Page 49 - MetalForming March 2012

Deep-Draw PPM Slashed from 300 to Zero
Tolerances on deep-drawn servo- motor and booster housings stamped at the Voit Automotive facility in El Salto, Jalisco, Mexico, average ±0.002- 0.003 in. That level of precision requires state-of-the-art process control, says engineering manager Raphael Zaz- zaretta, which is why he’s sold on using electronic pressure monitors at critical nitrogen-gas-spring die locations. The plant has the Dadco electronic pressure monitors installed on three presses, and will retrofit a fourth press this spring.
“Other types of pressure switches might function as-designed,” says Zaz- zaretta, “but the electronic monitors are the state of the art. In particular, the ability to program the gauges for upper and lower setpoints makes a big dif- ference in our ability to control our deep-drawing processes.”
Voit’s Jalisco plant operates six trans- fer presses and nine progressive-die presses, running 220 dies and 150 part numbers. Zazzaretta told us of one par- ticularly challenging part where the addition of electronic pressure moni- tors had an immediate and measurable payback—stamping of booster-motor housings. The housings, of galvanized steel sheet, run in a 12-station blank- fed transfer die, where concentricity tolerances are ±0.1 mm. “Even the slightest bit of nitrogen leaking causes out-of-tolerance draws from the very first die station,” says Zazzaretta. “Or, the parts can become stuck in the die and cause transfer problems.”
Responsibility for in-die pressure control falls to a set of 14 nitrogen-gas springs installed in the press table, not the die. “This setup makes it easier for us to achieve the required level of pre- cision compared to using a press cush- ion,” says Zazzaretta. Eight of the die’s 14 stations contain nitrogen springs. At the draw stations, the springs act as a cushion Each spring connects to an electronic pressure switch—14 sepa- rate switches connected to the nitrogen filling station.
“The pressure switches trigger a
press shutdown only once every few months,” says Zazzaretta, “due to out- of-tolerance gas pressure to the springs. This may not seem like much, but when you don’t immediately see a problem in the press, it causes quality issues. This is particularly important with these housings, since their design makes it difficult for operators to visu- ally identify tolerance issues—primarily varying wall thickness.”
Now, the firm no longer has to worry about nitrogen-gas pressure causing quality issues with this critical die. With part volumes approaching 3 million/yr., the die is running with a PPM of zero. This is a significant improvement com- pared to a PPM of 300 when the project first launched, without the electronic pressure monitors.
Zazzaretta continues to tell us of his latest project involving the electronic pressure monitors—a press that will run several different part numbers and dies. “Here, we’ll include the settings for each pressure monitor in the part pro-
Voit has electronic pressure monitors (shown here protected by green enclo- sures) installed on three presses, and will retrofit a fourth press this spring. The monitors gauge pressure to 14 nitrogen- gas springs installed at key stations in each press table.
gram, so that the PLC will automatically adjust the high- and low-pressure set- points based on the die,” says Zaz- zaretta, “without operator interven- tion. The program will automatically instruct the nitrogen filling station to provide the required gas pressure to each spring as programmed.” MF
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