## How to draw a Geneva drive using a compass and straightedge

**8.7.2013**

*Category: technical*

The Geneva drive is a gear mechanism that makes a wheel turn a specific angle at regular intervals. They're used in film projectors and mechanical watches and all sorts of doodads. Since drawing a simple four-slot Geneva drive doesn't involve squaring circles, doubling cubes or trisecting angles, the only tools we need are a compass, a straightedge (ruler), a pencil and a rubber (unless you are the God/Goddess of Draughting).

- Draw a circle with a line running through its centre. Label the two points where they intersect
`A`

and`B`

, and mark their midway points`C`

and`D`

: to do this, open your compass a bit further (doesn't matter how much) and mark the points where two circles centred on`A`

and`B`

would intersect. Draw an imaginary line through these two points and the places where it intersects the first circle are your midway points`C`

and`D`

. - Draw two circles centred on
`A`

and`B`

with a radius of`AC`

(the distance between`A`

and`C`

). - Using the technique from step 1, draw the midway line of the circle centred on
`B`

. - Draw a line through
`C`

and`B`

and one through`D`

and`B`

. Label the places where they touch the circle as`E`

and`F`

. - Draw a circle that centres on
`B`

and is tangent to circle`A`

. Note that it intersects the straight lines from step 4 at four points. - Using these four points as the centre, draw small circles with a radius of about 1/3 of the previous circle. Also do this for points
`A`

,`B`

,`C`

,`D`

,`E`

and`F`

. - Join these circles up with parallel lines to form 4 'spokes' radiating out from point
`B`

. - Draw a circle centred on
`B`

with a radius of`AB`

. Mark the points where it intersects the straight lines as`G`

,`H`

and`I`

. - Draw a straight line through
`A`

and`C`

and mark the length of`AB`

with point`J`

. - Draw a circle centred on
`A`

that is smaller than the existing large circle and doesn't touch the small circles centred on`C`

and`D`

. Using the same radius, draw circles (or just curves) centred on`G`

,`H`

and`I`

. - Using the radius of the largest circle centred on
`A`

, draw a circle (or curve) centred on`J`

. - Shade in and/or outline the Geneva drive.

### Why the Geneva drive?

I saw the compass-and-straightedge construction puzzle by Nico Disseldorp and thought, huh, I should make a tutorial! The most recent thing I drew using a compass and straightedge turned out to be a score keeper with a four-slot Geneva drive. (Not written up that project yet - watch this space.)

### Moarrr

- Can you draw a Geneva drive with five slots? Six slots? Etc.
- Can you draw an internal Geneva drive? A Geneva stop?
- If you were to make a working model out of your drawing, what other parts would you need?

### Disclaimer

Step 7 involves eyeballing because I couldn't be arsed to draw 4 extra lines...sorry purists.