John C. turned me on to these type of grinders. They are also known as rotary or flex shaft tools and are often used by jewelers. They are electric and pretty quiet though they can spin up pretty high. You hang the motor close to your work and use the flex shaft and tool holder to do the shaping. You can control them with a foot pedal.
An air grinder can keep a 5hp compressor very busy making lots of heat and noise. Very inefficient.
I bought a flexshaft setup and love it. I was too cheap to buy a complete FOREDOM setup (look 'em up). I bought the $50 HF flex shaft grinder with throttle pedal and 1/4hp motor and swapped the 3/16" die grinder for the Foredom 44T 1/4" grinder (another $50). The 44T is small and light and will fit in tight places while the HF motor makes plenty of torque/power. No ear protection required and the thing just hangs there, always ready to use. It's not as convenient as an air grinder for moving around the shop but any time there's more than a little to grind and I can take the work to the bench that's what I'll do.
