feed operations must handle such forces and stresses, and part fixturing as well must be designed to resist these high forces, too. Horsepower well beyond that of a standard grinding
machine is required for creep-feed grinding to generate and maintain the high-productivity forces needed to remove large amounts of material quickly. While a typical reciprocating grinder generates 20–50 hp (14.9–37.3 kW), a common creep- feed grinding machine ranges from 60–150 hp (44.7–112 kW) and more. One benchmark dictates 20 hp for every cubic inch of material removed per minute, per inch of wheel width. Ideal creep-feed grinding machines also sport direct-drive
spindles that eliminate variables such as slipping belts. Te drive system for their diamond roll dressers must also possess the capability to generate and withstand high levels of torque. A servomotor drives the dressing roll into the grinding wheel to reform it, but the dressing wheel needs to hold its speed and not impart accelerative forces to the grinding wheel itself. Creep-feed grinding requires that the workpiece be fed
slowly and precisely under the wheel—in general, the greater the depth of cut, the slower the table speed. Feed control is crucial to maintain part precision and also because any sud- den change in table feed can break a grinding wheel that is under heavy grinding forces, which is oſten the case in the creep-feed process.
in power usage occurs with in-process continuous-dressing, the machine initiates wheel dressing, and when consumption returns to normal, the dressing cycle stops. In the long run, such a strategy can optimize use of abrasives and save time by reducing the frequency of shutdowns for wheel changes.
Wheel Structure In light of renewed interest in creep-feed grinding, wheel
vendors have developed wheels that minimize wheel con- sumption in the continuous-dress creep-feed process. Grind- ing wheels engineered for creep-feed grinding applications feature open bond, “induced pore” structures. When the wheel is buried deep in the cut, the pores
provide a path for coolant, swarf and excess wheel material to escape the grinding zone. Aluminum oxide, for instance, is the most common abrasive material used in grinding wheels. New wheels designated as ceramics (actually premium-level alumi- num oxide) can grind up to three times faster than conven- tional abrasives, according to wheel makers, while the ceramic wheels’ durability enables them to process as many as three times as many parts per wheel. Te new wheels’ durability and productivity offset their required higher initial investment, wheelmakers say. Wheel durability as well as grinding results themselves are dependent on the proper application of coolant in creep-
Creep-feed grinding requires that the workpiece be fed slowly and precisely under the wheel—in general, the greater the depth of cut, the slower the table speed.
Variable-speed ballscrews, racks and pinions or electro-
mechanical table drives will maintain tight control over table speed and position. Tey provide this control minus any surg- ing that might occur with table speeds as slow as 0.5 ipm (12.7 mm/min), although more common speeds are in the range of 8–30 ipm (203–762 mm/min). Hydrostatic or linear-bearing ways then provide preload to eliminate slack and absorb vibra- tion for maximized grinding precision. A variation on the continuous-dress creep-feed process,
called in-process continuous-dress, can help control manufac- turing costs where possible. With the in-process strategy, the continuous-dressing operation is, in fact, not continuous, but switched on and off according to need. Based on the workpiece material, amount of stock being
removed, wheel configuration, and other factors, continuous- dressing on a long cut may result in grinding wheel overdress- ing. And while part dimensions remain unaffected, valuable wheel material is wasted. During the in-process dressing mode, the machine moni-
tors its power consumption and detects any increases or surg- es, which would indicate a dull grinding wheel. When a rise
feed grinding. Coolant prevents buildup of heat in the part and grinding wheel, and also removes swarf from the wheel contact zone to prevent marring of the workpiece. Sufficient coolant flow also prevents the pores of the grinding wheel from becoming filled with swarf, a condition that reduces the wheel’s cutting effectiveness and further reduces the coolant’s temperature-control benefit. When temperatures rise too high, the wheel may burn the workpiece and swarf may weld to the wheel, negatively affecting workpiece dimensions and finishes. Creep-feed grinding, and in many cases its continuous-
dress version, provide a variety of benefits when process- ing the growing selection of high-performance workpiece materials. Accuracy and repeatability are a given. Te speed of creep-feed grinding boosts productivity, and the elimination of many pre-grinding and post-process operations as well as part handling can significantly reduce overall part production times. Successful application of creep-feed grinding requires investment in appropriate equipment including specialized grinding machines and grinding wheels, but for the right situ- ations and materials, return on the investment will far exceed a creeping pace. ✈
Aerospace & Defense Manufacturing 2014 109
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