NEMA17 Hanging Wall Polargraph Drawbot

This is a hanging-string polargraph ("muralbot"). Two NEMA17 steppers sit at the top corners of a wall, whiteboard or plywood sheet. Each one winds braided fishing line onto a printed spool, and the two lines hold up a weighted pen gondola that the motors steer across the surface. It suits builders who want a drawing robot sized to their wall, not someone else's. The line spacing and drawing area come from where you screw the two backer plates, so the frame has no fixed size and needs no special pulleys, belts or beaded chain. It has no pen lift, so it is best at continuous-line art, TSP portraits, spirals and hatching. The spools use plain braided line and the motor mounts use the standard 31 mm NEMA17 bolt pattern.

The motors mount "body out": each motor faces the room, and its shaft points back toward the wall through a standoff cage. That puts the spool in the cage, about 25 mm off the wall, which is the same height as the gondola's line arms (the stack contract, I10). Because the lines run parallel to the wall, line tension doesn't lift the pen. The gondola is a pen sleeve standing on a sealed ballast trough. Two free-spinning line arms turn on the sleeve so both lines always point at the pen axis. One clamp collar holds the arms in place and grips the pen with a single screw.

Assembly order:

1. Press 4x M4 heat-set inserts into the post ends of each Motor Standoff Cage. Put 1x M3 insert radially into each Line Spool collar, 3x into the Gondola Pen Hub rim bosses and 1x into the Arm Clamp Collar.
2. Bolt each NEMA17 to the flat outer face of its cage plate with 4x M3x10. The screw heads go inside the cage and the motor body faces the room.
3. Tie a line through each spool's anchor hole, wind about 3 m on, slide the spool onto the shaft with its collar toward the motor, leave a 1 mm gap and tighten the M3 set screw on the shaft flat.
4. Fasten each Wall Backer Plate to its cage with 4x M4x10 from the wall side. Put the top web of the cage at the top, then screw the backers to the wall level with each other using #8 screws.
5. Fill the hub trough with about 250 g of steel shot or coins, screw the Ballast Trough Lid on, slide both Line Arms onto the sleeve, then the Arm Clamp Collar.
6. Put the pen in so its tip sticks out 1–2 mm past the base, and tighten the collar thumbscrew. It locks the collar and grips the pen together.
7. Tie each line to an arm eyelet. Start with the gondola parked at the calibrated home point.

Tuning notes: the 30 mm drum takes about 94.2 mm of line per turn. With 1/16 microstepping that is about 34 steps/mm. Calibrate by measuring real line length, because line build-up on the drum changes the effective diameter slightly. Keep the drawing area inboard of the motors so the lines stay less than about 70° from horizontal; the zones directly below each motor have poor tension and resolution anyway. Add ballast until the pen stays in contact without the gondola swinging. It runs on common polargraph firmware (Polargraph Server, Makelangelo-style firmware) on an Arduino with a CNC shield.