Errorproofing with in-die measurement
April 14, 2009
Ultra Tool & Manufacturing launches a program to tackle sensor technology in an effort to errorproof the pressroom.
Editor's Note: The following is the first article in a three-part series on in-die operations at Ultra Tool & Manufacturing. The first covers in-die measurement and springback compensation. Part II covers Ultra's in-die assembly capabilities, and Part III covers the company's in-die projection welding setups.
The done-in-one concept pervades manufacturing. Machine shop operators, for instance, fixture stock to a turn-mill center and remove a complex part finished to spec. Multistation progressive dies do something similar; coil stock enters the press and sheet metal components exit complete. But in recent years stampers have taken a step further with integrated in-die operations, including measurement, assembly, and welding.
Ultra Tool & Manufacturing today offers all three, a rare feat for a U.S. contract stamper, and it's a story that will take the next three months to cover. The Menomonee Falls, Wis.-based company has pursued the technology aggressively for almost a decade and, in doing so, has made great strides toward errorproofing, eliminating secondary operations, and drastically reducing the labor content for some seriously complex work.
Eight years ago Ultra President Terry Hansen challenged his staff to reduce unnecessary die repair costs by investigating in-die sensing technology. If sensors could measure part and process attributes, it would help eliminate crashes, improve part quality, and take a step toward errorproofing the operation. At that point the company launched its own sensor lab to tackle continuous improvement.
Bend-angle problems—what Brad Schmit, Ultra's manufacturing technology manager, called "a constant issue on the floor"—were the first to hit the team's radar. The upper and lower forming dies that create those bends always fought springback variances in the coil stock. Material properties varied slightly in both thickness and hardness throughout a coil as well as between coils, and those variances step to the footlights for precision jobs.
Schmit's team knew available analog technology could sense if a part's bend angle was in tolerance, but it still couldn't reduce the number of bad parts. Besides, a person checking parts at the press, albeit far less efficient, could accomplish the same thing; and out-of-tolerance parts still would require operators to shut down the operation and make the appropriate tooling adjustments."Until we could control those bend angles automatically, we weren't really helping ourselves," Schmit said.
At this time the team asked, What would it take to drive a mechanical mechanism—at the right time, velocity, and position—to adjust those bend angles on-the-fly without having to stop the press? For the answer, lab engineers needed to develop a multistation approach within the progressive die. The bend angle would form at the first station and then be measured in subsequent stations to determine if an altered restrike was necessary. The subsequent restrike station would use an integrated motor to move die elements a specific amount to bring that bend angle back into spec. A final sensor station then would verify the resulting part.
That's easier said than done. A typical analog sensor takes measurements and outputs a voltage or current that correlates to specific distances, but that's it. This application required a monitoring system that could make "intelligent" decisions about whether to move the motor in the restrike station at all and, if so, by how much. To attain this, the team built its own PLC system that used the analog signal to determine the part's actual angle. If the angle was out of tolerance, the controller signaled motors in the restrike die to move a certain distance to correct the variance.
After proving out the process, engineers put such a tool into production for a customer in the agriculture field (see Figure 1 and Figure 2). The job required bend angles to be ±0.004 in., which is no small task. For this, engineers integrated two sets of high- and low-tolerance limits within the control. The first set of high and low limits determined whether the part could be fixed at all; if the measurement was outside these limits, the part was scrapped. The second high and low limits (±0.002 in.) told the control to use the in-die motors to bring the part back into tolerance.
"The system forced us to become knowledgeable with all aspects of the programming and press stroke, as well as quality and timing," Schmit said."It helped us tie everything together and better understand the whole process."
This first effort in 2004 built the technological foundation for the in-die assembly and welding that was to come. Any in-die operation requires knowing at what point during the mechanical press stroke—that is, the exact crank angle—to initiate operations. Timing is everything.
With in-die measurement, for instance, the motors could move only with the dies open, but the measurement had to be taken at just the right moment during the press stroke for the process to work. After some trial and error, the team found they couldn't measure just after the hit, when the press frame and workpiece still had enough vibration to throw the sensors' voltage readings off. So the sensors measured after the press crank angle reached 180 degrees, with the die open slightly but the part still held securely and consistently by the spring-loaded stripper. This allowed sensors to output a stable voltage.
The part had three features, and to measure and verify all processes took 30 sensors within the tool: five analog along with 25 digital sensors. Digital sensors are either on or off; analog sensors work with various states of grays, outputting voltage or current readings that correlate to certain measurements. Sensors monitored stripper height, part-out, material thickness, material feed at the tool entrance and exit, and ensured scrap parts were separated from the good parts. If the sensors did indeed find a bad part, they would send a signal to pneumatically activate a scrap chute underneath the die where the part fell through. The chute would stay in position until the control verified there were good parts emerging from the tool again.
All this happened with real-time in-die measurement and springback compensation. Motors would adjust for any feature that drifted outside of the ±0.002-in. window. At the next station, sensors would verify the angles. If they were still out of tolerance, the motor would continue to adjust until the operation was brought back into spec. The result: Instead of producing piles of out-of-tolerance parts, the setup minimized scrap by adjusting for varied springback in real time, on-the-fly, without any human intervention.
"We could start this die up and walk away," Schmit said, "and we'd know that as long as that tool was running, it was making a good part. It's a wonderful way to be able to hold your tolerance and be assured you are making good product. In a nutshell, we now can produce a less expensive stamping that has a machined-part-type tolerance."