Norton’s Quantum VET promises cooler grinding of aerospace superalloys

Norton Quantum VET wheels have the capability to extract maximum advantage from the latest reciprocating grinding machines, which can generate high speed cutting conditions using dynamic linear drives, torque motors for the rotary axes, high wheel head speed and vibration-free structures. Applications for Norton Quantum VET wheels extend from the aerospace industry for producing turbine blades, vanes and blisks, to spiral flute cutter manufacture, and general production involving cylindrical and internal grinding. In most cases, metal removal rate (Q') matches or exceeds that of creep-feed grinding, reports Saint-Gobain. When grinding aerospace parts from nickel-based superalloys, which are tough, sticky to machine and have poor thermal conductivity, wheel porosity is vital to ensure that enough coolant reaches the point of cutting. Otherwise burning can occur, causing micro-cracking and thermal damage to the component. For some years it has been possible to manufacture highly porous vitreous grinding wheels in which aluminium oxide abrasive grains are held strongly. Such products have been used widely for creep-feed grinding of nickel, a process that involves a large contact area between component and wheel as well as heavy infeed forces. Reciprocating grinding, on the other hand, is not so aggressive to the surface of the component, as it uses a small depth of cut combined with fast movement of the workpiece back and forth. However, until now this process also had its drawbacks, making it suitable mainly for small batch sizes and relatively low stock removal rates. This was because the shallow cuts tended to prevent proper chip formation, rapidly wearing the abrasive grains. This in turn resulted in wheel glazing and a high level of friction, and hence the same problem of component burning and thermal damage. Moreover, the attendant increase in cutting force often caused detrimental chatter and vibration in machines of low rigidity which compromised accuracy. These difficulties on reciprocating grinders are eliminated by Norton Quantum VET, regardless of whether the wheels are used on simple 2-axis machines or models employing fully interpolating 3- and 4-axis motions for producing complex profiled parts. Norton has been able to formulate wheels with a porous bond strong enough to hold ceramic grain securely in place. Until now, such grain has tended to be easily dislodged from an open matrix, wasting material and shortening the life of the consumable. The bond material does not form a high proportion of wheel volume – resulting in free cutting – while the high bond strength maintains mechanical integrity of the wheel at peripheral speeds up to 100 m/sec. Furthermore, as the profile of the wheel edge is maintained for longer due to the abrasive retention and form-holding characteristics of the bond, more parts are produced before the wheel needs to be dressed. This minimises production downtime and extends the life of the consumable. The life of the dresser is consequently also prolonged. Unlike Norton's earlier seeded gel ceramic grains, which were hard and tough and would only micro-fracture under high grinding force, Norton Quantum grains have built-in fracture planes that allow sub-micron particles to break off at lower mechanical pressure and expose new, sharp grain edges much more readily. This microstructure lends itself to burn-free grinding of superalloys and other materials, even when water soluble coolant is used. Tests at Saint-Gobain Abrasives' research and development centre have shown that high speed rotation of a 300 mm-diameter Norton Quantum VET wheel, combined with a grinding machine table speed of 80 m/min and a stock removal rate of around 0.1 mm per pass, resulted in Q' of up to 133 mm3/mm/sec (133 mm3 of material removed per second for every 1 mm of grinding wheel width) in nickel alloys, equivalent to the performance of creep-feed grinding.