Cyclograph: fixed-angle rules sliding on two pins draw a circular arc — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/cyclograph-arc-mechanism/  (sheet CC BY 4.0; files Xem Studio Design License, https://xemstudio.com/license/)
Basis: Xem Studio design following Brown No. 403 (1868)
Catalog version: 2026-10-20

Sizes: see table below
- Two rules fastened at a fixed angle φ slide with their edges against two pins at the ends of a chord: the apex always sees the chord under the same angle, so (inscribed-angle theorem) it moves on a circular arc through the two pins — no centre needed, which is how flat arches of large radius are set out.
- Chord 100 mm and φ = 120° give R = c / (2 sin φ) = 57.74 mm and a rise of 28.87 mm. The rules' edges are tangent to the Ø3 pins, so the edge apex runs 1.73 mm inside the pin-centre arc. Drawn at the top of the arc, pins touching both edges.
- Brown's No. 403 (cyclograph: two rules from the chord's ends to the apex of the versed sine, braced, pencil at the angle).
- Parts (each has its own page): cyclograph-rule-150x15x4, cyclograph-brace-165-71, cyclograph-base-plate-180x100-c100, dowel-pins-metric.
- Brown's movement No. 403 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

parameter  value
chord c  100 mm
angle φ  120°
radius  57.74 mm
rise (versed sine)  28.87 mm
rules  2 × 150 × 15 × 4

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. 403: figure, his text and an animation — web — Publisher's page; dimensions are facts — https://507movements.com/mm_403.html

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