how do I make accurate male and female threads for 3d printing? or where can I get them?
Trial and error. For my printer I usually start with 0.020" clearance. Maybe more. You can also scale your .stl in your printer software. I usually start with 1%. Maybe more. I don't do both clearance and scaling at the same time. Attached is a 1/4-20 thread that works pretty good for me, although you might find it a little tight. I like it.
Depending on the threads you will be using and printer resolution, "accurate" threads may not be possible on the actual part and they may not fit or thread into each other well, more clearance may be required. What size threads do you plan to use?
To create the threads is not difficult. For screw threads, start with a shaft equal to the OD or the major diameter of the screw size you will use. Create a helix set to the correct pitch using a circle for the helix equal to the minor diameter of the threads. Create plane normal to the end of the helix, draw a triangle on this plane in the shape of a "V", bottom of the "V" coincident/pierce to the helix. Sweep this "V" sketch using the helix as it's path, done. You may need to create a new body for the portion where the threads are at to control the start/end of the threads differently or more easily so that the helix's start/end point doesn't cut into the part where you don't want it to.
To make threads on the nut, the ID will be it's minor diameter, do the above steps but make the "V" upside down and the major diameter will be the circle base for the helix. To make them fit or thread together, with printer parts, you may need to fudge or guess a bit and give more clearance in one part. Otherwise you'll be working things free by hand or it'll be difficult to turn.
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