A Spring 2023 Mechanical Design Project by Liza Gull, Cora Payne, Jordan Stone, Allison Tsai, and Gabriel Zwillinger
Project Background
Our goal for this project is to create a wood lathe that is powered entirely by a human. This has been done online using two predominant methods. First using a flywheel and treadle system and second using a bicycle. The flywheel treadle systems have been made very successfully and safely and there are a handful of internet guides that instuct you how to make one. There has been a lot less success using a bicycle and the one sucessful lathe we saw was incredibly unsafe. So, we wanted to challenge ourselves with making a bicycle powered lathe that is safe and effective. To do this, we will have one person be a designated biker while a second person is the lathe operator.
Requirements and Specifications
In order to make sure our lathe will work our group defined a series of requirements, specifications, and a handful of stretch goals. A list of those details is below:
We chose these requirements / specifications based on what we thought the integral parts of the human powered lathe while still emphasizing a concern for safety.
The base case assumptions of ideal lathe conditions are
sharp tools
softwood (easier to cut)
round stock (consistent contact is easier to cut)
Our first two requirements are core functions of our project, as if we cannot turn the softest wood under human power, we would not have a functional human powered lathe.The rest of the requirements arose from safety concerns.
Specifications were determined based on specifications of wood turning on a commercially available lathe, research on the average bikers cadence and force output, safety, and quality of life improvements.
Stretch goals were set by what would make our lathe as versatile and comfortable as possible, but not necessary to be safe and effective in our base use case (turning a pen).
Must be exclusively human powered
Must be able to turn soft wood
Must be safe to use for both the lathe operator and the cyclist
Must hold the work piece safely and securely
Must include a tool rest
Must include a tail stock
Be able to fixture various workpieces
Have a traditional emergency stop
Be able to turn hard woods or other materials
The Lathe needs to reach at least 1200 RPM
Powered input will be 200 N on each pedal at a cadence of 65-75 RPM
The lathe will use a regular bike in a bike stand rather than a stationary bike
The lathe will use the existing bike gear and a belt and pulley to transmit force between the pedals and the spindle
Tool Rest will be adjustable in height and position
Tail stock will be able to lock and be adjustable to provide adequate force
The lathe will sit on top of a table and be smaller than 12"x36"
Future Work
There are a number of improvements that could be made to the lathe in the future.
In the current configuration the belt is tensioned by pushing the table and bike apart and using sandbags to weight both the table and bike down. It would be nice if there was a more polished mechanism that could tension the belt and keep the table and bike fixed without the need for sandbags.
Currently the lathe operator and biker are very close in proximity. It would be more comfortable if the user and biker are further apart. While we were constrained by the length of our belt, in the future a longer belt and/or longer spindle shaft will help distance the user and biker and make the lathe more comfortable to use
The current tailstock on the lathe is quite small or while it does provide the necessary force to hold a work piece, it can be challenging to use due to its size. It also requires a tool in order to lock its position. Making or buying a larger, more robust tailstock that can be adjusted easily would make the user experience better.
Conclusions
Overall building the lathe was an incredibly fun and informative project. We had the opportunity to apply many of the analysis skills we learned about in mechanical design and the machine shop, as well as learn more in-depth analysis, part selection, machining, and project management skills. We had to apply fatigue, bolt, gear, and pulley analysis over the course of our project. We also had to learn how to do power analysis to compare our lathe with commercial products. We also had the opportunity to learn CNC turning to make our pulley and members of the group made a keyway and keyseat successfully. Finally, we learned some important estimation skills that saved us a lot of time for analysis. We calculated the stresses and factors of safety for the worst loading cases so that we could verify the structural integrity of our parts without needing to do a more complex analysis.
If we were to do the project again, there are a handful of details we would change. As mentioned earlier, it would have been nice if we had purchased a longer belt and a more rigid tailstock (although we were unsure of our budget constraints at the time). We also would have chosen to use a slightly smaller shaft for our spindle. Ultimately, we were constrained by the threaded feature on the chuck, but we still could have used a 0.75" aluminum shaft instead of a 1" aluminum shaft. We realized after purchasing our shaft that our initial fatigue analysis was incorrect and that a 0.75" shaft would have had a sufficient factor of safety.
Design Features
To see some of our awesome design features and the reasoning behind it, check out our "Features" tab. Linked here
Evaluation of Design
The math behind each of our components and design decisions can be found in the "Evaluation" tab of our website. Linked here