Mechanical Reverse Engineering Assistant

Turn an STL into an editable, manufacturable CAD model — by recovering the design intent, not by wrapping triangles.

This is not an STL-to-STEP converter. It's a desktop reverse-engineering workbench that looks at a mesh the way a mechanical engineer would: it finds the planes, cylinders, holes, pockets, bosses and walls the original designer intended, rebuilds them as true analytic B-Rep geometry (real circles, real arcs — never tessellation), and exports a clean AP242/AP214 STEP you can machine, laser-cut, or redesign.

Real result

The part that drove development: the backplate of an M5Stack Tab5, scanned from the product assembly STL.

Route	Quote (SendCutSend, qty 1)
3-axis CNC machining of the faithful reconstruction	$1,188
Flat sheet version produced by this tool + $3 of standoffs	$12.05

The sheet function measured the part's own towers (3.2 mm, Ø5.7), dropped all depth features, kept the profile and every through-hole as true arcs, and wrote a hole schedule telling the shop which holes serve which screws.

What it does

• Stage 1 – Repair: weld duplicate vertices, fix winding/normals, fill small holes, split multi-body assemblies (with a body picker in the GUI).
• Stage 2 – Recognize: segment the mesh into analytic surfaces (plane / cylinder / cone / sphere) with least-squares fits, normal-consistency gates, and per-patch confidence. Handles 100k+ triangle meshes in seconds.
• Stage 3 – Intent: infer the engineering features — base extrusion, holes (with pattern detection), pockets/terraces, pads/towers, fillets, wall thickness, mirror symmetry. Nearly-equal dimensions snap to common values; ambiguity becomes a question, never a silent guess.
• Stage 4 – Interactive: hole wizard (assign M2–M4 clearance / cut-tap / roll-tap purposes, or fill holes in), machining-cost savings menu (drop cosmetic recesses, eliminate whole CNC setups with tradeoffs spelled out), numbered feature list with click-to-highlight.
• Stage 5 – Rebuild: true B-Rep solids via OpenCascade — profile wires with recovered arcs and rounded rectangles, analytic cylinder holes, terrace pockets with islands, prismatic ribs for molded blends.
• Stage 6 – Validate: closed-solid check, open edges, slivers, volume cross-checked against the scan.
• Stage 7 – Export: STEP AP242/AP214 plus a machinist-facing *_holes.txt schedule (positions, diameters, thread purposes — because STEP geometry cannot carry thread callouts).
• Sheet Version: one click flattens any plate-class part to a laser/waterjet-cuttable sheet at your stock gauge.
• DFM cost model: per-feature machining cost estimate (setups and CAM programming dominate one-offs, not cutting time) with quantity amortization.

Install

Requires Python 3.12+ on Windows/Linux/macOS.

python -m venv .venv
.venv/Scripts/pip install -r requirements.txt   # Windows
# .venv/bin/pip install -r requirements.txt     # Linux/macOS

Run

python -m mra

(Windows: double-click MRA.bat.)

Workflow: Import STL → (Isolate Body) → Repair Mesh → Recognize Features → Reconstruct CAD (wizards ask about holes and cost tradeoffs) → Validate → Export STEP — or Sheet Version for the flat-cut variant.

Tests

.venv/Scripts/python -m pytest tests -q

85 tests cover every stage on synthetic parts built with boolean CSG, plus STEP round-trips verifying the output is analytic (real CIRCLE entities, never point soup).

Architecture

mra/
  core/            Datatypes: patches, features, tolerances, questions, loops
  meshproc/        Stage 1 — import, repair, stats, body splitting
  recognition/     Stage 2 — segmentation + surface fitting
  intent/          Stage 3 — feature/intent recovery
  reconstruction/  Stage 5 — B-Rep building, bridges, fillets, sheet
  validation/      Stage 6 — solid health checks
  export/          Stage 7 — STEP writer + hole schedules
  dfm.py           Machining cost model & simplification transforms
  gui/             PySide6 workbench (viewport, wizards, dialogs)

Every non-GUI module is importable and testable headless. PROGRESS.md is the full development log, including the dead ends and why they died.

Honest limitations

• Optimized for the extruded plate / enclosure part class (plates, brackets, covers, housings). Organic shapes reconstruct as honest FREEFORM regions and questions, not fake precision.
• Molded blends/gussets are converted to simple machinable ribs by design ("prefer manufacturable geometry over triangle fidelity").
• Complex thin-wall molded shells (wraparound enclosure bodies) are not yet reconstructable — the tool tells you so instead of producing garbage.

License

MIT — see LICENSE. Built for the makers.