Exercise 5. Redrawing an STL spool guide

Item	Detail
Input file	05-spool-guide.stl (a filament spool guide, 5,758 faces, with a male thread)
Time	5 min (25 s for analysis, rebuild and comparison together)
What you end up with	A document stl_test with the reference mesh plus a Body SpoolGuide (8 features, 1 thread, a parameter sheet)
What you learn	That a mesh (STL) has no faces and no solid, how to read bands, section circles and threads by measurement, and how to verify by deviation without booleans

An STL is a pile of triangles, so shape tools like hole finding and interference checking cannot read it. That makes the procedure different from STEP. You bring the mesh in as a transparent reference, fit circles to sections at various heights to get dimensions, draw a Body over it, and verify by surface deviation. In the original session all 2,863 mesh vertices landed within 0.03 mm of the model surface (0.003 mean), with a 0.014 % volume difference.

Original and finished

Original 05-spool-guide.stl read in as a Mesh object, isometric (pale because it is the default mesh colour) — 5,758 triangles

Finished SpoolGuide — 8 features (thread included), DoF 0, +0.014 % volume difference, maximum mesh vertex deviation 0.028 mm

	File	Description	Size
Original	05-spool-guide.stl	STL	288 KB
Finished	05-spool_guide-done.FCStd	Reference mesh included	327 KB
Finished	05-SpoolGuide-done.step	STEP	167 KB

The finished files are real

They were produced by running this page’s steps on FreeCAD 1.1.3, and the images are actual screen captures of those files opened in the FreeCAD 1.1.3 GUI (isometric, white background). Download them and you can look at the tree, the sketches and the Params directly.

Setup

1. Launch FreeCAD 1.1.3. You want [CAD X-ray] 서버 시작 in the Report view.
2. Put the STL at C:\cad\05-spool-guide.stl. The original STL’s name had spaces, but the release asset is short, so use it exactly as downloaded.
3. In PowerShell, move to C:\cad and open claude.
4. The AI creates the document. Close any that are already open.

Steps

1. Bring the mesh in and get the analysis table first

Say this

Load C:\cad\05-spool-guide.stl into a new document stl_test as a reference mesh and analyse it: closed or not, self-intersections, size, main axis, band boundaries, section circles per band, the common centre, the thread (pitch, major, root, handedness), and any section segments that don't fit lines or circles. Values read and assumed, as a table.

What the AI does — inserts the STL as a Mesh object at 70 % transparency without converting it. It checks closure (is_solid), non-manifold geometry and self-intersections plus bbox and volume, then finds the main axis (Z) and the band boundaries from planar patches and fits a least-squares circle to the section polyline at the mid-height of each band (within 0.01 mm of radius). When the angle of the maximum section radius rotates in proportion to z it judges it a thread (180° per 1 mm → pitch 2; in the original session this corrected an initial misreading of the feature as a cam lobe). The four arc slots and four windows are arcs plus lines rather than circles, so it detects corners in the z=1 section, fits arcs and lines per segment, and counts anything that does not fit as unsupported.

What the AI says back

Mesh added (transparency 70). 5,758 faces, 2,863 vertices, closed, no self-intersections, size Ø201 × 36, volume 56,195 mm³. Main axis Z. Band boundary z: 0 / 1.97 / 4.97 / 27.97 / 35.97. Common centre (0.54, 77.38). Section circles: plate R100.49, centre hole R18, rim wall R99.48–100.48 (1 wide, 3 tall), hub ring R18–25 (3 tall), hub R18.49–22.39 (z 5–28). Thread: major r 22.256, root r 21.038, pitch 2.006, z 28–36, right-hand → design values Ø44.5 / Ø42.1 / pitch 2 / length 8. z=1 section: four arc slots (r 67.5–87.5, 20-wide spokes, R10 outer corners), four windows (r 32.5–55). unsupported 0. Assumed: radii tidied to design values (integers and halves) within 0.01, thread root width 0.62 with a 60° V.

How to check — a semi-transparent mesh in the 3D view, and a table with five band boundaries, a thread pitch of 2 and unsupported 0. If is_solid is false, the AI warns you that the volume and deviation figures that follow cannot be trusted. If it says the axis is not Z, the STL was saved lying down: say “treat the axis as Z and redo it”, or ask for the Placement to be rotated.

2. Have it build the Body, thread included

Say this

Build a Body SpoolGuide from those values, thread included. Every dimension in Params, verify each feature and stop if one fails.

What the AI does — stacks four Pads and two Pockets band by band (plate → slot and window through-cuts → rim wall and hub ring → hub → thread cylinder), transferring the slot and window fit elements straight into sketches. Every circle is centred on the mesh’s common centre. For the thread it places the root section (a full pitch) on a plane through the axis and applies a helical cut. Because the helix starts at profile angle 0° and its first turn cuts nothing, it starts one pitch below (height = length + 2 pitches) and pads the z 25–28 ring wrongly cut into the hub back as PadRefill.

What the AI says back

PadPlate → PocketSlots → PocketWindows → PadRings → PadHub → PadThreadCyl → ThreadHelix → PadRefill. All 8 Valid, every sketch at DoF 0. Volume 56,202 mm³, +0.013 % against the mesh.

How to check — look in the 3D view at whether the mesh and the Body overlap. If the Body pokes outside the transparent mesh, or the mesh shows outside the Body, that band’s radius is wrong. Zoom into the thread cylinder and count four turns across the 8 mm length, and check there is no cut mark below the hub top face.

3. Verify by deviation and save

Say this

Compare the reference mesh with SpoolGuide: volume difference, surface deviation (max, mean, p95) and where the worst deviation is. If it's a match, show me iso and save to C:\cad\spool_guide.FCStd.

What the AI does — no booleans. It measures the distance from sample points on the Body’s faces along their normals to the mesh (deviation), and from the mesh vertices back to the Body (reverse deviation). The location of the worst deviation is exactly where you went wrong.

Never convert the mesh to a solid to compare

Run a boolean difference against a solid built by makeShapeFromMesh and the boolean holds FreeCAD’s main thread for over ten minutes and cannot be interrupted. Always have the comparison take the mesh by name.

What the AI says back

Volume difference +0.013 %. Deviation (740 samples): p95 0.044 mm, mean 0.021, max 0.89 — the large values are only four points at the thread start (around z 28). Reverse (2,863 vertices): max 0.028, mean 0.003, p95 0.008. Verdict match. Saved.

How to check — match is a success. A few large values at the thread start come from the helix start phase and are expected.

Checking your result

Criterion	Value
Features	4 Pads + 2 Pockets + 1 thread + 1 refill Pad, all Valid
Sketch DoF	all 0
Volume difference	0.014 %
Surface deviation	all 2,863 vertices within 0.03 mm (0.003 mean), only 3 points at the thread start at 0.4–0.9
Thread	male Ø44.5 × pitch 2 × length 8

When it doesn’t work

• is_solid is false — the mesh has holes. Get an STL closed by a slicer or a mesh repair tool. The volume and deviation figures cannot be trusted.
• FreeCAD froze — a boolean was run against a mesh converted to a solid. Wait for it or force-quit. When comparing, say explicitly “use the mesh by name”.
• A band radius is off against the mesh — the section circle fit value got rounded instead of used as-is. Ask again with “use the fit values exactly”.
• The thread ends up outside the cylinder, or runs the wrong way — a radius or z sign problem in the root section profile. Pin the coordinate system down: “the thread profile is radius 21.05–22.25 from the axis, and z is measured from the hub top face”.
• No thread is detected — the thread is too shallow or too short for the section to read as anything but a circle. Just state the pitch and major diameter yourself.

Going further

1. Change Params.thread_pitch to 1.5 and recompute. See whether the helix height (length + 2 pitches) and the refill Pad follow.
2. Run a print review on this Body in PETG with a 0.4 nozzle (Exercise 8). See how the thin 1 mm rim wall is judged.
3. Shrink the plate to Ø180 (Params.plate_r) to fit a smaller printer bed.

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What runs behind the scenes
Reading	import_mesh, analyze_mesh
Measuring	section_profile (mesh input, line/arc/circle fitting)
Building and verifying	build_features (wires, helix op), compare_shapes (mesh input, ray deviation)
Checking	get_screenshot, save_document

Original session log: https://github.com/WaveSimm/cadxray/blob/master/examples/stl_spool_guide_with_m7.py