Tips, Tools & Home Improvement

Degree of Hardness of Grinding Wheels

The most important utensil for grinding is the grinding wheel itself. Today, the grinding wheel comes in a wide variety of forms. Meanwhile, a distinction in Grinding in flap discs and fiber discs. The grinding discs belong to the older type. Today the focus is more on the fiber and serrated lock washers.

All of these disks have one thing in common: they represent an asymmetrical body that is made to rotate by the current. Grinding wheels are produced industrially. They consist of abrasive grain, binder and pores. The abrasive grain differs depending on the design. Minerals, crystals, corundum, silicon carbide, CNB and diamond are common in grinding wheels.

When a grinding wheel hits another object, it is abraded by the frictional force. This activity is called grinding. Other meanings are separating and sanding. Mostly you grind metal, wood and glass with a grinding wheel.

In order for the grinding project to be successful, it is important to find the right grinding machine. You also need the right grinder. If you don’t know your way around, it can quickly happen that you pick the wrong grinding wheel, risk damage or achieve a bad result.

It is interesting for the buyer to know that the hardness of a grinding wheel is determined by a sieving process. This has the advantage for the user that he does not have to determine the degree of hardness independently. This can save the user a lot of time. The manufacturers meet their customers and state the degree of hardness directly under the product properties of the respective grinding wheel.

When choosing a grinding machine, you should not only look at whether all the discs can really be attached to it. There are older grinding machines that are not compatible with all grinding wheels. Rather, you should also look at whether the grinding machine is also used with grinding pieces with the appropriate degree of hardness. Usually, most grinding machines should be highly compatible. However, there are always exceptions.

An important factor for the success of a grinding project is the grinding wheel hardness. Why this is so important is explained in more detail below:

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What does the degree of hardness of a grinding wheel mean?

Before you buy a grinding wheel, you should ask yourself the purpose for which you want to use the grinding wheel. Depending on the material that you want to process, you have to use a certain grain or a certain degree of hardness.

Basically, you should use a hard grinding wheel for the first rough grinding and for grinding work where you have to remove a lot of material quickly. As a craftsman, you should use a soft grinding wheel for fine sanding and polishing.

The degree of hardness of a grinding wheel reflects the resistance of the bond against the grinding grains breaking out. Interestingly, many tradesmen think that the degree of hardness indicates the hardness of the individual abrasive grains.

Degree of hardness for grinding wheels, pay attention to the following markings

extremely softA to D
very softE to G
softH to K
mediumL to O
hardP to S
extremely hardT to Z

As already mentioned, the grit number is decisive for the degree of hardness. It indicates the size of the abrasive grain. Their value is decisive for the removal rate. The size of the coarse grain is determined based on a standardized sieve. If the abrasive grain just falls through the sieve, the number of stitches is used as the grain number.

The higher this grain number, the finer and smaller the grain. The surface of the workpiece to be machined can also be machined the finer. There is an exception for diamond cutting discs. Here the higher number marking represents a coarser grain.

The grits and designations are as follows:

  • Rough: Designation P12 to P80
  • Medium: P100 to P280
  • Fine: P320 to P600
  • Very fine: P800 to P2500

All abrasives are divided into the areas “coarse”, “medium”, “fine” or “very fine” depending on their grain size. From the designation P240, the grain size is referred to as “micro-grain size”.

It is important to note that not all grinding wheel grits follow the principle of this classification. For example, the company 3M does not use any letters for its Cubitron grain. A plus sign is used instead.

Interestingly, the degree of hardness has little to do with the hardness of the abrasive used. The hardness of the grinding wheel based on the amount of bond it contains. The hardness increases with an increasing proportion of the bond. The individual abrasive grains are held together more and more firmly. The grains are firmly bonded together in a solid and hard abrasive wheel. You can, therefore, withstand very high grinding forces.

With a soft grinding wheel, the grains can easily be detached from the grain composite.


The type of bond has a significant influence on the structure of a grinding tool. The bond leads to very different grinding behavior. So grinding wheels with varying degrees of hardness can be used for various forms of materials. A distinction is made between the following types of binding, which apply to exceptional cases:

  • Magnesite bond: These discs are soft, elastic and water sensitive. The grinding wheels can be used for dry grinding and knife grinding.
  • Shellac binding: This type of binding is characterized by soft, elastic and water-sensitive properties. This type of bond is very common for saw and shape grinding and for regulating wheels for centerless grinding.
  • Metal bond: These discs are tight, insensitive to pressure and heat. They are mainly used for diamond tool grinding or wet grinding.
  • Ceramic bond: This type of bond is fired from 1000 to 1350 degrees. It is porous, brittle and insensitive to water, oil and heat. It is used for the preliminary and fine grinding of steel and corundum.
  • Synthetic resin binding: This type of binding is elastic, dense and porous. It is used for pre-grinding and cut-off.

The structure of the grinding wheel

The structure of the grinding wheel describes the proportion of pores. The pores together with the volume fraction of the abrasive and the bond always result in the 100 percent in this mixture. In addition, this structure also includes the size, shape and arrangement of the tie bars. This of course also determines the behavior of the grinding tool.


The size of the grain has an influence on the cutting performance of the grinding wheel and on the quality of the surface of the workpiece to be machined. The grain size is also selected so that it adapts to the surface roughness. The abrasive grain is classified by sieving during manufacture. The degree of hardness of the contact strip is also determined in this way. This process is of course not carried out by the buyer. The buyer can find out the hardness of a grinding wheel by looking at the properties of the product when buying it.

The grain size is also identified using a grain number. The grain size decreases with increasing grain number. The number of the grit corresponds to the number of the mesh, where the abrasive grain can just barely fall through the mesh. The number of the mesh corresponds to the number of stitches that the mesh has.

How do I find out the hardness of a grinding wheel?

The degree of hardness of a grinding wheel can be found under the product properties of a grinding wheel. The degree of hardness is determined using a standardized sieve. If the abrasive grain just falls through, the number of stitches is used as the value for the grit number.

However, this procedure does not have to be observed by the customer. Rather, the manufacturer finds out the degree of hardness in this way. The buyer then only sees the value on the product. Since the degree of hardness on a grinding wheel is so important for the success of the grinding process, the degree of hardness should also be displayed on each grinding wheel.

Where are hard grinding wheels used?

Grinding wheels can be used for a wide variety of purposes. Hard grinding wheels are primarily used for soft materials.

Where are medium-hard grinding wheels used?

Medium-hard grinding wheels are very suitable for surface grinding. Surface grinding is a grinding method that is carried out on hollow glasses. The surface grinding is used in the glass grinding shop. Glass grinders use medium-hard grinding wheels. They make engravings in glass and grind decorations into the glasses.

Where are soft grinding wheels used?

It is common in the industry to use soft grinding wheels for harder materials. The hardness of a disc is primarily determined by its binding. If hard materials are processed with soft grinding wheels, the abrasive grains break out. So there are new grains again. This is also known as the “self-sharpening mechanism”.

When choosing the right wheel, you should always bear in mind that the grinding wheel should be softer, the harder the material to be machined. The reason is that with regard to tool life it is crucial whether the cut surface of the workpiece to be hardened or not. If the workpiece is heated, the pane can be practically glazed. This can reduce the sleekness. If this happens, the hardness of the binding must be reduced and a softer disc used.

It also applies to grinding wheels that the grinding wheel bond should be softer if the material is hard. The processing purpose also plays an important role here. For example, corners and ridges are very sharp. For this reason, the abrasive grain can easily tear out of the bond. It is recommended to choose a hard binding. However, if you want to machine small welds, a disk that is too hard would dull too quickly. in this case, the disc could no longer grip so easily. Good service life can still be guaranteed in this case, however, the removal rate can suffer and the labor costs can be quickly increased.

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