Follow these simple rules when grinding metal, concrete, and more.
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An angle grinder is one of the most widely-used power tools, in part because it’s fun to use and in part because it’s so effective at removing material. Unfortunately, among power tools, it’s also among the most widely misunderstood and misused. People use them to cut metal with the wrong wheel. For example, they’ll use a grinding wheel not a cutting wheel or a combination grinding-cutting wheel. They may even use the same wheel to cut or grind concrete. They remove the grinder’s guard to expose more wheel surface. They do all kinds of crazy stuff with these tools, and usually they get away with it. But sometimes they get hurt. The rules for using the tool are few and simple—much like any other power tool.
The angle grinder is used primarily for grinding and cutting metal. However, with a masonry wheel mounted on it, it will also grind, cut, and score concrete, mortar, and brick. With a flap disc, it will blend and smooth out welds, making the welded assembly look like a single piece of steel. There are even wood grinding discs, if you’re so inclined. It’s a wonderful and versatile tool, so long as you use it correctly.
Not All Wheels and Discs Are the Same
The grinder’s motor shaft moves a gear assembly that turns the motor’s output 90 degrees to an output shaft or spindlet on which you mount a grinding wheel or disc. The wheel or disc is clamped between two flanges, one on the front of the wheel and one behind it. Once tightened between those two flanges, the wheel is locked securely to the spindle and spins.
Here’s where the trouble starts. Not all wheels and discs are the same. Fortunately, there’s no mystery in selecting a wheel (or disc). Everything you need to know, from its size, safe operating speed, and what it’s designed to do is printed on its package and on the accessory itself.
Below are two of the most common wheels used on a 4-1/2-inch angle grinder and an abrasive flap disc.
Here are the three wheels arrayed with their working surfaces face up.
Let’s look more closely at a wheel. All grinding wheels, combination wheels, and flap discs will have the following information printed on them: wheel type, size, opening diameter, maximum operating speed, and what materials and types of use the wheel or disc is rated for. The product text or an icon will identify whether the wheel is used for grinding or cutting (or both) and what kind of guard it needs to have in place for that operation.
Pay close attention to the icons on the grinding wheel or disc. In this case, here’s a closeup of the icons indicating safe operation of this wheel.
A flap disc is different than a wheel in that it is made up of layers of abrasive sheets arrayed around the wheel’s center.
Mount the Wheel or Disc
Grinding wheels and abrasive discs mount with a variety of methods. To mount or remove an accessory on the typical grinder, hold down the spindle lock button to keep it from turning.
Some grinders such as this Bosch feature tool-free accessory mounting and removal. Pull back the accessory-mounting lever to drop out the accessory (wheel or flap disc).
These tool-free grinders take a specialized wheel. In the case of the Bosch, its specialized spring-loaded locking mechanism must match up with a wheel or flap disc designed for it. Simply press a specialized Bosch grinding wheel onto the spring-loaded locking mechanism on the end of the motor spindle.
Orient the guard to allow the wheel full access to the workpiece and to direct sparks.
Thread the handle into the mounting hole on the left or right side of the grinder.
First, suit up and put on protective gear. Grinders are fun to use, but they are power tools, and like any power tool, they produce airborne debris and they can be dangerous if you misuse them. Grinder safety gear includes sturdy work shoes, coveralls or a long-sleeve cotton work shirt or a fire-retardant work shirt is even safer.
Let the grinder come up to speed and put the wheel’s leading edge against the work. Be careful not to bang the wheel and, until you’ve got the hang of using the tool, less pressure is more effective than leaning into the tool.
While you work, pay attention to where the sparks are going.
If you grind concrete or cut concrete blocks, you’ll also need a dust mask. The problem, of course, is that even vented dust masks can cause fogging of your safety glasses. And that’s dangerous when using a tool like an angle grinder. We’ve had decent results with 3M’s mask, and Stoggles safety glasses which come from the factory with a fog-resistant coating that needs to be re-applied with a spray-applied treatment occasionally.
Roy Berendsohn has worked for more than 25 years at Popular Mechanics, where he has written on carpentry, masonry, painting, plumbing, electrical, woodworking, blacksmithing, welding, lawn care, chainsaw use, and outdoor power equipment. When he’s not working on his own house, he volunteers with Sovereign Grace Church doing home repair for families in rural, suburban and urban locations throughout central and southern New Jersey.
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