Hirth Gear Coupling
myminifactory
Designed this with no idea that it was called a hirth gear/joint. Meant to accept 3/8" rods as a component of an articulating arm assembly. Holes normal to the 3/8" bore accept an arbitrarily dimensioned M3 brass insert that I bought online. Diameter of those holes measure 4.6 mm. Joint can be compressed via a spring/M4 nut+bolt assembly through the center bore of the joint, see image. Adjust tension accordingly. Printed on its flat face, the only areas requiring support are the center bore and possibly the 3/8" bores depending on your printer's capabilities.
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