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FreeCAD AI Modeling Guide

Redrawing STEP and STL

A vendor STEP is a pile of faces — there is no feature to grab when you want to change a single hole diameter. An STL is worse: nothing but triangles. Tell the AI “redraw it” and it measures the original, stacks the same shape into a new Body out of sketches, Pads, Grooves, Pockets and chamfers, then takes a boolean difference against the original to show you where it is wrong. The procedure is the same for STEP and STL; only the verification differs.

Things you can ask for

Section titled “Things you can ask for”
Say this What you get
“Rebuild this STEP part parametrically” Face classification → band heights → section tracing → feature stacking → boolean comparison against the original. A clamp jaw (44 faces) came out as 12 features, 0.0004 % volume difference
“Redraw this STL” The reference mesh is shown transparent and section circles are fitted per band, same procedure. A spool guide (5,758 faces) landed every vertex within 0.03 mm, 0.014 % volume difference, M44.5×2 male thread included
“Show me how far it is from the original” STEP: volume, area and bbox differences plus the location of missing and extra fragments, with a verdict (identical · match · different). STL: deviation max and p95 and the worst point
“Rebuild all 80 parts of this STEP assembly” One Body per part type, Link instances everywhere else. 20 types and 80 instances placed and grounded in 3.5 minutes, zero interference
“I just want to add holes on top of the original” No redraw — the original goes in as a BaseFeature and only Pockets are stacked on top (for parts with a lot of free-form surface)

A representative prompt

Section titled “A representative prompt”
Say this
Rebuild Part__Feature004 (the vendor clamp jaw) that's open in the Unnamed document as a parametric Body.
Build it in a new document called jaw_rebuild, and
- first show me the face classification (main axis, band heights, radii)
- then list which feature you'll treat each band as
- and stack it only after I confirm.
When it's done, compare against the original and tell me the verdict and where the wrong fragments are.

Once you confirm and it stacks and compares, you get a report like “verdict identical, volume difference +0.035 mm³ (0.0004 %), all 12 sketches at DoF 0”. If you get different, you also get the bboxes of the missing and extra fragments, so you only fix the feature at that location.

How the procedure runs

Section titled “How the procedure runs”
  1. Face classification — each face is judged plane, cylinder, cone, sphere or free-form. Faces that arrived as BSplines in the STEP get sampled and fitted, so “fake free-form” faces are filtered out. If there really is a lot of free-form surface, the AI proposes the hybrid approach (features on top of the original).
  2. Band heights — it reads the heights along the axis where the section changes. Only the datum points are read from vertices; the rest are tidied up to design values.
  3. Section tracing — for each band, the section outline is extracted as sketch lines, arcs and circles and padded. The coordinates never travel through the conversation, which saves tokens.
  4. Revolved grooves, holes, chamfers — ears that the revolved cut removed get padded back. Holes come from the hole analysis; chamfers from an edge radius filter.
  5. Comparison — fuzzy boolean difference for STEP, normal-ray deviation for STL. Either way you get coordinates for where it is wrong.

Things to know

Section titled “Things to know”
  • Radii are used exactly as measured. Round 33.620391 to 33.6204 and the mating faces are off by 9e-6, and the fuse loses 0.6 mm³. Leave the strange decimals in the parameters. Only the datum points get tidied to design values.
  • Check whether an STL is a closed mesh before trusting its volume. When the AI says is_solid is false, trust only the deviation comparison and treat the volume difference as informational.
  • Assemblies get one Body per part type. A STEP’s Placement is not the instance position, so the AI finds the rigid transform from the geometry and places Links. If mirrored instances are mixed in, it tells you separately.
  • Threads are not in a STEP. It only estimates M3 tapped or M6 clearance from the hole diameter. If an STL has actual thread geometry, it reads the pitch, major diameter and root and rebuilds it as a helix.
  • Separated Pads cannot go in one Body. That is why it stacks lower band → middle → upper band, overlapping.
  • You have to say “save it”.
Section titled “Related exercises”

What runs behind the scenes

Section titled “What runs behind the scenes”
Tool What it does
import_step · import_mesh Reads STEP/IGES and STL/OBJ/PLY/3MF (meshes become transparent reference objects)
classify_faces · analyze_mesh Face identity, main axis, band heights, radii / per-band section circles and threads for meshes
section_profile The section outline at a height, as sketch elements (STEP and mesh alike)
build_features Pad/Pocket/Groove/Chamfer/helix in order, tracing the original directly via profile.section
compare_shapes Boolean fragment bboxes (STEP) / ray deviation max and p95 (STL)
align_shapes Rigid transform between instances of the same part → Link placement