Prepping a tube for welding with an angle grinder.

A hand-held die grinder.

Bench grinder with 10 in. wheels.

Grinding burrs from a wheel with a robot.

When instructed to weld two parts together, no welder would pick up just any torch at hand and start laying a bead. The same consideration typically should extend to other shop equipment, such as grinders.

Grinders and grinding machines put abrasive into motion for removal of paint or scale, deburring, edge preparation, or reduction/ removal of weld beads and spatter for a smooth look on a product. Important specifications for grinders and grinding machines include rotary speed, surface or linear speed, power, and current. Some grinders and grinding machines use an orbital or planetary motion, others a vibratory or oscillatory motion. The choice will depend of the desired material removal speed or finish quality. In some grinders, motor speed can be adjusted continuously during grinding, or set at discrete speeds within an operating range.

The handheld option
The angle grinder enjoys a lot of popularity in shops for its portability and versatility. The grinders use abrasive discs ranging from 9 in. diameter down to 2 in. diameter, and shop machines can be electrically or pneumatically driven. The two-handed grip on larger grinders helps the operator maneuver the wheel into tight spaces and through odd shapes. Some manufacturers offer accessories for angle grinders such as wire wheels, buffing pads, saws and surfacing discs. Large bearings counter side forces generated during cutting tasks, as compared to a power drill, where the force is axial.

It should be noted, however, that many sources also consider angle grinders to be one of the most dangerous tools in a workplace. While most injuries are related to the operator not using safety glasses, an angle grinder is also prone to kick back. The discs are also prone to breakage due to rough handling or improper installation, resulting in pieces flying at high speeds in all directions. It cannot be repeated too often to let an angle grinder stop completely before setting it down; to grind the workpiece at an angle of 15 to 30 degrees to minimize the possibility of kick back; and to never use a grinder between the legs when sitting on a floor.

Die grinders are similar to angle grinders, but rather than discs, they use stones of abrasive bonded into shapes like cylinders, cones, balls and more. Most die grinders are offered with spindles axially aligned to the motor.

Flex-shaft grinders use stones like die grinders, but by shifting the motor away from the hand unit, there is less fatigue from supporting the motor's weight and greater ease of access to small areas. The flexible shaft that couples the motor to the handpiece tends to limit the amount of power that can be transmitted to the grinding tip.

Besides appropriate equipment such as safety glasses and dust/fume extraction systems, workers who use grinders for extended periods would benefit from special anti-vibration gloves, such as those that incorporate gel-filled pads.

Benchtop
Probably the first grinder anyone first experiences is one with a motor and one or two shaft stubs supporting abrasive wheels. Mounted on a bench or a stand, the bench grinder is best at dressing and sharpening tools. The machine is not recommended for removal of spatter and weld beads, owing to a limited visibility of the area being worked.

A better choice would be a bench-mounted belt grinder. Such units use a continuous abrasive belt, typically about 1 in. wide. The narrow width helps when working small articles, but it is necessary to have a secure grip on the item being worked, preferably NOT with the fingers: otherwise the belt can on snag the part, jamming the machine or turning the part into a projectile. Note that a few companies, such as Makita (www.makita.com), manufacture portable handheld narrow belt power sanders that can be useful in some metalworking detail applications.

Robotic grinding
With the growth of automated production systems have come robots and robot attachments designed for grinding welds and deburring castings. Some grinding attachments carry more than one abrasive, so that the robot performs both rough, then finish grinding with minimal interruption.

Manually grinding welds produces metal dust harmful to workers' eyes and lungs, besides being tough, dirty and noisy work. With a robot, a 96 man-hour-per-shift manual grinding job now takes less than 10 hours, with more consistent, high-quality results. Robots also remove the possibility of injuries that occur with constant use of grinders, as with white finger disease (also known as Raynaud's Phenomenon) that afflicts workers who deburr castings in foundries.

Some robotic systems incorporate a vision system or other sensors to determine orientation of the workpiece relative to the robot. Contact sensors, incorporated into a joint of the robot such as the wrist, help determine the shape, size, weight, or even the surface texture of an object. Additional force detectors and software enhancements can adjust the grinder to compensate for abrasive wear.

Ultimately, choosing a grinder follows the same rules as other shop equipment — what are the expected results of using the grinder, and how many workpieces are involved. From those questions comes the next step of examining catalogs, speaking to machine tool sales representatives, and purchasing what suits your shop's needs.

Images supplied by: Milwaukee Electric Tool Corp. (www.milwaukeetool.com), Baldor Electric Co. (www.baldor.com), and FANUC Robotics America, Inc. (www.fanucrobotics.com).