For this project my partner Ryan and I were tasked with creating a mechanical model that uses one of the examples in 507 Mechanical Movements. During the brainstorming process we had an idea to use an epicyclic train to demonstrate the earth and moon orbiting around the sun while also displaying the 12 monthly orbits the Moon does in a year.
Before creating our prototype we first needed to create sketches of our model to plan out what we would need to cut out and test during the prototyping phase. During this phase we did research to determine what gear ratio we should use to achieve the 12 revolutions in a year we wanted, and it turns out it depends on a lot of factors including how we defined a revolution! Since we were going to do testing in the near future we decided to go for a 1:12 gear ratio between the sun gear and the planetary gear, which meant the sun gear would be 144 teeth if the planetary gear was 12 teeth. We didn’t want to make the model too large so we planned on adjusting the number of teeth on the planetary gear to decrease the overall size before having to adjust the gear ratio.
Now that we had an idea of what we wanted to make we moved on to creating a low fidelity prototype to test out our mechanism and see what adjustment we needed to make. During this phase when creating the file to cut out the sun gear it turned out the size of a 144 teeth gear would have a diameter of at least 20 cm so we decide to reduce the planetary gear size to 6, which made it really small but we felt it would still be able to function properly. Once we had the file we created our prototype using cardboard and tested the main mechanism of our model.
Originally we wanted to use a carrier to move the planetary gear but we found difficulties make it work using only two gears so we switch to mechanism 55 since not only was it easy to add an outer ring to the model but there is an online planetary gear simulator that allowed us to predict the movement of our model by adjusting the parameters of the gears. In the end our prototype showed very promising results but required more testing with laser cut wood to make sure it would work.
Moving on to our medium fidelity model, we used most of the same files used previously to create our low fidelity model while adding other parts we hadn’t cut out before. We also decided to add a top disk that would help support the dowel going through the planetary gear to stay vertical while still allowing it to rotate, preventing it from wobbling too much.
We noticed a bit of difficulty turning the planetary gear but we determined that it was due to the use of hot glue to fix the dowel to the base of the model. Since we were already planning to double the layers of a few components including the base we didn’t think it would pose too much of a problem, so we continued to creating our final high fidelity prototype. We also noticed the number of revolutions the moon would make was 14, which as it turns out was caused by making the gear ratio between the sun and planetary gear 1:12 instead of the outer ring and planetary gear. Using the online simulator we adjusted the gears for our final version.
Finally we started working on the high fidelity model, which required more steps than the previous phases. We first laser cut out all the pieces we need for the model and made adjustments to the sun gears and top disk to include space for ball bearings.
Then once we had the pieces we cut out two rectangular metal pieces to use as gear connectors to ensure that both the top and bottom sun gear would rotate the same distance.
During this phase we were still making a few changes to make sure we avoided problems that would impact the performance of our model such as switching from using dowels to laser cut supports for the outer ring to ensure it was level with the base. We also originally planned on adding another piece to support the top disk but decided it wouldn’t be necessary for the final version.
Next we moved on to post processing. Originally we planned on using a dark stain for most of the model but due to limited time Ryan came up with the idea to spray paint the gears gold and the rest of the parts black to simulate the look of other orreries. We then added a clear coat and sanded lightly to prevent removing too much paint.
Finally we began bonding the parts together using wood glue and cutting out vinyl pieces to create stars and the continents on our model Earth. We waited until the near end of the project to decide how we would create the Sun, Moon, and Earth and we ended up using a wooden sphere and painting it blue to create the Earth and aluminum foil for the moon and Sun. For the sun we used paper towels to make a large sphere and covered it in thin layer of foil that we then spray painted gold.
In the end we were able to create an orrery of the Earth, Moon, and Sun using what we have learned so far in the course. Although there were a few problems along the way we are very satisfied with the end result.
Cost Analysis:
Materials:
- ⅛’’ Ply Wood – 5 sheets * $22/sheet = $110
- Vinyl – $6/roll = $6
- Aluminum – negligible cost, accounted for in equipment
- Spray paint – 4 types * $13/can = $52
- Hot glue – 2 sticks * $0.50/stick = $1
- Wood glue – $3
Equipment:
- Laser cutter – 10 hr * $20/hr = $200
- Plasma cutter – ½ hour * $30/hr = $15
- Vinyl cutter – 1 hr * $10/hr = $10
- Sand blaster – ½ hr * $30/hr = $15
Labor:
- 15 hours each * 2 team members * $22/hr for prototyping engineer = $660
Total Cost: $1072
