Plywood Bridge

This project is inspired by the "toothpick" bridge project from my high school days.

Requirements and constraints

You are to design and build a rubber-band powered car that can safely transport an egg. It must meet the following constraints:

• It must include a laser-cut wood part.
• It must include a 3d printed part that is involved in the functioning of the car.
• It must be powered by a rubber-band.
• It must fit within one cubic foot.
• It must include an openable hatch to enable inserting and removing an egg.
• It must be able to travel 10 feet on a hard surface floor while carrying an egg under its own rubber-band power.
• It must be able to travel 10 feet when rolled down a ramp, (with rubber band disengaged).
• The egg must survive when the cart is rolled down the same ramp, then colliding with a concrete block 2 feet away.

Hints: Because laser­ cutable materials are cheap, and 3D printing is expensive, you’ll likely want to save your 3D printing allotment for the wheels of your car or other small parts, so you car will likely be mostly laser­cut wood.

Materials

All of the following materials will be available to each student:

• 1x 12"x24"x 1/8" Craft Plywood:
  • http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXXJN2&P=ML
• 2x 3" #8­-32 threaded rod
  • http://www.mcmaster.com/#95475a335/
• 2x 4” #8-32 threaded rod
  • http://www.mcmaster.com/#95475a338/
• 4x #8­-32 hex lock nuts*
  • http://www.mcmaster.com/#90715a009/
• 4x #8­-32 hex nuts*
  • http://www.mcmaster.com/#90480a009/
• 4x #8 flat washers*
  • http://www.mcmaster.com/#90126a512/
• 4x O­Rings AS568A Dash Numbers 119, 125, 033
  • http://www.mcmaster.com/#9452k83/
• 1x threaded hook
  • http://www.mcmaster.com/#9587t21/
• 4x 7”x1/16” Rubber Bands
  • http://www.mcmaster.com/#12205t86/
• 4x 1/2” x 1/4” Rubber Bands
  • NOTE: Not ordered/not available at McMaster Carr
• 4x 2" x 1/8" Rubber Bands
  • http://www.mcmaster.com/#12205t76/
• Rubber tubing
  • http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXYXK8&P=7
• Cushioning foam: 5.5" x 7" x 1/4" piece, cut from:
  • http://www3.towerhobbies.com/cgi-bin/wti0001p?&P=9&I=LXE100
• 1 inch^3 of plastic on the 3D printer in the GVU Prototyping Lab (more may be available if 
requested)
• Use of the laser cutter in the GVU Prototyping Lab (or alternatively, the laser cutter in the 
ME Invention Studio)
• Any additional resources you decide to use.
• Glues:
  • http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXXJN2&P=ML
  • http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHS58&P=ML

Many parts above have CAD models available from McMaster Carr

Process

We will have at least the following milestones, which will be confirmed with “skill-demos”:

• Example hardcopy of something output from CAD software – confirming that you have successfully accessed some CAD platform.
• Initial 3D design of your car (not perfect, but initial).
• File suitable for lasercutting wood. And cut wood.
• File suitable for 3D printing, and a 3D part.

Details on these skill demos will be provided and refined via the course wiki and t-square.

3D Printing

You can export 3D part models as .stl files and print them on the GVU 3D printer. Most of you will be unfamiliar with the process, and can come to the GVU Prototyping Lab during the ‘lab hours’ or at other times when your TA is available to help you.

Laser Cutting

You can design a chassis as multiple flat parts that interlock and will be glued together. These parts can be designed as an assembly in your CAD software. Once you have them laid out, there are multiple ways to export them, but we recommend laying them all out at 1:1 scale in a drawing, and exporting that drawing as a DXF file. This can then be opened in Illustrator on the laser cutter computer. Again, for assistance laser cutting, come to the GVU Prototyping Lab during the ‘lab hours’ or at other times when your TA is available to help you. You may also consider use other laser cutters on campus, including the ones in the Invention Studio.

Iterate

There’s a fair chance that your first design may not work as expected. You may want to assemble your car with masking tape at first to test the design before using more permanent wood glue. If you’re pressed for time, you may also consider hot­melt glue as a weaker but faster option. Also consider being careful with your plywood usage in case you need to re­cut your entire car.