scara
A printed four-axis robot arm and the host-side stack that drives it: kinematics, path planning, calibration, and the serial driver.
Software stack written and tested, aimed at 0.1-0.3 mm repeatability over a 350 mm reach.
Why this geometry
Printed six-axis arms are floppy, and the reason is structural rather than fixable by tuning: every joint adds a compliant plastic interface in series, and the last one carries the accumulated error of all the others.
A SCARA puts both rotary axes vertical. Gravity then loads the bearings in compression instead of trying to bend the links, and stiffness in the working plane comes from link geometry you can actually print. The precision target is a consequence of that choice, not a hope attached to it.
Calibration as part of the software
A printed arm is never the arm in the model — link lengths are off, joints have offsets, the whole thing is slightly not what the drawing says. So geometric calibration is a first-class part of the stack rather than a spreadsheet someone keeps on the side, and the kinematics are property-tested rather than checked against a handful of poses.