I’ve printed many parts with threads and have made several observations about doing it. I can easily print 3/8-16 Bolts, Nuts and threaded Bosses, as shown below.
1. The resulting threads (for me) have always been good enough to mate with intended part (nuts, bosses, etc…).
The fit between mated parts of compatible threads also depends on Shrink and without some thread ‘chasing’ binding can occur. Optionally, the parts can be scaled up/down to compensate for the (common) 2% shrink (generally on the part with the internal thread).
2. The print-process parameters can be dialed-in to get ‘good enough’ parts, especially using ABS and PLA. PETg has some problems but, can also be dialed-in for a useful part.
3. If holding strength is needed, I can accomplish this by one of two way’s (or a combination of them): Increasing the number of Shells and/or Post-Processing with a real Thread-Tap or Thread-Die to ‘chase’ the threads.
Some dialing-in of Shells and Density is needed but, if planning on ‘Chasing’ the threads with Tap/Die, then I make that element of the part as a separate design and print with two processes (no point in printing aspects not needed on other features). They are printed at the same time (as two parts in their correct location in '3-D space'.
Use Concentric infill!
The square with center boss: Has two outer shell and the boss has 6 inner and 6 outer shells for cutting threads with a real Tap.
The screws/bolts, black mount with boss have printed threads as does the large Bolt/Nut.
3D Print Parts