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how to best design PLA screws and bolts (AA0.4)

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I am trying to make threaded bars and bolts from PLA with an UM S5 (Nozzles: AA0.4, Breakaway as support). What I have (unsurprisingly) found out, is that the components are perfectly flush in the design, they will not fit.


What kind of spacing would people use in situations like this one (M6 screw)? Before spending the rest of the day with trials-and-errors, I thought I might ask the cognoscendi...



Screenshot 2018-12-09 15.35.24.png

Edited by aag

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Pretty difficult to be prescriptive on this one. Not only is it required mechanical clearance but you have several other factors – plastic shrinkage, printing of circular geometry, dimensional accuracy of your printer, flow characteristics of your feed systems and settings plus potentially your filament.

I have found a clearance of 100 microns generally to be too tight when fitting a printed screw to a printed thread; so my starter is usually 200 microns. If too tight then I will add 100 microns until OK and then I will back of 50 microns to see if that is better.

Of course if you do reprints with different settings or different filament or different printer then you may need to resize.

It maybe that if you are printing screws/bolts to fit manufactured metal nuts etc. then you may find that once you have established your modifier it will always work, but I have only ever worked threads plastic to plastic so I do not know.

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I never got small threads to print reliably, without a lot of post processing. And also tapping threads in PLA didn't work: everything melted, even with good lubrication and very slow movements, and the threads were very weak anyway. It wasn't worth the effort for me.


So, now I usually design a slot in which I can drop a nylon nut, and use a nylon screw. Sometimes I provide a sort of retention, so that the nut can not fall out if the screw is totally removed. Needs no post processing at all. See the pics (these are M4).


Or if the thing needs to be locked only once, a snap fit also works. But it is likely to break if unlocked after some time, as the PLA gets harder and more brittle.


This is probably not the answer you are looking for, but it might be an alternative in some cases...









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Do you have the option to use a different material, for example PETG/CPE? Then you could easily post-tap the threads (print the threads anyway, this way you don't get problems like tapping into the infill, and beginning the cutting will be much easier). Actually, if I remember correctly, I even did this with PLA without too many problems.


Tolerance-wise, I'd say you have to experiment. Print small test-pieces to test out different clearances. One part I designed with a less-common thread (for pipe fittings) came out really well this way, and works reliably without any tapping. (The thread is much coarser than M6, of course).

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