Mostly posting this cuz its just funny.

[mrbi11]

I am tring to print something with female pipe threads because (long boring story).
So i downloaded a fitting cad file from mcmaster.com (who basically have cad files for all their fitings).
I needed a female fitting, so i downloaded a male and subtracted it from a solid block.
(why didn't i just download a female, you are thinking? I have no idea.)
So long boring odyssey to get data to printable shape, and behold, my first attempt looks like a threaded female part.
But my male fittings wont screw in.
Now, I downloaded a female part. Compared it to the male part, and the threads don't match (the fittings do match).
head scratching.

Punchline is, the male part has thread shaped circles, that dont spiral forward. The printed thread were just rings.
The female part had the actual spiral shape threads real fittings do.
Okay, only funny to other 3d printers.

Printed the female part and its fine. And was a lot easier too.

[Jimustanguitar]

What tolerance did you plan for? Is there a gap between the M and F threads?

A 1/4" peg won't fit into a 1/4" hole without engineering in a tolerance.