A Geneva Drive is defined by Wikipedia as “a gear mechanism that translates a continuous rotation into an intermittent rotary motion.” Although replaced by servo drives in many cases, these were once used in movie projectors and to power rotary tables in industrial assembly lines. This is where I first saw this type of drive, and after redoing practically the entirety of these machines, the venerable Geneva mechanism was still at the heart of it, working like a champ after 20+ years. Like most cam-type devices, they may not afford nearly the adjustability or the “easy” electrical programming of a servo of PLC controlled device, but they will run for literally decades on end with little maintenance.
So after that introduction, I recently machined one out of MDF on my router. I plan on making one that works a bit better, and is possibly motor-driven, but check out the video below for my prototype:
This is a crude model a this point, but I’ve included the G-code and DXFs for the mechanism at the end of the article. Everything was cut with a 5/16 inch bit that I’ve been using as a flycutter on my router, providing a relatively high pocketing speed. Unfortunately, it doesn’t provide for cutting small holes. This could be worked around by changing the tool, but it’s a prototype, so the middle hole was crudely marked with the cutter, then drilled with my manual mill. The cut inaccuracy is, I would assume, partly to blame for the reverse not working as it should in the video.
The design of this was done on Draftsight (see my review of this AutoCAD clone), with few problems. I’ve never designed one of these before, and there are some equations to use if you want to do it that way. Instead, I did it visually, drawing the basic circles first, making everything fit, and rotating each piece around the central axis. This is probably easiest to do with a four-stroke device like I made, but I’m sure other numbers could be done this way.
After I cut the two rotating elements out, I placed them both on a spare piece of MDF, spaced out by the circular cavity, and drilled pilot holes for the axes. I then put a few washers under the driven element and bolted them both down lightly. It works OK, but I plan to redo this design so that the drive gear, the driven “cross”, and the base can be cut out at one time with a CNC router. I’m sure I’ll have to do some finishing work, especially if I end up putting a motor on it (maybe recycled from the “rotary engine” model project). Be sure to check back on this project, or just do yourself a favor and subscribe to the RSS feed!