Abrasive Wheels Selection and Applications
White Cap has created a series of articles to help you select the right abrasive wheel for any job you undertake. We’ll guide you through the different aspects of selecting a wheel, explain its components, and how to achieve specific results. For an overview of abrasive wheels, their anatomy, and construction, read our article, Understanding Cutting Wheels.
How to Choose the Right Abrasive Wheel GradeAbrasive wheels are generally made up of two different major components — the grains that actually do the cutting, and a bonding agent that holds these grains together and provides support while they cut.
What is Abrasive Wheel Bond/Hardness?The bond of an abrasive wheel refers to the hardness and strength of the bond, or matrix, which holds the abrasive grains to the wheel. It does not refer to the abrasive grains, only the bond holding the grains. This bond is either “hard” or “soft.”
It may seem backwards, but you should use abrasive wheels with softer bonds on hard metals and harder bonds on soft metals. A soft bond is designed to break down faster for a consistent cut rate, exposes new grains quicker, and performs better on hard metals. However, a hard bond has a longer life span, grinds more smoothly, and performs better on softer metals.
The grading range is A to Z with A representing maximum softness and Z maximum hardness. Selecting the right grade is important. Wheels that are too soft for the materials that they are grinding will release grains too rapidly, and the wheel will wear quickly. Wheels that are too hard for the materials they are grinding will not release abrasive grains fast enough, and the dull grains remain bonded to the wheel causing a condition known as "glazing."
Abrasive wheel bonds can be classified as either “hard” or “soft.” Your choice of bond should directly relate to the type of job you’re doing. A wheel with a hard bond is used on high-horsepower machines and on small or narrow surfaces. Soft bonded wheels are used on jobs that require rapid stock removal, and on hard materials or large areas of contact.
A hard bond will resist the separation of grains, causing it to wear down more slowly, while a soft bond won’t last as long and will need more frequent replacements. The quality of the bond has a huge impact on the wheel performance, however. Too hard and the wheel won’t last and shatter; too soft and the wheel will wear down after limited cuts.
Factors for Selecting the Right Abrasive WheelThere are nine main factors to be considered when selecting an abrasive wheel for any application:
1. What material will you be grinding and how hard is it? The type of material affects the selection of abrasive, grit size and grade. As a general guide, the harder the material, the softer the grade of wheel required.
2. What stock needs to be removed from the material? High stock removal rates require coarse grit wheels, typically 12 to 24 mesh. Fine finishes and tight limits on finished workpiece geometry require finer grit sizes. Final surface finish is often achieved by ‘spark out’ where no further infeed is applied, and the wheel is allowed to grind until the majority of the grinding sparks cease.
3. Work out the shape of the material and the surface finish (or finishes) that are required.
4. What type of machine will you be using? Pay attention to its power and its conditions.
5. What wheel speeds and feeds will be involved? Ensure that the operating speed of the machine does not exceed the maximum operating speed as it’s marked on any given product.
6. Determine the size and hardness of the grinding contact area. The contact area between the wheel and work piece should also be considered.
7. Will your grinding operation be a wet or dry process?
8. What is the severity of the grinding required?
9. What is the dressing method?
Abrasive Wheel Markings
Whether it’s for purchasing or choosing which abrasive wheel to use on a project, it’s crucial to understand the markings and codes on a wheel’s label.
• Type - marked with an ANSI marking system number and indicates the wheel’s shape.
• Size - represents the wheel’s diameter x thickness x hole size. Marked as mm.
• Specification - letters and numbers indicating abrasive material, grit size, grade, structure and bond type. Use the general guide for examples.
• Maximum operating speed - marked in two ways: peripheral surface speed in meters per second and rotational speed in revolutions per minute (rpm). For high speeds, color-coded stripes are marked across the wheel.
• Restrictions for use - marked on the wheel using a code.
Other non-safety related information may include:
o Manufacturer’s trademark/nameo Test record number indicating it meets safety standardso Expiration date if it’s an organic bonded wheel (this will be 3 years from the date of manufacture)o Traceable code number indicating the source and manufacturing detailso Mounting arrow indicating the heaviest point of the wheel. The arrow should point downwards when the wheel is mounted.
Choosing an Abrasive Wheel for Concrete vs. Metal
Metal cutting wheels are made with aluminum oxide, and it is considered a soft abrasive. Aluminum oxide grains are initially tough and hard-wearing; however, after that sharp, fast first cut, the grains dull and lack the cut-rate and life span of other grains. Good for grinding metals such as steel, iron, bronze alloys, and stainless.
When abrasive particles are too hard for the material being cut, the wheels produce a lot of heat, and they can gum up. This means the abrasive should be matched to the material to be cut.
Abrasive Wheel Maintenance & Performance
Here are some best practices for maintaining your tools and how to get the best and longest-lasting performance.
- Use, handle, and mount abrasive wheels with caution using the proper safety measures.
- Store abrasive wheels so they are protected from banging and gouging. Your storage room should not be exposed to extreme weather conditions and temperature changes nor humidity which can damage the bond on some wheels.
- Immediately after unpacking, all new wheels should be closely inspected to be sure they have not been damaged.
- Whenever used wheels are returned to storage, they should be carefully inspected for damage, chips, and cracks.
- Handle wheels with care. Do not drop or bump which can lead to cracks, chips and other damage.
- Carry wheels to jobs; do not roll. For heavy wheels, securely cushion the wheel in a hand truck or forklift.
- Before mounting a vitrified wheel, ring test it. See ANSI B7.1 Safety Code for the Use, Care and Protection of Abrasive wheels. The ring test will detect cracks.
- Never use a cracked wheel.
- Make sure the spindle RPM doesn't exceed the maximum safe speed of the abrasive wheel.
- Use a wheel with a center hole size that fits snugly, yet freely, on the spindle without forcing it. Never try to alter the arbor hole.
- Use a matched pair of clean, recessed flanges at least one-third the diameter of the wheel. Flange bearing surfaces must be flat and free of any burrs or dirt buildup.
- Tighten the spindle nut just enough to hold the wheel firmly; do not use pressure to over-tighten. If mounting a directional wheel, look for the arrow marked on the wheel, and be sure it points in the direction of the spindle rotation.
- Always make sure that all wheel and machine guards are in place, and all covers are tightly closed before operating the machine.
- After securely mounting the wheel, and the guards are in place, turn on the machine, step out of the way, and let it run for at least one minute at operating speed before starting to grind. If a defective wheel is going to break, it’s likely to happen when it begins to turn.
- Grind only on the face of a straight wheel.
- Grind only on the side of a cylinder, cup, or segment wheel.
- Make grinding contact gently, without bumping or gouging.
- Never force grinding so that the motor slows noticeably, or the work gets hot.
- If a wheel breaks during use, carefully inspect the machine to be sure that protective hoods and guards have not been damaged. Also, check flanges, spindle, and mounting nuts to be sure they are not bent, sprung, or otherwise damaged.
Abrasive Wheel Safety Checklist
Here a few guidelines to help keep you and your team safe. And as always, consult OSHA or ANSI for detailed information or specific safety requirements.
- Always wear protective safety glasses or a face shield with impact-resistant lenses and side guards.
- Install shatterproof safety eye shields over all stationary grinders.
- Before mounting, check abrasive wheels for cracks, chips, and damage. Repair or replace as necessary.
- Make sure operating speed does not exceed rated maximum wheel speed.
- Inspect and ring test wheels for cracks before each use.
- Be sure the abrasive wheel arbor hole fits the machine arbor properly.
- Check that flanges are clean, flat, and the proper type and size for the wheel you’re mounting.
- Tighten nuts just enough to hold wheel firmly. Do not use excessive pressure.
- Standing to the side, run new wheels at full operating speed in a protected area at least one minute before grinding. Most defective wheels break when started.
- Use wheel guard provided with grinding machine.
- Grind a wheel at a rated speed LESS than rated grinder speed.
- Only mount ONE wheel per single arbor.
- Use wheels and grinding machines only for the purposes they are intended.
- Be aware of excessive vibration. It’s an indicator the wheel is out of round.
- Never grind towards yourself or another worker. Always try to grind towards a wall.
- Be sure the third wire is grounded to avoid shock.
- Make sure that a portable grinder has completely stopped before setting it down. Use a stand if possible. Rest grinder on its guard if there is no stand.
- ALL grinders should be used ONLY with a protective hood over the wheel.
- ALL grinders should be inspected every 30 days and necessary adjustments and repairs made.
OSHA Abrasive Wheels and Tools Code