December 03, 2007
Laser Scanning Makes Manufacturing Inroads
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Until recently, in-process inspection hasn’t been very practical in mold production, according to Bill Berry, President and CEO Die-Tech and Engineering Inc. in Wyoming, Michigan. “You’re not mass producing molds, so you can’t use the measurements as feedback for the next product,” he explains. Like any toolmaker, the best he and his staff could do was to measure the product after the fact and rework any deviations that might fall outside the specified tolerances. Quality control remained a post-production cost, rather than a value enhancement.
Berry didn’t give up, though. He remained committed to finding a way to implement in-process inspection. It was a principle of good manufacturing – one of principles that he learned while working for one of the Big Three automakers and that had made his 40-employee company successful at producing tools for plastic injection molding and aluminum and zinc die-casting. He has introduced as many of these techniques as he could, as well as the advanced technology to support them, since establishing the business back in 1984. “This industry has become more of a science than a craft,” Berry observes. “Even though we were a small company, we started employing the
most advanced production and computer technologies we could afford.”
Consequently, a full complement of CAD/CAM systems, CNC machine tools, and automated electrical discharge machines (EDMs) move both digital data and physical product through the 25,000 square feet facility, from conception to delivery. His steadfastness on in-process measurement was rewarded when Berry came across the LC50 laser scanner and the supporting Focus suite of software from Metris North America Inc. in Rochester Hills, Michigan, USA. The technology gave his quality-control experts the ability to scan the contours, sharp corners, and deep narrow pockets on the molds made by his company in a fraction of the time that it would take a coordinate measuring machine (CMM) fitted with
a conventional scanning touch-probe. Finally, he had found the cost-effective method that he needed to measure these features and elicit the appropriate feedback for correcting the process and avoiding costly and time-consuming rework.
CMMs for Inspecting Cavities
Despite his willingness to invest in high technology, Berry had never considered CMMs for process control and resisted buying one much longer than others in his position did. “It would have been purely an expense with no real pay off – just a quality control cost with no benefit other than record keeping or score keeping,” he says. “Without laser scanning, there was really no cost effective way to apply CMMs.” Because he could not justify the capital outlay, he sent his molds to measurement service whenever a customer required him to validate molds on a CMM. The problem with conventional CMM technology is that the touch probes and supporting software were
simply too slow and cumbersome for getting regular process feedback from molds. Even if the probes could reach into narrow recesses and sharp corners with radii as small as 1mm, gathering the large amount of datum points with conventional touch probes, even scanning ones, would require a significant programming effort. Then, interpreting the mountain of data collected by the CMM would be time-consuming and tedious.
After all that, scans compiled by probe would contain blind spots between touches, allowing anomalies there to go unnoticed. Moreover, measuring the molds after machining contributed nothing to in-process measurement. It was still inspect after the fact. Measuring the 10 to 50 electrodes typically used by the EDMs to finish the cavities was a slightly different story with the same ending. It would solve the problem of access because they are reverse images of the holes, cavities, ribs, and contours and, so, have no walls or other obstructions around them. It also would provide a kind of in-process feedback mechanism because any corrections to the electrodes occur before actually making
the finishing cuts on the cavity. The problem of collecting and processing millions of points and thousands of surfaces quickly and economically would remain unsolved, however.
The LC 50 laser scanning head and Focus software changed the situation radically, prompting Berry to invest in a CMM equipped with the Metris technology. With help from Focus Scan, the head can collect 19200 datum points per second along any 3D surface in its line of sight. Focus Inspection then compares the cloud of points to the CAD model and generates a color-coded map of deviations from nominal. In a few days, quality control can provide production a visual analysis of an entire surface. “Laser scanning changes the economics by gathering vastly more data in an extremely compressed time frame,” says Berry. The turnaround is fast enough for the shop to use this feedback to
produce perfect electrodes between the roughing and finishing processes while the roughed block is offsite at a heat-treatment facility for hardening. Meanwhile, the CAD department designs the electrodes, the machining department cuts them, and the quality control department scans them. “By inserting laser scanning into the process, we can use the electrode measurements and setup information to perform a kind of simulation of the process,” says Berry.
This simulation using Focus Inspection validates both the electrodes and the setup instructions for the EDMs, the two primary sources of variation in electro-erosion. The CAD technicians use the setup data to put the electrode into the specified position and orientation and then compare the electrode to the corresponding parts of the CAD model of the mold. Die-Tech’s CMM uses a Metris LC50 laser scanning head to measure an electrode that an EDM in the shop will use to finish an injection mold for producing housing pivots for windshield wipers. The color-coded map of the comparison leads machinists to any minute interference and clearance caused by an imprecisely machined or
positioned electrode for correction before any cutting takes place on the mold cavity. By the time the roughed mold block returns from heat-treating, the electrodes are within tolerance and the setup instructions are ready. Feedback from the simulation has neutralized the primary sources of variation, and the electroerosion process itself is extremely accurate and repeatable. “So we have reduced the dimensional errors in the production of our molds to the tolerances of our equipment, which normally exceed the requirements of customers,” reports Berry.
Consequently, EDM operators can have the confidence to run all of their jobs untended and as fast as allowable. “In the past, there were certain complex EDM operations that they hesitated from doing without being there because they were critical and had to be just right,” explains Berry. Now, the operators have the confidence not only to run their machines untended, either overnight or while they tend other machines during the day, but also at full speed. Berry estimates that the confidence created by Metris’ software squeezes about 10% from the lead-time, helping Die-Tech to keep its three to four week lead-times well below the industry’s more typical five to six weeks. As far as he is concerned, any scanning on the tool to prove that it adheres to customers’ specifications is free. “They might ask for 16 or so points for verification,” he says. “We can give them a million, proving overwhelmingly that the tool is incredibly accurate.” And now, that capability figures prominently in the business plan. “We don’t send anything out the door that we expect to come back for anything other than customer-paid engineering changes,” says Berry. “We haven’t budgeted for a mold’s coming back to make things right.” He also has confidence – the confidence to eliminate that line
from the budget. He knows that laser scanning has transformed measuring on the CMM from a cost into a value-added process that shortens lead-time, trims cost and guarantees quality. Chris Berry, the owner’s eldest son, uses Focus Inspection to check an electrode for finishing an injection mold for producing housing pivots for windshield wipers. He is comparing a cloud of measurement points collected by the laser scanner to the original CAD file.
Commentary By Jeffrey Rowe, Editor
Even though you have to dig a bit for it, this is actually an interesting application story about applying reverse engineering principles to inspection using the Metris LC50 laser probe and associated software.
The LC50 3D laser probe is a high-speed CMM scanner capable of scanning up to 19,200 points per second. It is mounted on a Renishaw PH10M (a motorized probe head) and transforms the 3-axis mechanical CMM into a 5-axis optical digitizing device. It is capable of quickly scanning complex forms such as turbine blades, and in this case, injection molds, for inspection and reverse engineering purposes. Non-contact CMM scanning technology such as this allows for accurately scanning flexible or fragile parts without risk of damage – more about that a bit later.
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-- Jeff Rowe, MCADCafe.com Contributing Editor.
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