mesh2cad

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Turn 3D meshes (STL/OBJ/PLY) into real, editable CAD (STEP) — fitted planes, cylinders, spheres, and cones instead of triangle soup. CLI + MCP server for AI agents. MIT, pure Python on top of OpenCascade.

Left: 21k-triangle mesh with 0.1 mm noise. Right: the STEP mesh2cad writes — 10 planes + 2 cylinders, max deviation 0.05 mm from the true part.

Install

pip install mesh2cad              # CLI + STEP
pip install "mesh2cad[mcp]"       # + MCP server
pip install "mesh2cad[cadquery]"  # + run/verify generated CadQuery scripts

Use

mesh2cad convert part.stl -o part.step --tol 0.1 --report report.md
mesh2cad report part.stl --json

--tol ≈ the noise / accuracy of your mesh in model units (default 0.1 % of the bounding-box diagonal).

Output: STEP + fidelity report — method (exact / faceted-hybrid / faceted), face types, solid validity, two-sided Hausdorff / RMS deviation, volume error, % area per surface type.

Editable parametric CadQuery script

For simple prismatic parts mesh2cad can also write a CadQuery Python script where every dimension is a named parameter, so you change a number and re-run instead of editing B-rep faces:

mesh2cad convert bracket.stl --cadquery bracket.py   # add -o bracket.step to also write STEP
python bracket.py                                    # -> bracket.step

# --- parameters ---
base_x           = 40.0  # base length (X)
base_y           = 30.0  # base width (Y)
base_t           = 6.0  # base thickness (Z)
...
hole_1_d         = 6.0  # diameter
hole_1_x         = 0.0
hole_1_y         = 0.0
hole_1_csk_d     = 12.0  # countersink diameter
hole_1_csk_angle = 90.0  # countersink angle

Make the hole 2 mm bigger: set hole_1_d = 8.0 and run it again.

Supported: Z-up box/plate base, rectangular pads on the top face, vertical holes (through, blind, countersunk) and vertical bosses (straight, drafted, chamfered). Values are rounded to clean numbers (5.997 → 6.0) when that stays within --tol. Before writing, the script is executed and compared with the mesh (needs cadquery installed); if the part is outside this vocabulary or the script deviates, nothing is written and the command exits with code 2 and the reason. Use STEP output for everything else.

MCP server (Claude Desktop, Cursor, …)

{ "mcpServers": { "mesh2cad": { "command": "mesh2cad", "args": ["mcp"] } } }

Tools: convert(input_path, output_path?, tol?, cylinders?, spheres?, cones?), analyze(input_path, tol?, cylinders?, spheres?, cones?), report(input_path, tol?, cylinders?, spheres?, cones?), to_cadquery(input_path, output_path?, tol?) (returns the parameters and measured deviation). spheres and cones default on.

Listed on Glama and the TensorBlock MCP catalog.

How it works

1. Load + clean mesh (trimesh); Taubin-smoothed copy for normals.
2. Region growing into planes; strip chains and leftovers are fitted as cylinders, spheres, or cones (normal-ray / algebraic / constant-normal-angle fits, robust nonlinear refinement). A coaxial cylinder + cone component (countersink, chamfered pin) is split by normal-to-axis angle first. Small noisy patches that tile a sphere are merged into one spherical region. Split/redundant regions are merged and absorbed.
3. Exact B-rep (OCP / OpenCascade): plane polygons from triple-plane intersections, circles from plane ∩ cylinder and plane ∩ sphere, plane ∩ cone, cylindrical faces, spherical caps and zones, conical frustums, sewn into a solid and validated.
4. Fallbacks: hybrid faceted solid (vertices snapped to fitted surfaces, coplanar triangles merged), then raw faceted solid.
5. Fidelity report against the input mesh.

Self-test benchmark

Ground-truth cadquery parts → tessellated + subdivided (≤ 2 mm edges) → Gaussian vertex noise → mesh2cad → compared with the truth. 3 seeds, worst case shown (python benchmarks/run.py); the same cases run in CI.

part	noise σ (mm)	exact	faces (plane/cyl[/sph[/cone]])	Hausdorff (mm)	RMS (mm)	vol err %	time s
cube	0.0	3/3	6/0	0.000	0.000	0.00	0.6
cube	0.05	3/3	6/0	0.019	0.006	0.07	0.6
cube	0.2	3/3	6/0	0.075	0.026	0.34	0.6
l_bracket	0.0	3/3	8/0	0.000	0.000	0.00	0.9
l_bracket	0.05	3/3	8/0	0.040	0.006	0.03	0.9
l_bracket	0.2	3/3	8/0	0.216	0.026	0.15	0.8
stepped_block	0.0	3/3	11/0	0.000	0.000	0.00	2.0
stepped_block	0.05	3/3	11/0	0.032	0.006	0.02	2.0
stepped_block	0.2	3/3	11/0	0.136	0.022	0.07	1.9
plate_with_hole	0.0	3/3	6/1	0.015	0.002	0.00	1.7
plate_with_hole	0.05	3/3	6/1	0.028	0.006	0.05	1.9
plate_with_hole	0.2	3/3	6/1	0.078	0.023	0.18	1.6
boss	0.0	3/3	7/1	0.027	0.006	0.01	1.8
boss	0.05	3/3	7/1	0.033	0.007	0.03	2.0
boss	0.2	3/3	7/1	0.090	0.017	0.07	3.3
ribbed_plate	0.0	3/3	10/2	0.013	0.002	0.00	2.2
ribbed_plate	0.05	3/3	10/2	0.028	0.005	0.03	2.1
ribbed_plate	0.2	3/3	10/2	0.108	0.020	0.12	2.1
spherical_dome	0.0	3/3	6/0/1	0.065	0.007	0.04	1.6
spherical_dome	0.05	3/3	6/0/1	0.067	0.008	0.06	1.6
spherical_dome	0.2	3/3	6/0/1	0.241	0.028	0.25	1.7
spherical_pocket	0.0	3/3	6/0/1	0.046	0.004	0.02	1.9
spherical_pocket	0.05	3/3	6/0/1	0.057	0.005	0.03	1.9
spherical_pocket	0.2	3/3	6/0/1	0.075	0.015	0.13	1.8
countersunk_plate	0.0	3/3	6/1/0/1	0.039	0.003	0.01	2.0
countersunk_plate	0.05	3/3	6/1/0/1	0.055	0.009	0.07	2.0
countersunk_plate	0.2	1/3	16342/0, 6/1/0/1	0.590	0.049	0.54	21.6
tapered_boss	0.0	3/3	7/0/0/1	0.086	0.011	0.05	1.6
tapered_boss	0.05	3/3	7/0/0/1	0.095	0.012	0.05	1.6
tapered_boss	0.2	3/3	7/0/0/1	0.123	0.025	0.22	1.5
chamfered_boss	0.0	3/3	7/1/0/1	0.053	0.005	0.02	2.4
chamfered_boss	0.05	3/3	7/1/0/1	0.096	0.012	0.08	2.3
chamfered_boss	0.2	3/3	7/1/0/1	0.511	0.059	0.49	2.1

"exact" = valid B-rep with the true face count. All rows are the worst of 3 seeds from python benchmarks/run.py. A row below 3/3 includes the faceted fallback (face count = triangles); that fallback is a valid solid. The 90° countersink (Ø6 hole, Ø12 head, 3 mm deep) at 0.2 mm noise is exact on 1 of 3 seeds. CI checks the cases that are exact, plus one fallback that must stay a valid solid.

Limits (v0.4.2)

Planes, cylinders, spheres, and cones. Cylinders must end on planes perpendicular to their axis (through-holes, bosses, pins). Spheres are exact when each boundary loop is a single plane ∩ sphere circle (domes, spherical bosses, spherical pockets, and zones between two parallel planes). A sphere that runs into an edge or a cylinder falls back to the hybrid solid — vertices are still snapped onto the fitted sphere, but the face stays faceted. Cones are exact when they run between two planes perpendicular to their axis: countersinks, chamfered holes and bosses, drafted/tapered bosses. Cones may also meet a coaxial cylinder (countersunk hole, chamfered pin). A cone that ends in its apex falls back to the hybrid solid. Countersinks and narrow chamfers are exact through about 0.05 mm of noise; a 2 mm chamfer and a wide drafted wall stay exact at 0.2 mm. The countersunk plate's 90° countersink (Ø6 hole, Ø12 head, 3 mm deep) at 0.2 mm noise is exact on one of three seeds. On the other two the conical band and the hole wall meet at a single vertex, so those seeds fall back to a valid faceted solid. No fillets or free-form yet.

Development

pip install -e .[dev]
pytest
python docs/make_demo.py   # regenerate images (needs matplotlib, pillow)

License: MIT