backing. Takes a day for the adhesive to cure but once it's stuck on it won't come off.

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