Cutting is often the first step in materialographic (or metallographic) preparation. Depending on the size and shape of the specimen, it may need to be cut (or sectioned). The plane of the cut should have as little deformation as possible to facilitate and speed up the next preparation step. The most suitable cutting method is therefore abrasive wet cutting, which causes the least damage relative to the time required.
Note: The selected sample must be representative of the properties of the parent piece from which it was cut.
Diamond/cubic boron nitride (CBN) cut-off wheels have long-term performance because the abrasive grains are extremely hard and the bond material holding them in place is very durable. Only a thin layer of abrasive is placed around the metal disc (continuous rim) — these are long-life cut-off wheels. Other abrasives, such as SiC and aluminium oxide, wear faster and are less expensive. Therefore, the entire body of the wheel consists of abrasive and bond material — these are consumable cut-off wheels.
Abrasive Wet Cutting
Abrasive wet cutting uses a cut-off wheel composed of abrasive particles and a bond material. Coolant flushes the wheel to prevent frictional heat from damaging the specimen. The coolant also removes debris from the cutting zone.
Cut-off Wheel Selection
Depending on the material to be cut, different wheel compositions may be required. The hardness and ductility of the material affect the choice of cut-off wheel.
- Ceramics or sintered carbides should be cut with diamond wheels bonded with metal or bakelite bond material.
- For ferrous metals, the typical approach is to use aluminium oxide (Al₂O₃) wheels bonded with bakelite. The use of CBN to cut harder ferrous metals is also increasingly common.
- Non-ferrous metals are cut with SiC wheels bonded with bakelite.
Cut-off Wheel Design
Apart from the abrasive type, the fundamental difference between diamond/CBN wheels and SiC/Al₂O₃ wheels lies in their design.
Cut-off Wheel Characteristics
1. Long-life Cut-off Wheels
Diamond cut-off wheels come in two types, differentiated by the bond material used to hold the abrasive grains: metal bond wheels and bakelite bond wheels. Both are used to cut extremely hard materials. Metal bond is used for cutting more brittle materials such as ceramics. Bakelite bond is used for cutting materials such as sintered carbides.
CBN cut-off wheels are available only with bakelite bond and are used to cut very hard metallic materials, such as white cast iron.
2. Consumable Cut-off Wheels
The cutting characteristics of consumable wheels vary with the nature of the bond material. Bond materials can be classified by their "hardness" or their ability to retain or release abrasive grains. A "hard" wheel retains abrasive grains better than a "soft" wheel. A "soft" wheel, because it continuously supplies fresh sharp abrasive grains as the softer bond breaks down, is used to cut hard, brittle materials. For cutting soft and ductile materials, a wheel with a harder bond material should be used. In addition, wheels with harder bond material are more economical because they wear more slowly.
Selecting the correct cut-off wheel for a specific material is critical. Only the appropriate wheel can ensure low deformation and a flat surface. A better surface after cutting will allow you to achieve the required preparation result more quickly.