Face spiral on a disc as a drill feed: scroll driving a rack — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/face-spiral-rack-feed-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 99 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- Resize: multiply every length by one factor k and the motion stays identical: angles, ratios, timing and the formulas below do not change, because a mechanism's motion depends only on the ratios of its lengths. Here, scales with k: feed per turn, rack height over the face; stays the same: disc, rack.
- Resize: fit clearances stay in mm (a slide fit is the same gap at any size); keep every printed wall and pin at least two nozzle widths; mass grows with k³ but part strength only with k², so at k = 2 parts are about 8× heavier and 4× stronger: check pins and thin links when scaling up.
- A spiral rib on the face of a disc engages a rack that slides radially over it: every turn of the disc moves the rack one pitch (3.14 mm) outward or inward — a slow, smooth feed from a continuously turning disc, the scroll-chuck principle used for the feed motion of drilling machines (the rack is on the drill's spindle sleeve).
- The rib's radial section is the module 1 basic rack; the feed rack's teeth were generated by the rib across the rack's 8 mm width, so they curve to follow the spiral and mesh without binding. With this 8.4-turn scroll (r 20 → 46.39) the 6-tooth rack stays fully on the rib over about ±3 mm of travel about the drawn position; a longer scroll gives a longer stroke. Checked at six positions over two turns: no overlap beyond contact.
- Brown's No. 99 (spiral guide on the face of a disc, drilling-machine feed). Drawn with the disc turned 90°, the rack's centre 31.78 mm from the disc's axis; the rack's guide is not drawn.
- Parts (each has its own page): face-spiral-disc-m1-r20-8-4turns-d100, face-spiral-feed-rack-m1-6t-w8, linear-shafts-h6.
- Brown's movement No. 99 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

parameter  value
disc  Ø100 × 6, spiral r 20 → 46.39 (8.4 turns)
feed per turn  3.14 mm
rack  6 T, module 1, 19.05 long
rack height over the face  pitch line 1.25 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. 99: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_099.html

Free files (Xem Studio Design License, Xem Studio; STEP + STL, 1 parts): https://xemstudio.com/files/hardware/face-spiral-rack-feed-mechanism/index.json
API: https://xemstudio.com/api/geometry?object=face-spiral-rack-feed-mechanism