For this Instructable I made a 3D-Printed Geneva Drive, which is a mechanism that converts constant rotational motion into rapid, indexed motion of a certain number of degrees. This is really hard to visualize, even from pictures, but Wikipedia has a great animation here: http://en.wikipedia.org/wiki/Geneva_drive . I originally made this Geneva Drive to be a present, but I ended up not needing to give it away, so I keep it around as a cool engineering project. I designed it in SolidWorks from pictures of Geneva Drives online, and had it 3D printed by Shapeways. You can see a video of me rotating it by hand here: http://www.youtube.com/watch?v=XuHOVUXu2yE
Step 1: Why are Geneva Drives Useful?
A Geneva Drive is a mechanical system used for many unusual applications. In a movie projector, if the driving part is rotated at 30 revolutions per second, the driven part will pause on each frame for 1/30 of a second, then rapidly switch to the next frame, rather than having the film strip continuously fly through the projection mechanism, which causes flickering. The Geneva Drive was originally invented in Geneva, Switzerland as a watch component. If one of the slots is filled in, the Geneva Drive becomes a Geneva Stop. When the driving gear hits the filled slot, it is unable to continue, allowing the driving gear to rotate only a fixed number of times in each direction. This is useful in watchmaking because if a watch spring is overwound or underwound, the spring will be permanently damaged. A properly designed Geneva Stop with the driving gear connected to the watch winder will prevent this from happening.
Step 2: SolidWorks
I love using Solidworks whenever I have the chance. I designed both parts in SolidWorks, making sure to design them so that they would fit together and work properly. I couldn't find any specific equations in Machinery's Handbook for designing Geneva Drives, so I looked at pictures online and designed the parts so they would fit together and operate the way I wanted them to. I then scaled them so that they fit within my price range, because 3D-printed parts are charged by the cubic centimeter of volume of the part.
Step 3: 3D Printing
This step was really cool, I had known about 3D printing for a while but this is the first time I actually had parts printed. I went to the Shapeways website: http://www.shapeways.com/ . They offer a wide range of materials, ranging from white plastic, which is $1.50 a cubic centimeter, to sterling silver, which is $20.00 a cm^3. I went with the white plastic, and tried to minimize the volume as much as possible, by cutting out as much material as I could from the back of the parts. On the bottom of the driving gear you can see that I cut some material away from the body and I designed it with some holes through some cylindrical parts of the gear. I designed it so that there was at least 1/8" of material everywhere, but I probably could have cut that down to 3/32" or 1/16". On the other hand, that little nub on the top of the driving gear used to be a little handle, but it snapped off when I had it in a bag I checked on an airplane. In hindsight, I probably should have made that handle a lot thicker and lower, maybe 1/4" in diameter and 1/4" high. The driver costs $18.81, and the driven part, which is called the maltese cross on there, costs $10.74. If you search for "Jared Humphreys" on Shapeways, you can view the parts on there.
Step 4: Wooden Base
This was a pretty easy step. To show off the function of the Geneva Drive, I drilled two holes into a piece of maple which were spaced apart so that the two parts would mesh properly. I determined the distance between holes by creating an assembly of the two parts in Solidworks and measuring the distance between the centers of the two parts.
Step 5: Functioning
The driving gear used to have a little handle on top, but it broke off, so now I have to rotate it by hand as best I can. You can see a video of it here: http://www.youtube.com/watch?v=XuHOVUXu2yE