Abrasive grains are available in a wide range of grit sizes and bonding agents. Determining which wheel is right for the job requires considering seven key factors: '
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The first thing to consider when selecting a grinding wheel specification is the workpiece material type and hardness. Is the material easy or difficult to grind? The relative ease of grinding is a major predictor of the appropriate abrasive type, grain attribute, grit size and bond type for the application.
By convention, aluminum oxide grains are used for grinding ferrous metals, and silicon carbide grains for non-metals and non-ferrous metals. Ceramic and superabrasive grains are compatible with all three types of materials, but are typically meant for specific circumstances where aluminum oxide and silicon carbide perform poorly.
With the grain type is established, material grindability determines many of the other necessary attributes for the grinding wheel. If the material is easy to grind, use a tough, durable grain. Since the material is easy to grind, grains shouldn't break down too soon or too easily, so the whole grain can be used to maximize wheel life. A coarser grit is best for these materials, as the grains can easily penetrate the material and maximize stock removal. A harder grade (that is, a harder bond between the grains and the wheel) also corresponds to easier grinding, as the bond will prevent the wheel from releasing the grains before they are consumed.
For hard-to-grind materials, reverse these recommendations. Mild, friable grades perform better on these materials, as they fracture more easily and stay sharper. Finer grit sizes improve the ability of the particles to penetrate hard materials and form chips. Because the grits will dull and cause metallurgical damage such as burning if held for too long, soft grades are necessary to release dulled grains and expose the material to sharper ones.
Users should also consider the grinding pressure, or force per grain. The higher the pressure, the more severe the operation, and the better ceramic and superabrasive grains are likely to perform. The severity of the operation also helps determine the attributes of the abrasive grain.
Tough, durable grains are able to tolerate higher levels of pressure and not break down prematurely. Coarser grits also assist the grains in holding up to grinding pressure. There may be times where distributing the pressure over more cutting points is best, but even that situation requires balance to prevent the pressure from turning the finer grains into dust. Heavy pressure also requires harder grades so grains can stay on the wheel long enough to perform the required grinding work.
In contrast, mild, friable grains perform better in light-pressure operations, since durable grains will only rub and dull. Finer grit sizes ensure that the grains can still fracture properly and self-sharpen, and softer grades release dulling grains before they begin to rub and burn the material undergoing grinding.
The unique specifications of different grinding wheels determine their optimal applications. However, factors like coolant use and the horsepower of the grinding machine can alter these optimal applications. All images courtesy of Norton | Saint-Gobain Abrasives.
Grinding wheels achieved their ubiquity due to their speed, form repeatability and ability to achieve desired finishes. When selecting a wheel, it is important to determine whether the application requires rapid stock removal or a fine finish. Equally important is whether the part will be simple and flat, or if there is a form to hold.
The required surface finish, dimensional tolerances, form holding requirements and stock removal rates factor into the appropriate grit size, grade and bond type.
For low-Ra finishes or close geometric tolerances, finer grits are helpful because they provide more points of contact between the work and wheel. This helps with precision finishes, which have a shallower scratch pattern and a lower micro-inch finish. Finer grains also aid in achieving and holding small-radius and complex forms. Coarser grits, by contrast, improve stock removal rates. Finding the optimal balance in grit size will decrease cutting cycle times.
Close geometric accuracy and form holding requires a harder grade. Harder grades enable the wheel to hold its profile longer and ensure the grains are held long enough to achieve the desired results.
This next recommendation may seem contradictory, but softer bonds are optimal for both finer finishes and higher stock removal. A wheel with a softer bond will easily release dull grains and keep newer, sharper grains in contact with the material. Sharper grains increase stock removal and improve finish by preventing dull abrasives from rubbing and burning the part during stock removal operations. Although the actual finish and stock removal rates from this operation are primarily dependent on grit size, keeping sharp grains in the grind zone benefits both.
Part requirements also determine the bond type. Vitrified wheels perform best for close tolerances and form holding, while organic and resin bonds are best for reflective and other fine finishes. Organic bonds, unlike vitrified bonds, have a little give to them. Some of the grinding forces go into the bond, reducing the chip size. Another benefit for fine finish grinding is the way organic bonds break down from the heat of the grind. They tend to hold grains a little longer, allowing them to run and dull. Planned plowing and sliding interactions that take place under these circumstances improve upon the initial scratch pattern formed during stock removal to generate a finer finish.
Area of contact is partly related to severity of operation in that it considers the area of contact between the work and the wheel. When a wheel is applied to the work, the force applied is distributed over all the cutting points in the grind zone. The larger the area of contact, the lower the force per grain. Conversely, the smaller the area, the higher the force per grain.
Operations with a small area of contact should use tough, durable grains that will not fracture too early or suffer premature wear under higher force per grain. Ceramic or superabrasive grains may even be necessary in these operations. Finer grit sizes are optimal for small areas of contact, because in addition to providing more abrasive points at the area of contact, the relative pressure or grinding forces will be split among many grains. The high forces of operations with smaller areas of contact also call for harder-grade wheels, as these will hold their shape against premature wheel wear.
When the area of contact increases and becomes larger, such for a Blanchard segment, milder grains are more appropriate. Due to the increased number of grains in contact with the work in the grind zone, the force per grain is lower and the grains will fracture and self-sharpen more easily. Coarse grits spread the pressure into fewer grains to ensure they will continue to penetrate the work. As the risk of burning from dulled grains is higher in these operations, softer wheels grades should be used so grains release before doing damage to the part.
Operations' wheel surface speeds can narrow down the bond type and wheel grade necessary to complete them. To calculate surface speed, use these equations:
Surface Speed (SFPM) = (π × Diameter (inch) × RPM) / 12
Surface Speed (m/s) = (π × Diameter (mm) × RPM) /
Wheel speed determines what bond type is best for the required speed, or if a special high-speed bond might be required.
As a general rule:
Wheels will also act differently based on their speed. For every 1,000 SFPM (5.08 m/s) the surface speed changes, the wheel will act one grade harder or softer. Slower wheel speeds equal softer performance, as the higher force per abrasive particle causes the grains and bond to break down quicker. Faster wheel speeds lead to harder performance, with the lower force per abrasive particle making the grains and bond act more durable.
Coolant has opposite effects on wheels with vitrified and organic bonds. Its presence makes vitrified wheels act softer, while organic wheels act harder.
Coolant in a grinding system affects vitrified and organic (resin) bonded wheels differently, and plans for its use must be considered when determining the wheel's grade or hardness.
For more information, please visit stone grinding tools manufacturer.
If coolant is used:
If no coolant is used:
The horsepower of the grinding machine can play a role in determining the grade of the bond or the hardness of the wheel.
Due to how many factors are involved in determining the starting specification for a grinding wheel, there will be situations when factors point in opposite directions. In such cases, look at where the majority of the factors are pointing, or place precedence on the most important factors for the application. For simple comparison, review the chart below:
When deciding on a wheel specification for a particular operation, it is important to consider all factors. In the event that the factors conflict, the user may need to choose a key factor to determine the final specification.
Selecting the right grinding wheel for your project is important and with an almost incomprehensible array of products to choose from, it can present a challenge. That's why we have put together this definitive overview to guide you into making the most informed choice possible.
This guide covers:
A grinding wheel is a precision tool with thousands of cutting points on its surface - abrasive grains that are held in place by a bond matrix (hence these are known as bonded abrasives) and separated by pores.
These pores are hollow spaces between abrasive grains and the bond , which help with the clearance of discarded metal chips in the grinding process.
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When the wheel is in use, the abrasive grains cut into the material that is being ground, removing the unwanted surface material in small chips.
During grinding, the cutting points on the abrasive grains are worn flat; becoming increasingly blunt. At the same time, the increased friction causes a build-up of heat, which fractures the abrasive grain and exposes new cutting edges or begins to disintegrate the bond bridges that hold the abrasive grains in place.
In normal vitrified grinding, the wheel has to be dressed using a dressing tool. By varying the properties of the abrasive, the type of bond, the wheel's construction, it is possible to produce grinding wheels with a vast range of different grinding characteristics.
There are 4 main types of abrasive grains available for grinding wheels, these are:
Often referred to as just 'Ceramic', Norton Quantum's patented ceramic form of aluminium oxide is harder and sharper than conventional abrasive grains.
This ceramic grain has a unique microcrystalline structure that is self-sharpening. This ultimately reduces the regularity that the grinding wheel needs dressing as well and providing a significantly cooler cutting action when in use.
Harder than standard aluminium oxide with a very sharp abrasive grain. It is a versatile material, recommended for grinding relatively soft metals such as aluminium or cast iron but can also be used on extremely hard materials such as cemented carbide.
For use in rough grinding applications where high stock removal is required. This grain is associated with high tech resin bonds.
Generally recommended for grinding materials of high tensile strength, such as stainless steel and tool steels but it can also be used on some high tensile aluminium and bronze alloys.
Aluminium Oxide is manufactured in varying qualities.
When selecting the perfect wheel for your project, it is important to know what the pictograms mean on the front blotter.
This guide will help you decipher they mean:
When to select a coarse grit size: where the neatness of the surface finish is not essential to the project, a coarse grit can be used. Also, the coarser the grit, the more rapid the stock removal, which makes it more suitable for large areas of contact than fine gritted abrasives.
They are also the best option for soft, ductile and stringy materials such as soft steel and aluminium.
When to select a fine grit size: fine grits are used when the finish is integral to the success of the project.
Also, choose a finer grit when the project requires a closer, more precise operation over a smaller area of contact. Hard and potentially brittle materials such as glass, tool steel, and cemented carbide are most suited to the finer gritted abrasive.
The grade indicates the relative holding power of the bond, which holds abrasive grains in a wheel.
Soft Grades:
' For hard materials such as hard tool steels & carbides
' For large areas of contact
' For rapid stock removal.
Hard Grades:
' For soft materials
' For small or narrow areas on contact
' For longer wheel life.
There are nine main factors to be considered when selecting a grinding wheel for any application:
1. What material will you be grinding and how hard is it?
2. What stock needs to be removed from the material?
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? (Norton products are designed and tested for certain applications and operating speeds.
In the interest of safety, please take the time to ensure that the operating speed of the machine does not exceed the maximum operating speed as it is marked on any given product.
6. Determine the size and hardness of the grinding contact area.
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?
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.
Hopefully this guide has helped you make in making the most informed choice possible.
In most cases (particularly within maintenance, repair and operations) you would typically use a grinding wheel on an angle grinder.
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