I wanted a mechanical switch to turn on and off my bench mounted vacuum system using only a single button so I designed this quick mechanism using a reciprocating rotary gear to turn rotary motion into reciprocating motion. It is printed in PLA (Base, Top, and Switch Holder) and ABS (Gear and Slider).
http://www.youtube.com/watch?v=TfMCX3Ipljo
Fits a Ridgid 6HP (16) Gallon Wet/Dry Vac (I'll have to check the "gallon" figure later)

Instructions

The switch uses a 3RPM Gear Motor (DFGA25RP-654i) and the motor mounts with (2) 3mm x 8 button head screws and the gear is tapped to accept (2) 6/32" set screws to fit the 4mm shaft. The bottom base and top plate are held together with (6) 2/56" x 1/2" screws and nuts. The switch itself is just press fit onto the vacuum and can be easily removed if needed. It is run off a 12 volt battery right now but I will add a wall power supply later on. The motor could be a bit faster as it takes about 7 seconds between on and off but it's what I had laying around at the time. (I have since switched to a 10RPM motor for faster response.)