How To: Make a Two Circle Wobbler from CDs

One of my favorite simple projects is building two circle wobblers. I love how such a simple object amazes with its motion. The two circle wobbler is an object made out of two circles connected to each other in such a way that the center of mass of the object doesn't move up or down as it rolls. This means that it will roll very easily down a slight incline. It will also roll for a significant distance on a level surface if you start it by giving it a small push or even by blowing on it!  

Here's a video of a two circle wobbler that I made last night. The table that I set it on is very slightly warped down towards the middle, and so you can watch the roller speed up as it moves towards the center of the table. If you look at it closely, you can see how it actually wobbles left and right as it moves, which led to its name.

You can make these from anything that is circular, but one of the best materials I have found are CDs. They are big enough that you can see exactly how they are built, but small enough that you can roll them just about anywhere. They also are pretty easy to cut notches in, which is necessary for this project. The notches must be cut so that they are 29% of the radius of the circle. For a full mathematical treatment of this object you can read the paper, "The Two-Disc-Roller – A Combination of Physics, Art and Mathematics".

Materials

• Downloadable template
• Scissors or exacto knife.
• At least 2 CDs
• Tape (I like packing tape if cutting multiple CDs at once)
• Utility knife
• Surface to cut on—Do NOT cut while holding in your hand!
• Optional: Scroll saw or jig saw to cut out multiple CDs at one time and speed up the cutting process.

Step 1 Download, Print, and Cut Out Template

Download the template and print it out at 100%. Do not scale to fit, this will not work. Once you have printed out the template, use scissors or exacto knife to cut the circle out of the template. It's important to be careful in this step.

Step 2 Place Template onto a CD and Tape It Down

Carefully place the template onto the CD so that it covers it symmetrically. Tape it onto the CD using multiple points of attachment so that the template will not slip as you are cutting the CD. 

If you are going to cut through multiple CDs at once using a scroll or jig saw, make sure you tape around the entire stack.

Step 3 Use a Cutting Tool to Cut the Slot out of the CD

Now carefully cut along the line on the template through the CD. You do not need to worry about the width of the slot yet, as long as you don't make it wider than the thickness of a CD. If using only a box knife, you are going to have to press hard and make numerous passes in order to make it through the CD.  

If using a scroll or a jig saw, make sure that you have a stack of at least 4 CDs. If you do not, you will probably break the CDs. All you have to do with these is cut down the center of the line and stop at the end of the line. A scroll saw is my tool of choice for this project because it is so easy, and can handle many CDs at the same time.  

Step 4 Widen Slots and Friction Fit the CDs

Remove the template and tape. Widen the slots by using the box knife to cut more on the sides of the cut and scrape the sides. Each time you make the slot wider, check to see if it is the right width by trying to slide an edge of the CD into the cut. You need to make sure that it is still quite tight since this is the only thing that is going to hold the object together. Once you have both CDs the correct width, slide them into each other.

The finished product should look like this:

Step 5 Amaze Your Friends and Show Off

These objects never cease to amaze me and most of the people I show them to.  If you and a friend make one, you can always have a race!

If you build one, let us know by posting a picture or video up on the corkboard.  If you have any ideas of how to make these better or faster, be sure to comment.

Next post, we'll look at 2 circle rollers made from other materials, and how to make different rollers that are even more funky.