Reducing labor with in-die welding
June 4, 2009
Ultra Tool & Manufacturing adds in-die projection welding to its capabilities, eliminating secondary operations and significantly reducing labor costs.
Editor's Note: The following is the final installment in a three-part series on in-die operations at Ultra Tool & Manufacturing. Part I covers in-die measurement and springback compensation. Part II covers in-die assembly.
Several years ago managers at Ultra Tool & Manufacturing knew some things on the shop floor had to change. By the end of 2006, the company had overcome the technical hurdles of two in-die operations. The first project measured and adjusted for out-of-tolerance bend angles. The second, an in-die assembly operation, involved a small, high-volume round part staked to another. And while each application was a resounding success, some company personnel had some warranted concerns. At the time hanging wires abounded around press tooling, and the shop was in dire need of simplification and organization.
So Ultra called in pressworking sensor guru George Keremedjiev of Tecknow Education Services. "We found that we were by no means world-class stampers when it came to sensors," said Brad Schmit, manufacturing technology manager for the Menomonee Falls, Wis., company.
Working with Keremedjiev, Ultra simplified its sensor setups, machining channels in die sets to run wires and bundling wires into standard umbilical cords outside the die. After the overhaul, the systems became robust enough to minimize the chance that an expensive in-die sensor could be inadvertently destroyed. After helping to streamline operations, Keremedjiev suggested the company pursue something new.
"After looking at in-die measurement and in-die assembly, he could see we were ready to do something more," Schmit recalled.
With confidence gained from the prior two projects, Ultra engineers took on a new challenge: in-die resistance projection welding. "Adding sophisticated electronics to these processes took some education," Schmit said. "We had to be safe, and we had to understand how these sophisticated electronics worked: voltage, current, resistance. Many things are happening inside of that press."
The company tackled the project simply to advance its technology offerings; it wasn't customer-driven. Because of this, managers were free to choose an existing application that would be the best fit for the process, one that was fairly simple and would allow the company to design the necessary weld stations and clearances into the tool.
The technology also had to work out from a business standpoint. For the job Ultra chose, the piece-part cost consisted of 37 percent labor, 73 percent of which was for a secondary welding operation. If Ultra could eliminate the secondary welding, it could eliminate most of the labor.
The overall design considered the heat welding generated as well as the space required to integrate the process in the die. The projection weld head and power cable had to be cooled. This made already large cables even bigger, with some reaching 3 inches in diameter. The design also had to isolate the current, which needed to pass through the electrode and two components being welded, and nothing else.
Specifically, personnel overcame two technical hurdles: They built a welding station into the tool and a table that could house all of the welding components, placed next to the operating press.
The table holds the bowl feeders containing the weld nuts; an inverter-based weld control; a weld transformer, which steps down a primary voltage to a secondary, lower voltage; and a chiller unit for cooling the weld head and cables. The cables are bolted to the in-die weld head using heavy-duty copper connections.
"It's imperative that the resistance values in those cables stay consistent," Schmit explained. "As you start welding, current passes through the cables, which will heat up. This changes the resistance values of the cables, which isn't ideal. We found that cooling the cables helped us maintain the weld quality."
The table also carries a weld analyzer that monitors current, pressure between the two welded components, and weld time during each press stroke. "If you can verify those three factors, you can verify weld quality," Schmit said. As in the previous in-die measurement and assembly applications, the in-die welding application uses sensors positioned, among other places, after the welding station to verify the nut was actually welded onto the part. This, in conjunction with the weld monitor, means each weld is double-checked for quality.
The progressive-die set had to be long enough to allow room for the welding process. As Schmit explained, a progressive-die setup has press bed size limitations, because the other stations—whether for trimming, stamping, or anything else—may not leave enough space to incorporate welding. The in-die welding operation required a certain amount of space for the weld head and associated cables and coolant lines, and it also need sufficient space between parts in the coil.
A separate PLC on the weld table controls a pneumatic tube system that shuttles two nuts to a specially built magazine, which can hold more than 10 nuts at a time and releases them at specific intervals for welding. Bowl feeders, Schmit explained, inherently do not feed at the same speed all the time, so the magazine system provides a buffer of sorts so the press doesn't have to wait for the next nut because of a hiccup at the bowl feeder. The nuts are fed in separate stations, after which is an idle station that allows enough room for the cabling and mechanisms required for the welding and shuttling.
This operation uses knowledge gained from the previous in-die operations, and like the operations before it, component timing hinges on specified press crank angles. The only element, in fact, not timed off press crank angle is the PLC that sends nuts up to the magazine.
To coordinate the operation, engineers had to work within the constraints of the press stroke. "You need the time to be able to make that weld happen," Schmit explained, so the stroke length has to be sufficient and meet the application's needs. There must be time to build up enough squeeze pressure and current for the weld to form. Time is also needed after the operation to allow the molten weld metal to solidify.
Now Ultra runs its press with in-die welding at about 30 strokes per minute. Without that welding tool, the press could run 50 SPM, Schmit said. But the fact that the in-die welding eliminates an entire secondary operation makes the whole process more efficient.
Before tackling any in-die operation—be it measurement, assembly, or welding—Schmit recommended that companies analyze their secondary operation from both a financial and lead-time perspective. Cutting out labor allows a stamper cut down lead-times, he said, and makes operations much more competitive.
Another key is standardization. For instance, Ultra found that it needed to purchase new process monitors for many of its presses. Uniform monitors not only gave engineers the ability to use a multitude of sensors, it also allowed the company to use shift-register logic (provided by Link Systems) that, as discussed in Part II, acts like a sequencer that counts every press stroke and coordinates motions based on those counts. Buying such a system, Schmit said, was far easier than building a custom control in-house.
Cabling, he added, also must be standardized, as well as digital sensing technology, from feed-in to part-out. Not only does standardization simplify the shop floor, it also makes planning future jobs easier. "Designers don't need to have lengthy conversations when they already know what the standards are," Schmit said. "They know we're going to use a specific 55-pin connector cable along with a metal die sensor box, where all the sensor [electronics] are routed." He added that standardization also makes the die-build process easier, and, once a job hits the floor, it allows different operators to run different presses.
Finally, Ultra found that standardization also applies to the table holding the welding equipment, particularly when it comes to justifying its cost. "The actual weld table that houses all the different components is a very expensive piece of machinery," Schmit said. "So we needed to build that in a generic fashion so we can run multiple parts with that unit." This meant the table needed a welding power source capable of handling various jobs, as well as standard cables that could be integrated with various die sets.
Schmit conceded that in-die operations forced the company to approach jobs differently. Although standardization has streamlined matters, the company still spends more time building dies with sensors than those without. But quality and efficiency have improved significantly.
"Before, die crashes were being fixed on second shift, so some people were not really seeing what was actually happening," he said. "In the end, we found we were spending a lot more than we figured on die repair."
Today everyone knows when crashes occur, in part because, thanks to the organization's in-die sensing developments, such events have become extremely rare.