Grinding Journal - Fall 2007 - (Page 20)

CBN: Alternatives for high volumes and complex forms Cubic boron nitride (CBN), a man-made crystal second only to natural diamond in hardness, is very effective when used as a grinding abrasive for superalloys. In addition to its hardness, CBN is an excellent conductor of heat and can help draw heat generated in grinding away from the part. There are two primary types of CBN tools: Vitrified and Electroplated employ resin, ceramic and vitreous binders, with vitrified materials being those most commonly applied to superalloy aerospace parts. Vitrified CBN: High volume production A Vitrified CBN wheel employs resin, ceramic or vitreous binders that holds the CBN grains to the wheel in the form of a dressable layer that is usually .375 to 1.00 inch thick. The core of the wheel is either aluminum or aluminum oxide. Typically, a vitrified CBN abrasive application involves grinding a large number of precision parts (10,000+) with light requirements for material removal. It requires a rigid machine with three CNC axes (or more, depending on part geometry requirements). Fixturing should be robust as well, to assure process accuracy. Water base coolant should be supplied at pressures from 250 to 1,000 PSI. A wheel cleaner, including low volume coolant injected into the wheel to clean out chips, is also recommended. Coolant delivery, important in all grinding applications, is critical in achieving maximum productivity and economy with vitrified CBN tools. The position of the flow and its consistency are important. Grinding machine manufacturers provide coolant nozzles engineered to generate laminar flow that eliminates all turbulence in the stream of coolant. Matching coolant velocity to the speed of the wheel also benefits CBN grinding productivity. The required coolant velocity in cubic inches-per-second can be determined by multiplying the wheel speed in inches-per-second times the nozzle area in square inches. Coolant pressures are in the area of 150-300 PSI. Investment is high in both the grinding machine and the wheels, which routinely are 20 to 50 times the cost of aluminum oxide wheels. The higher initial investment in wheels, however, is often outweighed by the resulting lower cost-per-part in high volume applications. It Fall/Winter 2007 should be noted that a vitrified CBN wheel generally is dedicated to a specific feature, making it difficult if not impossible to use that wheel for anything else. 3. Vitrified CBN Part, Feature: Turbine blade, dovetail form Machine: 3-axis CNC Wheel: 2"-wide vitrified CBN Wheel Speed: 9,000 SFM Roughing parameters: 0.025" DOC at 20 IPM Finish parameters: Creep feed 0.015" DOC and 10 IPM Coolant: Water base emulsion, 60 GPM, 250 PSI, via engineered nozzle; wheel cleaner 5 GPM, 1200 PSI Cycle time: 4 minutes Dressing method: Rotary diamond roller, formed or disk Workholding: Robust Q Prime: 6 mm3/sec Dressing is extremely critical and is typically measured in ten-thousandths of an inch. For example, a wheel used in grinding a turbine blade would typically be dressed 0.0002" every three to five parts. Such precision requires high rigidity on the part of both the dresser as well as the machine. Dressing in this case is not a grinding process but rather a crushing action intended to fracture the bonds of the grains in the wheel. A key to dressing CBN wheels is minimizing the dwell time after the dressing pass. A light in-feed of approximately 0.0002" would be followed by immediate retreat of the dresser. A dressing roll speed to wheel speed ratio of .3 -.5 is recommended for vitrified CBN wheels. High volume with low to medium stock removal Summarizing the application of vitrified CBN tools in regard to variables in the grinding process, initial investment in a rigid CNC machine and dressing system is relatively high. The CBN wheel, dedicated to a specific part feature, also represents a significant investment. However, CBN’s long life results in low abrasive cost-per-part. Some users run wheels up to six months or even twelve months between changes, which also minimizes downtime. The dedicated nature of the production process limits flexibility, so vitrified CBN grinding is best for high-volume production runs. Environmentally, water base coolant is easy to manage. Long tool life also enhances process control and achieving product design and tolerance requirements. Rigid workholding is necessary, with its complexity and expense depending largely on the geometry of the part

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Grinding Journal - Fall 2007 Contents From Virginia: Apparently no credit crunch for machine tools Why measure cutting tools Other perspectives on CNC tool measuring Productive grinding of superalloys SUPER service grinding BIG rolls Problem Solver

Grinding Journal - Fall 2007

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