By ROGER E. CLOUTIER, Senior Product Safety Engineer Carborundum Abrasives, Saint-Gobain North America
edited by RICHARD MANDEL, senior editor

Safely grinding material with a type 27 grinding wheel starts with properly assembling the tool.

Inadequate back support led to this wheel's failure.

OSHA diagram showing correct wheel mounting.

Placing 9 in. Type 27 wheel over adaptor back flange and hand tightening the adaptor nut.

Recently, engineers at Saint-Gobain Abrasives conducted investigations of wheel breakage involving seven-in. and nine-in. diameter, Type 27 depressed-center grinding wheels. (For those unfamiliar with depressed-center grinding wheels, an inner diameter of the wheel's face is concave for mounting to tools, letting the rest of the wheel's diameter to be used for grinding work without interference from the mounting nut. Grinding surfaces of Type 27 wheels are flat, while those of Type 28 wheels are concave. Convex surfaces are found on Type 29 wheels.) The conclusion indicated the breakage was occurring when wheels were mounted on machines with matched flanges, without the specially designed adaptor mounting system required for large Type 27 wheels. The matched flanges caused or contributed to the breakage by not providing adequate back support for the wheel.

ANSI (American National Standard Institute) and OSHA (Occupational Safety and Health Administration) have detailed specifications addressing the required supporting adaptor flange mounting system for seven-in. and nine-in. depressed-center grinding wheels. Additionally, CCOHS (Canadian Center for Occupational Health & Safety) has suggested the use of adaptor flanges on its web site.

Following is detailed mounting information from each of these organizations, followed by safety tips for using large, depressed-center wheels.

The relevant ANSI standard
The ANSI standard applicable to this situation is ANSI B7.5.1.2.2, which refers to "Types 27, 28 and 29 wheels larger than 5-in. diameter." The standard says that the types of grinding wheels referenced in the title require specially designed adaptors because of their shape and the ways in which they are used. Further, the adaptor or back flange should extend beyond the central hub (raised portion) and contact the wheel. The ANSI standard further explains that the purpose of this arrangement is to counteract the side pressure on the wheel while it is in use. To ensure that an adaptor does not interfere during side grinding, the standard also stipulates that the adaptor nut should be "less than the minimum one-third diameter of the wheel" and fit "in the depressed side of the wheel." This section of ANSI B7 concludes with an implicit warning that such adaptors should not be used with other types of grinding wheels: "These specially designed adaptors shall be used to mount only Types 27, 28 and 29 reinforced, organic-bonded wheels."

The corresponding OSHA standard
OSHA's 29 CFR Part 1910.215 elaborates upon the ANSI standard by explaining that the adaptor nut, which "is less than the minimum one-third diameter of [the] wheel" and "fits in the depressed side of [the] wheel...serves to drive the wheel by its clamping force against the depressed portion of the back flange." Explaining to some degree the final point of the ANSI standard, the OSHA standard says that it is because users normally apply side pressure on these wheels, that the adaptors should be limited to use with reinforced organic bonded wheels. The OSHA standard further notes: "Mounts which are affixed to the wheel by the manufacturer shall not be reused." The standard also emphasizes that Type 27 and Type 28 wheels "shall be used only with a safety guard located between [the] wheel and operator."

Advice from Canada
On the Canadian Center for Occupational Health and Safety's (CCOHS) Web site at (http://www.ccohs.ca/oshanswers/safety_ haz/abrasive_wheels/mounting.html) the text goes beyond the officious language associated with standards, instead making explicit recommendations for mounting depressedcenter wheels:

• Replace any worn or bent reusable adapters. A damaged adapter will not mount properly.
• Do not reuse "throw-away" adapters.
• Ensure that the grinder spindle shoulder runs true. The adapter must tighten against this shoulder. Use spacers provided with adapters if the spindle is too long.
• The wheel will wobble if the shoulder is not square with the spindle, or if the adapter does not tighten against the shoulder. This can result in wheel breakage and injuries to the operator or a nearby worker.

Advice from Saint-Gobain Abrasives
Our company's investigations into wheel failure, experience with difficult grinding situations, and the desire to keep our customers' employees safe, led us to the following list of DO's and DON'Ts regarding the use of depressed-center grinding wheels:

• Do store grinding wheels properly.
• Do check all wheels for cracks or other damage before using.
• Do use grinding wheels on machines that are designed and guarded for the grinding wheel.
• Do check machine speed before mounting a grinding wheel. Never overspeed a grinding wheel.
• Do use proper Type 27 grinding wheel guards.
• Do adjust grinding wheel guard properly. The guard must be between the operator and the wheel.
• Do mount grinding wheel properly.
• During new wheel start-ups or after storage, do always test wheel and machine by running it in a protected area at least one minute before grinding.
• Do wear required personal protective equipment (PPE) such as eye and face protection and other protection as required by the work being done.
• Do employ dust controls and/or protective measures appropriate to the materials being ground.
• Do grind using the proper grinding angles as shown on the grinding wheel.
• Do store wheel and machine properly. Use racks or hooks to support the machine when not in use.
• Do control the wheel's spark stream or shower. Care must be taken to shield any nearby flammable materials to prevent ignition from spark shower or latent sparks.
• Do be aware of bystanders. Shield or protect them from possible injury do to wheel breakage or spark stream/shower.
• Don't abuse wheels or their packages causing damage to the wheels.
• Don't use damaged wheels.
• Don't use wheel on pistol grip air sanders or any machine not specifically designed and guarded for Type 27 wheels. Never use on horizontal or inline grinders.
• Don't use these wheels without a properly adjusted wheel guard.
• Don't mount the wheels upside down.
• Don't reuse "non-reuseable" mounting flanges.
• Don't over-speed the grinding wheel.
• Don't drop or toss the wheel and machine. Wheel damage may occur!
• Don't handle machine carelessly.
• Don't use heavy side grinding pressure on thin Type 27 wheels.
• Don't "jam" the wheel into the work.
• Don't use the wheel flat. Use proper grinding angles.
• Don't use wheel and machine in a "pry bar" manner.

Type 27 grinding wheels have been successfully used for over 50 years. If you keep the proper safety rules in mind, this tool can provide you with high productivity and a safe working environment in welding and fabrication operations.