Grinding wheels is specialized tools that must be matched to the type of workpiece that you are grinding to ensure that the grinding process is success. If you use the incorrect type of grinding wheel for a specific type of workpiece, you can experience damage to your workpiece, your grinding tool, and your machine. To ensure that you use the correct grinding wheel for your workpiece, you must consider several variable of the grinding wheel.
The first variable to consider is the type of abrasive grain that will be used within the grinding wheel. Aluminum oxide abrasive grains is used most often for workpieces made of ferrous metal, like steel, because the aluminum oxide grains are strong and tend to self-sharpen as they grind the workpiece. Silicon carbide abrasive grain are used for non-ferrous metals, ceramics, and cast iron.
How to Choose a Grinding Wheel
For metals that are super hard, such as super hardened steels and carbides, the grinding wheel will contain superabrasives like cubic boron nitride or diamond because these superabrasives will maintain their cutting edge longer then abrasive grains like aluminum oxide or silicon carbide. The second variable to consider is the grit size of the grinding wheel. Coarse grit sizes is used to remove large amounts of material from the workpiece quickly but will leave a relatively rough texture on the workpiece.
Fine grit size will produce a relatively smooth finish on the workpiece but will require more time to remove the required amount of material from the workpiece. Medium grit sizes are the most common because they provide a good balance between the rate at which the workpiece can be ground and the quality of the finish that will be produced on the workpiece. The third variable to consider when selecting a grinding wheel is the hardness grade of the grinding wheel.
If the workpiece that you are grinding is relatively hard, it is best to use a grinding wheel whose bond is relatively soft so that it will prevent the grinding wheel from glazing. Hardness grades that are relatively hard are used for metals that are relatively soft or contain metallic components that act as gummy to the grinding process because they will prevent the workpiece from loading into the grinding wheel. The fourth variable to consider when selecting a grinding wheel is the structure of the grinding wheel.
Grinding wheels whose structures are relatively dense have less porosity and are used for form grinding operations. Open structures of grinding wheels have more porosity that allows for chips to be removed from the grinding wheel during deep cutting operations. The fifth variable in the selection of grinding wheels is the type of bond of the grinding wheel.
Vitrified bonds is rigid and porous so they are used in grinding wheels for precision grinding operations. Resinoid bonds are more flexible than vitrified bonds and are used for high-speed cutoff operations. Rubber bonds is used for thin grinding wheels because they will not crack when grinding workpieces of specific shapes.
The sixth and final variable to consider is the shape of the grinding wheel. Grinding wheels of different shapes are used in different machines and for specific grinding operations. Straight grinding wheels are used on bench and surface grinders.
Cup-shaped grinding wheels are used for sharpening of tools. Flared grinding wheels are used to grind complex relief angle on tools. Depressed-center or saucer-shaped grinding wheels are used for handheld angle grinders because they will prevent the grinding tool from gouging the workpiece if it comes into contact with flat stock.
Each grinding wheel will have specifications regarding the type of abrasive grain, grit size, hardness, structure, and bond indicated on the grinding wheel. These specifications will be indicated as a string of letters and numbers. For instance, if the code for the grinding wheel is A-60-K-5-V, the grinding wheel will contain aluminum oxide abrasive grains, have a grit size of 60, have a medium K hardness grade, have a moderate 5 structure, and have a vitrified V bond.
Another factor to consider when selecting a grinding wheel is the operating speed of the grinding wheel. The grinding machine must be set to a speed that does not exceed the maximum surface feet per minute that the grinding wheel can handle. If the speed of the grinding machine exceeds the safe speed for the grinding wheel, the centrifugal force of the spinning grinding wheel can cause it to break into fragments.
To calculate the safe speed of a grinding wheel, use the following equation: diameter times pi times RPM divided by 12. Last but not least, safety is a requirement for all grinding operations. Before setting the grinding wheel onto the machine, you must perform a ring test on the grinding wheel.
To perform a ring test on a grinding wheel, suspend the grinding wheel and tap it at a 45-degree angle to the grinding wheel. If the grinding wheel makes a clear ring when tapped, the grinding wheel is safe to use. However, if the grinding wheel makes a dull thud when tapped, it is cracked and must be discarded.
Additionally, grinding wheels can sometimes glaze or load with the workpiece that is being ground. If this is the case, you must use a diamond dresser to realign the grinding wheel to expose the fresh abrasive grain on the grinding wheel.
