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Hydrostatic guideways for the X (as shown in the drawing) and Z axes are a distinctive a feature of the Kel-Varia CNC grinding machines. Alternating pressure and vacuum zones in the hydrostatic guideways circulate the oil while maintaining a constant, even pressure. Controlling the temperature of the circulating oil is part of the overall strategy for thermal stability of the machine.
Putting pressurized hydrostatic ways on a CNC grinder can be costly for machine builders, but according to Kellenberger, the resulting accuracy and flexibility justify the expense. The Swiss company says that’s why it offers pressurized hydrostatic ways as a standard feature on the X and Z axes of its Kel-Varia universal grinders and other high-end models, even though this adds about 20 percent to the cost compared to conventional designs. The company, which is part of the Hardinge Group (Elmira, New York), also touts other benefits, including virtually frictionless (no-wear) axis motion, greater vibration damping, lower heat generation and improved crash protection.
It seems that grinding processes are always being pushed and pulled at the same time. The push comes from competing machining processes that might displace grinding. High speed milling, electrical discharge machining (EDM) and hard turning are examples of technologies that have edged out grinding in some applications. The pull comes from demand for grinders that can meet tighter tolerances, generate finer surface finishes, accommodate harder or more difficult workpiece materials and combine operations on a single machine platform.
For Kellenberger, hydrostatic ways are a key element in the strategy to keep grinding ahead of these trend forces, says Jeff Hilliard, manager of North American grinding sales at Hardinge. Almost all of the benefits of hydrostatic ways flow from the fact that a film of filtered, temperature-controlled oil prevents metal-to-metal contact between opposing surfaces when the axis is in motion. Because these way systems are pressurized, the oil actively lifts the upper way surface above the lower. This eliminates the tendency for the surfaces to exhibit “stick slip” or backlash, even when axis moves are programmed in increments of 0.1 µm.
On the Kellenberger grinders, the dual guideways on the Z (transverse) axis are constructed with flat-on-flat and mating-V configurations. Guideways on the X (wheel infeed) axis have a flat-on-flat configuration. On both axes, the moving way surfaces have shallow, milled pockets that create alternating pressure and vacuum zones. This enables oil to be continuously pumped in and drawn out. The system is controlled by sensors that monitor the pressure level and keep it even and constant. Although the oil recirculates, it is not a closed system, so it can function at low pressure to avoid the heat generated by high pressure hydraulic components. Oil drawn out of the system drains freely to a reservoir and passes through a filter and chiller before pumping back into the system.
Mr. Hilliard emphasizes that temperature control of the hydrostatic way system is complemented by recirculating coolant fluid through the spindle bearings, the motor housing, the wheelhead and the heat exchanger in the electrical cabinet. Likewise, the value of pressurized hydrostatic ways extends to vibration damping and crash protection. The film of oil absorbs energy to quell vibrations and isolate them from the grinding area. The oil film also acts a shock absorber if grinding components collide accidentally.
“The bottom line is hydrostatic axis ways significantly enhance grinding accuracy,” Mr. Hilliard concludes. “We have customers holding a tolerance of 0.000025 inch on diameter and shape,” he reports. He adds that the no-wear characteristics of hydrostatic construction enable maintaining such high accuracies over long periods of time.
Because the Kel-Varia is suitable for high-accuracy OD/ID cylindrical grinding, thread grinding, jig grinding, surface grinding and grinding of non-round or contoured surfaces, it has the flexibility for job shop and toolroom applications. At the same time, the virtual elimination of wear on axis guideway surfaces gives it the durability for production applications in automotive or oilfield shops. In all of these settings, the grinder lends itself to round-the-clock operation and 24/7 earning power. This potential makes justifying an investment in a hydrostatic-equipped grinder quite manageable, Mr. Hilliard says. Kellenberger’s Kel-Viva models are also equipped with hydrostatic ways as standard features.
At IMTS 2012, Kellenberger introduced an optional pressurized hydrostatic system for wheelheads with a rotary B axis that works in conjunction with the integral direct-drive motor. Hydrostatic ways on the wheelhead axis enable smoother and faster wheel indexing. Wheel to wheel time decreases from 8 to 2 seconds. Rotational positioning repeatability also improves from 5 arcseconds to less than 1, which enables more accurate grinding of tapers on toolholder bodies, roller bearings, spindle components and other critical workpieces with high-precision tapers.
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