Hey guys,
For the midterm project the task was to create a complex machine assembly, based on the 2D drawing done in the third assignment.
If you remember from my third assignment post, the mechanism I chose was the Geneva Wheel – (here is the post in case you forgot). In this project, tough, I made an external and a internal Geneva Wheel, pictures below:
External Geneva Wheel
Internal Geneva Wheel (more specifically the type of internal wheel I based my design on)
A Geneva wheel (internal or external) is a gear mechanism that translates continuous rotation motion into intermittent rotation motion. Its main application used to be watches and also movie projectors, but it has been substituted by electronic control. To learn more click here. Both of the mechanisms are illustrated below:
External Geneva Wheel
Internal Geneva Wheel
To achieve our goal and produce a well design, fabricated, assembled and finished mechanism, we divided it in stages – suggested by Dr. Wettergreen:
- Low fidelity prototyping;
- Medium fidelity prototyping;
- Final working model.
- Low fidelity prototyping
The first step I and my partner, Brian, took was to make a low fidelity prototyping of the machine to understand how the parts interact and how they are assembled. So we did our first prototype, the external wheel only (the idea for the internal wheel would come later). It was made by hand cutting cardboard and using wooden dowels as handles and shafts, then we glued and taped the parts together and assembled the mechanism.
Despite the lack of precision to accurately produce the parts, resulting in a prototype that did not work, it was very important to understand some aspects of the wheel, such as the different layers of the machine and how the moving parts interact with each other.
After talking to Dr. Wettergreen about the low fidelity prototyping he advised me to increase the complexity of the machine, by for example adding another wheel. Then I had the idea to make an external and an internal wheel.
2. Medium fidelity prototyping
After it, we modified the 2D drawings we have done for the third assignment, adapting them to be used in the laser cutter, also we added a box joint to connect the base and back plates. In addition, I designed and made the drawings for the internal mechanism, using Solidworks instead of Illustrator.
Then we built the medium fidelity prototype. The parts were made by laser cutting clipboard (a better quality cardboard), then we glued the different layers and assembled the parts using some wood dowels. Both wheels worked this time but the mechanism was quite hard and difficult to turn. This prototype gave me important information on the optimum gaps between parts should be to allow movement and how interferences could be used to limit movement.
Parts cut using the laser cutter
Front of the machine, showing the external wheel
Back of the machine, showing the internal wheel
After analyzing my machine I thought it was too small and decided to make it bigger, thus I had to scale my drawings. The only limitation was the size of the dowels, we had available only 1/8″, 1/4″ and 1/2″, therefore I could only double the size of the parts if I did not want to redo all drawings. At the end I doubled the size of the external Geneva and redraw the internal Geneva with a scale of 1.5.
3. High fidelity prototyping
With the scaled drawings we started making our high fidelity prototype. At the beginning, we wanted to do some parts using acrylic, but since there was not enough material for what we intended all parts were made from wood. Again the parts were cut using the laser cutter
Parts being cut in the laser cutter
The final files I used to make the part are these:
Geneva_Wheel_external – External wheel
Geneva_drw and Internal_geneva_90 – Internal wheel
After it we glued the layers of the parts using wood glue and clamped them to make pressure.
Glued back plate clamped
After having all parts glued I started post processing them. My first idea was to use sand blasting to remove the irregularities of the wood, but as you can see below it did not work out the way I imagined:
Sand blasted wood piece
The sand blaster was too strong to be used on wood, so instead of just removing a little bit of material on the surface to remove defects, it actually carved the wood.
After this failed test I decided to use the orbital sander to smooth the sides of the parts. Having done that I chose to stain the back and base plate and paint the remaining pieces. The stain gave a really good look to the plates, as you can see below:
Stained back and base plates
For the painted parts I decided to use red for the driven parts on both mechanisms and white for the driver parts in both mechanisms and the lid of the internal wheel. After painting the parts looked like this:
Painted parts
After painting, we moved to the next step: Cutting and gluing the dowels at their respective places. To get the desired result our procedure was to first measure the required size of the dowel, mark it and cut it a little bit longer using a hand saw. Next we would position the dowel and test the mechanism. If any ajusts were necessary, we would reduce the length of the pegs by sanding them. The tests were done to ensure they were performing correctly.
During the assembly we applied lithium grease as lubricant in the moving parts to reduce friction.
A wooden ball was used to limit the translational movement of the driven external Geneva wheel. To place the ball I drilled a hole of it with the diameter of the dowel, (1/2″ in that case) and glued it to the end of the dowel.
After all dowels and parts were assembled the machine was finished and it looks like this:
We have a video of the final prototype working:
The model works very well despite having 2 minor defects. The first one is the driver part of the external Geneva wheel (the one with the handle) which is a little wobbly, that happened because when we glued the central dowel on this part it came inclined, not perpendicular, to the part. That’s why it moves back and forth when it is rotated. The second small problem was the dowel used to transmit rotation on the internal Geneva wheel, in the initial plan it would have 1/4″ diameter, but when we tested it the mechanism was very hard and difficult to turn, because the gap was not big enough, as a solution we decided to use a 1/8″ dowel. This change made the wheel much more smooth to turn but it made it more fragile too.
Before concluding, there is one aspect of the mechanism’s design I would like to address. After the machine was assembled it could not be disassembled, no part could be removed. That’s because all parts were glued in their positions. Therefore it is impossible to perform maintenance on this equipment in case it breaks or requires more lubrication. That is why for future designs or versions of these mechanisms I recommend changing the design of the machine to use bolts, screws and pins as a non-permanent assembly method, thus making it maintainable.
Overall I really liked my machine because it has a smooth functioning and a very nice look (the contrast of red and white worked as expected). I have learned a lot in this project, in special how important is the correct design of gaps and how the fine adjustments done during assembly can mitigate fabrication and design defects, allowing the mechanism to perform as designed.
Thanks for reading and see you on the next post!
