# Proportional compasses (sliding pivot in slotted legs)

> How do proportional compasses scale a measurement?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 409 (1868)
- Page: https://xemstudio.com/objects/proportional-compasses-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=proportional-compasses-mechanism · Plain text: https://xemstudio.com/objects/proportional-compasses-mechanism/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); files Xem Studio Design License (free to use and sell from, not to redistribute as files: https://xemstudio.com/license/)
- Use in every chat: add https://xemstudio.com/mcp as a connector (https://xemstudio.com/use-with-ai/)

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- Two pointed legs cross on a pivot that sits in a slide block running in a slot in each leg: the points at the two ends open in the ratio of the arm lengths either side of the pivot, so a distance taken with one pair of points is copied at a fixed scale by the other.
- Drawn with the pivot 26.67 mm from the legs' centres: arms 106.67 and 53.33 mm, ratio 2 : 1, legs at 20°. Moving the slide to the centre gives 1 : 1; a set screw (not drawn) locks it.
- Brown's No. 409 (proportional compasses with a sliding pivot and scale).
- Parts (each has its own page): proportional-compass-leg-160-slot6, slide-block-12x6, dowel-pins-metric.
- Brown's movement No. 409 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| legs | 2 × 160 × 14 × 5, slotted |
| pivot offset drawn | 26.67 mm |
| ratio drawn | 2 : 1 |
| slide blocks | 2 × 12 × 6.2 (catalog) |

## Where the files are

- Henry T. Brown (1868, public domain): [Five Hundred and Seven Mechanical Movements — scan on archive.org](https://archive.org/details/fivehundredseven00browiala) — web — Publisher's page; dimensions are facts
- 507movements.com: [Brown's movement No. 409: figure, his text and an animation](https://507movements.com/mm_409.html) — web — Publisher's page; dimensions are facts

## Free STEP/STL (1 parts, Xem Studio Design License, Xem Studio)

Index: https://xemstudio.com/files/hardware/proportional-compasses-mechanism/index.json · license: https://xemstudio.com/files/hardware/proportional-compasses-mechanism/LICENSE.md

- proportional-compasses-mechanism-assembly: [STEP](https://xemstudio.com/files/hardware/proportional-compasses-mechanism/proportional-compasses-mechanism-assembly.step) · [STL](https://xemstudio.com/files/hardware/proportional-compasses-mechanism/proportional-compasses-mechanism-assembly.stl) — Assembled mechanism, all parts placed at one pose (STEP keeps each part separate)

## Questions

**How do proportional compasses scale a measurement?**

Two pointed legs cross on a pivot that sits in a slide block running in a slot in each leg: the points at the two ends open in the ratio of the arm lengths either side of the pivot, so a distance taken with one pair of points is copied at a fixed scale by the other.

## Fits with

- [Proportional compass leg 160 × 14 × 5, pointed, 6.2 mm slot 100 long](https://xemstudio.com/objects/proportional-compass-leg-160-slot6/)
- [Slide block 12 × 6.2 mm for a 6.4 mm slot](https://xemstudio.com/objects/slide-block-12x6/)
- [Dowel pins, 2–6 mm (ISO 2338 / DIN 6325)](https://xemstudio.com/objects/dowel-pins-metric/)