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Original Date: 10/07/2002
Revision Date: 01/18/2007
Best Practice : Deep Pocket Milling
General Tool Company used high speed machining technologies to reduce the machining time by approximately 75% and improved the part quality, which reduced the downstream finishing and assembly operations by 50%.
General Tool Company (GTC) recently completed the fabrication of a series of large components for the National Ignition Facility at the Lawrence Livermore National Laboratory (LLNL). These 60 x 80 x 19 inch components were machined from a 6061 aluminum alloy casting with approximately 0.75 of an inch of machining stock. The average wall thickness on the 10 through-windows was 0.390 of an inch, with an accompanying opening tolerance of +/- 0.009 of an inch and a surface finish requirement of 32 RMS.
The initial fabrication process was based on a horizontal-boring mill with a 1,100-rpm spindle. This approach required multiple setups, elaborate clamping and vibration devices, and a rough machining stage. The best machining time achieved was 80 hours per part and was accompanied by inconsistent size control, a poor finish, excessive cycle time, and cost overruns in down stream finishing and assembly operations.
GTC resolved these problems by moving the job to a machine with an 8,000-rpm spindle. A 19 inch cutter with polished, round inserts (see Figure 2-1) was used to finish machine the window sections in a single set up (after the corners were cleared in a preliminary operation). The machine tool path was a waterline or thin-to-thick configuration (see Figure 2-2) with a relatively light depth of cut (0.090 of an inch) and a 393 inch-per-minute feed rate.
GTC’s high speed machining technologies reduced machining time by approximately 75%. This throughput increase has allowed GTC to fulfill its commitments to customers. The high speed machining also improved part quality, which reduced downstream finishing and assembly operations by 50%. These machining practices can also be applied to similar future parts.
Figure 2-1. High-Speed Cutter
Figure 2-2. Machine Tool Path
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