Page 48 - MetalForming March 2012

Tooling Technology
By “too complicated” Segovia means that the load cells were touchy and over-sensitive to pressroom conditions. In less than two years the company spent $65,000 replacing broken load cells; experienced press downtime to the tune of $680,000, caused by faulty load cells; and paid out another $55,000 to its electrical-engineering depart- ment to recalibrate the machines every time a load cell failed.
“We had to find a better, most cost- effective, reliable and accurate solution for monitoring the spring forces,” says Segovia. To replace the load cells, Segovia opted to install electronic pressure mon- itors (from Dadco). Each of the 12 BAMs that have been upgraded for the Green Ammo program now boasts four pres- sure transmitters connected to nitro- gen-gas springs, and a central control panel mounted on the press ram. The pressure monitors provide a 22-lbf res- olution, offering significantly more process control compared to the 316-lbf resolution of the load cells.
To fit ATK’s presses, Dadco cus- tomized the die springs to mount the M6 port in the center rather than on the side, “a key factor to the success of the project,” says Segovia. “Our press rams are designed such as there is no room to go left or right—they’re all set up for vertical motion. All of the lines going to the control panel had to go through the top of the ram. We drilled a hole through the top of each ram to accom- modate the lines.”
Since installing the electronic pres- sure monitors, maintenance issues are nonexistent, maximizing press uptime. And the facility is busy preparing a dozen or so additional BAMs similarly to continue to support the Army’s EPR programs.
Perfect In-Die Weld Pressure
Resistance-weld quality hinges on three key process variables—time, pres- sure and current. When welding inside a stamping die, pressure often proves the most difficult variable to control. Metal stamper GR Spring & Stamping, understands this well—well-versed in in-die welding, 30 of its dies include in-
Metal stamper GR Spring & Stamping is well-versed in in-die welding—30 of its dies include in-die nut-welding stations. On larger dies, the firm pipes nitrogen- gas springs together so it can pump up and dial down weld pressure as needed. The firm has retrofitted electronic pres- sure monitors on two dies running on a 200-ton press at its Kentucky plant, to gauge weld force applied by the gas springs.
46 MetalForming/March 2012
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die nut-welding stations. On larger dies, the firm pipes nitrogen-gas springs together so it can pump up and dial down weld pressure as need- ed. Measuring and monitoring gas pressure initially fell to analog switch- es, which would trigger press shut- downs if nitrogen pressure dropped beyond an allowable setpoint, perhaps due to a worn spring or a leaky tubing connection.
“While the pressure switches were better than nothing,” says toolmaker Wade Pennington, “hysteresis on the switches often was significant. We needed a narrower tolerance range on gas pressure to ensure good-quality welds.”
To ensure zero-PPM parts delivered to customers, typically GR Spring & Stamping, based in Grand Rapids, MI, and with additional facilities in Chi- huahua, Mexico, and Richmond, KY, requires operators to perform push- off tests on in-die welds once every hour. Should a weld fail such a test, an hour’s worth of parts is quarantined for further testing. To avoid these unproductive quality checks and the costly quarantine of suspect parts, the firm has retrofitted Dadco electronic pressure monitors on two dies running
on a 200-ton press at its Kentucky plant. The monitors, which gauge weld force applied by nitrogen gas springs, have allowed the plant to adjust its schedule for performing weld push-off tests to just once per shift, a huge productivity gainer.
“We intend to replace analog switch- es with electronic on all of the larger dies in which we run in-die welding operations,” says Pennington, who notes that the retrofit process is “simple and straightforward.”
In addition to less-frequent destruc- tive testing and greater assurance that the in-die welding process is perform- ing as designed, using the electronic nitrogen-pressure monitors has eased the troubleshooting process.
“Because the tolerance window was so large in the past, as well as the hys- teresis, we’d waste a lot of time trying to pinpoint the exact cause of a shut- down. We would, for example, spend time adjusting welding parameters, leading to a long and tedious trial-and- error process that often was unneces- sary. Now we can look at the digital readouts of the pressure monitors and know instantly if gas-spring pressure is the problem, or not, and address it immediately.”