NEMA17 Cantilever Pen Plotter

This is a desktop drawing machine that runs GRBL. It turns SVG and G-code into ink on paper. There are two axes. The Y axis is a pair of 400 mm, 8 mm rods resting on two end blocks on the desk, and a sled rides them on four LM8UU bearings. An X arm is bolted to that sled: two 350 mm rods reach out over the paper, and a small roller foot at the far end holds the tip level. A pen shuttle runs along the arm. Its pen slides on two 4 mm steel guide pins and is lifted by an SG90, so the pen lands square and straight every time and doesn't swing on a servo horn. It's built for Arduino Uno + CNC-shield owners who want a frame that stays square and a real pen-lift bracket.

Both axes use the same drive: a NEMA17 bolted to a printed wall turns a GT2 20T pulley 16 mm off its faceplate. At the far end, a 608 bearing on an 8 mm pin works as the idler. The idler axle sits 4.65 mm lower than the pulley axle, so the top run of the belt, which is the one clamped to the moving part, stays perfectly level. Every rod end sits in the same 25 mm pinch-clamp socket. To tension a belt, slide the idler block along its rods and tighten the pinch screws.

Assembly order:

1. Press 4 LM8UU into the Y sled, then melt 4 M3 heat-set inserts into its top deck. Do the same on the pen shuttle (2 LM8UU, 4 inserts on its front face).
2. Put the Y rods through the sled and into the Y motor end block and the Y idler end block. Bolt the Y NEMA17 to its wall, then fit the pulley with its belt teeth centred on x = 30.
3. Fit the 608 on its 8 mm pin between the Y idler block's cheeks and lock it with the M3 screw. Run the belt around the pulley and idler and anchor both ends on the sled's belt posts. Slide the idler block outward to tension the belt, then tighten the rod pinch screws.
4. Bolt the X arm root block to the sled (4x M3x10). Mount the X NEMA17 and pulley, then clamp the two X rods in with the pen shuttle already threaded onto them.
5. Clamp the X tip block onto the rod ends and fit its 608 idler. Mount the 624 roller on the M4 axle, then slide the axle up or down in its slot until the arm sits level. Belt the X axis the same way you did Y.
6. Press the 4 mm guide pins into the pen-lift frame with the clamp slider threaded onto them. Screw the SG90 into its window, then bolt the frame to the shuttle.
7. Wire the Arduino Uno with the CNC shield and A4988 drivers. Put the servo on the spindle-PWM pin using a servo-capable GRBL build.

Tuning notes: GT2 with 20T pulleys at 1/16 microstepping works out to 80 steps/mm on both axes. Clamp the pen so that when it touches the paper, the slider's lift tab rests about 2 mm above the servo horn. Then set the pen-up angle so the pen lifts 4–6 mm. Keep the arm feed rate under about 3000 mm/min, because the cantilever tip rings at high accelerations. Everything prints in PLA at 3 perimeters and 25–30 % infill.