Machine tool 101: Part 7, Machine tools of the future

Manufacturing Engineering, Jul 1994 by Aronson, Robert B

Predictions of economic growth for the machine tool industry are conservative, but positive. A new spirit of cooperation exists among industry, academia, and government to develop programs that best utilize research resources to help industry resolve problems in the way of progress. There are even signs that training, a long neglected issue for the machine tool industry, is getting fresh attention. Meanwhile, pressures for lower cost, greater productivity, and improved quality are forcing even the most conservative machine-tool users to re-evaluate their equipment and procedures.

In response, the machine tool industry is experiencing both evolutionary and revolutionary change. Key components such as spindles and motors are improving incrementally while researchers propose radical new designs for bearings and machine tool configurations.

Short production runs and just-in-time delivery are pressuring machine tool builders to make each machine do more. Now, for example, about 25% of all lathes have some milling capability; by the year 2000 80% may have that capability.

WHO NEEDS HIGH SPEED?

Of all the proposed and actual innovation, high-speed machining is the most talked about. Increased spindle and feed speeds for milling operations have inspired many of the changes, but all machining operations are involved.

Higher speeds can increase production and improve quality, yet despite all the talk of high speed, it is still not widely accepted. Most companies see 10,000 rpm as the maximum spindle speed needed. Two areas where it has become important are in cutting aluminum for aerospace applications and die making.

Oddly, the defense contract cutbacks caused a big increase in machining operations. When a major aerospace company built 800 planes a year, it fabricated parts using forming dies. Now that 100 planes a year is a big order, machining parts is more cost effective.

Jerry Halley a senior project engineer at McDonnell Douglas (St. Louis), says they began moving away from dies and fabricated parts to machining five years ago. For example, an aircraft section initially made of 44 parts needed 53 die sets. Today, the same section is done in six segments with no dies.

Diemakers like high speed because it means a smoother surface and less hand refinishing. It's this savings in time and quality improvement that will make high-speed machining more popular.

SPINDLE CHANGES

"Around the corner, we will see spindle speeds of 250,000 rpm and cutting at over 1000 ipm (not necessarily both in the same application). But that will start in more specialized applications such as nonferrous and nonmetal machining, not conventional ferrous metal milling," says Jochen Zenker, chief engineer, The Precise Corp. He believes that the very high spindle speeds will initially be limited to specialty applications such as drilling composites for electronic circuit boards or polishing injector nozzles (not typically milling and routing). "But even conventional shops will be seeing speeds gradually increase an average of 5000 rpm over the next few years," he predicts.

At the same time, other spindle builders say we will probably see more dc brushless motors because they have more power and ceramic bearings will become more common as cost comes down. Now, about 20% of spindle bearings are ceramic, and that could soon go to 50%.

Active damping will be used as speeds go up. This could be via one of several design changes that can influence damping such as shifting a mass or changing lubricant flow to a hydrostatic bearing.

The technology exists for complete monitoring of spindle performance. It can range from a simple sensor that tells you the tool is still there to continuous monitoring of loads, vibration, thermal growth, and motor current. From this sensor information you can not only predict error, but compensate for it. "We are putting more sensors on the spindle," says Zenker, "but the question then becomes, how much data can you handle and do something useful with?"

LINEAR MOTION

Ballscrew speeds are limited by physics. Their mass limits acceleration and deceleration and past a certain length, they whip, which interferes with accuracy. The industry is waiting for a practical, low-cost linear motor. The linear motor is a reality, but it may never be as widely used as a ballscrew. Where translation speed is more important than stiffness, however, it is a serious contender.

For now, it's used only in special applications and needs a lot of power and special control to work effectively. The new Giddings & Lewis piston-turning machine uses a "voice coil" linear motor with only an inch of travel, which is all that is needed to move the cutting tool in that special application. In addition, Ex-Cell-O Machine Tools Inc. recently offered a high-speed machining center, the XHC 240, which has Indramat linear motors for both high acceleration and deceleration and precise positioning.

BETTER BEARINGS

Bearing development is another critical area. For now, the ceramic hybrid remains the top-of-the-line bearing for high speed with ceramic rolling elements and hardened steel raceways. Some stiffer ceramic materials will be used but require tighter manufacturing tolerances and greater care in application.