Modern Machine Shop

OCT 2018

Modern Machine Shop is focused on all aspects of metalworking technology - Providing the new product technologies; process solutions; supplier listings; business management; networking; and event information that companies need to be competitive.

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Page 90 of 147

Machining High-Temperature Materials Modern Machine Shop 89 intermittent dressing. Metal-bond superabrasive wheels using diamond or cubic boron nitride (CBN) grit have been used in cutting tool manu- facturing for grinding composite, cermet and ceramic tools. Based on the similarity of mate- rial properties, Norton Saint-Gobain engineers believe these wheels could also efficiently grind ceramic-matrix composite and gamma titanium aluminide parts for aerospace. One useful feature of these wheels is their porosity. For grinding wheels in general that are engineered for creep- feed grinding, material grains are spaced widely to create microscopic porosity allowing coolant to infiltrate the wheel. In a superabrasive wheel such as Norton's Winter Paradigm product line, the metal bond allows for a wheel porosity ranging to 46 percent. In some cases, superabrasive wheels also can be used without any dressing. Single-layer metal-bond superabrasive wheels designed for no dressing have been applied to realize creep-feed grinding on CNC milling machines. 8. Broaching now has a low-footprint rival. Milling is not the only competitor to creep-feed grinding. Another is broaching, specifically the broaching that is applied to realize the fir-tree forms in aircraft-engine disks made of super- alloy. A form such as this can be generated through creep-feed grinding. The result might be considerable f loor-space savings. Because of the long linear travel it requires, the broaching machine for this operation could easily be 30 to 40 feet long. Creep-feed grinding offers the chance to perform the same machining within a standard-size machine tool. 9. In aerospace, the MRR can match that of milling. Dr. Varghese stresses again: The view that grinding is a finishing process and the final touch applied to a machined part to realize dimensional and surface tolerances—that is, the historical role of grinding—is a view that will become less and less inclusive of all that grinding can do as advanced workpiece materials are used more While each grit on the grinding wheel experiences a low force vs. other modes of grinding, the force to the machine overall is likely to be high. Down grinding is the preferred choice for creep-feed grinding. The alternate approach subjects the wheel to friction and heating before it begins to remove material. To choose down grinding is to grind with a cooler wheel. Up Grind Down Grind

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