Rubber Band Car

objective

The objective of the Rubber Band Car was to learn how to manage my time using a Gantt chart and manufacture a rubber band car using the CNC machine and 3D printer while applying engineering concepts. Concepts that I wanted to include in this car was a gear ratio, flywheel, and to use rubber bands as a source to store and exert energy.

Purpose

The reason why I decided to create a rubber band car was because I wanted a project where I could use both the CNC machine and 3D printer to manufacture parts. In addition I wanted a project where I could combine several interesting engineering concepts. For example, this car would use elastic energy that would power a gearbox. The gearbox would generate rotational energy. The rotational energy would then be stored in flywheels. When the elastic energy ran ran out the flywheels would keep the car rotating for a little bit longer because of their stored energy.

research

I looked into several different areas pertaining to the rubber band car. I looked into the aerodynamics. I found out that cars/truck with curves are able to reduce drag which makes them require less energy to move. To transfer this finding to the rubber band car I modeled a “shield” in Solidworks that would have made the rubber band car more aerodynamic. I modeled this shield off of the front of Mack trucks, since the rubber band car has a profile of a Mack truck due to the gear box sticking approximately 2 inches out of the body. Another area I researched into was gear ratios. I used a gear simulator to figure out the pitch diameter, number of teeth, diametral pitch, and pressure angle. I did this so the gears would mesh perfectly. (I did not download the SVG I modeled these gears in solidworks.)

analysis

Analysis Overall: Overall I was mostly happy with the outcome of my project. It was able to accomplish what it was meant to do. Also I learned the importance of a Gantt chart and I was able to stick with it pretty well.

Analysis Research: For the first couple of days I researched the rubber band cars and its components. There was not that much information about rubber band cars specifically so I looked into cars/trucks/mack trucks. I also looked into gears and what to modify to make them mesh properly. I feel that I did a good job with this part because the gears mesh extremely well and I had a curved aerodynamic shield I could have 3D printed.

Analysis Design: When I was designing I initially wanted to use flywheels, an aerodynamic shield, and what I called axle circles. When it came to building the car I used the vex axle spacers instead of 3D printing an axle circle. I did this because the vex pieces cause much less friction than my 3D printed piece would have. In addition while I was designing the flywheels I decided to stop designing them because I decided to use the medium size vex gears as my flywheels. I did this because the gears have several holes on them which I could put a screw though and put a rubber band around the screw(I would have the option to change which hole I wanted the screw in easily). Also I choose the vex gears because they had a good weight to them and could act as a flywheel. In addition to not printing the flywheel and the axle circle I did not print the aerodynamic shield because I felt that since the car did not move very fast(~1m/s) the shield would have a very small effect that I could not measure. Also due to time constraints, the limited amount of space available on the 3D printer, and the amount of material it would require I felt that the shield was not worth it.

Special features that I put into the parts that I designed were, in the body I cut out several holes that were larger than the nuts so the nuts could sit flush. I also cut out spaces in the body so the additional parts can “snap” in. I found out that I should have added fillets to the 3D printed parts so they could “snap” in more easily. I felt that I designed the gears very well because they meshed well. The only issues with the gears was that after an extended period of use(me putting the gears in the gearbox, then sticking it inside the drill press so it would rotate a couple hundred times per minutes) the square hole would become circle. To solve this I could CNC the gears out of a harder material, make them thicker, or attack the vex axle locks to them.

Analysis CNC Machine: This was the biggest learning curve in the process. First I had to learn and figure out what axis’s I needed to set. I figured out that the xy, a, and z at center or origin should be set. Another thing I learned was since my design needed to be cut on all sides I had to add roughing and finalizing tool paths to the cutting tool path. Also I learned that there are different bits that can be used with the CNC machine. For example the square bit makes very nice corners vs a round bit. In addition I learned to never use the yellow plastic because it has a mesh in it that clogs the drill. Also I learned that if a design has holes in it the bit needs to be measured to make sure the shoulder of the CNC machine will not dig into the material while it is cutting.

Analysis Materials: My initial goal after I processed the foam on the CNC machine was to make the body out of the yellow plastic. I tried this and it did not work out great on the CNC machine because the yellow plastic has a mesh in it. Next I decided I wanted to cut the body out of chemical wood. The body cut out nicely but the material was not strong enough. Finally I decided to cut the body out of pine wood.

overall

Overall: Overall I felt that the car worked well. It accomplished what it needed to. I felt that I did good research, created good designs, the parts were manufactured well, and the car was assembled nicely. If I had more time the improvements I would have made would be adding fillets to parts that were meant to “snap” into the body. I would make the body longer. If the body was longer I would have been able to add more rubber bands and use the longer rubber bands to their full potential. The problems that I had with the large rubber bands was that they would get bunched up due to the amount of slack. This would cause the rubber band car to slow down or stop. I would have figured out what material instead of rubber bands could create more torque to have a 4:1 gear ratio. I would have experimented with using a solar panel and a vex motor to power the car.