Linear motors
drive aerospace machining
to higher speeds

by Richard Mandel

Trade shows are always an excellent time to witness advanced thinking -- a chance to peer into concepts and products that are "coming down the pike." The annual Consumer Electronics Show in Las Vegas is one of those, and another is the National Plastics Exhibition, held earlier this year after a 360-month absence. Of course, there's the annual Manufacturing Week in March. And this year also saw the semi-annual International Machine Tool Show (IMTS) in September.

It was at the latter that Cincinnati Machine, Cincinnati, OH, displayed their prototype linear motor HyperMach technology machine on the show floor. Under development since 1995, the machining technology was created to address how aircraft of the future may be fabricated.

Re-engineering aircraft design

Aerospace engineers have long understood the possibilities and advantages of making "monolithic" parts -- structural components hogged out of single billets of metal (usually aluminum). As an example, most wing ribs and floor spars on the majority of aircraft today are made of stamped sheet metal cut to a contour. Extrusion stock is then riveted to one or both faces to add strength. And these are large parts -- a small wing rib could be 2- x 10- x 55-in. in dimension, while floor spars can be up to 3 ft. wide and more than 20 ft. in length. When one considers that a typical commercial jetliner has around 100 wing ribs and another 100 floor spars, the time in labor, as well as the cumulative weight of the components, tends to add up.

Making these parts, as well as other sheet-and-extrusion assemblies such as fuselage frames, from single aluminum pieces would cost about the same in material as sheet-metal assembly. The real savings would be in the elimination of hundreds of fasteners, increased production speed, elimination of tooling (and the production of those tools) -- ultimately reducing the overall cost per aircraft. The challenge has been how to machine these large parts with speed, efficiency and repeatable accuracy.

Components in a commercial jet that can be replaced with monolithic parts

The HyperMach program was, from the beginning, a collaborative development with Boeing, McDonnell Douglas (still a distinct company at the time), United Technologies and the US Air Force as primary partners. In 1996, the group identified the user requirements, which included not only the machining process parameters (such as tooling speed), but also the volumetric matter of just how big the machine was supposed to be. The following year saw testing and a self-education process with linear motors and CNC servo systems, to develop useful data that would be incorporated into the design. Detailed design of the first working module occurred in 1998, and the first test cutting with the module took place in 1999. That first module was named ZAC, in reference to the three axes of motion. A and C are the rotating axes, and since the unit was designed for horizontal machining, the Z axis refers to motion perpendicular to the face of the aluminum billet, with about 20 in. of travel. The X axis would correspond, then, to travel along the length of the billet parallel to the face. For demonstration purposes at the IMTS show, the module was added to another linear motor on a vertical (Y) column.

Linear motor-ized

For this type of machining operation to be economically viable for the aerospace industry, the HyperMach technology would have to take a quantum leap past current constraints on acceleration and velocity. While the spindle speed was targeted at 60,000 RPM with a 100 HP motor -- four times the speed of spindles in today's high-speed machining, and nearly the same leap in power -- the need for fast feedrates was the primary reason to explore using linear motors in the X and Z axes. Ultimately, the use of linear motors allowed the HyperMach testbed to generate contours at feedrates of 2400 ipm, with rapid rates of 4000 ipm. The machine would have the potential of changing direction at 2 Gs -- almost 800 in./sec².

For the prototype HyperMach module, the designers selected motors from Kollmorgen Corporation, Waltham, MAs, the motors for the Z axis were mounted in a V-shaped form in relation to one another, along their axis of movement, perpendicular to the work floor. It was discovered that the attractive magnetic force of these motors, which is about three times the motive force of the slide, tended to help counter the stresses on the assembly's bearing caused by the weight of the module. The vee bed of the motors produced attractive magnetic forces that, rather than being directed straight through the structure, was transmitted in a vector. Literally, the magnetic force helped the assembly counter the pull of gravity.

ZAC module ­ note vee bed of linear motor magnets on "Z" axis

Model 840D Digital CNC/Digital Servo drives made by Siemens, Elk Grove Village, IL, were used because their open architecture permitted the use of motors from other companies. Also, at the time the prototype was being developed, Siemens was not yet offering their own linear motors.

While the machine isn't used in ferrous metal operations, it is not because of the potential chance of contamination of the motors by chips and debris. Rather, the primary reason is because the speeds that aluminum can be processed are much higher than those for ferrous materials. Steel or cast iron require "brute force" machines that apply high thrust forces against the workpiece. An additional benefit over conventional machines, besides the faster rate of production, is that at the extreme milling speeds of the HyperMach, the heat produced is taken away by the chips themselves, leaving the workpiece cool and stable. The high rate also keeps forces low, permitting the generation of thin gages with minimal distortion.

Making chips fly

In tests before IMTS, the HyperMach machine milled out, in less than 30 minutes, thin-walled aluminum structures from a single billet that typically would have taken at least three hours to produce on a state-of-the-art, high-speed CNC machine. A conventional CNC mill could have taken as long as eight hours on the same part.

Already, aircraft companies are starting to use monolithic aluminum components in development and production vehicles. Virtually all of the wing ribs of the next generation 737 commercial jet are designed to be monolithic components. Military aircraft projects like the Joint Strike Fighter and an unmanned combat aircraft program are also potential targets for a switch to aluminum. Other industries that use aluminum components, as with the increase seen in automobiles over the last five years, may have to wait for another version of this production technology because aircraft-grade aluminum has different properties that lends itself better to this kind of extreme speed machining.

Many thanks to Randall Von Moll, HyperMach production manager, for his assistance in preparing this article.

For more information:

Circle 410 - Cincinnati Machine or
connect directly to their website via the
Online Reader Service Program at http://www.1rs.com/011-df410

Circle 411 - Kollmorgen or connect
directly to http://www.1rs.com/011-df411

Circle 412 - Anorad Corp. or connect
directly to http://www.1rs.com/011-df412

Circle 413 - Siemens or connect
directly to http://www.1rs.com/011-df413