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Machining center goes on the fast track.

Fast Track is the successor to a three-year Cincinnati Milacron/Lawrence Livermore National Laboratories partnership that recommended ways to enhance the accuracy of general-purpose machine tools. Its goal was to improve accuracy by 3x in the X and Y axes. Improvement features were applied and proven on Maxim 500 horizontal machining centers (HMCs) from Cincinnati Milacron, Cincinnati, OH, at customer plants. Production areas in two customer plants received Maxim HMCs with identical upgrades, which fell into two categories: 1) precision machine setup and 2) fluid temperature control.

Technicians used interferometers, electronic levels, and precision alignment software written by Milacron to measure axis squareness and pitch/yaw/roll values, and adjusted the machines to bring these values to within a third of the generally accepted specification. Error was addressed in the control using laser interferometry and CNC error compensation. Bill Stegmuller, Milacron R&D project manager says: "We also measured the movement of the foundation directly beneath the machines. Measuring its movement as-sured us that the machine performance would not be degraded by external influences."

For fluid temperature control, two chillers were retrofitted to each machine one to hold the part coolant to a set temperature and the other to do the same for the machine's hydraulic oil. "We're concerned with more than just the expansion of the machine, which is why the coolant chiller is key. Accurate positioning requires heat rejection from the part, tool, and fixture - all of which is flooded with the temperature-controlled coolant. Thermal compensation can help to some degree, but with variations in material thermal expansion, the best solution is minimizing thermal change," says Mr Stegmuller.

The machine improvements were tested on ANSI/ASME B5.54 aluminum parts offering an 11 1/2[inches] x 5.9[inches] face to the spindle. The HMC placed a pattern of 36 bores in the parts. Speed and feedrates were accelerated from ANSI values to be more in line with CNC production operations. Comparisons of bore locations machined before and after the improvements revealed location errors at the extreme corners of the test parts, which had averaged about 15[Mu] in the X axis and 10.4[Mu] in the Y axis, dropped to an average of 3.6[Mu] as a result of the precision setup and fluid temperature control.

All hardware and procedures proven through Fast Track are available as options for any Milacron HMC. Mr Stegmuller says that instead of using an environmental room for bore mill operations, the goal is to control the machine environment with a sheet metal "cold suit" for the machining center. An enclosed roof and special ducting system will improve thermal stability by allowing temperature-controlled air to circulate to machine structural components.

For more information from Cincinnati Milacron, Cincinnati, OH.
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Publication:Tooling & Production
Date:Nov 1, 1997
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