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Ballast Gondola Pen Hub
This is the main gondola body for the 'NEMA17 Hanging Wall Polargraph Drawbot' kit. It is a 90 mm dia x 46 mm hub with a 22 mm OD pen sleeve and a 14 mm through-bore that takes pens 8–13.5 mm across. The sleeve sits on the axis of an annular ballast trough, which holds about 120–150 g of shot or coins to give steady pen pressure. The trough is closed by the kit's screw lid, fixed with three M3x8 screws into M3 heat-set inserts (4.0 mm bores) at r = 40 mm. A 3.6 mm slot in the sleeve lets the collar's M3x16 screw clamp the pen, and a 1 mm chamfer on the base helps it glide over paper.
🧩 Part of the NEMA17 Hanging Wall Polargraph Drawbot (pen sleeve on a weighted ballast trough) kit.
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