Spur gear, module 1, 21 T, profile shift -0.46, Ø6 bore (meshes a 30 T at 25 mm) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/spur-gear-m1-21t-xm046-b6/  (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 22.0, width 22.06, height 8.0
- Module 1, 20° involute, 21 teeth with a negative profile shift of -0.46 m: thinner teeth, tip at Ø22.08, so that it meshes with an unshifted 30-tooth gear at 25 mm instead of its natural 25.5 mm. Below the base circle (Ø19.73) the flank is radial; the mating idler's tips are shortened to stay clear of it. 8 mm face, Ø6 bore.
- Xem Studio design following Brown's figure; true size, no printing allowance (apply your own to the mating features).

dimension  value
module  1
teeth  21
profile shift  -0.46 m
tip Ø  22.08 mm
centre distance with 30 T  25 mm
bore  Ø6

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. 504: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_504.html
- Xem Studio: drawing — web — Publisher's page; dimensions are facts — https://xemstudio.com/objects/fergusons-paradox-mechanism/

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/spur-gear-m1-21t-xm046-b6/index.json
API: https://xemstudio.com/api/geometry?object=spur-gear-m1-21t-xm046-b6