Grinding Journal - Fall 2007 - (Page 19)

In-feed axis 2. Continuous Dress Creep Feed (CDCF) Part, Feature: Land-based power generation turbine blade, dovetail form Machine: 4-axis CNC Wheel: 6"-wide aluminum oxide, special bonds Wheel Speed: 4,500 SFM Roughing parameters: Creep feed 0.06" DOC at 30 PM Finish parameters: Creep feed 0.002" DOC and 50 IPM Coolant: Water base emulsion or synthetic, 120 GPM at 100 PSI Cycle time: 7 minutes per side Dressing method: Continuous Workholding: Robust hydraulic clamping with work support Q Prime: 19 mm3/sec Rotary drive Invented for grinding superalloys, continuous dressing maintains a sharp wheel with an accurate form throughout the job. It also achieves the highest volumes of stock removal. As illustrated below, the rotary dresser moves on a CNC axis to compensate for the changing diameter of the grinding wheel. Continuous Dress Creep Feed (CDCF) grinding applications typically involve long part runs and dedicated equipment, while providing high-volume stock removal. The heavy cuts involved require robust clamping, work supports, and stiff machines ranging from 50 to well over 100 HP. Wheel is always sharp This application features a 4-axis CNC grinder with an overhead dresser that feeds down and dresses the wheel while grinding proceeds. The result is a constantly sharp grinding wheel that can produce highly accurate forms at very high metal removal rates. The wheel remains open and clean so it keeps the cut cool and works exceptionally well with difficult-to-grind materials such as superalloys. In fact, the creep feed process was originally developed for such hard-to-machine materials. Constant dressing assures a stable process and continual compensation for changing wheel size. The special bonded wheels used here feature a large amount of induced porosity, permitting heavy volumes of coolant to flow into the cut and large volumes of chips to be carried out. Continuous dressing reduces cycle time because it eliminates time spent moving to a dressing table. Wheel consumption does increase, but newer machines offer the ability to dress intermittently, with dressing rate increased in proportion to the load generated as the wheel dulls. The high metal removal capability of CDCF grinding makes it an excellent choice for processing large parts, especially land turbine components. High investment is a limitation of CDCF. Heavy-duty 4-axis machines are expensive. Additionally, the formed diamond roller and overhead dresser limit flexibility in terms of wheel movement around the part compared to other processes. The aggressive nature of the process makes strong work support and clamping critical to assure that the operation is stable and vibration-free. Fastest of the four methods A key advantage of CDCF is low cycle time; it is the fastest grinding process reviewed in this discussion. In terms of our analysis of process variables, CDCF grinding involves a significant investment in a heavy duty, multi-axis machine. The abrasive cost per part is high because continuous dressing results in high wheel consumption. Dedicated wheels and dressing equipment limit the flexibility of the process, but enhance its productivity for shops handling long production runs requiring aggressive grinding. Use of water-based coolant is environmentally acceptable, while large volumes of grinding swarf may pose disposal challenges. From the part design and tolerance aspect, continuous dressing produces high and consistent quality because the wheel is always sharp and open. Depending on the complexity and amount of design work necessary to create workholding for the CDCF operations, workholding considerations are moderate. Grinding Journal 19

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Grinding Journal - Fall 2007 Contents From Virginia: Apparently no credit crunch for machine tools Why measure cutting tools Other perspectives on CNC tool measuring Productive grinding of superalloys SUPER service grinding BIG rolls Problem Solver

Grinding Journal - Fall 2007

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