Retrofitted Laser-Scale Systems Improve Linear Accuracy Of Gantry Mills

When aerospace machining specialist Alson Manufacturing of Compton, California, produced its first parts more than 30 years ago, allowable tolerances on a 160-inch part might have been 0.030 to 0.040 inch. Today, thanks to an industry-wide narrowing of tolerances, that same part has to be machined to within ±0.004 inch of the printed specification.

According to Ray Sanchez, manufacturing engineer at Alson, aircraft manufacturers have upped the ante for accuracy and precision in the last few years. "Reduced variation in parts is an industry-wide goal," he says. The objective is reduced assembly time, effort and tooling—and ultimately, a move to automated assembly.

"Customers want to eliminate shimming, which adds weight to the product and time to the process. And they want to minimize fixturing," he adds. "Instead of using fixtures and shims to bring parts into assembly tolerance, they want the parts themselves to meet the tolerances."

Weight is also an issue. Less variance on the hundreds of parts that make up a plane can cut dozens of pounds from the aircraft's final weight, enabling aircraft manufacturers to achieve greater capacity, range and fuel efficiency.

The bottom line for Alson was, and is, meeting the customer's specifications. "Until a few years ago, we had not been dealing with extremely close tolerances," says Mr. Sanchez. "Customers expect much more today. The game has changed."

Alson anticipated the changes in the game and began installing Renishaw (Hoffman Estates, Illinois) HS-10 laser scales and HC-10 temperature compensation systems on its large machines in the mid 1990s.

Alson improved the accuracy of its existing machines nearly 70 percent by installing the laser scales and temperature compensation systems. With five laser-positioned machines now online and an impressive list of happy customers, Alson Vice President Don Schoellerman looks backs and says, "For the parts and the kind of work we specialize in—close-tolerance five-axis milling and drilling—we believe very strongly this is the only way to meet the new aerospace industry requirements."

Alson's laser-positioned machines are all "long axis" Cincinnati Machine units. Two are dual-gantry, three-spindle, five-axis mills with travels of 164 inches by 1,000 inches. The third machine is a rail-type five-axis mill with travels of 70 inches by 336 inches.

These machines had been factory-equipped with standard technology for linear position feedback: shaft-mounted resolvers or encoders on the rack-and-pinion drive on the X axis and a leadscrew on the Y. "We had a lot of experience with CNC machines and rotary encoders/resolvers," Mr. Sanchez says. "While they were ‘good enough' at one time and still are for short axes—a number of variables degrade their measurement accuracy."

Alson's engineers used—and continue to use—all available means to compensate for sources of linear position error. Machines are still regularly calibrated with a laser interferometer to ensure they deliver the best accuracy possible. To compensate for thermal expansion/contraction in the parts—which, for aluminum can produce linear errors ten times greater than those of the machine—Alson had monitored ambient temperatures and adjusted the time of day for performing an operation, but these temperature compensation efforts were "very crude," according to Mr. Sanchez.

The HS-10 scale uses the same technology—laser interferometry—that machine calibration specialists use for linear error compensation, except it is designed to be a permanent scale on the machine, replacing resolvers and encoders. The HS-10 functions as an independent measurement reference, unaffected by backlash in the drive. That is, it measures the actual position of the machine gantry (X axis) and spindle (Y axis) for the CNC. Traditional axis position feedback systems—optical and magnetic encoders or resolvers on the servomotor shaft driving the ballscrew—can only tell the CNC what the servomotor is doing. It is assumed that the tool point is moving in perfect synchronization with the motor, but this is rarely possible. Unlike a scale, a laser has no short-term errors that can "stack up" on long axes, and its 2,400-ipm measuring speed accommodates today's fast-moving machine tools.

The HC-10 temperature compensation system establishes all machining action in relation to 68°F. The shop isn't air-conditioned, and temperatures can range from 50°F to 90°F, according to Mr. Sanchez. "We use the lasers to comp the two longest axes of the machine," says Mr. Sanchez. "The only thing that changes is the reference point where we feel the thermal compensation needs to emanate from—that is, from a hard stop or from the center of the part. Some setups use a hard stop and, therefore, the part is growing from that point; other setups are designed to allow the part to grow equally from the center."

To date, Alson has installed laser scales on the X and Y axes of all five of its gantry machines. "The beauty of laser positioning and temperature compensation is that you can retrofit it to an existing machine and get a near-instant upgrade of accuracy, plus you eliminate environmental conditions as an error source without going to an air conditioned shop," says Mr. Sanchez. "Based on extensive experience, we know we can hold 0.005 inch true position on a production basis on a part 135 inches long, which translates to about ±0.002 inch. We're actually probably better, but this takes into account all the other variables in the process, such as the way the operator loads the part, spindle runout and so on."

The exceptional positioning accuracy of these machines also allows them to function as giant CMMs, using Renishaw spindle probes. Final surface finish of parts is improved, too, because laser positioning eliminates any ripple effect of the mill.

"Laser scales are an integral part of our competitive strategy," adds Schoellerman. "It's the most cost effective way to deliver the higher tolerances our industry demands today."