- The abrasive grains must bring a certain hardness, toughness, thermal resistance and chemical resistance to a material removal process. Abravise can be used in conjunction with bonds and pore formers in fixed abrasive grinding wheels which will determine their wear resistance.
- Conventional abrasives are based on corundum, the crystalline form of aluminium oxide (Al2O3), and carborundum, the crystalline form of silicon carbide (SiC). Aluminium oxide is generally coded A in fixed abrasive wheel manufacturing while silicon carbide is generally coded C.
- The corundums are divided into:
- The silicon carbides are only available in the colors green and black. Green SiC is used for precision grinding tasks, Black SiC is used in rough machining, such e.g. Cast brushing, rough rough grinding tasks
- Corundum has a Knoop hardness between 1,900-2,200 N/mm². The heat resistance is between 1,750-2,000 ° C.
- Silicon carbide has Knoop hardness between 2,400-3,000N/mm². The heat resistance of about 1,370 ° C.
- Super abrasives are based on cubic boron nitride (c-BN) and diamond (C). Cubic boron nitride is generally coded B in fixed abrasive wheel manufacturing while diamond is coded D. For reference, Borazon was the brand name of cubic boron nitride when first produced by General Electric in 1950s.
- Diamond is the hardest material with a Knoop hardness of 7,000-8,000 N/mm², but the thermal resistance is very low. At approx. 750 - 800 ° C, the diamond loses its hardness through oxidation. Diamond tends to dissolve in iron bassed alloys at high temperatures essentially carburizing the alloys and making the abrasives less effective.
- CBN has a Knoop hardness of 4,500-4,700 N/mm². The heat resistance of CBN is between 1,300-1,400 ° C.
- Cerium Oxide (CeO2) offers an enhanced polishing performance for optical flats, as well as other applications like filters and prisms. Cerium oxide has long lasting polishing efficiency providing excellent surface quality. Cerium oxide can retain its effectiveness over longer periods, even when used under demanding polishing conditions. It’s used for optical pitch applications and with a variety of pads.
- Boron Carbide (B4C) is one of the hardest materials known after Diamond and cubic Boron Nitrade with a Knoop hardness of 3,000-3,500N/mm². It is commonly used for lapping of Sapphire and other high hardness materials.
Basic summary that applies to most abrasives:
- The abrasive must be very hard and as tough as possible, so that the workpiece material removal rate is higher than the fixed abrasive wheel material removal rate. Ideally, the abrasive maintains its sharpness to keep the cutting stable over a long period of time.
- The abrasives should have sufficient thermal resistance, so that the grain can withstand both the high processing temperatures and the rapid temperature changes.
- The abrasives should be chemically resistant, so that at higher pressures and temperatures as well as when in contract with air, coolant or material of the workpiece no chemical compounds weaken the grain.
- The grinding wheel bond has to fulfill the following tasks:
- The most important bonds when using the conventional abrasives are the vitrified or glass and resin or polymer (also known as bakelite) bonds. Vitrified bonds are generally catagorized as V while Resin bond are generally catagorized as B by fixed abrasive wheel manufactures.
- The vitrified bonds have the following positive as well as negative properties:
- Resin bonds on the other hand, are resistant to shocks and impacts as well as lateral pressure. The chemical resistance to cooling lubricants such as oil and emulsion is worse than with the vitrified bond. Shelf life is a maximum of 3 years according to the FEPA (Federation of the European Producers of Abrasives) standard. The reason for this is the constituents of the resin bond tend to degrade overtime. This applies to both the conventional and the super-abrasive grinding wheels.
- In addition to the vitrified and resin bonds, the metal bond (generally catagorized as M), the galvanic or electro plated bond (generally catagorized as G), and the hybrid bond (generally catagorized as H). Hybrid bonds are as the name suggests are mostly mixtures of resin and metal bond.
- Disadvantages of the bonds of super abrasive grinding wheels are:
At the moment, the EDCM (Electro Chemical Discharge Machining) dressing method is favored for the metal bonds as well as the galvanic bond. There is also a hybrid
Technology that consists of an electrochemical and spark erosive process.
- Another catagory of bond is Elastomer or E which are generally used for polishing application and are made from natural or synthetic rubbers.
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