feat(cad): add a parametric bottom cover and chair mounts - #49

What

Add a standard parametric BottomCover for FS0307, SG90, and HD1370A, with four recessed desk fasteners and two new reinforced chair mounts. It follows the existing underside outline, preserves the square chair corners, and keeps the specified screw heads above the flat bottom face.

Related #48. Follows the new CAD contributor workflow.

• TinyEngineer.f3d is the only editable source. The cover uses linked desk geometry and shared parameters, is jointed into the main assembly, and has linked, jointed copies in PRINT_LAYOUT and black PartsSetB.
• Regenerate BottomCover, Chair, and PartsSetB in 3MF/STL/STEP for every preset: 27 exports. No hand-edited release geometry. The old fixed-size accessory sources and helper scripts are removed.
• Use six M2×8 thread-forming pan/button-head screws, heads up to Ø4 × 2 mm. Update the parts table, assembly order, black aggregate documentation, and M2×8 shopping quantity from 21 to 27. Keep ScrewSizingTest guidance, CERN-OHL-S, stable source attribution, and unchanged AiEmblem.
• Open the four desk-corner head pockets to avoid a 0.85 mm thin rim. Keep a 2.2 mm bearing layer, two round chair recesses, and 0.3 mm head-to-floor clearance. Fully constrained pocket sketches preserve their shape when presets change.

Compatibility: the two new chair mounts require the updated Chair print. An older chair without those pilots must be reprinted for the six-point attachment. This revision has CAD validation only; no physical print, thread-grip, harness-fit, or assembled stability test is claimed.

Design notes, dimensions, and downloads · Recorded validation and file hashes

Previews

Rendered from the revised CAD/export geometry; these are not photographs of a physical print.

Checks

• Conventional commit and PR title: feat(cad): add a parametric bottom cover and chair mounts.
• Native master saved and reopened; all three servo presets rebuild the whole timeline with zero errors/warnings. All new cover sketches are fully constrained.
• Correct component/export names, feature blocks, assembly joint, print-origin joint, and same-component black-set copy with a placement joint. Print copies sit at Z = 0 with pockets up.
• Zero cover interference with other main assembly parts or other PartsSetB members; zero measured excess or missing mating-outline area.
• All six hole axes align within 0.00001 mm calculation tolerance. New chair pilots clear existing bores by 4.10 mm minimum, add no material inside those bores or the checked central wire passage, and leave the original side screws accessible before cover installation.
• Opposing desk screws have 12.2 mm minimum conservative tip separation. All 18 nominal head-fit checks pass: zero seated collision, positive shoulder contact after a 0.1 mm upward move, 0.3 mm floor clearance.
• All 27 exports checked independently: valid STEP solids, watertight positive-volume 3MF/STL meshes, expected solid counts, matching bounds/volumes, correct bed orientation.
• Assembly/BOM/shopping/color-group docs, NOTICE and REUSE updated; archive, hashes, links, and text whitespace checks passed. Generated STEP whitespace is preserved verbatim.
• CAD and documentation only relative to upstream; firmware/package/HTTP/settings/pin/PCB checks are not applicable. Servo motion limits are unchanged.
• Hardware status stated: no physical test of this revision. No secrets or device configuration included.

The largest PartsSetB is about 235 × 332 mm; home printers with smaller beds should use the individual files. Physical perimeter fit, cable routing, screw grip, and flush seating remain to be checked on a printed assembly.