Safety in Use of Abrasive Wheels

Note: The author has contributed his grinding knowledge to an interactive e-learning course on the safe handling of abrasives wheels. This course dives in deeper than this article could ever do. Access to this course, and other engineering courses, can be obtained from THORS eLearning Solutions. Click on the image below:

The recommendations, as set out in this document, are only a summary of the most important precautions. All users must strictly comply with the safety instructions existing in the laws, regulations, and technical standards of their country.

 United States of America

American National Standards Institute (ANSI), Inc., NY, "Safety requirements for the use, care, and protection of abrasive wheels," Standard B7.1. www.ansi.org, Also, consult the Occupational Health and Safety Act of 1970 (OSHA).

 India

India adheres to the US standards. Equivalence: ANSI B.7.1, Bureau of Indian Standards, (BIS), Production and General Engineering, Section Name: Abrasives (PGD 9), Designator of Legally Binding Document: IS 1991-4, "Safety requirements for the use, care, and protection of abrasive grinding wheels."

United Kingdom

"Safety in theUse of Abrasive Wheels". Revised in line with the Provision and Use of Work Equipment Regulations 1998 (PUWER 98). This is a free-to-download, web-friendly version of HSG17 (Third edition, published 2000). It has been adapted for online use from HSE's current printed version. The books are available at www.hsebooks.co.uk and many bookshops., ISBN 978 0 7176 1739 5 https://www.hse.gov.uk/pubns/priced/hsg17.pdf

Europe in general:

FEPA Safety Code, obtainable from: FEPA, 20, Avenue Reille, 75014 Paris, France, www.fepa-abrasives.or

Safety Guides, MSDS and Wheel Warning Messages

Many wheel suppliers provide information about the safe use of grinding wheels. This information should be read carefully. Contact the wheel supplier for copies of MSDS (Material Safety Data Sheets).

 Attributing Safety Responsibilities

When attributing responsibilities for the overall safety issue pertaining to grinding wheels, we can distinguish between three groups:

 1.   The grinding machine builder

2.   The grinding wheel manufacturer

3.   The user of both machine tool and grinding wheels

 Each of these groups has specific responsibilities:

1.   The Grinding Machine Builder:

• Grinding flange design in line with regulations
• Automatically locking doors
• Software controlled speed limitation
• Above 63 m/s (12,500 sfpm), the machine requires an independent safety authorization process

2.   The Grinding Wheel Manufacturer

• X-fold safety against wheel bursting
• Speed testing at the plant with higher than the operating speed
• Bursting tests at the plant
• Marking with maximum permissible operating speed (and/or RPM)
• Ensuring adequate packing for shipping

3.   The User of Machine Tool and Grinding Wheels

• Proper storage
• Ring test of the grinding wheel before mounting
• Use proper plastic, paper, or aluminum blotters
• Use proper wheel flanges
• Adequate mounting (using a torque wrench)
• Balance the grinding wheel (if necessary)
• Check the maximum permissible RPM
• Idle wheel for two minutes before use
• Wear safety glasses

Safety Aspects of the Machine Tool

Modern CNC machine tools have many features to protect operators from harm. Primarily, the CNC machine tool design must prevent the user from operating the machine tool with open doors and with safety guards removed. For this purpose, machine tool builders fit so-called "interlocking devices" on doors and guards. These devices are covered by the standard EN ISO 14119. In the interest of greater clarity, these interlocking devices are also called safety guards. The term "interlocking" is still somewhat misleading because interlocking does not refer to the safety door as such but instead refers to the interlocking of the CNC unit to prevent the machine from starting. This means that the machine cannot start until the safety door is closed. Inversely, on opening, the machine must stop automatically. Modern safety guard switches cannot easily be outmaneuvered to make the machine control accept that the door is closed when, in fact, the operator keeps it open to work within the machine area.

Emergency Stop

Machine tools are also fitted with an emergency stop, which upon pressing, will retract the grinding wheel from the workpiece and shuts down the machine tool. 

Illustration 1: Emergency stop (Source: Reishauer)

Illustration 2: Safety Guard switches on the machine tool (Source: Reishauer)

Machine Lockout

For more complex machine systems that may include robots for material handling, the machine is subject to a machine lockout plan. This lookout plan stipulates which steps have to be taken before any maintenance can be initiated. The lockout procedure identifies and locates energy sources such as electricity, water, and air. Furthermore, the procedure clearly describes the tasks that have to be performed.

Safety Shields on Machine Tools

Safety shields on machine tools are designed to offer the operator visibility of the machining process while protecting the operator from potential injuries caused by grinding wheel fragments or workpieces flying out of the machine's working zone. Polycarbonate proves to be a suitable material for vision panels because of its high-energy absorption capacity. The impact resistance of 8-mm thick polycarbonate pane is roughly equal to that of a 3-mm thick steel sheet. The limited aging stability, however, makes it necessary to protect polycarbonate against cooling lubricants by using additional panes on both sides, or by replacing polycarbonate safety shields every two years as illustrated below:

Illustration 3: Polycarbonate shields deterioration over time (Source: Fritz Studer AG)

CO2 Fire Extinguishing Unit

Grinding machines that use pure oil as their grinding fluid (coolant) pose a fire hazard. To protect operators and the machine tool, CO2 fire extinguishing units are integrated into the machine design. These units are equipped with sensors that would automatically trigger the release of CO2 gas into the machine to extinguish the fire. As CO2 gas is also a health hazard, the triggering of the gas release must sound an alarm to evacuate the building immediately.

Illustration 4: CO2 fire suppression unit (Source: Kraft & Bauer)

As automatic fire extinguishing units tend to be very costly, many production plants rely on the operator using a manual CO2 fire extinguisher. For this purpose, the machine tool builder has to fit the machine with a flap door through which the CO2 gas can be released manually into the machine's operating space.

Illustration 5: Flap door for manually inserting fire extinguisher (Source: Reishauer)

Marking of Grinding Wheels

The manufacturer must mark grinding wheels so that they are easily identifiable. This may be direct printing on the grinding wheel body with a special inkjet printer or marked with stencils. If a grinding wheel has been supplied with a paper blotter glued on, it may carry the required information on the blotter. The following shows such a blotter listing all the legally required information: 

Illustration 6: Information supplied on a wheel blotter

Before delving into the "dos and don'ts" of grinding wheel usage, we should first define what a grinding wheel is. For this purpose, we primarily look at grinding wheels (bonded abrasives) for precision grinding machines, either cylindrical or surface grinding. This means that the wheels described in this course are not suitable for grinding with hand-held machines. Most grinding wheels used on high precision cylindrical or surface grinding machines are vitrified bonded wheels. A vitrified bonded wheel consists of abrasive grains held together by a vitrified bond. A third element that makes up a vitrified wheel are the pores. These create space for the chip removal during grinding and the transport of coolant into the grinding zone. The bonding material, mostly special glasses, must hold the wheel together during the grinding process at high surface speeds between 35 and 50 m/s (6,890 to 9,843 sfpm) for conventional grinding. When grinding with CBN, surface speeds may go up to 140 m/s (27,559 sfpm).

Illustration 7: Components of a vitrified bonded grinding wheel

Grinding wheel speeds are given in meters per second (m/s) or surface feet per minute (sfpm).

The conversion factor of converting m/s to sfpm = 196.85. For example, 50 m/s = how many sfpm?   50 x 196.85 = 9,440 (rounded-off). To calculate the wheel's RPM from the wheel's surface speed, use the following formula:

ds = wheel diameter (mm)

ns = wheel RPM

vc = wheel surface speed in m/s

Over-speeding Factor

To ensure the safety of the operator and the machine, all grinding wheels are tested with an over-speeding factor. For example, in the USA, this over-speeding factor is equal to 1.5 times the permissible operating speed. These over-speeding factors vary according to countries. Europe has a lower over-speeding factor, which is 1.1 for fully enclosed machines and a factor of 1.3 for open machines. Japan, on the other hand, has the highest over-speeding factor, which is two times the operating speed. Many European grinding wheel manufacturers tend to utilize the American over-speeding factor of 1.5 operating speed.

That is to say, that a wheel used at a maximum permissible operating speed of 50 m/s (9,845 sfpm) must be subjected to a testing speed of 75 m/s (14,765 sfpm), which the wheel must withstand without bursting. The wheel manufacturer must regularly monitor busting speeds by over-speeding the wheels until they burst. This procedure ensures that the wheel safety remains constant. The following graphic illustrates the different speeds applied.

Illustration 8: Operating, testing, and bursting speed of grinding wheels

 The grinding wheels must be marked with the maximum surface speed at which they can be used. The wheels marking must contain the following information, either on the wheel body itself or in the case of smaller wheels, on the paper blotters that are glued onto the wheels.

 Shelf life and Proper Storage of Grinding Wheels

As mentioned before, cylindrical grinding wheels are generally vitrified bonded abrasives. However, there are exceptions, such as resinoid bonded wheels. Regarding shelf life, resinoid bonded wheels should be used within two years from the date of manufacture. Theoretically, vitrified bonded wheels have an unlimited shelf life, which, according to FEPA, is limited to 10 years. However, to be on the safe side, even vitrified bonded wheels should be inspected again after two years and be re-speed tested if they are older than two years. Never use an abrasive product beyond its expiry date where marked. In line with FEPA guidelines, observe the recommended shelf life for the following specific products:

•  resinoid and shellac products: 3 years
• rubber products: 5 years
• vitrified products: 10 years.

To minimize deterioration, wheels must be stored in a dry room that is not subject to extreme temperatures or moisture. It is recommended that wheels should be marked with the date they are received from the supplier. Older wheels should be used before newer wheels, and if there is any doubt, or if wheels have been in stock for more than three years, the manufacturer should be consulted about their suitability for use.

 Suitable racks should be provided to accommodate the various types of wheels used. Most plain and tapered wheels are best supported on their edges or on a central support. When the wheels are placed on their periphery, the support should take the form of a cradle to prevent rolling, with a sufficient number of partitions to prevent wheels from falling over. See illustration below: 

Illustration 9: Proper storage of grinding wheels

The Ring Test

Vitrified wheels must subject to the so-called “ring-test” before being mounted on a wheel flange and before being used for grinding. This testing technique has its origin in the craft of bell making. After a bell was cast and had cooled down, the master bell maker would tap the bell with a hammer, and if there was a clear ringing tone, he knew the bell was sound. The same principle applies to grinding wheels. The ring sound comes from the glassy structure of the bonded material and its ability to transmit sound across the wheel matrix and thus producing a clear ringing sound. If the wheel is cracked, the sound waves stop at the crack, and the sound we hear is dull with no ringing to it. Please note that the ring test does not apply to resinoid bonded wheels as these do not emit a clear bell-like sound even if they are not cracked. 

Illustration 10: Ring testing a vitrified grinding wheel

Procedure:

The vitrified grinding wheel should be lightly tapped with a non-metallic hammer to the right and the left of the centerline. Light and small wheels should be held on a finger or supporter by a mandrel. Heavier wheels should be placed on the floor, standing upright on their peripheral face. In the case of concrete floors, place a wooden board between the floor and the wheel . A crack-free vitrified wheel emits a clear, bell-like ringing sound, whereas a cracked wheel sounds "dull and dead."

Please note: Cracks are often invisible to the eye!

The "DOs" and "DON'Ts" of Grinding Wheel Usage

Always:

• Read the safety rules pertaining to your country. Consult the grinding wheel supplier's main catalog and any safety leaflet which may be included in grinding wheel shipments
• Inspect products for defects when receiving and again before mounting. 
• Store grinding wheels in suitable rooms fitted with appropriate racks. 
• Ensure that permissible RPM and/or surface speed in m/s or sfpm cor-responds to the grinding wheel and the machine intended for use.
•  Use paper or plastic blotters between the wheel and the steel flange.
•  Use a torque wrench when tightening flange nuts.
•  Inspect all guards and other safety features for proper functioning.
•  Wear safety glasses.
•  When starting the grinding machine, step out of the way, and run the machine at idle speed for 1 minute before cutting metal.
• Operate machine according to machine and wheel instructions.

NEVER

Never run a grinding wheel at higher surface speeds (m/s or sfpm) than allowed by the wheel manufacturer and marked on the wheel

Never mount a vitrified grinding on a flange without a prior ring test. If a dull sound occurs, discard the wheel and make sure no one else uses it.

Do not force a grinding wheel onto a flange, shaft, or arbor. Should the wheel bore be too tight, discard the wheel or return it to the supplier. 

Do not operate a grinding machine with its safety features removed

Do not use a grinding wheel without wearing safety glasses

 If in any doubt, contact the wheel supplier!

Changing Wheel Geometry by Adding Recesses

Straight Type 1 wheels can be recessed on one side to create a Type 5 wheels shape or recessed on both sides to create a Type 7 wheel. It is important to note that the total recess must never exceed 50 % of the wheel width" T": 

Illustration 11: Wheel shapes and safety aspects

The Ratio of Steel Flange Diameter to Grinding Wheel Diameter

The outer flange diameter must, as a minimum, measure one third (1/3) of the new grinding wheel diameter. This ensures proper and safe clamping of the grinding wheel.

Illustration 12: Ratio between the grinding wheel and flange diameter

Mounting of Grinding Wheels

Before mounting, ring the wheel to ensure that it is not cracked. Always use blotters between wheel and steel flange. For surface speeds of 50 m /s (9,845 sfpm) or higher, plastic blotters (polypropylene 0.5 mm (0.020") thick should be used. If paper blotters are used, moisten them with coolant before flange assembly. Paper blotters may require re-tightening after one day.

Illustration 13: Paper or plastic blotters between steel flange and grinding wheel

The Flange Assembly

The flange assembly consists of the grinding wheel, a metal flange made of mild steel, and paper or plastic blotters between the flange plates to compensate any wheel irregularities. Ensure that the metal flange does not have any nicks or burrs and that it is not deformed.

Tightening the flange assembly bolts

Always use a torque wrench for tightening flange screws. As per ANSI B7.1-1988, Chapter 6.10.2, applied torque should not exceed 20 foot-pound (ft-lb) or 27 Newton-meters (Nm) unless specifically recommended by the wheel manufacturer.

1 Newton meter (Nm) = 0.737 foot-pound (ft-lb), 1 foot-pound (ft-lb) = 1.336 Newton meter (Nm). Use the tightening sequence as illustrated below: 

Illustration 14: Tightening sequence of flange bolts

Balancing a Grinding Wheel

There are two distinct forms of balancing:

1. Static balancing outside the machine

2. Dynamic balancing on the machine

Grinding wheels are statically balanced if the machine does not feature an integrated balancing unit inside its grinding spindle. Static balancing takes place on a separate balancing unit. The balancing process is performed with the grinding wheel upright, freely rolling on support rails. The balancing unit must be aligned with a spirit level, and the grinding wheel must be dry. The balancing arbor and its mounting surface, and all other contact surfaces must be clean. The balancing unit must sit on a firm base.

 Balancing Procedure

• Remove all three counterweights.
• Place the grinding wheel on the balancing stand and allow coming to rest.
• The heaviest spot of the wheel is now at the bottom. Counterweight No. 1 is mounted exactly directly opposite the heavy spot and is not moved again. 

Illustration 15: Static balancing of a grinding wheel (Step 1) 

Distribute the other two counterweights, No. 2 and No. 3, symmetrically over approximately 120°. Now rotate the grinding wheel by 90° and stop it. If the counterweight No. 1 pulls down, push the other two weights symmetrically away from No. 1, or towards No. 1. When the wheel stays stationary in any position, it is perfectly balanced.

Illustration 16: Static balancing of a grinding wheel (Step 2)

Many modern CNC cylindrical grinding machines are equipped with automatic wheel balancing units integrated into the grinding wheel's spindle. In this instance, for a high-quality grinding wheels, static balancing is generally not necessary.

Illustration 17: Automatic wheel balancing system

Grind safely!

 Walter Graf, Copyright September 2020, The Philosopher’s Grindstone