For the past few weeks, I have been working on the midterm project for ENGI 210. The goal of this project was to make a working mechanical model based on ideas in the “507 Mechanical Movements” book, ideally making it into a “story.” This is the first group project I have done for ENGI 210 – my partner for this project was Brandon Peoples. This blog post will cover my process, areas for improvement, and cost analysis for the mechanical model.
Figures 1 & 2: The front and back of our final mechanical model, “ENGI 225.”
Our Process
First, Brandon and I began by looking through the “507 Mechanical Movements” online and brainstorming ideas for the “story” that our model could tell. Some of our early ideas included making a “treadmill” that would have a continuously moving track, and a “gondola” for a marble. However, the two of us have been going to the gym pretty consistently this semester, and decided that we wanted our model’s “story” to be a person bench-pressing.
Figure 3: A man bench-pressing.
Having selected our “story,” we began to search for the perfect mechanical movement to use for our model. We wanted a movement that could transform rotational motion into linear motion (since the bench-press is a vertical, linear movement), and could be performed continuously without the user resetting the model or changing how they manipulate it. Eventually, we found a movement that not only satisfied these items, but also had the additional feature that one direction of the linear motion was performed slower than the other, similar to how people tend to lower the bar on a bench-press faster than they can push it up, especially when it is a heavy weight for them. This movement was Mechanical Movement #131, which involves a continuously rotating crankpin that fits into a slotted arm, rotating it one direction and then the other, but one direction faster than the other (the faster direction is the direction the arm moves when the crankpin is closer to the inner part of the arm), which causes the linear motion of a straight, toothed rack.
Figure 4: Mechanical Movement #131 (Source: “507 Mechanical Movements, 131”).
Having settled on our final idea, we got to work to make it a reality. We began by making early prototypes of the gears. Initially, we envisioned our model being much bigger than it ended up being, which is why the gear prototypes were so large in comparison to our final product. We created the teeth for the slotted arm using Gear Generator – we generated a gear, took 1/4 of the file, and put it on the end of the slotted arm. We then copied the teeth onto the rack. As you can see in the early prototype in Figure 5, the rack had the same curved teeth as the slotted arm. A later prototype we made featured curved teeth for the slotted arm but teeth with straight lines for the rack, which we found to work much better than the aforementioned design. Thus, this is the design we moved forward with (but with the pieces being much smaller).
Figure 5: Early gear prototypes.
To get a sense of how everything would fit together, we made a test box out of cardboard with inner dimensions of 12 inches (width) by 8 inches (height), by roughly 4 inches (depth). By doing this, we could lay out our gears and see how much room they would take up, where they would need to be fixed to the box, and where they would connect with each other. After this, we were able to make our box out of wood for our final product, which was considerably thinner than this test box.
Figure 6: A corner fit test for our box.
While this process was happening, we also had to design the body of the person who was going to be doing the bench-pressing, as well as his arms, which we wanted to move freely about shoulder and elbow joints as the “barbell” component moves. Another notable requirement for this project is that we had to incorporate at least one sticker made with the vinyl cutter somewhere in our project. We decided to implement this in our person’s clothes, making one sticker for his shirt, one for his shorts, one for his shoes, one to cover up his shoulder joint, and an arrow for the back of the model indicating which direction to turn the crank (if turned the wrong direction, he will push the bar up faster than he brings the bar down, which does not make sense because it is much easier to let 225 pounds come downwards than push it upwards). We also designed the two 45-pound plates that are on the end of the barbell. These components can be seen in Figures 1 & 2.
Before attaching the person, we stained and clear-coated our box to give it a dark walnut color that would stand out against the person and finishing that would help protect the wood.
Figure 7: The box after staining.
Once we were done putting finishes on the box, we attached the person and our project was complete. Below are videos of the mechanism and a video of me demonstrating the model, as well as Brandon and I’s slide deck.
Mechanical Movement 131 in real life.
Areas for Improvement
Although I am really proud of how our final product turned out, there definitely are parts of the model that could be improved.
One part of the project that could have been done better was the person’s arms. When I designed the arms in Adobe Illustrator, I added a bit of extra length so I did not accidentally make them so short that they would keep the bar from going high enough for the crankpin to complete a full rotation. However, the arms came out being a bit too long, making it so that the person is not benching with anywhere near a full range of motion. While this is somewhat of an aesthetic imperfection, I was successful in ensuring that the arms would not interfere with the functioning of the mechanism.
Another area where this project could have been improved is with the smoothness of its operation. There are some parts of the crankpin’s rotation that are somewhat stickier than others, making the model a bit less pleasurable to use than it could have been. This problem could easily have been solved by sanding down the teeth and the slot in the slotted arm. When we were running our tests, however, all the parts seemed to rub against one another smoothly – it was not until the box was fully assembled that we noticed a bit of roughness. It is worth noting, though, that the final product has never been so rough that it was difficult to make it move; it is noticeably sticky, but still takes very little effort to operate even in its sticky areas.
Besides these items, there are some other basic, minor flaws, such as some paint marks on the bottom of the box from residue on the spray-painting table outside and some marks on the barbell and the handle from when we stained the box.
Cost Analysis
For this project, the elements that could have accrued cost are the wood, the clear coat, the wood stain, time spent using the woodworking tools, time spent on the laser cutter and vinyl cutter, and the opportunity cost of the time that Brandon and I spent working on this project.
Wood
The wood we used was a single “0.25” thick” (0.2 inch thick in reality) 32″ x 24″ birch plywood sheet. At the OEDK, which was where this project was made, these sheets cost $5 per sheet.
Clear Coat
Most retailers seem to sell a can of the clear coat we used for around $5. We did not use anywhere near the entire can, but it is impossible to know how much of the can we used since we cannot visually see how much of the contents are left. Therefore, I will make a generous estimate of $1 for the amount of clear coat that we used.
Wood Stain
The wood stain we used ranges in price between $5-9 per can on a simple Google search. As a midrange estimate, I will estimate the cost of the entire can to be $7. However, we did not use the whole can. While I would say that 1/7th of the can is still a bit of an overestimate for how much we used, for purposes of making my numbers nice, I will use this approximation so I can likewise approximate that we used $1 worth of this stain.
Woodworking Tools Time
For this project, we used a miter saw, a belt sander, and sandpaper. These materials would all be present in a standard woodshop, but it is difficult to find a standard price online for an hour in a woodshop. For this reason, I’ll estimate a cost of $10/hour to work in a woodshop. We were working with these tools for about half an hour, so this would accrue a cost of $5 for the project as a whole.
Laser Cutter/Vinyl Cutter Time
For this project, between making test cuts and engravings as well as producing our final pieces, I would say that we spent roughly three and a half hours using the laser cutters and vinyl cutter. Finding the cost of using a laser cutter for an hour was difficult, but I found a blog online (Dahlstrom Roll Form) that estimated the cost of running a laser cutter for an hour to be between $13 and $20. Since a business would want to profit from someone using their laser cutter, they would charge a customer more to use it than it would cost the company for them to use it. For this reason, I’ll estimate the cost of us using the laser cutters and vinyl cutters for an hour to be $15 (a midrange estimate) x 3.5 = $52.5.
Opportunity Cost of Time
One assumption that I will make for this calculation is that Brandon and I spent the same amount of time working on this project. While there were times where one of us would be working on it without the other present, these amounts of time are roughly the same for both of us, and we did the majority of the project while were both present anyway. I will estimate the amount of time that we each spent working on this project is around 15 hours. I will also assume the same value for Brandon’s time as that of my time, which I estimate to be the value of my time working as a Tech TA at Rice University ($12/hour). Therefore, the opportunity cost of our time was $12 x 15 x 2 = $360.
All together, the total cost of this project is $5 + $1 + $1 + $5 + $52.5 + $360 = $424.5. As usual, a large portion of the cost of this project (97%) is associated with the time Brandon and I spent working on it. However, if we were to make this same product again, I think we could have dramatically reduced the cost now that we have our files prepared and are familiar with the fabrication process. If we did this again, I anticipate that we could lower our opportunity cost of time by at least $240 and the cost of our laser cutter time by at least $30, meaning we could reduce our overall cost to at most $154.5, which is only 36% of our original cost.
