I believe what I am attempting to do is not possible but here goes. I want to design a fixed overall length coil which I can manually adjust the pitch on. Optimally one would be able to grab the coil and by twisting it in place would be able to change it's pitch by changing its diameter but not changing its overall length. One end of this coil is going to be connected to a motor and the length needs to remain constant, really only the diameter can change though maybe there is another way? Now you know why I think this is not physically possible. Is it? Thanks!!