December 1, 2016 | Aerospace | Test & Measurement
High-Resolution Surface Measurement
Inspectors who are assessing the severity of defects and the dimensions of small features on precision machined surfaces are limited by the capabilities of existing measurement techniques. A new type of instrument has been developed that enables direct, non-contact inspection of precision surfaces in shop floor environments. By combining the resolution of optical techniques with the portability of a handheld gauge, the new technology gives inspectors an important tool for finding and quantifying critical features on machined components.
Figure 1. A wiregrid micropolarizer array divides light into four polarization angles, enabling simultaneous acquisition of all data for vibration-immune measurement.
Precision components for aerospace, automotive, heavy equipment, and other applications are inspected frequently during production for potential damage, and throughout their life spans for wear, corrosion, and defects. Excessive defects will, of course, inhibit component performance. In many applications, however, undetected defects can lead to costly component failure, and even potential loss of life.
Inspectors face challenges in determining the severity of defects or the depth of desired features such as laser marks or dot peening. Pits or scratches just tens of microns deep may be sufficient to cause part failure, and imaging such small features requires high-resolution metrology. Similarly, markings that are too shallow will wear off, losing part serialization, while markings too deep can compromise part longevity.
Figure 2. Measuring a surface defect on a large aircraft engine component with a PSL-based gauge. By enabling in-situ measurement on the large component, the measurement time was reduced from an hour to less than 10 seconds. (Image courtesy StandardAero)
High-resolution metrology options, including optical and stylus profilometers, are available and widely used. These systems, however, are typically too expensive, slow, and overly susceptible to environmental factors for use on the shop floor. They are also typically mounted in workstations that limit their capability to measure features on large components or in hard-to-reach areas. Inspectors must create plastic/rubber replicas of these surfaces, which can then be sectioned and measured on an optical comparator or other instrument. This time-consuming operation may require an hour's effort for a single measurement, and may still not provide a high level of quantification due to variability in the replication or sectioning processes.
In lieu of high-resolution metrology, inspectors most commonly assess feature heights using visual inspection and comparison to sample surfaces of known quality or methods such as scribe checking. These methods, however, are neither precise nor repeatable, and are highly subject to interpretation. This overall lack of precision leads to an abundance of caution in interpreting the results — which, in turn, results in millions of dollars lost to the rejection of components that actually meet specification.
Another significant shortcoming of visual inspection is that many areas of large and complex components are difficult or impossible to reach for thorough inspection. Areas under flanges, inside of bores, between blades, etc., are difficult or impossible to inspect visually. Slow and messy replication is again the only option for measuring these areas.
Figure 3. A PSL testbed gauge measuring difficult-to-access areas of an aircraft engine fan disk.
Polarized Structured Light
The lack of metrology options has been a challenge to various industries, including aerospace and automotive, for many years. The challenge to metrology providers has been to provide sufficient resolution in a portable instrument that can be used in the dirty, vibration-prone environments of manufacturing and repair facilities.
A new technology has now been implemented that enables handheld measurement with micrometer resolution. The technology is based in interferometry, in which a light beam is split and directed both to a test surface and to a perfect reference surface. When recombined, the beams form a pattern that indicates the difference between the two surfaces. One surface is then shifted relative to the other, and multiple images are obtained. From this data, the test surface's shape can be determined, with nanometer-level resolution.
The technique has two significant limitations. First, because acquisition time is relatively long, interferometers are highly susceptible to vibration and air turbulence. Second, the technique can only measure surfaces with pixel-to-pixel height differences of a few hundred nanometers, limiting its usefulness for measuring rough surfaces, curved areas, and larger defects.
Over the past decade, a technique called Dynamic Interferometry, pioneered by 4D Technology Corporation, was developed to overcome the susceptibility to vibration. In a dynamic system, a wiregrid mask (Figure 1) exploits the polarization of light to split the light beam into multiple phases such that all measurement data can be acquired simultaneously. Acquisition takes only tens of microseconds, enabling the instrument to measure accurately in noisy environments.
A new technique uses polarized structured light (PSL) to extend the vertical range of Dynamic Interferometry from hundreds of nanometers to thousands of microns. The non-contact technique provides multiple benefits that make it ideal for measuring precision surface features:
Figure 4. Measuring corrosion pits on the flange of an engine shaft. The black-tipped shaft is an alignment aid that sets the gauge at the correct measurement distance.
Sub-micrometer resolution is sufficient for imaging defects from machining, wear, and corrosion.
The vertical range is much higher than with interferometry, enabling measurement of features from 2.5 - 2500 microns (0.1 to 100 mils).
Vibration immunity enables the system to be handheld for far greater inspection flexibility.
Measurement results are instantaneous, reducing measurement time from many minutes to several seconds.
Results are quantifiable and can be tied to traceable standards.
A large focus range makes it possible to align the system by hand and measure defects on flat or curved surfaces.
Areas without direct line of sight can be inspected without replication.
A PSL-based gauge can be handheld to access tight corners or to directly sample the surfaces of large components (Figure 2). When coupled with a mobile computer and battery power, the instrument can be moved throughout a shop to measure large components in-situ. Bringing the metrology to the component, rather than vice versa, saves time and avoids potential handling damage to large, expensive components.
The PSL technique has been demonstrated successfully for accurately measuring defects such as pits, scratches, nicks, dents, and bumps on flat or curved surfaces. Figure 3 shows a testbed PSL gauge being used to measure difficult-to-reach areas of an aircraft engine fan disk. A mirror attachment is being used to bend the test beam 90 degrees for borescope-like measurement of hidden areas.
Surface features, such as laser marking and dot peening for part identification, can also be precisely characterized. A PSL gauge can measure the depth of each laser mark or dot peen strike to ensure compliance with specifications. Rivet depth and shape, the radius of small corners, and precision features such as solder and adhesive bonds can also be assessed.
Rapid, portable, high-resolution metrology is a key driver for decreasing scrap and improving reliability of components in a wide variety of industries. By enabling in-situ measurement of surface defects and features, PSL provides manufacturing and repair facilities with a cost-effective option for accurate, traceable defect and feature inspection.
This article was written by Mike Zecchino, Manager of Technical Communication, at 4D Technology Corporation, Tucson, AZ. For more information, visit https://info.hotims.com/61068-422.
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