Bill of materials

A full build is about $127 in commodity parts. Nothing here is bespoke — the arm is a consumer pen plotter and the cameras are ordinary USB webcams.

Component	Item	Approx. price
GRBL arm	Paixi pen plotter P25 (X/Y gantry + Z servo)	~$80
Top camera	UGREEN 1080P USB camera, fixed focus	~$14
Side camera	UGREEN 1080P USB camera (same model)	~$14
Stylus	Capacitive stylus, conductive fiber tip 8–10 mm	~$1.50
Camera mounts	Gooseneck desk clamp, metal, 50 cm (×2)	~$4
Phone mount	Anti-slip pad + L-shaped blocks	~$1.20
USB hub	Powered USB 3.0 hub	~$13

Notes on parts

• The arm is the only component that matters for precision. Any GRBL-compatible X/Y/Z plotter works; the P25 is cheap, rigid enough, and runs standard G-code.
• Both cameras are identical on purpose — one firmware quirk to learn, one driver, interchangeable.
• The stylus tip should be soft conductive fiber, 8–10 mm. Hard rubber tips register less reliably and scratch screens.

Assembly at a glance

1. Mount the phone in the cradle on the plotter bed, screen up, held by the anti-slip pad and blocks.
2. Replace the plotter’s pen with the capacitive stylus in the Z holder.
3. Clamp the top camera on a gooseneck directly above, looking straight down at the screen.
4. Clamp the side camera at ~45° so it sees the stylus tip and the glass surface.
5. Wire everything to the powered USB hub.

Rigidity matters

Flex in the camera goosenecks is the most common cause of drift. Tighten the clamps, keep the arms short, and re-calibrate if anything is bumped.

Next: install the server and run calibration.