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I created a two piece case for an electronics project. The pieces are joined with M3 screws. The screw enters a 3d printed cylinder.

In order for the screw to secure itself, it connects to a nut I have installed at the base of the cylinder. When you look at these pictures, you will understand.

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Is there a way to omit the nut installation and secure the screw in the cylinder directly? If yes, I guess there should be an internal thread on the 3d printed cylinder.

What is the best way to create such thread? Should the thread be in the model itself? Should it be created later? If yes, how? With a tap? With a drill perhaps?

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There are metallic threaded inserts which function like the nut in your design. They are embedded into the 3d print by melting the plastic (using an equipment like a soldering iron). Apparently they are called heat set inserts (I am linking the serch result page since most results seem to be supplier sites).

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  • $\begingroup$ I don't know if it is the ideal way to do it since it involves additional post-printing-effort. $\endgroup$
    – AJN
    Nov 6, 2023 at 12:00
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Internal threads can be printed directly, but the result will vary based on print method, thread size, etc. Depending on the application, this may or may not be acceptable. I have seen limited success directly 3D printing threads out of PA12GB using MJF, but other methods may not work as well.

Typically what I do is print the hole to the standard pre-tap drill size and then tap after printing. A cheap tap should be fine for this because likely anything you are printing will be quite soft and the tap should cut through it easily. I do this all the time for low load applications like sensor brackets.

Heat-staked threaded inserts can be another option, this type of thing would be used when the loading is expected to be too much for plastic threads.

Another option that can work well is to use only bolted connections, print hexagonal cavities to captivate the nuts and just thread everything together. Adhesives can work well on captive nuts too in these types of scenarios.

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you can:

  1. Print thread geometry. This has a tendency to not work out that well for something like m3 on a FDM printer, but works fine for m5 and up. On the other hand a resin printer probably is ok.

  2. You can tap a 3D print. For this I suggest adding more walls around your hole. Sure this is post printing effort but an option. You can combine this with method 1 to fix any problems.

  3. You can drive the screw in a hole made for tapping, without tapping. This likely does not have a very long life in opening and closing. There are screws for this use called thread forming or self tapping screws (though also wood screws are thread forming). Although ive have had some success in driving normal machine screws in also.

  4. You can modify your printer so it picks up and inserts the captive nut into the print cavity. Ive had some success with this but its definitely not the easy way out.

  5. you can use press inserts (not heat set), this way you can attach the insert to the screw mounting and press them all in in one go with whatever your attaching.

  6. use heat set inserts

  7. Do not use screws instead make holes for plastic rivets. Or use another snap fit connector.

  8. Use a expansion fitting that gets expanded by screw.

  9. Misc other like Velcro strips, 3D printed pieces that's wedge the shape in place, 3D printed rivet studs etc.

Thats about all i have tested.

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