measurement: Tolerances and Inspection Go Hand-in-Hand

April 1, 2005
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Optical gages that measure parts also scan the conical seat, including roundness, straightness, roughness and more.

One of Zygo’s laser interferometers measures a 210-micron long conical seat on a fuel-injector part in seconds. Meanwhile, it also checks roundness and deviation from form. Source: Zygo Corp.

Tight tolerances and rapid form-and-surface inspection might not have gotten along very well in the past. But they certainly seem to be going hand-in-hand much more often these days-if developments in fuel-injector manufacturing are any indication. Suppliers to the automobile industry are using laser and white light interferometers to measure mating surfaces inside the injectors so they can guarantee that the metal-to-metal seals will withstand pressures greater than 30,000 psi.

Not only are these optical gages measuring the roughness and flatness of the parts, they also are measuring their conical forms. "They can scan the whole working surface of the conical seat in about five to 10 seconds," says Jack Clark, applications development manager, Zygo Corp. (Middlefield, CT). That includes roundness, straightness, roughness, angles and deviation from form.

Although fuel-injector manufacturers are showing great interest in these gages, they are one of a growing number of businesses favoring them over the contact devices that have dominated the field in the past. Manufacturers like that modern optical gages can take a variety of precise measurements very fast in three dimensions because they get the information needed to make good decisions for quality and process control.

Information from optical gages is many times more useful than measurements from conventional devices because it is a 3-D picture of the feature being measured. The problem with a stylus-based surface gage, for example, is that it traces the profile of peaks and valleys of only a line across the surface. An inspector is not quite sure whether a valley is a trough or a pit in the surface or whether a high point is a ridge running across an area or just debris. On the other hand, the true nature of these readings becomes evident in 3-D scans.

Interferometers make checking such features practical on all of the parts coming off a production line. They also are inherently more accurate than contact gages. "When you touch the part, the part could bend, deflect or become damaged," explains Dr. Thomas Dunn, engineering manager at Corning Tropel Corp. (Fairport, NY). "Optical measurements avoid all of those problems." They also eliminate the problems caused by the friction, wear and vibration present in any mechanical system.



Scanning-frequency interferometry developed by Corning Tropel can use one or two optical heads. The single-head arrangement checks parallelism, flatness, depth and roughness. The extra head in the dual-head arrangement can gage thickness in addition to checking these parameters on both sides of the workpiece. Source: Corning Tropel

Better Gages

Although the optical gages have been offering many of these advantages for decades, now they have become practical on a wide scale only in about the last five years. Part of the reason is the better optics and data analyses that were developed. Another part is the applications engineering that has gone into deploying interferometry. "Also, the scanners have improved so that they are faster and far more repeatable and predictable," Zygo's Clark adds.

As with everything, inexpensive computing power has made tremendous contributions too. Today's microprocessors not only give the gages the capacity to handle large amounts of data in short periods of time but allow builders to develop sophisticated algorithms for extracting the desired information quickly enough for use in production. "A normal data set for 3-D gaging contains between 300,000 and 400,000 points," Clark explains. "So the raw data from an interferometer can be 25 MB. The microprocessors can accept those data, apply software filters to reduce them into megabyte-size files, and report the desired information within 10 seconds."

This ability allows the gages to process images of surfaces and perform inspections done traditionally by scanning electron microscopes and CCD cameras. "We not only can produce a height map but use a gray-level picture for such things as lateral measurements, edge radii and hole positions," Clark says. The computing power available today also is key for integrating surface and form measurement into untended automated lines.

Advancements in laser diodes also are contributing to the rising popularity of optical gaging. Better diodes developed by the telecommunications industry coupled with today's computing power have allowed Corning Tropel to develop a new kind of interferometer. Based on a technique called scanning-frequency interferometry, the device differs from conventional interferometers in that it uses a tunable laser that generates a beam of varying wavelengths. The laser in a conventional interferometer generates a beam of only one wavelength.

The advantage of the tunable laser is that it can measure to depths as great as 20 millimeters. So a gage using one can measure rough surfaces, recesses and contours. "It takes a picture of the whole surface at one wavelength and then takes another picture at a different one," Dunn explains. "It does that a number of times, and the data are processed to develop a 3-D picture of the part." The range of wavelengths used depends on the depth of the feature and the required resolution for the job.

The process takes approximately five seconds to measure surfaces throughout the full 20-millimeter depth in its 40-millimeter by 40-millimeter measuring range. "It can measure form and distance tolerances to better than 0.1 micron," Dunn says. "Obviously, if you have a shallower depth, smaller area or a looser tolerance, it will be faster."



Automatic Feedback

Such speeds are fast enough in some applications to provide real-time feedback to manufacturing processes. At a beta site, engineers at Corning Tropel are working with their counterparts-from a manufacturer of fuel injectors and a builder of double-sided grinders-to install a dual-headed version of the concept. Called LightGage, the device will measure the thickness, flatness and parallelism of washers as they emerge from the grinder. The goal of the project, however, is to establish a closed feedback loop so the grinder can adjust its speeds and feeds automatically to avoid drifting out of tolerance.

Another in-process application of scanning-frequency interferometry is combating poor fits in precise assemblies. In these cases, "You can make Parts A, B and C perfectly to within the tolerances in the blueprint, but they won't fit when you try to assemble them because of tolerance stack up," Dunn says. Rather than eliminating the problem by making the components to even tighter tolerances, he wonders about an alternative: Measuring Parts A and C to create profiles of both, calculating the necessary size of Part B between them and then either selecting or producing Part B to fit.

Another fuel-injector manufacturer is wondering about the efficacy of the technique too. It is exploring this option at another Corning Tropel beta site. Engineers there hope to use the builder's scanning-frequency interferometer to create profiles of two components in one of its fuel injectors and then calculate the appropriate size of spacer that fits between them. "Very small differences matter when you're trying to create a good seal," Dunn says. "At submicron levels, it becomes that much more critical."

Although the goal is to do 100% inspection with this technique, research still is in the initial stages. Right now, a team of engineers from the fuel-injector manufacturer and Corning Tropel is looking for an efficient way of measuring the tilt of the spacer and correlating the measurements to leakage from the injectors.

Dunn anticipates that he will work with more manufacturers in the coming years because contact tools simply cannot provide the same performance as optical gages. "We get blueprints for parts that specify roundness to within half a micron," he says. "When you talk about making parts with that level of precision, you really need to look at new optical technologies." Q



sidebars: Tech tips

• Optical gages measure the roughness and flatness of the parts. They also measure their conical forms.

• Information from optical gages is more useful than the measurements from conventional devices.

• A data set for 3-D gaging contains between 300,000 and 400,000 points. The raw data can be 25 MB.

• The goal is to do 100% inspection with the technique. Still, research is in the initial stages.



Report on Roughness Standards

Given the growing popularity of 3-D optical gages for checking surfaces, the industry is busy defining a set of surface-roughness standards for 3-D measurements. Members of the B.46 Surface Standards Committee of the American Society of Mechanical Engineers (New York)

are helping a working group of the International Organization for Standardization (ISO, Geneva, Switzerland) to formulate that standard. Committee member Jack Clark of Zygo Corp. (Middlefield, CT) reports that a draft standard exists and that many gage builders already are following it. All that remains is for the ISO to finish and adopt it.

"ASME B.46 pretty much decided to follow ISO," Clark says. "So there will be little deviation from ISO coming from B.46." The committee changed its mind a few years ago about deviating from ISO partly because the international organization began issuing badly needed standards on 3-D surface measurements. Earlier, ISO was lagging in meeting the needs of early innovators of the technology in the United States.

The new draft standard incorporates many of the original Birmingham parameters that extrapolate the conventional 2-D roughness parameters into three dimensions. It also includes new parameters that exploit the capabilities of optical technology. It does not, however, keep pace with the filters that gage builders used to clean and segment the data.

"Because new data filtering methods have been developed and the ISO standard has not yet published or adopted some of them, the actual number that would come from filtered data could be quite different from instrument manufacturer to manufacturer and not correlate," Clark says. Users, therefore, must be aware that the processing of raw data affects the relationship of the parameters reported and must consider its effects on the function of the parts or assemblies inspected.

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