NEMA17 Belt-Drive SCARA Drawing Arm

A two-joint SCARA arm that sits on your desk and draws with a pen, marker or fineliner on the table around it. The arm reaches 260 mm (upper arm 140 mm plus forearm 120 mm). Two NEMA17 steppers drive the shoulder and elbow through 3:1 GT2 belt reductions. A small 20T pulley on each motor drives a 60T pulley printed into the joint, which gives 9600 microsteps per joint revolution at 1/16 stepping. Each joint turns on a pair of 608 bearings riding on a fixed 8 mm steel rod. The elbow motor rides on the upper arm, so each motor drives only its own joint and the maths stays simple: two-link inverse kinematics with no coupling correction. An SG90 at the tip lifts a sliding pen carrier, and gravity puts the pen back down. It's aimed at hobbyists who want a real SCARA to practise inverse kinematics, G-code streaming and belt tuning on, using parts they probably already have.

The base plate carries the shoulder tower and the hanging shoulder-motor bracket. The shoulder pulley hub drops onto the tower rod and the upper arm beam bolts to its top. The forearm hangs under the far end of the beam on the elbow rod, and a shaft collar holds it on. The pen tool head bolts under the forearm tip.

Assembly order:

1. Press M3 heat-set inserts into the base (4), the shoulder hub top flange (4) and the forearm tool pad (4).
2. Slide the 100 mm rod into the base tower and lock it with the M3 grub screw. Bolt the motor bracket to the base, then hang the shoulder motor under it with its 20T pulley on top.
3. Press two 608s into the shoulder hub and drop it onto the rod. Bolt the upper arm beam on top with the beam pointing the same way as the hub's belt-anchor lug.
4. Mount the elbow motor on top of the beam with its shaft pointing down, and fit a 20T pulley so its teeth line up with the elbow belt plane (z 86 to 94).
5. Cut the second rod to 70 mm and grub it into the elbow boss. Press two 608s into the forearm, slide it onto the rod from below, and lock it with the shaft collar pushing up on the lower bearing's inner race.
6. Wrap each open GT2 belt around its motor pulley and 60T pulley, loop both ends around the M3 pins in the anchor lug, then slide the motor away along its slots to tension the belt and tighten it.
7. Screw the SG90 into the tool head, slide the pen slider in from below, add the M3 retaining screw, and bolt the head under the forearm pad. Clamp a pen so its tip sits about 2 mm below where the slider would rest on the retaining screw.

Tuning: home the arm by hand at startup (arm straight along +X), or add endstops of your choice. The belt anchors limit each joint to about ±150°, and the forearm should be kept from folding back past about ±135° or it hits the shoulder pulley. Any GRBL/Marlin SCARA build or a small Python IK streamer can drive it with a CNC shield. Screw the base down, or clamp it to the table: at full reach the arm puts a real bending moment on the base.