Ring nut M8 (DIN582)

Printing

Printing this part is less problematic than one might think. I had it printed successfully on a Ender3Pro from PLA and on a Prusa Mini from PETG, both without supports or brim. 

Sure, the surface quality at the top part of the rings inner side is not perfect, but I doubt that it would be much better with supports.

Mechanical tolerances

Printing metric threads with their 60° flank angle is always a bit of a challenge. The thread fits slightly tighter on a metal M8 screw, when printed on the Ender3Pro from PLA, compared to a metal M8 nut. 

Following a pattern that I have observed before, printed on Pursa printers, holes and female threads end up very slightly smaller than on the Ender3Pro (talking about roughly 0.2mm here). Consequently the thread is noticeably tighter on the part that I printed on a Prusa Mini from PETG. Still I believe it is a good choice like this, as a tighter printed thread can carry more load.

The thread wears in very quickly, so it is better to print it slightly too tight rather than slightly too loose. 

Design workflow

For those that are interested how I did this part. It is made using purely open source software. The main form was designed in FreeCAD 0.19. The thread was created on a RaspberryPi with stl_cmd. The command used was: 

stl_threads -f -D 8.5 -P 1.25 -a 60 -h 6.5 filename.stl

That results in a female thread with 8.5mm nominal diameter. When being printed, that gives a thread with exactly the above described tolerances. 

I did not even bother to convert the .stl thread part into a solid to meld it together with the rest of the part in FreeCAD. I simply selected both parts in FreeCAD and exported them into one .amf file. This file I loaded into Prusaslicer and took things from there. 

The uploaded .stl file is an export from Prusaslicer. To upload only parts here which I exported from PrusaSlicer, has proven to result in no troubles with corrupted .stl files. 

Happy printing and stay safe everyone!