One of the most vexing catch-22s in large-part machining appears to have finally been caught. Rapid volumetric compensation for high accuracy in large-scale five-axis machining is now within the grasp of the manufacturing community.
Why is this significant? Large components are often painstakingly crafted from numerous smaller components because the best machining technologies are not accurate enough to achieve the tight tolerances on large monolithic parts that could reduce part count and assembly time. Even with the best thermal control and machine design, manufacturers aiming to use ever-larger monolithic parts have found themselves stuck in a costly quicksand of workarounds: make-to-fit; shim-as-needed; manual drilling with patterns, jigs and fixtures; custom finishing and rework. Accuracies that are considered routine for small parts become much more difficult to achieve over the longer distances in larger parts. It is the same “interchangeable parts” challenge faced by gun manufacturers in the early 1800s, amplified to a much greater scale today.
The defense industry, in particular, is behind the demand for larger monolithic parts—such as aircraft skins/frames, land vehicle hulls, and submarine propellers—largely to reduce manufacturing cost and time. Required cutting tool position and orientation tolerances approaching the thickness of a human hair anywhere within a 6 x 3 x 1.5 m or larger work envelope are significant challenges just to measure, let alone achieve, consistently and reliably. That's not to say it is impossible to volumetrically compensate a large machine tool for this level of performance; it just is not yet practical if it takes days and weeks. That’s too long for such a valuable machine tool to be out of production.
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 Inc. (API), Boeing, and Siemens—recently completed development of a rapid, and hence game-changing, approach to volumetric error compensation. Created especially for large and multi-axis machines, the system achieves the goals set by the design consortium: to reduce 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.
This volumetric error compensation (VEC) method, developed by Boeing, uniquely considers the full interrelated effects resulting from the kinematic stack-up of all the machine tool axes. Where conventional approaches to volumetric compensation focus on the first 21 error sources associated with three orthogonal linear axes, this VEC method compensates any arbitrary stack of linear and rotary axes, addressing the 43 (or more) kinematic errors associated with a five-axis machine. The holistic methodology determines a specific volumetric error compensation solution for every tool position and orientation combination inside the work volume. VEC takes a good machine and makes it that much better.
VEC offers game-changing technology in a simple, proven package. It gives plant management a rapid and affordable way to raise a machine’s process capability, meet tighter accuracy requirements, and explore new manufacturing strategies.
An official from Boeing, which prototyped and proved out VEC on three different machine tool configurations as a project partner, estimates the breakthrough could save the company $100 million a year by dramatically reducing assembly and fitting costs on large programs such as the F-18 or 700 series commercial aircraft.
The metrology system uses innovative laser technology from Automated Precision. A laser source, the T3 Laser Tracker, is placed in the workpiece 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 some 200 statistically randomized multi-axis "poses" within the machine’s work envelope. Programmed poses are compared to measured positions over three NC program runs with two different tool-length dimensions for the active target. Software processes the gathered data to determine the holistic compensation solution.
The volumetric compensation file is entered into the control and activated by program statements. Then the CNC’s “compile cycle” technology integrates the compensations into the real-time path interpolation algorithms. It is worth emphasizing that these compensations are applied as simultaneous equations in the control, so it is actually the path performance. As the machine tool moves in five-axis space, the compensations are applied within the interpolation loop of the CNC, which is unique and extremely valuable.
The machine tool partner for the VEC development consortium, Mag offers VEC as an option on new machines and as an upgrade on customers’ legacy machines. It is also available through API.
Jim Dallam, Product Manager, Volumetric Error Compensation, Mag Industrial Automation Systems, wrote this article for Aerospace Engineering.