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Metrology helps manufacturers repeat good parts of the past

Bruce Morey
By Bruce Morey Senior Technical Editor, SME Media

Scanning metrology providers respond to varied needs; software improves to help engineers with scan data

The need for reverse engineering in manufacturing arises from a variety of situations, Creaform’s Dan Brown said. Old parts that remain useful may have been designed before CAD existed. Others deviate from their original CAD designs during their lifetime. And, in many cases, CAD exists but is inaccessible for a variety of business and competitive reasons. Benchmarking and competitive analysis is an important use of reverse engineering.

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Converting point cloud data captured in a scanning metrology device into polygons is an important but first step in reverse engineering. (Photo courtesy Creaform)

The backdrop is QA (quality assurance): Oldies but goodies demand replication.

Reverse engineering comes in handy in aerospace, in particular. A common reason is modifying existing planes for new purposes. A common task is retrofitting a plane for an aerial-photography camera. In that case, engineers need to scan the fuselage and create CAD data to design modifications to accept the camera. Another example is reverse engineering cockpit designs for virtual training for as-built conditions in aircraft, using the data in simulations.

“It is especially useful in tooling for molds, because molds are often modified by hand after the process of making the original,” he said. “At some point they realize the original CAD data is out of date and use reverse engineering to modify it to the as-built condition.”

Even in the high-tech world of jet engines, Creaform scanned a complete engine for a manufacturer that only had 2D drawings and needed complete CAD to keep up with the times, he added.

Process and procedures

“3D scanners, we have found, are ideal for reverse engineering,” Brown said, noting that his firm’s HandySCAN product is highly portable. Creaform’s MetraSCAN product is larger, but it also provides more accurate data faster than its HandySCAN product.

Brown is also careful to point out that, even with fast 3D scanners, reverse engineering is an involved process.

Customers often “think it is a process of scanning the data, pushing a button, and creating a CAD model,” he said. “It is much more involved than that.”

That is why the company offers its VXmodel software to bridge the gap between the scan and the CAD software.

After collecting data, which exists as a cloud of points, the first step is creating a closed mesh of polygons and aligning it in a common reference frame. “Frankly, there are always some problems with that point data that you need to clean up. The first mesh may contain reflections or holes, for instance,” he said.

The next step in reverse engineering is not reproducing a clean mesh file; it is capturing design intent, using the mesh as a reference. “For example, when you extract from the part a 1.9° draft angle, it is probably 2°,” he said. A critical dimension measured at 1.98 m is probably 2 m. That is why you need a human to analyze the mesh in creating a CAD model: Human interpretation is needed.

He also stressed that the VXmodel software does not actually create the CAD model. Rather it provides a direct transfer of parametric geometric entities into a customer’s favorite package—which usually has the capability of creating its own native CAD format design from that input.

Parts and probes

“The choice of metrology device is largely dependent on the part that is to be reverse engineered,” said Steve DeRemer at Capture 3D, the exclusive North American distributor for GOM GmbH. “Parts that are very basic with only a few holes in a very prismatic shape with straightforward features—cylinders, planes, circles—then a CMM is ideal for that.”

However, shapes with multiple curves and organic shapes, commonly found in aerospace parts, need a metrology device capable of “blanketing every element of that part with data to capture it effectively for adequate reverse engineering,” he said. “That is one of the huge strengths of an optical system like the ATOS Triple Scan that we provide, because it is going to provide what we call full field definition of the part. It is going to capture everything.”

DeRemer also said the resulting data is both “highly organized” and in a relatively smooth, continuous field. “High-quality data improves the output of the reverse engineering process.”

The ATOS Triple Scan series is offered in three different resolutions: 5, 8 and 16 million points per scan. Matching the needs of the application to the right resolution is important. A stamping die may need a lower resolution system, but a highly detailed engine part with tight tolerances may need the highest resolution.

Capture 3D provides software to convert the point cloud data into a polygonised mesh, but it relies on third-party providers to take the data to the next step and create a CAD model. Two that are popular in the industry, he said, are PolyWorks from InnovMetric and Geomagic Design X from 3D Systems.

DeRemer has seen the software that takes their output and creates a CAD model evolve and improve. “The mainstream CAD vendors are offering ever better add-on modules to their core products, which are reverse engineering friendly. They are designed to take in a polygon mesh and allow you to do more with it than just rotate it around on your screen. You can cut sections, fit curves, and loft surfaces and start to build up a CAD model manually,” he said. This has been especially aided by the growth in power of feature recognition algorithms that identify critical features in the polygon mesh.

Additive manufacturing

Additive manufacturing is another application where reverse engineering can play a vital role.

“There are various applications of AM, from making special tooling and fixtures to creating single or prototype parts,” Alicona CEO Stefan Scherer said. “But an important emerging field is where engineers can scan a crucial but damaged part, scan it, reverse engineer it and then manufacture it again using an additive process.”

The popularity of the STL file format is especially useful in this context, since there is no need to recreate a CAD file: STL is the file format invented for AM.

This skirts some of the problems in generating a CAD file from polygon data (though cleaning up the polygon mesh for reflections and holes would still be needed to produce a valid part program for an additive machine).

“Creating a CAD model beyond clean-up requires special knowledge on the part of the operator. That is because CAD is based on a description of primitive shapes such as, cones, spheres, cylinders, not polygonal meshes,” Scherer said.

Alicona’s measurement systems are based on using focus variation to provide high-resolution surface data of solid objects. While it is similar in concept to confocal techniques, it differs in that it is entirely passive, using only reflected light.

“Our technology is competitive whenever high precision or relatively small features need to be measured in tolerances of 20 microns or less,” he said. Alicona’s systems are capable of measuring dimensions and surface roughness, with its best applications in sculptured surfaces of critical but small parts, like turbine blades.

Scherer believes Alicona’s Advanced Real3D Rotation Unit is especially useful in reverse engineering. Equipped with a motorized tilt axis and motorized rotation axes, it acts as a five-axis machine, allowing its focus variation technique to measure parts in true 3D.

There are other applications outside of additive.

“We have the technology to scan two parts and then virtually marry the two parts together through our Real3D Rotation technology. The system optically registers those two parts with respect to each other,” he said.

A good example of where this might be useful is in matching extremely small parts where the tolerances needed to mate any two parts is impossible to create. This could be in microgear wheels, for example.

Verification and simulation

Reverse engineering can entail at least two different concepts, Renishaw’s Michael Litwin said.

In one instance, it is analyzing the features of a part so that a blueprint or CAD model can be recreated, and the part can be manufactured from that extracted information. In another sense, it can also be a way of verifying that a part was made to a manufacturer’s specifications using engineering analysis software such as finite element or computational fluid dynamics simulations.

“In this sense of reverse engineering, you already have a CAD model and you’ve manufactured the part. But now you’re looking at understanding what you manufactured and how that impacts the performance,” using engineering analysis tools on the actual part as opposed to the theoretical CAD nominals, Litwin said.

Renishaw offers this capability with SurfitBlade, a downstream add-on application to its APEXBlade and MODUS point cloud sectioner packages. The APEXBlade application is a software tool for path planning and generating DMIS-compatible part programs using five-axis REVO sweep scanning of the blade geometries. The REVO probe can be attached or retrofitted to most CMMs.

SurfitBlade generates blade surface forms for use in engineering analysis, the firm said. Complementing APEXBlade, SurfitBlade creates a fully reconstructed surface from tip-center sweep scan data from the REVO probe. By using tip center data and mathematically offsetting the point cloud by the ball radius, Surfit nixes cosine errors common in traditional CMM blade cross sectional analysis.

“Being able to analyze the actual manufactured blade is important because you are able to calculate an engine’s performance and efficiency, prior to assembly and testing,” Litwin said. “The same thing can be done from nominal CAD data, but, that analysis is based on a theoretically perfect part. Theory and what you actually produce aren’t always the same thing.” This requires complete data capture of the blade, he said, noting that the software creates an IGES compatible CAD model.

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