Laser scanning casts savings into foundry's bottom line

When your livelihood depends on an inventory of tools worth millions of dollars, you’re going to play it smart. You’re going to take care of that inventory. At least, you will if you’re Grede Foundries Inc., a Milwaukee based company specializing in ferrous castings for various industries. While its plant in Reedsburg, Wisconsin, USA, casts suspension parts, differential cases, crankshafts, and like parts, inspectors there check hundreds of tools regularly with a coordinate measuring machine (CMM) retrofitted with an LC50 laser scanner from Metris USA Inc. in Rochester Hills, Michigan. Their goal is to prevent the inevitable wear on the surface of the tool from progressing to the point that it causes quality problems and damage that is expensive to repair. In casting, a tool called a pattern creates impressions in sand to create a mold into which molten steel is poured. The constant pushing of the pattern against the sand causes a grinding action that abrades the surface and wears away important details. To retard this wear, Grede protects the surfaces by applying hard, abrasion-resistant chromium based coatings chosen carefully for each job. In time, though, the sand eventually wears them away too. “After a specified number of cycles or when the operator can see wear, a pattern has to be inspected,” says Bernie Bill, Grede’s Layout Supervisor in charge of the Quality Laboratory. “We will rescan it and compare the measurements to the baseline.” The baseline is the scan of a pattern that has been proven to produce good castings. “We don’t compare measurements to the original CAD model of the part because the pattern has to vary from it slightly to accommodate shrinkage,” says Bill. “We have to tweak the pattern here and there to get the castings to meet customer specifications.” Once the patterns are able to make good castings and the customer approves them, Bill’s team scans the tool and stores the cloud of points as an STL file. Checking patterns for wear goes rather quickly. Although writing the macro for an inspection routine takes between 4 to 16 hours, depending on the size and complexity of the pattern, the programming occurs during the approval process and is ready to use again. The inspector aligns the pattern to a jig mounted on the CMM, retrieves the program used to create the baseline STL model of the pattern, and lets the CMM inspect the tool. The inspection routine usually takes between a half an hour and one hour, depending on the size and complexity of the pattern. Some patterns contain several cavities to produce molds that can make half a dozen or more parts at a time. Then, Bill uses Metris’ Focus Inspect software to compare the cloud of measurement points to the baseline and generate a color-coded map of the part. “You can have results within 15 minutes to a half an hour,” he says. Because each color represents a deviation from nominal, production can see at a glance where wear is occurring and how much has occurred. “The results tell them what the plan for the pattern is going to be,” says Bill. “They know that they might be able to get by with running 5000 more cycles before sending the tool out for stripping and recoating.” Or they might pull the tool immediately to prevent further wear that would require welding and grinding the tool to bring it back into specification. If, however, they were to find that they were too late and that repairs were necessary, then the scanner would check the repairs afterward against the baseline to ensure that they returned the pattern to the approved specifications. Monitoring wear is not the only use of the laser scanner and baseline scans. Scanning also comes in handy for helping engineering to troubleshoot problems, either to provide a value-added service for customers or to solve problems that creep into any manufacturing process from time to time. “If production has a problem, they’ll often schedule a tool for a scan to rule out the tool before going to engineering,” says Bill. For example, scanning can help them to diagnose an alignment problem that might prevent the two halves of the mold from fitting together just right to create a good seal. Without enough clearance, the two sandbanks on the outer edges of the two halves of the mold will crush each other, which can cause some sand to fall into the cavity. Iron forms around the sand, creating holes in the casting. Too much clearance, on the other hand, will let some molten metal leak from the parting line. The resulting thin, but hard flashing must be cut and ground away. “So we scan both patterns, put the scans together, and check for clearance and crush electronically,” says Bill. “When we put it on the screen, we can see whether it’s a pattern problem and, if it is, exactly what they’ve got to fix.” Not only do the color maps eliminate the need to pour over tables of measurement data, but they also can be attached to work orders to show the problem clearly and exactly to toolmakers in the pattern shop. In the past, the toolmakers would have had to weld and grind the patterns based upon their experience and those tables of measurements, which were taken by touch probe from a sample casting. Then manufacturing would have to interrupt production on one of its molding machines to install the repaired pattern and produce another sample for inspection. Sometimes, the toolmakers would be lucky the first time, but most of the time, four to six iterations would be necessary to correct the problem. With laser scanning, however, diagnosing problems and repairing patterns is no longer a trial-and-error process. Because scanning collects more data in less time and presents it in a format that can be read intuitively, it eliminates guesswork. “It may be erasing three to five samples,” says Bill, “which means eliminating three to four shifts of work in the pattern shop, production, and inspection to make the necessary adjustments.” Most of the time now, the pattern shop is making the right correction the first time on the first try. Moreover, scrap rates are way down. A good example is a set of tools for making a bracket for automobile brakes. Laser scanning helped engineering to find not only some clearance in the patterns but also some variation in the machine that exacerbated the problem and caused a lot of scrap. Based on the information gleaned from the color maps, engineering was able to reduce a 5.2% scrap rate down to 1.0%, thereby saving the company $48,000 a year on that job. Although the savings that Grede reaped from eliminating trial and error is impossible to calculate, the savings from reducing the scrap rates is known. After using the scanner for six months, Bill estimated that using the laser scanner only one shift a day would save the company about $81,000 during the first year by reducing scrap alone – and that was after paying for the Metris laser scanner and software. Almost six months later, he could see that he was going to have to revise his estimate upwards. So plans are to play it even smarter –to scale up and run the scanner another shift.

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