I’ve typically used a tap to clean out printed threads.
I recently bought a Qidi X-Max 3, and threads printed with it are usable as-is, which wasn’t true with my previous printers. So clearly a well-tuned printer is capable of tight enough tolerances to print threads.
So it might be valuable to run through this tuning guide:
Alternatively, the relatively new OrcaSlicer has tuning help built in:
It is a fork of Bambu Studio so it has a UI that is substantially different from PrusaSlicer.
But with the printer as-is, you might be able to work around this. In FreeCAD, when you model threads in holes, you can choose the tolerance class for the threads. Try higher numbers in G tolerance class and see if that helps?
Years ago I found a percentages table for different plastics, similar to when casting metals and such, however nowadays I just add 0.2mm to the diameter of an internal hole (or subtract if some sort of pin), sometimes a little more if not such a tight fit is required.
If it is an actual screw going to be screwed into a hole I either leave as is or even make smaller by the same amount… I also technically know I should look up at my hand written thread tapping table and see how big the threads should be for that particular size, but hey ho…
Clearly its important for the 3D printer to be properly tuned to achieve accurate, precise parts. And I agree with the recommendation to check the printer tuning and calibration first.
However I’m still not convinced that tuning is the full story… In my original post, when I adjusted the printer and print settings to print the inside diameter (ID) of holes accurately enough to take shafts, the outer diameter (OD) of circles on the same part printed at the same time with exactly the same settings and filament turned out to be oversize. If I adjusted to print the circles accurately then the holes would be undersize.
In the end I gave up and just drew the outer diameter undersized so that both the inner and outer diameter would come out right. (If you recall I didn’t want to drill the hole to size because that introduced wobble on the sander hub that I was printing).
I also conducted a test on a small 5x25x25 square with 2, 4, 6, 8, and 10 mm holes in it and checked the accuracy of the diameter of the holes with drill bits. I found that tightness of fit was randomly dependent on the diameter of the hole. Some of the drills were a loose fit some a tight fit and one did not fit at all. I am not certain why that is occurring but I suspect it has to do with print resolution. I’ve since bought different size print nozzles, but haven’t explored the problem further as I moved on to other mysteries.
Finally I hope the everyone realises that a 6mm rod/hex will not fit into a 6mm hole. I knew but it didn’t occur to me that I needed to add a tolerance until I threw a few parts in the bin.