# Rolling parallel ruler (blade on two notched rollers)

> How does a rolling parallel ruler stay parallel?

- Category: Mechanisms › Linkages
- Kind: Hardware
- Basis: Xem Studio design following Brown No. 323 (1868)
- Page: https://xemstudio.com/objects/rolling-parallel-ruler-mechanism/ · JSON: https://xemstudio.com/api/geometry?object=rolling-parallel-ruler-mechanism · Plain text: https://xemstudio.com/objects/rolling-parallel-ruler-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

- A straight-edge carries an axle with a roller at each end; the rollers are the same size and fixed to the axle, so as the ruler rolls every point of the blade moves the same distance at right angles to the axle — the blade stays parallel to its starting line.
- The notched rims (24 V-notches, 0.85 mm deep) bite the paper so neither roller slips; the rollers stand 1 mm proud of the underside through openings in the blade. Drawn with a Ø3 × 150 axle, Ø18 × 4 rollers at ±60 mm and a 200 mm blade.
- Brown's No. 323 (rolling parallel ruler); the navigator's parallel rule.
- Parts (each has its own page): rolling-ruler-blade-200-d18, notched-roller-d18x4-b3, linear-shafts-h6.
- Brown's movement No. 323 (1868) shows it; this model is a Xem Studio design following his figure, every part at true size.

## Sizes

| parameter | value |
|---|---|
| blade | 200 × 24 × 6 mm |
| rollers | 2 × Ø18 × 4, notched |
| axle | Ø3 × 150 (linear shaft) |
| roller spacing | 120 mm |

## 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. 323: figure, his text and an animation](https://507movements.com/mm_323.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/rolling-parallel-ruler-mechanism/index.json · license: https://xemstudio.com/files/hardware/rolling-parallel-ruler-mechanism/LICENSE.md

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

## Questions

**How does a rolling parallel ruler stay parallel?**

A straight-edge carries an axle with a roller at each end; the rollers are the same size and fixed to the axle, so as the ruler rolls every point of the blade moves the same distance at right angles to the axle — the blade stays parallel to its starting line.

**Why does a rolling ruler need equal rollers?**

Unequal rollers would make the blade swing about the smaller one; equal, fixed rollers make every point travel the same distance.

## Fits with

- [Rolling parallel ruler blade 200 × 24 × 6 with lugs for a Ø3 axle](https://xemstudio.com/objects/rolling-ruler-blade-200-d18/)
- [Notched roller Ø18 × 4, Ø3 bore, 24 V-notches](https://xemstudio.com/objects/notched-roller-d18x4-b3/)
- [Linear shafts Ø3 to Ø25 mm (h6), 100 to 1000 mm long](https://xemstudio.com/objects/linear-shafts-h6/)