Arm Clamp Pen Collar
This collar is part of the 'NEMA17 Hanging Wall Polargraph Drawbot' kit. It is a 34 mm OD x 10 mm knurled ring, and its 22.4 mm bore slides over the gondola hub's 22 mm pen sleeve to hold the two line arms in place. One M3 heat-set insert goes in a 4.0 mm radial bore in a flat 14 mm pad. An M3x16 thumbscrew in that insert passes through the sleeve slot to clamp the pen and lock the collar with one turn.
mechanism kit polargraph pen-holder