Flying high with optical metrology

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4D Technology’s technical communication manager, Mike Zecchino looks at how you can reduce repair/rework through the use of high-resolution, shopfloor defect measurement.

When is a scratch or dent critical to function, and when is it just a cosmetic issue? Will wear in a bore lead to excessive heat and possible failure? Are corrosion pits deeper, or more numerous, than specifications permit?

Inspectors in maintenance, repair, and overhaul (MRO) facilities face these types of questions daily. They must be able to accurately gauge the severity of surface defects in order to decide whether components should be accepted, refurbished or scrapped.

Yet, measurement methods for rapid, reliable defect measurement have, to date, been lacking. At one end are methods such as visual comparison or scribe checks which, while fast, lack repeatability and are highly subjective. At the other end of the spectrum are high resolution metrology systems which provide quantifiable results but are typically too expensive, slow and susceptible to vibration for defect measurement on the shopfloor.

Lacking repeatable measurement methods, inspectors err far on the side of caution when judging the severity of defects in order to avoid accepting potentially unsafe components. The net result is millions of dollars lost due to the rejection of components that actually meet specification.

A PSL gauge measuring a) a metal fan disk, b) a composite panel, and c) the interior of a plastic component

Let there be light

A new measurement technique known as Polarised Structured Light (PSL) has been developed and productionised to meet the need for accurate, quantitative defect measurement. A PSL gauge provides handheld, 3D measurement of surface defects and features, with micrometre resolution. Focus, alignment and measurement are accomplished in under 10 seconds.

The PSL technique combines two measurement technologies: fringe projection (structured light) and dynamic interferometry. In the structured light method, a pattern is projected optically over the test surface and then imaged at a different angle. The surface’s shape can be reconstructed based on the distortion of the projected pattern. The method is somewhat slow and is too susceptible to vibration for use in a handheld system.

Dynamic interferometry, another non-contact, optical measurement technique, provides high-resolution measurement despite vibration. The method makes use of the polarisation of light to simultaneously obtain all necessary information for 3D reconstruction. Acquisition time takes only tens of microseconds, enabling measurement in noisy environments or with a handheld instrument. The method has traditionally been used to measure extremely smooth surfaces, and the method alone has not been capable of measuring larger steps or deep features.

The PSL technique is a combination of the two methods. PSL provides non-contact, vibration-immune, high speed measurement of surface defects and features from 0.0002” to 0.1” (5micron – 2.5mm) deep or tall. Handheld PSL gauges provide much higher resolution and repeatability than visual inspection, and much higher speed and flexibility than stationary metrology instruments. Sub-micrometre resolution enables imaging of defects caused by machining, wear and corrosion, as well as measurement of features such as part marking and radii. Using a PSL gauge, the measurement cycle is reduced from many minutes to several seconds. Results are quantifiable and can be tied to traceable standards.

In addition to defects a PSL gauge can measure features such as, a) paint thickness and b) rivet depth

Handle with care

A handheld, PSL-based gauge can be used to inspect both simple and complex part geometries. In Figure 1, a PSL-based gauge is shown measuring wear on a fan disk (left) and on an engine casing (right). The instrument includes software that automatically removes the overall curvature of the surface such that the depth of each feature can be accurately gauged. The handheld format makes it the only available method for high resolution measurement of large components such as these. The gauge’s portability and vibration immunity make it possible to measure components in-situ on the factory floor, reducing the potential for handling damage while moving components to and from a metrology laboratory.

The PSL technique can be used to measure defects and features on a wide variety of materials with varying reflectivity and roughness. Figure 2 shows a PSL gauge measuring, a) a metal fan disk, b) a composite panel, and c) the interior of a plastic component. Fold mirrors and alignment aids help an inspector to access areas inside of bores, between teeth, or under flanges.

In addition to defects, the SPL gauge can also be used to measure fine surface features. A PSL gauge can quantify paint step thickness (Figure 3a), rivet depth and shape (Figure 3b), the depth of laser markings or dot peen strikes, the radii of small corners, and other precision features.

A PSL gauge can measure multiple defects simultaneously, such as quantifying the dimension of many corrosion pits to ensure compliance with specification. A measurement and analysis of a corroded area takes only a few seconds and reports the depth, volume, area, and location of each pit in the field of view. Thresholds sliders let the inspector include only pits that are deeper or taller than specification. All of the data is available for immediate reporting and uploading to quality control systems.

Rapid, portable, high-resolution metrology is essential to maintaining component performance and safety, and to reducing the amount of unnecessary scrapping and reworking of expensive components in a wide range of industries. By enabling in situ, high resolution, non-contact measurement of surface defects and features, a PSL gauge provides manufacturing and repair facilities with a cost-effective option for accurate inspection of surface defects and features.

www.4dtechnology.com

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4D Technology

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