The Tuning Slide
I used two different designs to act as the tuning slide. Both of these have problems, the largest for both designs being the disturbance to the smooth inner surface of the tube. This can mess with the flutes tone and playability. This isnít a huge deal but I have noticed the flutes I have with a tuning slides do play differently from my non-tunable flutes. On the plus side, this type of tunable system also (and probably more usefully) allows you to mix and match head joints and bodies. You may want to only make one embouchure and use it to play a family of flutes in different keys or experiment with different embouchure diameters on the same flute for comparison.
The easier method uses the 1/2 inch connectors already available for the pipes. To add one of these, it only requires that you use two sections of tubing for the flute. One section of the flute, the head joint, has the embouchure; the other has the finger holes. The only tricky part of this design is getting the head joint the right length so it is within range of the correct pitch, while still near the center of travel in the connector. This can be difficult due to the small range over which the tuning slide can move.
Although more time consuming, the acetone method can make for a better tuning slide since it can be as long as needed. The acetone softens the PVC so it can be stretched to fit on another piece of PVC. To start this method you will need acetone, which is sold as a solvent in the paint section of home centers. You also need a container that wonít degrade sitting with acetone in it for a few days. Glass works well for this. The container should be as tall and narrow as possible so as to not waste acetone (once the acetone is used once, it is best that you dispose of it properly or store it separately rather than it back into its container since it leaches all sorts of unknown chemicals from the PVC.)
To make the slide, I started with enough acetone in my container to submerge as much of the PVC as I wanted to stretch. The PVC sat for at least 24 hours before it was soft enough. At this point I took out the acetone soaked pipe and stretched it over another piece of pipe. I let the pipe dry in this position for another 24 plus hours before it was solid again at which point you can finish the flute as usual. You can stretch either the head joint or the body but it would be best to do this labor intensive process to which ever part you plan on sharing the most between other complimentary sections.