The grinding wheel hardness grade are a measurement of how tightly the bond hold the abrasive grains. The hardness grade of the grinding wheel will determine how the grinding wheel perform during the grinding operation. If the hardness grade that is selected for the grinding wheel are incorrect for the material that is being ground, then the material may burn or the grinding wheel may disintegrate.
However, if the selected hardness grade is appropriate for the material, then the cut will remain clean and the parts will remain within there established tolerances. Grinding wheel hardness grades does not refer to the abrasive grains that is contained within the grinding wheel. Instead, the grades refer to the strength of the bond that holds the abrasive grains.
Grinding Wheel Hardness and How to Choose the Right One
Grinding wheels with soft grades will allow the abrasive grain to break free from the bond that holds them to the grinding wheel. These breaking grains will expose a new cutting edges of the grains. Grinding wheels with hard grades will hold the abrasive grains to the grinding wheel for longer periods.
Hard grades is used for materials that do not offer alot of resistance to the grinding wheel. The grades range from A to Z, with A being the softest and Z being the hardest. Grades A through E is considered very soft, whereas grades R through T is considered hard.
For grinding wheels and work pieces, there is a rule of opposites to follow. Hard work pieces require soft grinding wheels. This is because if the work piece is hard, then the grains on the grinding wheel will rapid become dulled.
Using a soft grinding wheel allow the grains to shed to prevent the buildup of heat on the work piece. Soft work pieces, on the other hand, require hard grinding wheels. This is due to the fact that soft work pieces do not provide a lot of resistance for grinding to wear down the grinding wheel with the work piece.
Hard grinding wheels will allow the work pieces to last longer with the grinding process. Internal grinding processes create a larger area of contact between the grinding wheel and work piece. Due to the increased contact area, heat can be trapped between the two component.
For this reason, softer grades of grinding wheels are required for internal grinding processes. External cylindrical grinding processes create a smaller zone of contact between the grinding wheel and work piece. For these processes, firmer grades of grinding wheels can be used.
Different materials require different type of grinding wheels with specific combinations of hardness grades and abrasive grains. For instance, mild steel uses medium-soft K-L grade with aluminum oxide grains. For hardened tool steel, users use softer H-J grades with CBN abrasives so that the tool steel will not burn during the grinding process.
For aluminum metals, soft G-H grades with silicon carbide grains are used because silicon carbide help prevent loading of aluminum on the grinding wheel. If the hardness grade of the grinding wheel is too hard for the material that is being ground, then the abrasive grains will not be able to shed from the bond. As a result, the grains will glaze over the surface that is being ground.
Grinding wheels that glaze will generate more heat during the grinding operation. The excessive heat can lead to chatter marks or thermal crack on the work piece. For the same reasons, if the hardness grade of the grinding wheel is too soft for the material that is being ground, the grinding wheel will shed its abrasive grains too quick.
Eventually, the grinding wheel will lose its shape. As a result, the dimensions of the work piece will drift from the established specification. Additionally, the grinding wheel will have to be replaced, increasing the cost of using that grinding wheel.
One method for adjusting the grinding wheel to allow the grinding process to perform better is by dressing the grinding wheel. By performing a fine dressing pass on the grinding wheel, it will close the pores of the grinding wheel. By closing these pores, the grinding wheel will act as if it has a harder grade for the grinding process.
An aggressive dressing pass will open the structure of the grinding wheel. By making the structure of the grinding wheel more open, the grinding wheel will act as if it have a softer grade. Additionally, the type of bond that is used with the grinding wheel will affect how the hardness grade of the grinding wheel behave.
For example, grinding wheels with vitrified bond are rigid in structure. Users use vitrified bonds with grinding wheels of the H-P grades for precision task. Resinoid bonds for grinding wheels are flexible in their structure.
These type of bonds will act as if they are softer than there letter grade suggests. The structure of the grinding wheel also affect the performance of the hardness grade of the grinding wheel. The structure of a grinding wheel range from number 1 to 13.
Number 1 is for a dense structure of the grinding wheel and 13 is for an open structure. Grinding wheels with a dense structure are able to increase the apparent hardness grade of the grinding wheel. The dense structure is used to hold the work piece to a specific form.
For grinding wheels with an open structure, the pores will allow chip to be cleared from the grinding zone of the grinding wheel. For metals like stainless steel, hard grades of grinding wheels with open structures will allow for better management of the heat that is generated during the grinding process. Finally, there are various test that can be used to test the condition of the grinding wheel.
For example, the tap test can be used to test the sound of the grinding wheel. If the grinding wheel produce a clear ring when tapped, the grinding wheel is sound. However, if the sound that is produced is a dull thud, it suggests that the grinding wheel have cracks in its structure.
You should of checked the wheels first. It is actualy important to recieve proper results.
