Improving Machine Tool Linear Accuracy With Laser Scales

Where does machine tool linear accuracy come from? Many things contribute to it, but the device your machine tool probably uses to measure its linear position is actually rotary in naturean optical or magnetic encoder or resolver on the servomotor shaft that drives the ballscrew. The computer numerical control (CNC) unit effectively "associates" a certain number of counts from this rotational device with linear positions for the spindle.

This would be fine in a perfect world where screw leads were true and drives were backlash-free. But in the real world, it is an inherent source of linear error that machine tool makers compensate for with laser calibration and special software in the CNC. For axes of about a meter, this works very well until the ballscrews, bearings, rack-and-pinion or other components wear a little and get sloppy. A typical 600-mm pallet horizontal machining center with this setup can deliver "new" accuracy of around ±0.0002 inch (±0.005 mm), and this can often be halved by going to a costly, optional linear glass scale.

However, in the aerospace industry, machine axes are often longer than a meter, and glass scales, because of cost and physical limitations, are not a practical option for accuracy enhancement. Thus, the aerospace industry has driven the development of a laser-based linear position feedback system for large machines. However, in aerospace, it's not just very large machines that must be considered, it's also very large workpieces--workpieces that undergo substantial changes in dimension due to fluctuations in temperature. For this reason, advances in positioning systems are being complemented by a thermal compensation system that allows large aluminum parts to be measured for temperature, and machined accurately in a non-temperature-controlled environment.

This combination of technologies is yielding accuracy improvements of 20 to 30 times for large machine tools and assembly systems, according to reports from the field. It brings accuracy down to the repeatability of the machine--check out your machine's specs, and you'll appreciate what this means--approximately one part per million (±1 micron per meter) on axes out to 250 feet (75 meters), or approximately ±0.0004 inch for a 40-foot axis.

Because this system measures the true position of a machine component, such as a column, it can be retrofitted to existing, older machines. Given the cost of the large machine tools typically found in aerospace, preserving this investment is a major consideration, especially under the economic pressures this industry faces. But economic pressure is certainly not limited to aerospace shops. A version of this same technology is in OEM use on new "standard" machines, too, making the benefits available to shops of all sizes.

Aerospace Driven

As with many significant advances in machine tools, it is the aerospace industry pushing the accuracy and technology envelope. Why? Because the size, shape and temperature sensitivity of parts in the aerospace industry make it extremely difficult to achieve print tolerances. Part costs are high, and part runs are so short there is no "trend" to monitor with statistical methods.

If machining variances can be reduced even 0.001 inch on the myriad of parts in an airframe, the cumulative effect can be a measurable weight reduction in the final product. Weight variances are an obsession in the aerospace industry.

But weight alone is not the issue. Aircraft makers are striving to enjoy one of the cost economies that many other makers of machined parts take for granted: automated assembly. Greater adherence to print tolerance reduces the amount of shimming in final assembly. Improved part accuracy also opens the door to greater use of automated assembly machines. The stacking error in conventionally machined wing components, for example, currently makes automated wing assembly very difficult.

Two Sources Of Error

Machine inaccuracies tend to come from two sources:

• The axis position measuring system: Aerospace machines--often equipped with rack-and-pinion drives, and encoders or traditional scales for position feedback--cannot deliver ultra-high accuracies even when new.
• Fluctuating environmental conditions cause dimensional changes in the part during the cut.

The laser scale and compensation unit address both these problems. The scale is based on technology used for laser calibration of machine tools, widely known as linear error compensation. Theoretical accuracy for a laser calibration system is ±0.1 micron per meter. The scale uses the same principle of interferometry, but is packaged for the machining environment. Beam strength, optical sensitivity and other features have all been enhanced over a calibration laser to achieve high accuracy in an actual machining environment.

The scale functions as an independent measurement reference, unaffected by backlash in the drive. That is, it measures the actual position of the machine 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 glass scale, a laser has no short term errors that can "stack up" on long axes, and its 2400-ipm measuring speed accommodates fast-moving machine tools.

The laser scale integrates with CNCs capable of reading an A/B quadrature signal (RS-485 port), which is the standard signal produced by linear and rotary encoders. Only the laser source and two small optical elements mount on the machine. Protection from the machining environment can range from nothing to minimal shielding with simple sheet metal covers. Plastic pipe has been used effectively as "protection."

Temperature Compensation

A large, asymmetrical work-piece of thermally responsive aluminum is literally a moving target during the machining process. Spars and other aluminum workpieces can change length by 0.1 inch or more in a few hours, responding to changes in plant environment.

This behavior is predictable, however, and can be mapped in CATIA or other CAD/CAM systems. The thermal compensation unit, linked between the scale and the CNC, uses that prediction, along with input from temperature, pressure, and humidity sensors, to adjust the laser scale's signal. The temperature of the machine, workpiece and air are all accounted for, the latter being needed, along with humidity and pressure, to compensate for the refractive index of air, which affects the laser beam.

The compensation unit is an industrial, Pentium-based, rack personal computer (PC). It can compensate as many as six axes, with inputs for both a scale and reference mark switches on each axis. Compensation can be dynamic, for thermally sensitive materials that are known to change and expand during machining; or simply "frozen," for large, thermally stable workpieces that are well fixtured on vacuum tables.

The compensation unit can apply a simple textbook coefficient of expansion for a workpiece and adjust all axes with the same factor. Or, much more can be done to achieve the highest accuracies on thermally sensitive, asymmetrical parts, such as a wedge shape, where expansion and contraction rates vary dramatically from one area of a part to another. The variation can be further complicated by the way the part is fixtured. However, a programmer can "see" all of this behavior as the part model is "heated" in CAD/CAM software.

Using model information, aerospace CNC programmers have:

• Calculated a new, arbitrary coefficient of expansion that reflects more accurately the behavior of the part,
• Calculated multiple coefficients of expansion that account for the different directions of part expansion/contraction in relation to fixturing points and reference mark signals, and
• Created bounded temperature "zones" on the part with site-specific compensation data.

A creative programmer can virtually "linearize" the expansion behavior of highly asymmetrical parts, such as those with varying thicknesses, shapes, holes, pockets, honeycombs, laminations and so on--any part that can be modeled in CAD/CAM software.

Not Just For Aerospace

The technology needed to satisfy the demands of aerospace machining ultimately migrates to all machine tools, much as servocontrol and NC did. That migration will very likely occur much faster for laser scales and improved thermal compensation.

Cost-effective systems for standard machines are being developed. And unlike NC and servocontrol, the laser scale won't be limited to new machines. These systems can be retrofitted to machines already in place to give them accuracy that will be nearly equal to the best new machines.

Click here for more information about the author and Renishaw, Inc.