D.I.Y. Making Centering Rings, Part 2

Depending on the thickness of the mat board, you cut probably on the outside line using scissors.

Check against the end of the larger tube. Chances are this diameter will be to large.

Here's why you cut out four rings - 

You'll need the thickness and stiffness of all the rings to sand to the correct diameter. One or two rings by themselves would flop around when sanding to size.

On the left I'm sanding the ring edges with a rotating motion. Check the fit, sand some more until you get a friction fit.

Here's a good friction fit of the ring in the BT-50 tube.

I originally was going to have the build powered by 13mm  diameter mini motors. After studying the design I decided to go with 18mm standard motors.

I won't need these rings now, but will hang onto them for another time.