Quality Test & Inspection: Working with 3-D Data Can Be Easy
At the Grede Foundries plant in Reedsburg, WI, the two coordinate measuring machines (CMMs) working side by side in the quality lab extract 3-D data from castings in very different ways. The Brown & Sharpe Excel 7-10-7 from Hexagon Metrology (North Kingstown, RI) makes basic prismatic measurements with conventional PH10 touch probes supplied by Renishaw Inc. (Hoffman Estates, IL). And the other CMM, a Brown & Sharpe Validator, scans complex free-form surfaces with an LC50 laser scanner from Metris North America Inc. (Rochester Hills, MI).
Because the scanner can collect 19,200 points per second and easily generate a million points for each part, Grede had to buy another software package, Metris' Focus Inspection, to process the data. The conventional CMM software that the plant had been using before retrofitting the scanner to the Validator simply could not handle the volume generated. Consequently, Grede Foundries became one of a growing number of manufacturers learning that two worlds of software have emerged for managing and using 3-D inspection data.
"CMMs have always worked with 3-D data," says Jim Clark, vice president at Metris. "It's just that they've always worked with small data sets. With scanners, however, you're talking about millions of points. Conventional CMM software was never built to handle that many. It was designed for an era where data typically didn't exceed thousands of points." So a new class of software was necessary.
By being able to handle the huge volume of points, this new class of software has opened CMMs to an entirely new category of measurement-the precise measurement of complex surfaces, according to Bernie Bill, Grede's layout supervisor. Although CMMs technically did have the ability to make these measurements by touch probe, the process was too time-consuming to be practical for all but expensive parts or those that are mass produced. Scanning changed the economics, freeing Grede and others to check free-form surfaces.
Bill stresses that the LC50 scanner and Focus software are just another set of inspection tools in his toolbox. They let him and his staff compare surfaces to computer-aided design (CAD) models, but touch probes and conventional software, such as Hexagon's PC-DMIS, remain their best tools for making basic prismatic measurements and comparing points and small, simple curves to CAD models.
Using Points and CloudsTo explain why, Bill points to the jobs that were on the CMMs while he was talking to Quality Magazine. Touch probes and PC-DMIS were the correct tools for checking a sample of 40 automotive steering linkages that were on the Excel. The operator was measuring a set of preprogrammed points to verify certain dimensions. "On this part, we make about 100 hits with the touch probe," says Bill. "The program might take only 4 minutes per part. With the scanner, it would probably take a half hour per part."
Here, the relatively small amount of data collected works to Grede's advantage in another way, too. There are fewer points to manage and store. The large number of points generated by laser scanning would have not only been overkill for this job but also added an unnecessary complication.
Conversely, the scanner and Focus software were better suited for inspecting the upper control arms for automotive suspension systems that were on the Validator. The operator was checking the 3-D contours of the casting's walls, comparing them to the CAD file to ensure that their thickness was adequate. Here, the huge number of points necessary for an accurate and complete analysis made the scanner faster than touch probes.
The CMM can move in a continuous motion as the scanner sweeps the part's surface. It need not go from point to point, slowing as it approaches each one, stopping as it registers a touch, and backing away before moving to the next point, as it would have to do if it were using a touch probe. Even if the machine was using an analog-scanning probe, it would still have to make an approach and register a touch before continuing with a trace.
Not only would the measuring speed be too slow, but the programming also would take much too long to be practical. Conventional CMM software measures from a defined origin along specified axes. So a programmer must begin by defining the origin and axes and then continue by defining both the X, Y, Z coordinates of each point to be measured and the I, J, K components of the normal approach vector. This process is much more tedious and time-consuming than programming a laser scanner to move in a series of straight lines or arcs that keep the target points within the laser's line of sight.
Although most software for analyzing mass point clouds does not require defining an origin and axes at the beginning, it allows doing it at any time-even after scanning. Focus Inspection also allows importing a coordinate system, or alignment, from PC-DMIS to superimpose a scan onto work already done in that package. Or operators can bypass defining the coordinate system altogether and use the feature that aligns the point cloud with the CAD model through a best-fit analysis.
A Better BreedGrede Foundries is not the only company adding laser scanning to its toolbox of inspection devices. Metris and other manufacturers of scanners and supporting software report that the technology is catching on among leading-edge mold and die makers for checking the forming surfaces and parting lines of their tools. It also seems to be gathering new momentum among the automakers and their top-tier suppliers for gap-and-flush checks and other inspection work.
One reason offered for the heightened interest in scanning lately is improvements to the scanners themselves. Metris' LC50 scanner, for example, collects 19,200 points per second. "Years ago, scanners were collecting only a few thousand of points," says Alberto Griffa, senior product manager at Geomagic Inc. (Research Triangle Park, NC), a developer of computer-aided inspection software. "Today, collecting a million points is commonplace." He suspects that it will not be long before his and his competitors' software is handling billions of points.
The improvements also have boosted accuracy. "Laser heads had been used mainly for reverse engineering," says Griffa. "As they became more accurate and precise, though, laser heads were used more regularly in inspection." For this reason, laser scanning has encroached on applications previously reserved for touch probes, the more accurate of the two technologies.
Today, the dividing line between the two technologies tends to be tolerances that are somewhere around 0.0002 inch, depending on the application. Checking cross-sections on jet-engine turbines is an example of an application that still requires touch probes. Tolerances are usually 0.0002 inch or better. In mold production, however, the difference between the accuracy of a touch probe and a laser scanner is usually negligible for the tolerances typical for that industry.
Better performance notwithstanding, the advancements would be for naught if it were not for the accompanying enhancements to the software that collects and interprets the data. The latest generations of the software make it practical to inspect large surfaces and assemblies. "Now people are starting to inspect and evaluate the entire car as an assembly, rather than just evaluating pieces of it," says Griffa. Aerospace manufacturers are doing the same with various scanners that work independently of CMMs.
Another big development in software has been its ease of use. Based on feedback from early users, developers have automated as much of the process as possible and have simplified their user interfaces. Most of today's interfaces have a series of tabs, icons or combination of the two across the top to lead users through the process step by step. When the user clicks on a tab or icon, the right toolbars and menus appear on the screen for the task at hand.
Get the Right ComputerAs often is the case today, the capabilities available from the software have depended on the strides made in computing technology. Collecting one to five million points and comparing them to CAD files within a few minutes takes a lot of horsepower. "And the processing speed of computers simply wasn't available five to eight years ago," says Peter Frieling, a regional sales manager with Metris.
Given the processing power and memory capacity now available at reasonable prices, today's high-end PCs are more than sufficient for working with mass point clouds. In fact, the software can run on the same PC running PC-DMIS and other brands of conventional CMM software. Both sets of software use CAD models and so, must run on PCs that have high-end processing capacity. "You can't use a commodity PC that you order online for $700," says Frieling. "You're going to have to spend a few thousand dollars."
The PC should have a relatively large random access memory (RAM) in the 1- to 2-gigabyte range, and its video card should have at least 128 megabytes of RAM. The central processing unit can be any microprocessor that is equivalent to a Pentium 4 or better. Although the typical monitor used for CAD software will be adequate for both kinds of inspection software, a manufacturer will want a color printer, if he does not have one already. Color is necessary because the reports are maps that denote deviations from nominal through various shades of color.
For better performance, Griffa at Geomagic recommends a computer with two microprocessors to take advantage of the multithreading capacity available in his company's software. In multithreading, the software divides a set of computations between two microprocessors and performs them in parallel, thereby doing the computations in half the time and improving the response of the computer. Software developers commonly apply multithreading to such computationally intensive tasks as comparing point clouds to CAD models and performing best-fit alignments on large files.
Consider the power of the technique on a job that entails comparing about a million measurement points to the corresponding points in the CAD model. Conventional software and computers would compute the deviations serially, point by point, until they process all one million points. Multithreading software, on the other hand, completes the computations is half the time.
"If the computer has two processors, then Processor One processes the first set of points, while Processor Two processes the second," says Griffa. When they complete those tasks, "Processor One continues to the third set of points, and Process Two moves to the fourth, and so on, until all one million points have been compared. Rather than processing a million points, each is processing only a half million points."
Griffa warns users against false expectations that multithreading will let them complete a comparison in half the time. "It cuts the processing time in half for that particular computation, not for the entire process," he says. "Some operations in the job, such as inputs, occur without any multithreading."
Nevertheless, it does speed processing and contributes to streamlining the manipulation of large clouds of 3-D data. It and the other innovations in software, computers and scanners make working with 3-D data much easier than it has been in the past. So, having a lot of data is not a problem anymore. Q
For more information on the companies mentioned in this article, visit their Web sites:
Geomagic Inc., www.geomagic.com;
Hexagon Metrology Inc., www.hexagonmetrology.us; and
Tech Tips• CMMs have always worked with small sets of 3-D data. With scanners, millions of points of data are collected.
• By being able to handle the huge volume of points, a new class of software has opened CMMs to a new category of measurement-the precise measurement of complex surfaces.
• Although most software for analyzing mass point clouds does not require defining an origin and axes at the beginning, it allows doing it at any time-even after scanning.
• In multithreading, the software divides a set of computations between two microprocessors and performs them in parallel, thereby doing the computations in half the time and improving the response of the computer.
Sidebar: The next generation of 3-d scanners By David GagneIn the past 20 years, noncontact 3-D scanning technology entered the industrial market as state-of-the-art technology. This technology allowed 3-D data acquisition of parts for reverse engineering where point clouds are generated to accurately reproduce the surfaces.
Originally, 3-D scanning systems were only available with big coordinate measuring machines (CMMs) at a cost reaching more than $200,000. Only a few companies were able to afford this technology. Moreover, experienced metrologists were the only people able to operate those devices on CMMs because of the complexity and setup procedures, as well as its calibration process.
Years later, a second generation of 3-D scanners was born and solved some of the weaknesses of older systems. This second generation brought smaller equipment, including laser scanners adapted to portable CMM arms. Many industries took advantage of this technology and implemented 3-D scanners for use in a wide range of applications both on- and off-site. Still, the complexity of operation of the systems of this generation made it usable only by specialists.
Today, the third generation of 3-D scanners is entering the market. This new generation of scanners already equals the scanning quality and precision of bigger systems from previous generations. This new generation of scanners offers a number of new features that allow its use in a wide range of applications including:
• Self-positioning, eliminating the need for external positioning devices such as an arm, CMM or tracker
• Part scanning of any size without limitation
• Object part can be moved while being scanned
• It can be used by anyone, eliminating the need for highly trained staff
• Fully portable-some systems fit as a carry-on in an airplane
• Superior data acquisition speed
• It creates a STL file instead of point clouds
• Real-time rendering
New ApplicationsWith the introduction of this latest generation of laser scanners, new application trends are being established for quality inspection. To qualify parts, some companies are removing conventional checking fixtures from the production process and using 3-D scanning technology with full-color map analysis from nominal computer-aided design data. Inspections are now more accurate over 360 degrees and results are calculated rapidly. These state-of-the-art scanners can be used for production part approval process and for statistical process control in various production runs.
Scanners for the third generation and their data acquisition process also are being used for other industrial applications including design analysis, finite element analysis and prototyping.
Technological BreakthroughOne of the breakthroughs with this third generation of 3-D scanners is the fact that it does not require an external tracking device. For example, one method uses alternative systems such as highly reflective dots randomly positioned on the part to scan. The scanner uses the dots to position itself in the space by triangulation much like GPS systems. This way, there is no need for external positioning. Therefore, this change in the scan process has driven costs down.
Those systems exhibit results or precisions comparable to those achieved by a CMM, which is about 50 microns. Those results are sometimes obtained just by best fitting a scanned part over nominal CAD data.
Finally, the third generation of 3-D scanners is changing and improving some processes in many industries. With this scanning technology being more accessible to everyone, many manufacturers can now take advantage of it for the design of their new products or to inspect different processes.
David Gagne is a program manager at Creaform Inc. (Quebec, Canada). For more information, call (418) 833-4446 ext. 288, e-mail firstname.lastname@example.org or visit www.creaform3d.com.