Some prep work is needed before moving on to making the rest of this launcher.
In this step we will prepare the DC and servo motors so that they will be easy to install later on.
Make a Piston
Glue a small square of cardboard to the end of a 4" length of straw.
Poke a hole in the other end with a piece of wire or paper clip.
Cut a straight length of metal from a paper clip.
Introduce a 90 degree bend to one end.
Stick this through a hole around the outer edge of the servo wheel and glue in place.
Stick the straw onto the wheel.
Create a small elevated platform for the servo motor to sit on.
Use a square of double sided tape to stick the servo motor on top.
You may notice this small platform is able to wobble back and forth. This is to our benefit.
Positioning the piston perfectly can be hard. This small wobble will allow for fine tuning the position of the piston even after it's stuck to the platform.
Use a Brace
Adjust position of servo motor as necessary so that ping pong balls are just able to fall into place when the piston is pulled back as far as it will go.
Only when you are 100% certain of the position of the piston should you glue the brace to the adjustable platform.
Prepare the Launch Wheel
These wheels come with a grippy rubber ring, which will reliably grab and launch the ping pong balls.
The wheel centers itself conveniently on top of the CD/DVD motor. Place it on the motor to feel where it engages.
Use hot glue to connect the wheel to the top of the motor. Once the glue has cooled a bit, spin it to make sure it's centered and well-balanced.
At the speeds this will be going, a wobbly wheel is no good.