feat(cad): add an optional HD1370A bottom cover

3d_models/NOTICE

@@ -12,6 +12,12 @@ Source Location
 
 https://github.com/jamro/tiny-engineer/tree/main/3d_models
 
+Optional HD1370A bottom-cover modification (Hanson Wen, 2026):
+https://github.com/Hilo-Hilo/tiny-engineer/tree/feat/hd1370a-bottom-cover/3d_models
+
+The modified master includes a hidden HD1370A-only cover reference. Its editable
+source, print exports and verification scripts are included in this source tree.
+
 When distributing a specific revision, use the corresponding tagged
 release or commit URL.
 

3d_models/README.md

@@ -15,6 +15,10 @@ Two ways to get the mechanical set (same place in the build path):
 - **Print yourself** — use the **`.3mf` files in `parts/sg90/3mf/`**, **`parts/fs0307/3mf/`**, or **`parts/hd1370a/3mf/`**. Ready to print in an orientation that does not need supports. **PLA** or **PETG**. Start with [Print first](#print-first).
 - **No printer?** — order the aggregated sets from a third-party service: [order printed parts](../docs/3d/order-parts.md). Orders are at your own risk; fit can vary by provider and design revision.
 
+### Optional HD1370A bottom cover
+
+An optional [T-shaped underside cover](parts/hd1370a/bottom-cover/README.md) is available as an editable Fusion design and [printable 3MF](parts/hd1370a/3mf/BottomCover.3mf). It follows the desk/chair perimeter and has four recessed M2 head seats with a flat exterior. **Prototype: CAD geometry checked; physical fit and screw retention unverified.** HD1370A only; not included in the normal part-set exports.
+
 ### Print first
 
 **Home printers only.** Print these two small testers from that folder **before** the rest of the set. Do not print the full robot until both fit. Skip this step when ordering from a service (testers alone are rarely worth the min fee + shipping).

3d_models/cad/bottom-cover/README.md

@@ -0,0 +1,43 @@
+# HD1370A bottom-cover source
+
+See the [part documentation](../../parts/hd1370a/bottom-cover/README.md) for geometry, previews and the unresolved screw-retention limitation.
+
+`BottomCover_HD1370A.f3d` is the editable native cover. `BottomCover_FitCheck_HD1370A.f3d` contains the source desk/chair fixture and four reference screw envelopes. The master `../TinyEngineer.f3d` contains a hidden `OPTIONAL_BOTTOM_COVER_HD1370A` reference outside `PRINT_LAYOUT`; it must only be shown with the HD1370A preset.
+
+The fixed perimeter comes from `underside_profile.py`, serialized as exact lines/arcs in `underside-profile.json`. `source-alignment.json` records source occurrence transforms in the HD1370A frame. The source mating plane is mapped to Z=4.5 mm; the desk print export needs a 180° rotation about X and the chair a −59.3 mm translation in X.
+
+## Rebuild and check
+
+Use a clean checkout. Install the CAD dependencies into an isolated Python environment:
+
+```sh
+python3 -m venv /tmp/tiny-engineer-cad-venv
+/tmp/tiny-engineer-cad-venv/bin/pip install -r 3d_models/cad/bottom-cover/requirements.txt
+/tmp/tiny-engineer-cad-venv/bin/python 3d_models/cad/bottom-cover/build_and_check.py
+```
+
+This writes the canonical STEP/STL/3MF exports, the aligned fixture and geometric checks. It first produces a CadQuery mesh; the following Fusion step replaces the 3MF with the native export. Do not confuse the full inspection assembly with the single printable cover.
+
+In Fusion's Text Commands panel, use **Txt** mode and run these scripts in order, substituting the absolute checkout path:
+
+```text
+Python.RunScript "/absolute/checkout/3d_models/cad/bottom-cover/create_in_fusion.py"
+Python.RunScript "/absolute/checkout/3d_models/cad/bottom-cover/verify_in_fusion.py"
+Python.RunScript "/absolute/checkout/3d_models/cad/bottom-cover/present_in_fusion.py"
+Python.RunScript "/absolute/checkout/3d_models/cad/bottom-cover/integrate_master_in_fusion.py"
+```
+
+Use a fresh Fusion session for a full rebuild: the verification/presentation scripts select the documents created by the first script. `integrate_master_in_fusion.py` replaces the optional reference in the master, checks the original parameters and geometry before saving. It reopens the saved archive, verifies preservation, then checks the HD1370A fit in that temporary inspection copy without saving preset changes.
+
+Then run:
+
+```sh
+/tmp/tiny-engineer-cad-venv/bin/python 3d_models/cad/bottom-cover/verify_master_mates.py
+/tmp/tiny-engineer-cad-venv/bin/python 3d_models/cad/bottom-cover/inspect_perimeter.py
+/tmp/tiny-engineer-cad-venv/bin/python 3d_models/cad/bottom-cover/verify_print.py
+/tmp/tiny-engineer-cad-venv/bin/python 3d_models/cad/bottom-cover/verify_head_seats.py
+```
+
+The JSON files are CAD/mesh evidence, not physical load or thread-engagement tests. Previews are generated from the imported CAD and actual print-file section. The original HD1370A mating STEP files are unchanged from the baseline revision recorded in `provenance.json`.
+
+CAD sources, scripts and exports in these CAD directories are CERN-OHL-S-2.0; retain [NOTICE](../../NOTICE).

3d_models/cad/bottom-cover/build_and_check.py

@@ -0,0 +1,230 @@
+# SPDX-FileCopyrightText: 2026 Hanson Wen
+# SPDX-License-Identifier: CERN-OHL-S-2.0
+"""Rebuild and inspect HD1370A bottom cover with CadQuery; dimensions in mm."""
+from pathlib import Path
+import json, math
+import cadquery as cq
+import trimesh
+from underside_profile import derive_profile, wire_from_segments
+
+ROOT = Path(__file__).resolve().parents[3]
+OUT = ROOT / "3d_models/parts/hd1370a/bottom-cover"
+SOURCES = ROOT / "3d_models/parts/hd1370a/step"
+PARAMS = {
+    "thickness": 4.5,
+    "clearance": 0.3,
+    "hole_diameter": 2.3,
+    "head_diameter": 4.3,
+    "head_depth": 2.3,
+}
+HOLES = [(-30, -61.6), (26, -61.6), (-30, 61.6), (26, 61.6)]
+
+
+def common_volume(a, b):
+    return a.intersect(b).Volume()
+
+
+def mesh(s):
+    v, f = s.tessellate(0.025, 0.1)
+    return trimesh.Trimesh(vertices=[p.toTuple() for p in v], faces=f, process=True)
+
+
+def build():
+    OUT.mkdir(parents=True, exist_ok=True)
+    desk = (
+        cq.importers.importStep(str(SOURCES / "Desk.step"))
+        .val()
+        .rotate((0, 0, 0), (1, 0, 0), 180)
+        .translate((0, 0, 38.5))
+    )
+    chair = (
+        cq.importers.importStep(str(SOURCES / "Chair.step"))
+        .val()
+        .translate((-59.3, 0, 4.5))
+    )
+    source_wire, segments, source_edges, chair_rear = derive_profile(desk, chair)
+    (Path(__file__).parent / "underside-profile.json").write_text(
+        json.dumps(
+            {
+                "revision": 2,
+                "source": "Actual bottom planar rim of assembled HD1370A Desk and Chair",
+                "perimeter_offset_mm": 0,
+                "segments": segments,
+            },
+            indent=2,
+        )
+        + "\n"
+    )
+    outline = wire_from_segments(segments)
+    p = cq.Workplane("XY").newObject(
+        [cq.Solid.extrudeLinear(outline, [], cq.Vector(0, 0, PARAMS["thickness"]))]
+    )
+    for x, y in HOLES:
+        p = p.cut(
+            cq.Workplane("XY")
+            .center(x, y)
+            .circle(PARAMS["hole_diameter"] / 2)
+            .extrude(PARAMS["thickness"])
+        )
+        p = p.cut(
+            cq.Workplane("XY")
+            .center(x, y)
+            .circle(PARAMS["head_diameter"] / 2)
+            .extrude(PARAMS["head_depth"])
+        )
+    plate = p.val()
+    cq.exporters.export(plate, str(OUT.parent / "step/BottomCover.step"))
+    # Print top mating face on bed; counterbores open upward, with no bridges.
+    printable = plate.rotate((0, 0, 0), (1, 0, 0), 180).translate((0, 0, 4.5))
+    m = mesh(printable)
+    m.metadata["units"] = "mm"
+    m.export(str(OUT.parent / "stl/BottomCover.stl"))
+    (OUT.parent / "3mf/BottomCover.3mf").write_bytes(
+        trimesh.exchange.threemf.export_3MF(trimesh.Scene(m))
+    )
+    desk = (
+        cq.importers.importStep(str(SOURCES / "Desk.step"))
+        .val()
+        .rotate((0, 0, 0), (1, 0, 0), 180)
+        .translate((0, 0, 38.5))
+    )
+    chair = (
+        cq.importers.importStep(str(SOURCES / "Chair.step"))
+        .val()
+        .translate((-59.3, 0, 4.5))
+    )
+    top = (
+        cq.importers.importStep(str(SOURCES / "DeskTop.step"))
+        .val()
+        .translate((0, 0, 40.0))
+    )
+    pad = (
+        cq.importers.importStep(str(SOURCES / "DeskPad.step"))
+        .val()
+        .translate((-2.0, 0.0000082699, 38.5))
+    )
+    ass = cq.Assembly(name="BottomCover_FitCheck_HD1370A")
+    for n, s, c in [
+        ("BottomCover", plate, (0.08, 0.6, 0.72)),
+        ("Desk", desk, (0.35, 0.35, 0.38)),
+        ("Chair", chair, (0.48, 0.48, 0.5)),
+        ("DeskTop", top, (0.28, 0.28, 0.3)),
+        ("DeskPad", pad, (0.65, 0.5, 0.28)),
+    ]:
+        ass.add(s, name=n, color=cq.Color(*c))
+    ass.export(str(OUT / "BottomCover_FitCheck.step"))
+    # Normalize exporter whitespace without changing STEP records.
+    for path in [
+        OUT.parent / "step/BottomCover.step",
+        OUT / "BottomCover_FitCheck.step",
+    ]:
+        path.write_text(
+            "\n".join(line.rstrip() for line in path.read_text().splitlines()) + "\n"
+        )
+    report = {
+        "parameters_mm": PARAMS,
+        "outline_segments_mm": segments,
+        "revision": 2,
+        "hole_centres_mm": HOLES,
+        "scope": "HD1370A only; CAD verified, physical print not yet fit-tested",
+        "assembly_transforms": {
+            "Desk": "rotate X 180 deg; translate (0,0,38.5) mm",
+            "Chair": "translate (-59.3,0,4.5) mm",
+            "DeskTop": "translate (0,0,40.0) mm",
+            "DeskPad": "translate (-2,0.0000082699,38.5) mm",
+        },
+        "valid_solid": plate.isValid(),
+        "solid_count": len(plate.Solids()),
+        "mesh_watertight": m.is_watertight,
+        "mesh_winding_consistent": m.is_winding_consistent,
+        "mesh_euler_number": int(m.euler_number),
+        "volume_mm3": plate.Volume(),
+        "bounds_mm": m.bounds.tolist(),
+        "part_interference_mm3": {},
+        "head_checks": [],
+    }
+    for n, s in [("Desk", desk), ("Chair", chair), ("DeskTop", top), ("DeskPad", pad)]:
+        report["part_interference_mm3"][n] = common_volume(plate, s)
+    # M2 conservative envelope from native source: 4 mm diameter x 2 mm head.
+    for x, y in HOLES:
+        head = cq.Solid.makeCylinder(2.0, 2.0, cq.Vector(x, y, 0.3))
+        shaft = cq.Solid.makeCylinder(1.0, 6.0, cq.Vector(x, y, 2.3))
+        report["head_checks"].append(
+            {
+                "centre_mm": [x, y],
+                "head_to_floor_clearance_mm": 0.3,
+                "radial_head_clearance_mm": 0.15,
+                "radial_shank_clearance_mm": 0.15,
+                "remaining_plate_bearing_mm": 2.2,
+                "M2x6_insertion_into_desk_mm": 3.8,
+                "head_plate_interference_mm3": common_volume(plate, head),
+                "shank_plate_interference_mm3": common_volume(plate, shaft),
+                "shank_desk_interference_mm3": common_volume(desk, shaft),
+            }
+        )
+    # Compare the finished cover's actual mating-face exterior to the source
+    # underside, independently of the chosen hole geometry.
+    mating = (
+        max(
+            [
+                f
+                for f in plate.Faces()
+                if abs(f.BoundingBox().zmin - 4.5) < 1e-6
+                and abs(f.BoundingBox().zmax - 4.5) < 1e-6
+            ],
+            key=lambda f: f.Area(),
+        )
+        .outerWire()
+        .translate((0, 0, -4.5))
+    )
+    expected_face = cq.Face.makeFromWires(source_wire)
+    actual_face = cq.Face.makeFromWires(mating)
+    overhang = actual_face.cut(expected_face).Area()
+    undercoverage = expected_face.cut(actual_face).Area()
+    dev = max(
+        mating.distance(cq.Vertex.makeVertex(*e.positionAt(i / 100).toTuple()))
+        for e in source_wire.Edges()
+        for i in range(101)
+    )
+    report["perimeter_fit"] = {
+        "perimeter_offset_mm": 0,
+        "excess_footprint_area_mm2": overhang,
+        "missing_footprint_area_mm2": undercoverage,
+        "max_sampled_source_edge_deviation_mm": dev,
+        "copied_desk_edge_count": len(source_edges),
+        "radii_mm": [s["radius_mm"] for s in segments if s["type"] == "CIRCLE"],
+        "square_rear_corners_mm": [[-75.3, -35.6], [-75.3, 35.6]],
+        "rear_seam_bridges_mm": [0.3, 0.3],
+        "note": "Straight rear closure coincides with chair and desk rear planes and bridges only their two existing 0.3 mm assembly seams.",
+    }
+    assert overhang < 1e-6 and undercoverage < 1e-6 and dev < 1e-6
+    # Exact STEP round trip plus print-3MF checks.
+    reread = cq.importers.importStep(str(OUT.parent / "step/BottomCover.step")).val()
+    scene = trimesh.load_scene(OUT.parent / "3mf/BottomCover.3mf")
+    report["step_roundtrip_valid"] = reread.isValid()
+    report["step_roundtrip_volume_error_mm3"] = abs(reread.Volume() - plate.Volume())
+    report["3mf_mesh_count"] = len(scene.geometry)
+    report["3mf_bounds_mm"] = scene.bounds.tolist()
+    assert (
+        plate.isValid()
+        and len(plate.Solids()) == 1
+        and m.is_watertight
+        and m.euler_number == -6
+    )
+    assert all(abs(v) < 1e-6 for v in report["part_interference_mm3"].values())
+    assert all(
+        abs(h[k]) < 1e-6
+        for h in report["head_checks"]
+        for k in [
+            "head_plate_interference_mm3",
+            "shank_plate_interference_mm3",
+            "shank_desk_interference_mm3",
+        ]
+    )
+    (OUT / "verification.json").write_text(json.dumps(report, indent=2) + "\n")
+    print(json.dumps(report, indent=2))
+    return plate, desk, chair
+
+
+if __name__ == "__main__":
+    build()