Recently I built a Geneva drive model that could be hand cranked. Obviously this would be better with a motor, so one was added. I had a gear reduced motor that was originally slated for what became a “radial engine model”, which worked well to drive the assembly. I also remade the Maltese cross and the plate that everything sat on with my CNC router. Things worked a bit better this time, but the drive is by no means a refined product.
The following is a video of the drive’s development so far. You may recognize the ending segment from my EL lights review. Not sure where I got the idea for the flashing EL lights, but it seemed as good a way as any to try them out.
One thing that is a challenge with motorizing something like this that is made out of MDF (medium density fiberboard – think a more ground-up and uniform particle board), is that using a set screw isn’t as easy as with more robust materials.
The solution came during a rare visit to Walmart, where I tend to wander around the tools/toys/electronics section while my wife shops for whatever is actually needed, I happened to find a tube of Loctite Repair Putty that claimed to be able to bond to wood or ceramic and then be painted afterwards. My thought upon seeing this is that it could be used as a type of “insert” into MDF to allow it to be more easily tapped.
As seen in the picture gallery, I milled out a slot heading to the center hole and filled it with this putty. I then drilled a 3/16 inch hole through the middle for the motor shaft to fit into, and drilled a hole parallel in this material for a set-screw. Interestingly, what ended up happening was that when the shaft was pushed into the hole, the drill selected made it just tight enough to form a nice press fit, eliminating the need for the planned set-screw.
I didn’t actually have a set screw that would work nicely, so I was planning to instead counterbore so a 6-32 screw would fit below the cylinder’s surface, however this wasn’t necessary. If you’re wondering, I did tap for the 6-32 screw, and was able to securely fasten a screw to it. Sure, it’s not cold rolled steel, but for MDF, it could be an acceptable tapping option in some situations.
The newly-machined Maltese cross was fastened down with a somewhat loose 10-24 screw acting as the axis of rotation. The gear-reduced motor was glued to the base piece, where the driving mechanism was then pressed on. After this, a 9 volt battery was attached as well as a switch. Although the screw has a tendency to tighten when rotated, this crank mechanism generally works well, and could be called a success, considering it’s only purpose was to see if it could be done.
As far as I know, this is the word’s only working Geneva Mechanism made out of MDF. If I’m wrong, please let me know in the comments! I can add that to my collection of world wonders, including what I believe to be the most advanced tripod-mount system for the Env2 dumbphone!
If you’d like more details on the hand-cranked version of the mechanism, check out this post to see how it was made, including the Gcode and DXFs they were based off of.