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Large part machining gets accuracy boost

Extra-large parts such as submarine propellers and aircraft frames are often crafted from many smaller components because even the best machining technologies are not yet accurate enough for large, one-piece parts to meet required tolerances. Manufacturers aiming to use ever-larger parts often find themselves stuck with costly workarounds — make parts to fit, shim as needed, and worst of all, rework. Accuracies considered routine for small parts become much harder to achieve over the longer distances in larger parts. In some cases, cutting-tool position and orientation tolerances are a few thousands of an inch — difficult to consistently achieve for huge parts.

A joint project of the National Center for Manufacturing Science called Volumetric Accuracy for Large Machine Tools (VALMT), involving MAG Industrial Automation Systems, Automated Precision, Boeing, and Siemens, recently developed a promising approach to volumetric error compensation (VEC). Created for large and multi-axis machines, the suite of hardware and software reduces the downtime needed to determine necessary volumetric compensations from weeks to a day or less, via a simple automated process that improves a machine tool's volumetric performance by 50% or more.

The method, developed at Boeing, considers the full effects resulting from the kinematic stack-up of all machine tool axes. Where conventional approaches to volumetric compensation focus on the first 21 error sources associated with three orthogonal linear axes, the new method compensates any arbitrary stack of linear and rotary axes, addressing the 43 (or more) kinematic errors associated with a five-axis machine. A specific VEC solution is determined for every tool position and orientation combination inside the work volume. One official from Boeing estimates the breakthrough could save the company $100 million a year by reducing assembly and fitting costs on large programs like the F-18 or 700 series commercial aircraft.

The metrology system uses laser technology from Automated Precision Inc., Rockville, Md. A laser source, the T3 Laser Tracker, is placed in the work piece position. It directs a laser beam to the “active target,” mounted in the machine tool's spindle. These devices interact to maintain a metrology “beam lock” during data gathering. The procedure captures position data for a cloud of 200 statistically randomized multi-axis “poses” within the work envelope. Programmed poses are compared to measured positions over three program runs with two different tool-length dimensions for the active target. Software processes the data to determine the compensation solution.

The volumetric compensation file is then entered into the control and activated by program statements. Next, the CNC's “compile cycle” technology integrates the compensations into the realtime path interpolation algorithms. As the machine tool moves in five-axis space, compensations are applied within the interpolation loop of the CNC. For more information, visit mag-ias.com.

Information courtesy of Jim Dallam, MAG Industrial Automation Systems