Wheel for a face spiral, module 1.5, 24 teeth, generated by the scroll at radius 39 — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/face-spiral-wheel-m1-5-24t-for-scroll-r39/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown's figure (1868); true size, no printing allowance
Catalog version: 2026-10-20

Overall (mm): length 38.85, width 38.85, height 8.0
- 24-tooth wheel of module 1.5 (pitch Ø36, tip Ø39, root Ø32.25, 8 mm face, Ø6 bore) whose teeth were generated by the spiral rib itself: in every plane across the face the rib's section (a rack tooth that is slightly wider and farther out away from the disc's centre line) was swept past the turning wheel and the envelope taken, then the seven sections lofted (spline fit within 0.108 mm). The result is an involute-like tooth that is very slightly curved and thinner across the face — the exact mate of the rib at radius 39, where a plain straight spur gear would bind at the face edges.
- One tooth per turn of the disc; the wheel's axis lies parallel to the disc face, tangent to the spiral at the mesh.
- Xem Studio design following Brown's figure; true size, no printing allowance (apply your own to the mating features).

dimension  value
module  1.5
teeth  24
pitch Ø  36 mm
tip / root Ø  39 / 32.25 mm
face  8 mm
bore  Ø6
mesh radius on the disc  39 mm

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Henry T. Brown (1868, public domain): Five Hundred and Seven Mechanical Movements — scan on archive.org — web — Publisher's page; dimensions are facts — https://archive.org/details/fivehundredseven00browiala
- 507movements.com: Brown's movement No. 29: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_029.html
- Xem Studio: drawing — web — Publisher's page; dimensions are facts — https://xemstudio.com/objects/face-spiral-disc-and-wheel-mechanism/

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/face-spiral-wheel-m1-5-24t-for-scroll-r39/index.json
API: https://xemstudio.com/api/geometry?object=face-spiral-wheel-m1-5-24t-for-scroll-r39