When I made the move from doing woodworking to working with metal a lot of the tools were new to me. In the days before building an effective tumbler I used wire wheels to remove fire scale. I used wire wheels mounted on a bench grinder and wire cup brushes on an angle grinder.
It didn't take long to discover the annoying and dangerous tendency to shed wire strands which will penetrate clothing. I used heavy gloves and a full face shield in addition to safety glasses. Still that didn't save me when an angle grinder kicked back and the knot wire cup brush grabbed my tee shirt and plowed across the skin of my midsection. Not serious but scary and painful enough to get me going on designing a better tumbler.
An experienced blacksmith gave me the tip of using a variable speed polisher/sander instead of an angle grinder. I got an inexpensive one and it did work well. Most of the time my work doesn’t require high speed operation. The tool is pretty heavy and more awkward to use than an angle grinder. I use it infrequently so it has lasted ten years so far.
I was well acquainted with a wood router and had a speed control unit so I tried it on an angle grinder and found that setup works better. My work can be accomplished at speeds well below where wire breakage is likely and the velocity of a broken wire would not likely cause a serious injury.
This is the model I used.
http://www.harborfreight.com/router-speed-control-43060.html I notice it now sells for $19 but was $15 when I bought it.
I gave up the use of the wire wheels on the bench grinder altogether and never tried a speed control on it.
The tumbler has largely replaced the use of the cup brushes too but on the occasions when I need one I prefer the stainless steel knotted cup brushes.
Civilization owes a lot to the inventors of the wheel and the related beneficial applications of rotary motion but they also introduced the hazards associated with the kinetic energy of a rotating mass. It's our responsibility to utilize the benefits and subdue the dangers.