An air-powered die grinder, outfitted with a cut-off wheel, is one of my most-used tools. Cut-off wheels are thin abrasive disks, 3 inches in diameter. When you put them on a die grinder they become mini-hacksaws, bolt cutters, polishers, slag removers and perform all sorts of handy tasks.
Last week I had to cut a large, irregular housing made of plastic resin. Too irregular to get an electric cut-off saw into, and the cuts were too deep to clear the frame on a hacksaw. Using my variable-speed die grinder with cut-off wheel I was able to cut a straight line across all the irregularities and accomplish the job.
The list of chores you can accomplish with a die grinder/cut-off wheel is long. I've ground the heads off rivets, I've "freshened" the worn slots in screws and I've cut links out of roller chains. I've made large-diameter circular cuts in sheet metal, I've cut conduit and exhaust pipes, and use my die grinder/cut-off wheel to smooth ragged edges and remove paint from the edges before I weld broken pieces back together.
I highly recommend a reversible die grinder because it allows the user to control which way sparks come off the cut-off wheel. I also like grinders with adjustable air discharges in the handle, so I can direct the oily blast of air away from my face, or keep it from blasting dustor metal filings into my eyes.
Face shields/safety goggles and ear protection are mandatory when using die grinders for any purpose. I used to be macho and do short, quick jobs without taking time to don eye and ear protection. I've learned my lesson.
Add a set of carbide bits to your toolbox and an air power die grinder becomes a double threat. Carbide bits in straight, cone-shabed, burr-tipped and other designs allow you to quickly ream out or enlarge holes in metal, and do all sorts of things that would take hours with a rat tail file.
Air-powered die grinders aren't especially expensive, but you get what you pay for. A $50 grinder will have low power and stall out frequently. A $200 die grinder is much easier to use because of it's extra torque and power. Cut-off disks cost only pennies a piece; and arbor shaft to attach a cut-off wheel to a die grinders is less than $10. A set of carbide bits can range in price from $50 to $200, and once again, you get what you pay for. I paid $75 for my set of carbide bits, and wish I had paid more.