HOW TO CHOOSE A GRINDING WHEEL
Many of us have used grinding wheels as part of our regular, daily work functions, but most of us don’t know why we use the wheel we do, or even if the wheel we are using the right one for the job.
There are many different types of abrasive grains, available in a whole range of grit sizes and held together by different bonding agents. How do we know which to use?
It's simpler than we might think. A grinding operation is a system, and as a whole, there are many parts to consider. One key component is the wheel. To determine where to start, consider the seven operational factors.
The Material Being Ground
The Severity of the Operation
Required Finish and Form Accuracy
Area of Contact
Wheel Speed
Coolant Use
Machine/Spindle Horse Power
By taking each of these seven factors into consideration, it is possible to narrow the field down to a smaller list of options to start with for any grinding operation.
1. THE MATERIAL BEING GROUND
The first thing to consider when selecting a grinding wheel specification is what are we grinding? What is the material, and how hard is it? Is it easy to grind or difficult? By reviewing these elements, we can select the correct abrasive type, the grain’s attributes, the appropriate grit size, and bond type.
Knowing the properties of the material we are working with helps us select the proper abrasive grain and its attributes. By convention, we use aluminum oxide grains for grinding ferrous metals and silicon carbide for non-metals and non-ferrous metals. Ceramic and superabrasive grains can be used on either but generally under specific circumstances where the material being ground requires these types of grains or when we are looking to optimize process performance.
Once we know which grain type to start with, we can look at the material grindability. If the material is easy to grind, we will want to use a tough/durable grain. Since the material is easy to grind, the grain shouldn’t break down too soon or too easily, so the whole grain can be used to maximize wheel life. For materials that are hard to grind, we will want to use a mild/friable grain, which fractures more easily, stays sharper, and actually grinds the material.
For easy to grind materials, we would want a coarser grit. This is because the grain can easily penetrate the material, make and remove chips. Using a larger or coarser grit maximizes the stock removal, reducing cycle time.
Another aspect of the wheel we can dial in on based on the material being ground is the grade or hardness of the bond. If the material is considered easy to grind, we can use a harder grade, which ensures that the wheel doesn’t release the grain before it is consumed. Knowing that we want to use the abrasive grain for as long as we can, we want the bond to hold that grain in the wheel for as long as possible.
If the material is difficult or hard to grind and we use a blocky, tough, and durable grain, we run the risk of simply dulling the grain and opening the door to finish issues, such as burning, because the grain will rub and not grind.
The material being ground also helps us determine the grit size. For hard to grind materials, we would recommend a finer grit size because a smaller particle will penetrate hard materials and form a chip easier than a larger blockier one. Difficult or hard to grind materials are abusive to the abrasive grain and can cause them to blunt or dull. Since we need more sharp points to penetrate the material, we want to ensure the grains are being released before they become too dull and cause metallurgical damage. With hard materials, a softer grade should be used so that the material is constantly being exposed to sharp grains. The grade needs to be soft enough to release the dulled grains and keep exposing new sharper grains to the work.
2. THE SEVERITY OF THE OPERATION
Here we consider how much or how heavy the grinding pressure will be in the grind zone. The higher the grinding pressure or force per grain, the more severe the operation. It is operations like these where today’s ceramic and superabrasive grains do well. Much like the material we are grinding, severity of operation helps us determine the attributes of the abrasive grain.
For operations with heavy pressure or high force per grain, we will use a tough/durable grain because this type of grain can tolerate the large amount of pressure generated during the operation. Tough/durable abrasives are better able to withstand the pressure and not break down too soon, allowing them to perform the required work.
Severity of operation also helps determine the grit size. For operations that are more severe or have heavy pressure, we want to use a coarser grit so that the grain will hold up to the grinding pressure. There may be times where we want to distribute the force/pressure over more cutting points, but even in that situation, we need the grain to be as coarse as possible to tolerate the pressure without turning to dust.
The last part of the wheel spec that severity of operation helps us determine is the grade of the bond or hardness of the wheel. Where we have heavier pressure, we naturally go with a harder grade for the wheel because we want the wheel to hold up under the forces of the grind operation - to hold on to the grain long enough for it to do the work we need it to do. In short, a harder bond is required so that it will hold the grains long enough to be used fully and not released too soon.
For operations with light pressure or lower force per grain, we use a mild/friable grain. When the severity of operation is low, we don’t want a durable grain that will only rub and dull. We need one that will continue to break down to expose new sharp cutting points, and mild or friable abrasives do this better, keeping sharp grains in contact with the material.
For light pressure operations, we use finer grit sizes. Since the pressure per grain will be lower overall, we need to make sure the grain is still able to fracture properly; if it's too coarse, the grain may not break down and self-sharpen at all.
When working under light pressure, a softer grade can be used because we need the wheel to break down and release the dull grain before it starts to rub and heat or burn. We also want the wheel to break down to bring new sharp grains to the cutting surface so we can perform the required work and get the desired performance from the grain.
3. REQUIRED FINISH AND FORM ACCURACY
We use abrasive products such as grinding wheels because of their speed, ability to repeat form, and achieve finish. When trying to select the correct wheel specification, we need to look at the operation and establish whether we are looking for rapid stock removal or a finer finish. Whether the part is simple/flat or if there is a form to hold. Knowing these requirements will help us select the correct grinding wheel for the process.
Again, we need to consider the required surface finish, dimensional tolerances, form holding requirements, and stock removal rates. By examining these, we can determine the appropriate grit size. What we need to achieve with the wheel will also help us determine the grade of bond or hardness of the wheel.
For low Ra finishes and/or close geometric tolerances, we naturally want to use a finer grit because the actual grit size of the grain provides for more points of contact between the work and wheel. This helps with precision finishes, which have a shallower scratch pattern, resulting in a lower micro-inch finish. It is also the physical size of the grain that allows us to achieve and hold small radius and complex forms better than we could with larger or coarser grit sizes.
When we require fast stock removal rates or when form and finish are not as critical, we want to use a coarse grit size. Whether we need a specific finish or form, we always want to use the coarsest grit size we can. A coarser grit will take a larger chip, and as such, increase stock removal cutting cycle time.
When we need close geometric accuracy and form holding, we need to use a harder grade. Going as hard as we can allows the wheel to hold the profile/form longer, as well as ensuring the grains are held long enough to achieve the desired results.
This next comment may seem conflicting, but when we want finer finishes and higher stock removal, we can use a softer bond. A wheel with a softer bond will easily release dull grains and keep newer sharp grains in contact with the material. This in turn means that sharper grains are continually exposed to the work, increasing stock removal as well as helping with the required finish by preventing dull abrasives from rubbing and burning the part. Although the actual finish is more a factor of grit size, keeping sharp grain in the grind zone helps provide the required finish.
The final aspect of the wheel spec determined by looking at the part requirements is the bond type. When we require close tolerances and form holding, we will want a vitrified product. Vitrified wheels hold their form/shape better than organic or resin bonded wheels, but organic bonds tend to finish better. For reflective and other finer finishes, the operation should consider using an organic or resin bond. Organic bonds, unlike vitrified bonds, have a little give to them and some of the grinding forces are going into the wheel/bond, reducing the chip size. Another benefit of using an organic bond for fine finish grinding is that organic bonded wheels break down from the heat of the grind, and they tend to hold the grain a little longer, allowing it to run and dull. It is this dulling and rubbing that helps generate the finer finish.
4. AREA OF CONTACT
Area of contact, the forth factor we consider, is in part related to the second factor: severity of operation, in that it considers the amount (or area) of contact between the work and the wheel. This factor looks at how the force applied to make a chip will be distributed through the grind zone in much the same way surface area is related to pressure in a hydraulic system. 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 gain. Conversely, the smaller the area, the higher the force per grain.
When we have a small area of contact, we would want a tough/durable grain. A small area of contact equates to a higher force per grain, so we need an abrasive grain that can hold up to these forces without fracturing too early and suffering premature wear. Again, when the force per grain is higher due to smaller contact areas, a ceramic or superabrasive grain may be a good choice.
Knowing we have a small area of contact, we will want to use a finer grit size because in addition to providing more abrasive points at the area of contact, it will also ensure that the relative pressure or grinding forces will be split among many grains.
A smaller area of contact typically also calls for using harder graded wheels. Because the forces are higher in smaller areas of contact, we need the wheel to hold its shape and not release grains too soon. Since the relative grinding pressure at the point of contact would be high, a harder grain is used to prevent premature wheel wear.
When the area of contact increases and becomes larger, like that of a blanchard segment, we need a milder, more friable grain. Due to the increased number of grain in contact with the work in the grind zone, the force per grain is lower and the grain can fracture and self-sharpen more easily.
In operations with a large area of contact, we want a coarser grit so that the grinding forces (which would be lower because the area is large) are spread over fewer grains, thus increasing the pressure per grain so that work can be done more efficiently. The forces will help the grain penetrate the work and assist in making the grain breakdown/fracture as needed.
In operations with large areas of contact, we run the risk of dulling the grain. This is due to the lower force per grain we typically see in operations with a large area of contact. To offset the possibility of burning related to dulling grains, we want a softer grade for our wheel so the grain can be released and replaced before any damage is done to our part.
The final factors, although important, only help fine tune or narrow down the wheel specification options.
5. WHEEL SPEED
The fifth factor to consider is the wheel speed. We have to consider the operating speed of the wheel in surface speed. To calculate surface speed, use these equations or try our iGrind app.
Wheel speed determines what bond type is most suited for the required speed or if a special high speed bond might be required.
As a general rule:
For surface speeds of 8500 SFPM (43 M/s) and below, either a vitrified or an organic bond can be used; although, most common vitrified wheels are designed for 6500 SFPM (33 M/s) and lower.
For surface speeds over 8500 SFPM (43 M/s), we usually recommend an organic bond be used for safety reasons. We should note though that some of today’s newer vitrified bonds can run at speeds over 8500 SFPM (43 M/s) but typically require a special rating.
One final note on wheel speed. Wheels will act differently based on their speed. It is a commonly accepted practice that for every 1000 SFPM (5.08 M/s) the surface speed changes, the wheel will act one grade harder or softer depending upon how the speed changes.
Slower = Softer. At slower wheel speeds, there is a higher force per abrasive particle causing the grain and/or bond to break down quicker.
Faster = Harder. At higher wheel speeds, there is lower force per abrasive particle making both the grain and bond seem more durable and resistant to breaking down as designed.
This can be useful in troubleshooting a wheel specification and/or to dial in the wheel grade.
6. COOLANT USE
The sixth factor we consider is coolant use. Coolant in a grinding system affects vitrified and organic (resin) bonded wheels differently and is considered when determining the wheel's grade or hardness.
If Coolant is Used:
Vitrified bonded wheels will act softer because the lubricity of the coolant helps reduce the friction/energy in the grind zone, which can help keep the grain from dulling. This keeps it sharper and freer cutting.
Organic (resin) bonded wheels will act harder because the coolant reduces the heat in the grinding zone. It is the heat of the grind that softens the wheel, allowing it to self-sharpen; if the heat is eliminated or reduced, the wheel will not break down as designed.
If No Coolant Used:
In dry operations, vitrified bonds will act harder because the grain will rub and dull more, creating more heat in the grind zone and potentially leading to burning and/or other damage.
Organic (resin) bonded wheels will act softer due to the ways in which the bonds work. More heat in the grind zone will soften the wheel quicker, possibly leading to early wheel wear and premature life.
7. MACHINE/SPINDLE HORSE POWER
The seventh and final factor to consider is horse power. We have to consider the horse power of the grinding machine to determine the grade of the bond or hardness of the wheel.
High Horse Power – When a machine has a higher horse power available at the spindle, we should use a harder wheel grade. We make the wheel harder so it will hold form and hold on to the grain as long as possible under the higher power/force conditions. We can also use a more durable grain knowing there should be sufficient force/energy available to fracture the grain and get it to self-sharpen.
Low Horse Power - We know that grinding energy causes the wheel and abrasive grains to break down and perform as designed. When the machine is low power or is under powered at the spindle, we may not be able generate enough force to break the wheel down as needed, resulting in grain dulling leading to burn and other surface damage. To mitigate this, we need to use a softer grade for our wheels. We may also want to look at more friable grains to help with this.
As you can see, there are several things to consider when trying to determine a starting specification for a grinding wheel. There may be situations when one factor may point you in one direction, while a second may point you in the opposite direction. In cases like that, it is best to look at where the majority of the factors are pointing you and/or to consider what you feel are the most important factors of your operation and use those factors to determine where to start. The chart below lays out all of the factors on a single sheet, making it easier to reference. All seven are listed along with their considerations and what aspects of the wheel specification they affect. FACTORABRASIVE TYPE & CHARACTERISTICGRIT SIZEGRADEBOND TYPE1. MATERIAL BEING GROUND Consider the material's properties (Metal/[Ferrous/Non-Ferrous] or Non-Metal, Hardness, Grindability, Etc.)Ferrous Metals: AI-O or cBN Non-Ferrous/Non-Metal: SiC or Diamond Easy to Grind: Tough/Durable Grain Hard to Grind: Mild/Friable GrainEasy to Grind: Coarser Grit Hard to Grind: Finer GritEasy to Grind: Harder Grade Hard to Grind: Softer Grade 2. SEVERITY OF OPERATION Consider the force per grain, grinding pressure in the grind zoneHeavier Pressure: Tough/Durable Grain Lighter Pressure: Mild/Friable GrainHeavier Pressure: Coarser Grit Lighter Pressure: Finer GritHeavier Pressure: Harder Bond Lighter Pressure: Softer Bond 3. REQUIRED FINISH AND/OR FORM ACCURACY Consider the required surface finish, dimensional tolerances, form holding, and stock removal requirements Higher Stock removal, Basic Form and/or Rougher Finish: Coarser Grit Size More Complex Form, Close Tolerance and/or Finer Finish: Finer Grit SizeMore Complex Form, Close Tolerance and/or Better Form Holding: Harder Grade Higher Stock Removal and/or Finer Finish: Softer GradeBetter Form Holding: Vitrified Bond More Reflective/Better Finish: Resin Bond4. AREA OF CONTACT Consider the amount or area being contacted between work and wheel in the grinding zoneSmall Area of Contact: Tough/Durable Large Area of Contact: Mild/FriableSmall Area of Contact: Finer Grit Large Area of Contact: Coarser GritSmall Area of Contact: Harder Grade Large Area of Contact: Softer Grade 5. WHEEL SPEED Consider the required/desired wheel speed in surface speed (SFPM = 0.2618 x Whl Dia x Spindle RPM)For every 1000 SFPM a wheel speed changes UP and DOWN, the 'effective' grade of the wheel changes harder (faster) or softer (slower). This can be useful when troubleshooting or "dialing in" a wheel specification.For speeds of 6500 SFPM or lower: Vitrified (Vitrified can run up to 8500 SFPM under certain conditions) Speeds over 8500 SFPM: Organic/Resin6. COOLANT USE Consider whether or not coolant will be usedFor Vitrified Wheels, we consider the material and its effects on the material For Organic/Resin, we consider the wheelIf coolant is used: Vitrified: Can go 1 grade harder Organic/Resin: Should go one grade softer 7. MACHINE/SPINDLE HORSEPOWER Consider the available spindle power of the grinder/operationSpindle Motor Sizing: BALLPARK - 15 to 20 HP per inch of wheel widthHigh HP: Harder Grade Low HP: Softer Grade NOTE: When deciding on a wheel specification for a particular operation, all factors need to be considered; however, it is possible that they may conflict. If that should happen, it is important to use any key factor(s) to determine the final starting specification. Related / Latest Stories Tips for Improved Surface Grinding Create the most productive surface grinding process with these grinding wheel selection and parameter recommendations. Read more 5 Grinding Considerations for Improving Surface Finish See how making adjustments to operational parameters, wheel dressing, grit size, coolant delivery, and machine condition can improve surface finish. Read more 5 Reasons to Upgrade to Vit-cBN Learn when to upgrade your cylindrical grinding process and what factors to consider when making the switch from a conventional vitrified wheel to a vitrified cubic boron nitride, or vit-cBN, wheel. Read more
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