CNC Positioning | Laser Accuracy Systems

How capable is your machine tool?

Gregorig, Ian

Machine tool builders routinely use laserinterferometry to check equipment during assembly and automatically compensate for CNC machine and coordinate measuring machine (CMM) positioning errors. Now, some companies working toward ISO 9000 certification, even those with as few as 10 CNC machine tools, are using it to "map" their shop-floor process capability.

Though ISO 9000 doesn't specify how often to check machine tools, manufacturing engineers must monitor equipment process capabilities on an ongoing basis. Demands for high part quality often preclude using conventional gages, making laser interferometry or a more costly calibration technique the only option. Measurement intervals will vary. Some machines must be calibrated monthly or quarterly, others annually. Timing depends on the equipment's working hours, stability, required accuracies, and historical processing performance.

Laser interferometry also allows grading machine tools based on accuracy. This strategy allows manufacturing engineers to assign high-precision work only to the most accurate machines, using the others for routine applications. It also reduces scrap.

WAVELENGTH ACCURACY

Coupled with application-specific PC software and appropriate optics, a laser interferometer can provide accuracy and convenience unmatched by gages or indicators. Common applications include calibrating machine tools and CMMs, quality control, preventive maintenance, error mapping, automatic linear error compensation, and a range of geometric measurements. Unlike a laser alignment system, which measures beam position on a target with resolution near 0.0001" (0.003 mm), the interferometer can measure linear distance to an accuracy of +/- 1.1 ppm.

Interferometry is made possible by the fact that a laser beam is coherent light, with all rays at the same wavelength and phase (peaks and valleys are in sync). During linear-distance measuring, the equipment shoots a beam through a small optical device made up of a beamsplitter and a retroreflector, which always reflects a beam parallel to the source beam. The beamsplitter sends a reference beam back to a detector on the laser source and allows a second beam to strike another retro-reflector, which returns another beam to the detector. The detector counts how many wavelengths of light one optical element moves relative to another and displays the corresponding distance measurement on a PC screen.

When measuring a long axis, the interferometer usually is stationary, and the second retroreflector is on a moving element such as a machine spindle. The interferometer is separated from the laser source to prevent distortion due to heat buildup. As a result, the measurement is independent of laser source position.

A typical interferometer uses a stabilized helium-neon laser with a 0.633-mum nominal wavelength and a long-term wavelength accuracy (in vacuum) better than +/- 0.1 ppm. The laser light wavelength depends on the laser tube length, which quickly heats to a specified temperature (stabilizes), allowing full accuracy ten minutes after laser startup.

Though a beam of light never wears out and laser frequency is constant, the air surrounding the beam can cause measurement error. While environmental conditions have little impact on accurate measurement of angle, velocity, or flatness, they must be compensated for to achieve accurate linear displacement measurements. For example, there will be an error of about 1 ppm for each of the following changes: 1.8deg F in air temperature, 0.1" (3 mm) Hg in air pressure, and 30% in relative humidity. The machine tool's temperature also impacts laser accuracy. A change of just 1deg F in hardened steel, for instance, can introduce error of +/- 7 ppm.

To correct for extraneous influences, the interferometer's environmental compensation unit continually monitors all factors affecting the refractive index of air, automatically compensating for them. The system also has inputs for up to three material temperature sensors to normalize the machine under test to 68deg F (20deg C).

Historically, straightness has been difficult to measure due to sensitivity to air turbulence and poor optical design of the measuring equipment. Interferometer optics are now designed to reduce this sensitivity. While similar in principle to linear optics, they allow measuring straightness in almost the same time required for linear measurements. A typical system, including an interferometer and reflector, can measure straightness of travel, parallelism, and perpendicularity of machine axes. The straightness retroreflector forms an optical straight edge, allowing measurements in the two orthogonal planes perpendicular to the travel axis. A squareness pentaprism allows measuring in two axes nominally at 90deg to each other, allowing perpendicularity assessment. By measuring straightness of colinear axes such as Z and W with a common optical straight edge, operators can also derive parallelism measurements.

Straightness can be measured by stopping the machine and recording the data point or by taking data automatically while the machine is moving. The second case reduces machine tool downtime because the calibration test time is now limited by velocity capabilities of the machine tool or CMM.

Systems can provide short or long-range straightness measurement, but there is a slight tradeoff in accuracy for the second option. For instance, with a straightness range of 0.1-4 m, the measurement range is +/- 2.5 mm, resolution is 0.01 mum, and accuracy is a +/- O.5%. For a system with a straightness range of 1-30 m, measurement range is +/- 2.5 mm, resolution is 0.1 m, and accuracy is +/- 2.5%.

DISPELLING MYTHS

Two misconceptions about lasers persist today. One is the science fiction image of the laser as a weapon of mass destruction. However, the interferometer's Class II He-Ne laser source only uses a maximum power of 1 mW. It also conforms to federal safety regulations. Although operators shouldn't stare directly into the beam, they can safely view it from the side. In normal use, a shutter mechanism on the front of the laser source can automatically shut off the beam when the system isn't measuring.

The second misconception is that a PhD is needed to operate a laser. The laser interferometer is often easier to use than the machine tool it calibrates. A trained operator can unpack a portable laser and begin' taking measurements in 15 min. Its PC-based control provides simple, menu-driven interfaces (often the user simply buys an expansion card for an IBM-compatible PC already in house). The PC links directly to the CNC, allowing the operator to communicate with both the machine tool and the laser.

These software features also facilitate laser interferometer operation:

* Automatic data capture. In a typical linear displacement measuring run, the operator aligns the laser with two optical components on the machine and keys in the start and stop points. The laser software then monitors the machine's programmed, incremental moves, taking measurements at each position. The software then automatically analyzes and plots the results according to various international standards, including ISO 230, ANSI, BSI, B89, and AMT.

* Automatic linear error compensation. The software, which supports Fanuc, Siemens, Allen-Bradley, Heidenhain, and Acramatic controls, can generate and download an "exercise" program to a CNC, monitor the machine tool moves, capture measurement data automatically, revise the CNC's internal error compensation values as necessary, and retest the machine. This evaluation step now takes just a few minutes instead of hours and requires little operator intervention.

* Automatic beam-break recovery. Much like automatic tool recovery on a CNC machine tool, this feature allows the laser to resume a calibration run after a beam break without having to start over.

JUSTIFYING LASER TECHNOLOGY

A complete laser calibration system, including a kit of linear measurement optics, the laser source, environmental compensation unit, and software to run the user's PC costs about $20,000-$25,000. A system can be disassembled and carried in two or three carrying cases rugged enough to be checked as airline baggage, facilitating its use in manufacturing or the research lab, as well as at another manufacturing location. Portability also allows some companies to expand laser calibration into a profit center by offering calibration services to external customers.

Measurements are traceable to national standards. To calibrate the laser and environmental control unit, Renishaw uses a certified iodine laser calibration system and certified temperature, pressure, and humidity measurement equipment. Where traceability to a national standard is required, laser manufacturers can arrange calibration at the National Institute of Standards and Technology (Gaithersburg, MD) or another accredited calibration facility.

Copyright Society of Manufacturing Engineers Feb 1994
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