Modern Machine Shop

DEC 2016

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82 MMS December 2016 FEATURE A small area of contact means both tangential and normal grinding forces are low. In addition, as wheel speed increases, both of these forces decrease exponentially. For example, when run- ning at a peripheral wheel speed of 35 m/sec. with a material-removal rate of 1.0 mm 3 /mm, the tangential force would be 1 N/mm. On the other hand, running the wheel at 100 mm 3 /mm and the same material removal rate would result in a tangential force of only 0.2 N/mm. Low grinding forces mean that long, relatively small-diameter workpieces might not require a steady rest for added support during peel grind- ing. Plus, the heat created during peel grinding is evacuated with the chips, which prevents workpieces from burning and also facilitates faster grinding speeds. Mr. Maurer says peel grinding's reduced grind- ing forces mean wheels require dressing less frequently. Wheels used for peel grinding carbide typically require dressing after 500 to 1,000 parts. That's good, because dressing metal bond wheels even using the S41's silicon carbide rotary dress- ing disc can take upwards of a day. That said, proper coolant selection and deliv- ery is also important for effective high-speed peel grinding. Complete Grinding Solutions use s straight oil for coolant. Mr. Maurer says this is necessary to provide sufficient lubrication at a grinding wheel's cutting edges, which isn't attain- able with oil/water emulsions. In addition, peel grinding requires coolant delivery at high pressure to ensure that the coolant gets to the area of c ontact b et we e n the w he e l a nd wor k pie c e. Although a grinding machine with a conventional low-pressure system might provide 5-bar (72.5- psi) pressure and have a 100-gallon tank, the S41 Complete Grinding Solutions commonly uses metal bond wheels measuring 5 mm wide and 400 mm in diameter for high-speed peel grinding. The periphery of these wheels has a 10-degree lead angle where roughing is performed followed by a flat profile for finishing. Most wheel wear occurs at the corner where those two areas transition, and wear rates are essentially the same for shallow and deep passes. Wheel Width 5mm Workpiece Finishing Roughing Wheel Traverse Direction 10°

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