Machine tools turn to linear motors

Linear motor designs replace the entire mechanical transmission with a magnetic circuit fixed between guide rails. Because motor and the ball screw inertia is eliminated, the main advantage is increased acceleration. While conventional transmissions achieve up to 10 m/sec² accelerations, linear motors take off at up to 100 m/sec². Displacement speeds are doubled (100 m/min instead of 50 m/min on conventional machines), and the positioning precision is increased due to simplified system dynamics. The drawbacks (apart from the price) are low thrust, heat generation, and the magnitude of magnetic fields, which is a nuisance for swarf.

When compared to designs of just a few years ago, today's linear motor driven machine tools achieve greater feed forces, accelerations, and traverse speeds thanks to better motor designs, new machine architectures, and improved controls. Result: less non-productive time. Precisely the benefit three manufacturers of high-speed linear motor machining centers claim for their new machines. The companies Deckel Maho, Cincinnati Machine, and Renault Automation Comauall use linear motors, servo drives, and CNC control. Each features different application-specific structural designs, and each claims noteworthy advantages over conventional ball-screw machines in areas of speed, power consumption, and accuracy.

Linear motors go vertical

Linear motors are visibly making inroads in high-end HMC (Horizontal Machining Center) designs where costs are more easily justified. But until now, use in lower-cost VMC (Vertical Machining Center) applications was another story. VMCs typically cost half the price of an HMC, making cost justification more challenging. But Bielefeld, Germany-based Deckel Maho's DMC 85 V, the first linear-driven vertical machine, may be setting the stage for even more linear motor machine tool applications.

Today's direct-linear drives combine 2G accelerations (20m/sec^{2), 120m/min rapid traverse speeds, and 8,000 N feed forces, while maintaining or even increasing precision. But you can't just replace ballscrews with linear motors and expect to improve productivity. The machine's structure must be designed to help manage the linear drive's heat and magnetic-field generation in order to prevent structural deformation, machining errors, and motor contamination.}

Just ask engineers at Deckel Maho (Pfronten, Germany). They know, because they recently designed a line of machining centers that incorporate the latest digital linear-motor technology, which was introduced at EMO Paris 1999. The exhibition, focused entirely on machine tools and machine components, was held May 5th through 12th just north of the French capital in Villepinte.

DMC 85 V construction uses a highly dynamic, 3-axes gantry. A heavy bed in monoblock design works to suppress structural deformation and vibration, and increase precision, cutting capacity, and tool life, says Uli Schaupp, milling product manager at Deckel Maho's Charlotte, North Carolina facility. Linear drives mount to the side of a massive block of cast iron that helps dissipate the heat they generate. Based on the method of "finite elements," the rigid machine bed-in-block design ensures precision and stability.

To prevent contamination from chips and cutting fluid, the linear drive motors are isolated from the work area. "We call it a box-in-a-box design," says Schaupp. High performance milling spindles accommodate speed ranges of up to 30,000 rpm. Control options include: PC-NC Siemens 840 D or Heidenhain TNC 430. Both Intel/Windows-based control systems command spindle motion in all axes, while the workpiece remains stationary. Combined with numerous other intelligent details, Schaupp adds, the bottom line is even more performance during high-speed cutting in manufacturing or moldmaking operations.

Milling monolithic parts

If engineers at Cincinnati Machine have their way, airframe designers may soon be creating new types of floor beams, wing ribs, and fuselage frames. The giant machine tool manufacturer is nearing the finish of a five-year development program on a five-axis mill that could enable such parts to be created as single-piece, monolithic structures.

Currently, airplane manufacturers build floor beams, wing ribs, fuselage frames and other parts as an assembly of hundreds of pieces, including rivets, bolts, plates, channels, and steel angles.

But technology developed on the new machine, known as HYPER-MACH, should eventually enable them to create enormous single-piece parts simply by hogging them out of huge billets of aluminum. The reason: HYPER-MACH offers a combination of size, speed, and power that's never before been available in a machine tool. A test stand model has operated at speeds of 63.5m/min with 2 G's of acceleration.

Engineers from Boeing, which initially proposed the machine to Cincinnati Machine, believe HYPER-MACH's technology could yield huge benefits for airframe makers. "When you machine a part out of a plate of material, instead of assembling it, you realize a lot of advantages," says George Neilson, an Associate Technical Fellow in Boeing's manufacturing research and development area. "You don't have to make tooling to hold the pieces together. You don't have to pay someone to pound rivets. And you don't have distortion of the part after you've loaded all those rivets into it."

The key to the machine's exceptionally high speed is its use of linear motors. Three of the test stand axes are powered by linear motors, supplied by Anorad Corp. (Hauppauge, New York), and Kollmorgen (Radford, Virginia). A gimbaled spindle operates at speeds up to 60,000 rpm and is rated at 60 KW. The test stand uses a non-contact AGM 101 Grid Encoder, made by Heidenhain Corp. (Schaumburg, Illinois) that allows comparison of the tool's measured path to its actual path, and a CNC motion package from Siemens Energy & Automation.

A prototype of the machine, which is not yet completed, looks unlike a conventional mill. Its movable portion resembles a structural truss, and its high-speed spindle is cantilevered out over the work area. Engineers say that the unconventional appearance, however, was done for good reason. "Historically, machine tool makers have over-designed their machines," says Chip Storie, aerospace industry market manager for Cincinnati Machine. "The solution was always to add mass. But that doesn't work if you want to go fast and turn on a dime."

A key focus of the Cincinnati Machine-Boeing team effort was the development of a "virtual prototype" that enabled engineers to test the machine's performance before hardware was built. The process was similar to the one used in the famed paperless design of the Boeing 777 aircraft.

The team's engineers used the virtual prototype to deal with speed-related issues, such as vibration and position accuracy. By combining customized dynamic simulation software with a virtual servo system and CNC code, they were able to create a machine that offers greater accuracy than conventional mills, even when operating at speeds in excess of 50.8 m/min.

In tests, HYPER-MACH has milled out a thin-walled, honeycomb-shaped part out of an aluminum billet in less than 30 minutes. Engineers say that the same process would take approximately three hours on a state-of-the-art high speed machine, or as long as seven or eight hours on a conventional mill.

For airframe manufacturers, the existence of machines like Hyper-Mach is expected to bring dramatic changes. "Because labor is so expensive, the whole industry is moving in the direction of monolithic parts," says Neilson. "But for us to really get to that point, we need to have high-acceleration, high-velocity machine tools."

Parallel structure boosts linear drive acceleration

The Urane SX concept machine combines linear motor technology with a parallel machine architecture to boost acceleration. For Renault Automation Comau (RAC), the Franco-Italian company that showed the Urane SX at the Paris EMO, linear motor technology has become routine. At least 100 linear-motor driven Urane 20 and 25 machining centers are currently being delivered or are on order.

The parallel structure is what gives Urane SX its exceptionally high accelerations of up to 35 m/sec2. "A parallel structure has very little inertia because of the light weight and few number of moving parts. It is the only means of achieving such a high acceleration," comments Jean-Paul Bugaud, executive vice president machining.

Robots with parallel structures have been used for some years. The Urane SX is, in fact, based on the Delta design which came out of the Ecole Polytechnique Federale de Lausanne in the early 80s. Its applications up till now are for handling very light loads at very high speed. RAC has now taken out patents to use the same design but with linear motors and for machining tasks.

The Delta structure resembles a tripod in which each of the three "legs" is composed of a parallelogram. At the point where the three legs come together, they are attached to the tool or gripper that is being manipulated. At the other end, each leg is attached to a drive system. By carefully controlling the displacement of each leg at this end, it is possible to achieve a precise displacement of the tool or gripper at the other end.

In the case of the Urane SX, the tripod is mounted horizontally inside the machine structure and the tool that is manipulated by the legs is an electric spindle. The current machine is fitted with a 15kW electric spindle