Part II: Sensors prevent crashes, clear way for growth
September 11, 2007
Automotive Tier II supplier of ride control parts Tennessee Stampings makes extensive use of sensors to mistake-proof its stamping, and clear the way for its 23 percent annual growth. Before the dies ever hit the presses, they are properly sensored so you don't incur downtime associated with troubleshooting the sensor in the press.
Editor's Note: This is the second article in a three-part series on Tennessee Stampings LLC. Part Iexplored the company's lean practices. Part II discusses how the company's use of sensors and mistakeproofing led to its growth. Part III, which will appear in the October issue, examines the company's facility expansion, addition of a transfer press, and installation of an underground conveyor system.
When you stamp 100 million automotive parts annually, as does Tennessee Stampings LLC, Portland, Tenn., you might accept die collisions as just part of the "traffic statistics."
Not so, according to Greg Cornett, plant manager of the Tier 2 supplier of ride control components (struts and shocks).
"What we've probably done better than most companies is we've realized the importance of mistakeproofing our dies to avoid crashes," Cornett said.
Tennessee Stampings has continuously increased its number of customers and the volume of work from current customers. The company just recently completed a 40,000-square-foot expansion with an underground conveyor system—its third expansion since 1993—and purchased a new transfer press to increase capacity. Sales have grown by approximately 23 percent per year for three years. Cornett credits much of that to their efforts to eliminate die crashes.
"The main reasons our growth has been so great are mistakeproofing and in-die sensor systems," Cornett said.
Yes, the company has a mistakeproofing lab, developed about four years ago. "The goal of the mistakeproofing lab is to ensure that before the dies ever hit the presses, they are properly sensored so we don't incur downtime associated with troubleshooting the sensor in the press," Cornett said.
Bill Graves, the mistakeproofing department manager, said, "Seven years ago, we were crashing dies twice a week. If you crash a die, you can do easily $10,000 to $15,000 [worth of] damage in one stroke. Now, with the proper sensors, it might be six months before we have a die crash. In fact, we haven't had a major die crash since November of '05," Graves said.
Occasionally crashes do occur during setup, because the sensors are not enabled during setup mode, Cornett said. "Attempting to set up a die with the sensors enabled creates excessive nuisance stops, so the sensors are bypassed in the setup mode. The operators must pay attention to every stroke to prevent the occasional mis-hit—that's where most of our crashes come from.
"It is easy for a new hire to accidentally mislocate the strip in the die during setup. For this reason, we have an extensive in-house training program for all our new hires," Cornett said.
Recently the company has experienced sales growth as a result of "takeover" work. Dies, often in poor condition, arrive at TSL with no sensors. "We won't run a die without specific sensors being installed first—even on a temporary basis," Cornett said.
Tennessee Stampings does not build its own dies, but its sensor department designs the sensor system for new tools based on tooling CAD and sends the design to the tool builder before the dies are built. In addition, the stamper has a fully capable toolroom that can retrofit takeover dies with the required sensors. Sensors are sourced from Banner, Turck, and Balluff.
"A sensor can last for more than 2 million strokes if it's buffered well," Cornett said, "but it would not last a stroke if it's not protected right."
It is primarily the wires going to each sensor that need protection, Cornett said. "When we first started our sensoring program, if you took a picture of the press, it would look like a rat's nest of wires." At first the wires were encased in square tube to protect them from getting inadvertently cut—as a result of what Cornett calls the gorilla effect. "If something can be damaged or broken, it will be."
Now, for most dies, channels are milled into the die to embed the wires. "It's part of our die standards," Cornett said. "If it's a high-volume die, we will mill out pockets and try to embed them for increased protection."
More recently the sensors are fitted into a 41-pin military connector that acts as the umbilical cord from the die to the press control (see Figure 1). "So every time we set up this die, it's ready to go. In the past it was like a jigsaw puzzle, trying to determine where sensor wires go, but now we just come in, plug up, and go," Cornett said.
Cornett said that every die has a minimum of four sensors:
"You're going to have a misfeed sensor, indicating that the material fed up to the right location each time; you're going to have an overfeed sensor to ensure that you didn't overfeed it; you've got the part-exit sensor, which makes sure a part exits that die every stroke, because if it stays in there, you're going to crash this die; and then we use what's called a stripper sensor," Cornett said. A stripper sensor monitors the distance between the upper stripper plate and the bottom die section at 180 degrees. If a slug stays in the die, which would damage parts, the stripper distance will change. The stripper sensor would detect this change, stop the press, and alert the operator, thereby preventing defective parts, Cornett said.
TSL's press tonnages are from 75 to 800. The stamper places a higher priority on sensing up dies for the larger presses first, but even small dies can be equipped with many sensors, such as a small deep-draw die for stamping small internal components on a press running from 150 to 250 strokes per minute (SPM). "A lot of dies have nitrogen gas in them," Cornett said. "If gas cylinders leak and nitrogen pressure changes, that can affect part quality. So we monitor the pressure in our nitrogen cylinders. If the pressure exceeds or drops below a certain set limit, the sensor shuts off the press.
"You can do the same sensoring on smaller dies as you do on the larger ones. We just have a different type of plug, since you don't have as much on some die shoes to mount 41 pin connectors."
A typical sensor sends out a fishtail signal and measures what is inside that fishtail, Graves said. "If you've got what we call an XYZ coordinate machine, you can set up a sensor and actually measure what you're getting inside that field.
"So you know when you're in that die, you need to be 2.5 millimeters from where you're measuring."
Graves said he uses Labview software, also used by NASA, NASCAR®, and Xbox®, to track the signal.
"Every press stroke produces a signal, kind of like an EKG of a heart. If the punch is dull or broken, it takes longer to break through the material, so the line gets longer. Or if it's not there at all, you're going to flatline, it's going to give you a fault," Graves said.
"Before, if you're running 100 SPM, and you have a broken punch and it's been five minutes since you checked your parts, that's quite a few parts that you're going to be scrapping. But this type of technology detects the broken punch, shut the press off, and alerts the operator," Graves explained.
"We try to apply all the technology that we can find and create so the dies get better parts, faster, and without having quality issues. We want to be able to track what we're doing," Graves said.
Resolver-based press controls are required to use sensors, Graves said. "If you didn't have something telling you where you are in the stroke, your sensors wouldn't know where to cut off," he explained.
The stamper, working with its vendors, developed a unique mistake-proofing solution to an internal problem it had with the distribution of sensors. It had been difficult to determine who used a sensor and for which die. Often sensors were in short supply even right after a full supply came in.
"We found that toolmakers like to hoard sensors. They wanted to make sure there would be a sensor there when they needed it, so they'd put three or four in a toolbox, and then we couldn't find them at inventory. Our sensor cost really shot up. These packages are $250 to $300," Cornett said.
Now the sensors are sold on consignment from the sensor manufacturer, accessible only from a vending machine sitting in the shop. Each toolmaker has a credit card to insert in the vending machine. He selects the sensor he needs, and the sensor comes out of a slot at the bottom. The sensor manufacturer ties the inventory data into the phone line and knows when the supply is close to being depleted, and the company knows "who got what," Cornett said. "We aren't charged until the toolmaker pulls the sensor."
Problem solved. The system worked so well, the company plans to expand the concept to other small consumables, such as welding electrodes and safety devices.
"When I ran a press, 15 to 18 years ago, I had to sit there and watch every stroke with my hand on the stop button, watching for that material to short-feed or for a part to not come off," Cornett said. "You couldn't leave it running when you went on break. Now, with our sensor program, some operators will let the press run until the coil runs out. Or maybe the shift will end and we don't have an operator; if it's near the end of the coil, we just let it run and finish the coil out. Sensors have just added productivity.
"When I first came onboard here in 2000, tooling maintenance represented 9 percent of total sales dollars. Last year we got that down to 4 percent. So in dollar numbers, it's pretty substantial," Cornett said.
Greg Cornett will give the keynote presentation at the Stamping Journal® Forum on Sept. 27 in Hopkinsville, Ky., entitled "Lean Manufacturing—Creating Flow on the Shop Floor." Cornett will share his insights on lean practices derived from real shop floor experiences at Tennessee Stampings. Topics include: