Depressed-center grinding wheel safety—test your knowledge

June 1, 2012
By: Dave Long, Roger Cloutier

Do you use portable grinders with depressed-center wheels? Are you following the proper safety guidelines? Take this 10-question test and find out.

Portable Grinders

All power tools have inherent dangers and can be hazardous when used improperly. Careful attention to safety guidelines is paramount when using these tools.

Portable grinders with Type 27 depressed-center wheels are among the power tools used in metal fabricating. The following test is designed to assess your knowledge of depressed-center grinding wheel safety. Are you skilled using these wheels and tools, or do you need more training? Take the test and find out.


1. What are the proper grinding angles for a Type 27 depressed-center wheel?

a. Wheels can be used at any angle.

b. Wheels must be used flat.

c. Wheels must be used on their outer periphery only.

d. Depending on the thickness and application of the wheel, it may be used at a 30-degree or 90-degree angle as indicated by icons on the wheel.

2. What is the recommended shelf life of a Type 27 grinding wheel?

a. Grinding wheels are “rocks” or “stones” and do not have a shelf life.

b. Grinding wheels should be used within one year of their date of manufacture.

c. Properly stored grinding wheels should be used within two years of their date of manufacture.

3. What is the guarding requirement for a Type 27 depressed-center wheel?

a. If the tool does not have a wheel guard and one is not supplied by the tool manufacturer, then none is required.

b. If I am wearing personal protective equipment (PPE), no guards are required.

c. The correct size and type of wheel guard must be used at all times.

d. A wheel guard may be removed if it interferes with grinding the workpiece.

4. If a grinding wheel has an RPM rating of 6,600, what is the maximum RPM rating of the tool it can be used on?

a. It’s common knowledge that these wheels are tested at 1.5 times their rated speed, so as long as the tool’s speed rating is below 9,900 RPM it is safe.

b. It is safe to use this wheel on a tool that has an RPM rating of 6,600, even if there is a warning on the tool that says, “All accessories must be rated at least 7,000 RPM.”

c. The speed of the tool must be the same as or below the speed of the wheel.

d. Wheel speed is unimportant; I can ignore it.

5. Is it safe to remove the side handle on vertical and angle grinders?

a. No, it is not safe. ANSI B7.1 requires that all portable vertical and angle grinders 4 in. and larger be equipped with a side handle.

b. Side handles help you control the tool and make grinding more comfortable, but they are not required and have no safety value.

c. Side handles may be removed when they get in my way.

d. Side handles are a problem for left-handed people, who are allowed to remove them.

6. How do I “ring test” a Type 27 depressed-center wheel?

a. Tap the wheel about 45 degrees each side of the vertical centerline and about 1 or 2 in. from the periphery. Rotate the wheel 45 degrees and repeat the test.

b. Gently tap the wheel with a light, nonmetallic implement, such as the handle of a screw driver.

c. They cannot be ring-tested, because they are made of resin with fiberglass reinforcement. I should carefully inspect the wheel for any type of damage. After mounting the wheel on the tool, I should run the wheel at operating speed with the safety guard in place and in a protected enclosure (e.g., in a barrel or under a workbench) for one full minute before I start grinding.

d. Type 27 depressed center wheels are tough and do not need any testing.

7. How often does ANSI B7.1 recommend that the speed of pneumatic (air driven), portable, governor-controlled, vertical and angle grinders be checked?

a. The tool’s speed should be checked with a tachometer or similar device after every

20 hours of actual use or once per week, whichever comes first, and after any tool maintenance or repair.

b. The tool’s speed should be checked only when I notice a change in the tool’s speed.

c. The tool’s speed needs to be checked only after repairs or maintenance.

d. The tool’s speed is unimportant to safety and does not need to be checked.

8. Per ANSI B7.1, the proper mounting of Type 27 depressed-center wheels (without discard mounts or mounts affixed by the manufacturer) requires the following flange setup:

a. All wheels must be mounted between matched flanges.

b. All wheels must be mounted between flanges of at least one-third the wheel’s diameter.

c. All wheels must be mounted between flanges of at least one-fourth the wheel’s diameter.

d. Wheels with diameters greater than 6 in. must be mounted with a special supporting back adaptor and inside flange nut. Wheels 6 in. and smaller may be used with matching flanges or the adapter.

9. Type 27 depressed-center wheels must be stored under the following conditions:

a. Being wheels are resin products, they can be stored under any weather condition.

b. Wheels should not be stored where they are exposed to water or other solvents; in any temperature or humidity condition that causes condensation on the wheels, and freezing temperatures.

c. Wheels should not be stored where they are exposed to water and freezing temperatures only.

d. Wheels should not be stored outside.

10. When using Type 27 depressed-center wheels, I and bystanders should wear the following PPE:

a. Because most grinders are well-guarded, PPE is not required.

b. Eye protection, and dust and hearing protection when needed.

c. Faceshield, eye protection, dust protection (when needed), hearing protection, gloves, apron, safety shoes and long pants.


1. D

2. C

3. C

4. C

5. A

6. C

7. A

8. D

9. B

10. C

If you scored 90 percent or higher, congratulations; you earned a gold star. If you scored 70 percent to 90 percent, you earned a silver star. If you scored below 70 percent, you need a refresher.

The answers to these questions were taken from a combination of sources, such as ANSI B7.1, OSHA, and industry best practices.

For additional information on this topic or if you need any other abrasive safety information, please review ANSI, OSHA, and all literature provided by the coated abrasive and machine manufacturer.

Dave Long, Director of Marketing & Strategy, Norton Abrasives
Roger Cloutier, Senior Product Safety Engineer, Saint-Gobain Abrasives Inc.

Dave Long

Director of Marketing & Strategy
Norton Abrasives
1 New Bond St
Worcester, MA 01606
Phone: 800-446-1119

Roger Cloutier

Senior Product Safety Engineer
Saint-Gobain Abrasives Inc.
One New Bond St.
P.O. Box 15008, M.S. 301-411
Worcester, MA 01615-0008
Phone: 508-795-5000

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