Glenn, I am very impressed by your methodical approach to the problem. Many times someone will come to one with the culprit rather than with the problem. The fact that the robot could not speak for itself was probably the reason in this case. The QC manager should have just said the parts were "bent" and left it to someone like yourself to determine what happened.
I ran into something similar with a piece of networking software. It turned out that it was the application design that was the culprit. The customer had put a network analyzer on their system and tried to lead me in that direction. The only problem was that there were almost no parameters that could be adjusted in this particular software. Once they took my suggestions about how to use existing mechanisms to restructure the software, performance was greatly improved. I even gained a reputation as the one who had solved a big problem for another company (who was using the same software). Really, it was only a couple of hours of work.
Glen, not only is this a tale of good detective work, it is also about the only reasonable way the investigation could have been done. A close-up examination as the process was running would have been both difficult and dangerous. Even plastic injection molding, a process sort of similar to the metal high pressure casting process, has a few hazards to be aware of and avoid.
Doing all of the trimming by hand, rather gently, is the way to avoid use of the trim press, which would have been my first guess as the "bending" culprit. A trim press is able to cause damage if the parts are not placed in the dies exactly right.
William K; The trim press could also have been deforming the part. The trim press was part of the automated process, so it was easier to take the part completely out of the automated robot and trim press cycle and use the band saw.
I don't recall ever naming robots or other machines, although I have worked in shops where machines had been given names. And my father alays told me that machines are female, so no, she (the robot) didn't drink beer.
We have a large Fanuc packing robot that we named Big Bird. We also have a work station that flashes a red andon light when it breaks down. It stops enough that the operators have nicknamed her Roxanne.
I once worked at a place that had a strict policy against naming robots. The idea was that if robots had names, people would attribute humanlike characteristics to them, and that this could result in people blaming a robot for bending parts (for example). If robots don't have names, it's easier to remember that they are machines that do what they're programmed to do. At least, that was the theory.
This topic was an area of study for one young man's H.S. Science Fair entry, and proved interesting to the judges... just not quite interesting enough to go to ISEF. I watched a relatively low-end academic performer garner a TON of attention from the judging staff as they had NEVER considered what could happen when anthropomorphism creeps into society. Your post should inspire further study as the engineers in my workplace ALWAYS "name" their prototypes. One was named Daisy Duke and we had huge expectations....
We have a pair of Fanuc palletizing robots on a gantry system, and the German company that installed them, labeled one Tom and the other Jerry. Apparently, somebody is a fan of cartoons.
Their names are actually labeled on the robots with large lettering, and the software on the HMI station refers to them by name. It is useful to identify them, since there are two robots next to each other on the same system.
Great story, Glenn. Sounds like a die design problem. "Die spit" (molten metal shooting out of the die) shouldn't happen if the die is properly vented, and "dirt" shouldn't build up on the die surfaces if it has sufficient overflows. Diecasting is a complex process with a lot of variables, but it all starts with good tool design. (One question: shouldn't something building up on the die make the parts thinner, rather than thicker?)
Dave Palmer; The dirt build-up was on the flat mating surfaces, rather than inside the cavity which forms the part. Another problem with die-casting is die deformation from heat stresses. This die had deformed by a few thousandths of an inch, which was enough that there was not enough material on a critical face. The dirt on the mating surfaces spaced the two halves apart by a few thousandths of an inch, making the part a little thicker, which brought that face into specification.
@GlennA: The clamp force on most diecasting machines is measured in the thousands of tons, so I have a hard time believing that you could build up a few thousandths of an inch of dirt on the die blocks. You were there, and I wasn't, but I'm still a little skeptical. In-cavity buildup is one thing, but on the surfaces of the die blocks? I'd have to see it for myself to believe it.
I'd be more inclined to believe that the die was being held open by excessive parting line flash, due to inadequate venting. The amount of flash could increase over time as the die heats up, which would fit with your theory.
As you say, thermal control is extremely important in diecasting. Were they pre-heating the cavities? Did they have thermocouples to measure the cavity temperature in different places? Did they have water lines in place? These are things that are mandatory in order to make good die castings.
Anyway, I think we both agree that the problem had nothing to do with the robot, and that the solution to the problem ultimately lay in the toolroom. Unless the tool was designed by a robot, robots were not to blame here.
Dave Palmer; I think the lock-up on this machine was 250 tons, and it was the biggest in the plant. There were no thermocouples for temperature monitoring. The initial die heating was done with a radiant heating element placed between the die halves. After several hours, several cold shots were taken. The first cold shot parts were only partial as the molten aluminum was still heating the die. When the die was hot enough that the parts were full, the automatic cycle of the robot unloading the part was started. The die was water-cooled. The operator adjusted the water flow until it looked 'right'. There was talk of implementing a vacuum system, a thermal oil heat control system, and a Nicolet shot monitoring system. None happened while I was there.
The build-up was causing the parting line to thicken. These dies were about 4 feet wide and 3 feet tall. The mating surfaces at the parting line were probably about 8 square feet.
I have seen a robot throw a part quite a distance. The part in question was part of a clutch-pack assembly being removed from a leak-test operation in a transmission plant, and the throwing problem was created by a lack of adequate air pressure for the part gripper. The tooling designer had not included a positive grasp means on the fingers that gripped the part, and instead relied exclusively on a friction gripping to retain the part for the unloading sequence. The problem developed as a combination of a change in the unloading path and a reduction of gripper clamping air pressure, which was intended to reduce compressed air usage as a cost reduction. So at one point in the unload path the change in direction allowed the three pound part to slip out of the gripper's fingers and fly about ten feet through the air. It was very fortunate that nobody was in the area where the parts flew, since an injury would undoubtedly been inflicted.
The solution included an adjustment of the path and an increase in clamping air supply pressure.
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