How to draw a Geneva drive using a compass and straightedge


How to draw a Geneva drive using a compass and straightedge

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).

  1. 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.
  2. Draw two circles centred on A and B with a radius of AC (the distance between A and C).
  3. Using the technique from step 1, draw the midway line of the circle centred on B.
  4. 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.
  5. 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.
  6. 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.
  7. Join these circles up with parallel lines to form 4 'spokes' radiating out from point B.
  8. 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.
  9. Draw a straight line through A and C and mark the length of AB with point J.
  10. 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.
  11. Using the radius of the largest circle centred on A, draw a circle (or curve) centred on J.
  12. 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

Disclaimer

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


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