Case Study: CMM Flies Through Flywheels
To upgrade a manufacturing line for more flexibility during lean times, QS-9000-certified engine-component supplier DACO Inc. (North Aurora, IL) re-engineered a machining cell to integrate an existing coordinate measuring machine (CMM) with part-handling robots and six machining centers, automating the entire production and inspection process for multiple part numbers.
The family-run business has about 90 employees and essentially two customers, one of which is a major diesel engine manufacturer. The company's long-standing customer relationships are based on contracts that run much longer than a few months and this gives the company confidence, says vice president Bruce Lindgren, to "invest in things like robotics and CMMs, and rearrange our plant to give a total commitment."
The need for flexibility in the shop is clear. The company's cast-iron flywheels range from 16 inches in diameter and 40 pounds in weight to about 42 inches and 2,000 pounds. Moreover, the volumes for these parts can vary radically. "We might make one a year or we might make 50,000 a year," says Lindgren. As a value-added service, the company not only ships the flywheels just-in-time but also packs them in the correct build sequence for assembly, sometimes knowing only a day ahead of time what is needed. To improve flexibility, DACO had decided to re-engineer a focused machining line to handle additional parts of various sizes, weights and complexity. "That's the challenge right now-to redirect that equipment to lower volume and build more flexibility into the cell," says Lindgren.
The first step toward complete cellular manufacturing took place in 1999, when the company installed three Fanuc robots for automatically loading and unloading Okuma lathes and six horizontal machining centers containing 12 pallets. Each machining center can handle a single flywheel part number, and each robot knows exactly which part it is handling.
However, after the initial cell automation project, inspection of the flywheels' threaded holes and dimensions remained a mundane manual process, tying up one or more inspectors, says Steve Lindgren, engineering manager. "We spent a lot of time taking manufacturing labor out of a part," he says, "but inspection is labor too, so why not take the labor out from the inspection standpoint?"
Finding a CMM to integrate with the automated cells was not a problem. The company had just brought in a CMM to measure larger part sizes, replacing an older machine. The older CMM was put at the end of a production line in an enclosure to protect it from the environment.
For integration, both software and hardware upgrades were needed for the CMM, so DACO turned to its CMM supplier, Mitutoyo America Corp. (Aurora, IL). "We gravitated toward Mitutoyo's GEOMeasure 6000 software, which offers a higher-level language," Steve Lindgren says. This advanced version of GEOMeasure provides the programmability for inspecting a variety of part numbers, and "we could use it to interface with the outside world of robots."
It was a challenge to get the robots and CMM to "talk" with each other to allow the system to handle parts appropriately without human involvement. "If you're going to replace the human, you have to also account for all of the useful things that the human does," says Steve Lindgren. "The human does a lot of visual checking and inspecting and so forth. The other thing a human does is to use some common sense."
Thus, the upgrade had to provide the mind set and reasoning of a human inspector. For example, "if we check the diameter of 10 tapped holes in a single plane with a spherical probe, we're going to find some that are out of tolerance. This is due to the nature of tapped holes in cast iron, and the burrs that form on the thread," he says. "If we checked the same ‘bad' hole in a different plane, we may find that the hole passes inspection. An experienced inspector knows this and can make an educated determination that the hole is good based on the measurement results from different planes within the cylinder. Therefore, the software had to be programmed to make the proper judgments about what percentages of good holes in a pattern are necessary for acceptable parts. So we can throw out the ones that are ‘bad,' and if the majority are ‘good,' we can make the assumption that the pattern is good."
But there was a question of whether this kind of reasoning would work for the automated line. "When we took this on, I knew it could be done, [but] I just didn't know that it hadn't ever been done," Steve Lindgren adds.
Mitutoyo installed its Universal Inspection Interface (UII) to allow the CMM's software and the manufacturing cell's Programmable Logic Controller (PLC) to communicate. The link allows a robot to pull a part off the line, send it to the CMM, and then send a signal to the PLC, through the UII, instructing the GEOMeasure software to run one of up to 2,400 inspection programs. After the inspection is complete, the software signals the UII and PLC with the inspection results, and the PLC directs the robot to handle the part accordingly. If the part is good, production continues; if bad, a second part is checked and if it is bad, the system flags that machine tool for service.
The robots confirm the part number and instruct the GEOMeasure software to run the specific measurement program for that part. The selection of parts for inspection is random, except that the frequency of inspection can be controlled, allowing the CMM to serve as a qualification tool for specific machine tools.
Besides eliminating human inspection duties, CMM integration also reduces scrap by catching machine tool problems 24 hours a day-problems that might not have been caught with manual inspection, explains Steve Lindgren. "We had a case, for instance, where a single trailing flute on a drill chipped and started making oversized minor diameters through the threaded hole." Because the hole on the part looked OK with visual inspection, the problem did not become apparent until a bolt was inserted on the customer's assembly line. Now, with the more rigorous and reliable integrated CMM, the off-
diameter hole would raise an immediate action flag, he adds.
Automating the CMM within the six machining centers has reduced the quality department's burden by about 25%. This allows the cell team more time to develop other types of projects and new ways of inspecting parts and cutting scrap. For example, they've noticed excessive CMM idle time, so they've set out to increase the number of inspections, up from the current minimum 4.8% audit rate-48 pieces per day. Now, the shop's operators are determining how to optimize the part handling to give the robots even more opportunities for delivering particular part numbers for inspection, with a goal of doubling the audit rate.
Mitutoyo America Corp.