A workhorse in any stone studio, the standard 4 to 5 inch angle grinder spins up around 10,000 rpm. This is pretty much designed speed for metal and masonry cutoff and grinding discs. Diamond blades and cup wheels for dry operation are engineered for this speed and rely on the higher rpm for proper cooling. Silicon carbide cups, Zec discs and similar abrasives are rated at the higher speed, but perform smoother and longer around 5000 rpm. Finishing and polishing materials are happier around 3000 rpm. So, three different uses at three different speeds; go buy three different grinders?
The frugal stone carver soon discovered that plugging into a router speed control would let one grinder do an adequate job at three different speed requirements. The router speed control works similar to a fan rheostat or a light dimmer. Moving a dial varies the amount of a.c. current available to the connected device; less current means less power and slower rpm. So really, what you get is a variable power grinder that runs fast under no load and slows down to some slower speed dependent upon how hard you push on the stone. Tool manufacturers saw $$$ in the demand for this flexibility and incorporated the handful of electronics for current control into “variable speed” grinders. Despite their shortcomings, these are a viable all-around tool. Hang on tight when you hit the start switch. Ease onto the stone for slower speed operations. Pay attention to what the attachment is doing on the stone and vary pressure accordingly to control the speed. After a while it all seems normal and you don’t even consider getting along without one.
Enter a new breed of grinders; advertised with “electronic speed control maintains desired speed under load”. These are a grand step forward in multi-function tools. They use an electronic feedback circuit to monitor the shaft rpm and vary the current to hold the speed steady regardless of the load (well, at least within the current/power rating of the tool). Dialing up a speed, like setting a cruise control, lets the sculptor attend to form and technique with less distraction imposed by the tool. Smooth operators they are, with repeatable and consistent performance during cutting, grinding, shaping and sanding. A few models also use the electronics to reduce the current for sudden drops in rpm, like wedging a disc in a corner, thus preventing motor burnout and reducing damage to attachments. Some also incorporate “soft start” features, slowly and smoothly bringing the tool up to speed instead of instantly torquing against the hands and wrists. Most of these new grinders have some type of shaft clutch to protect the gears and the sculptor from kickback caused by serious jamming.
If you are in the market for a new variable speed grinder, or maybe you’re just another tool hog like me, check these new machines out. They don’t have much history. They do seem to perform at a level different from their predecessors. I’m interested in any feedback? Oh, yeah . . .gloves and goggles and muffs, oh yeah. Mail/email your comments to us at NWSSA.