Tooling Article



In-Die Sensing Makes Good Sense

Thursday, May 1, 2008
What once were called trends—lean manufacturing, just-in-time production, etc.—now are facts of life for most metalformers. Stampers make parts today for shipment later the same day, which then make it to customers’ assembly lines the following day.

There is no room in the supply chain for unplanned downtime, unless you, the supplier, enjoy shutting down the entire supply chain.

To keep a press running—and making good parts—rather than experiencing downtime due to misfortunate incidents in the die—in-die sensors play a critical role. The alternative to sensing for protecting the supply chain: slowing the press down and throwing more people at it, a choice few metalformers can afford to make.

This progressive stamping die, equipped with digital and analog sensors runs at Waukesha Tool & Stamping.
This progressive stamping die, equipped with digital and analog sensors from Turck, runs at Waukesha Tool & Stamping, Sussex, WI, stamping parts for the electronics industry. The firm’s advanced die-sensor program creates mistake-proof dies and allows it to automate in-die press operations such as assembly, welding and measurement.

Sensors Cover for the Imperfect Die

“Initial tool tryout can create problems that never get solved before a die enters production,” says Drew Stevens, die-protection specialist with sensor manufacturer Balluff Inc., Florence, KY. “These problems—excessively sharp corners on a rail, for example—can prevent tools from running at their designed stroke rate and at optimum quality.

“But the trend today is increased interest in getting new tools through the PPAP process and into production,” Stevens continues, “and hoping for the best. In this scenario, stampers must be able to detect events in the tools more than ever, to prevent bad parts from being shipped and, in the worst-case situation, crashing the die.”

Too many dies in production right now run at less than optimum speed just to keep them running at all, says Dave Bird, Balluff’s business development manager, and often management does not know this fact. Nor does it understand how a simple and affordable investment in die sensors can allow for increased production rate as well as quality.

“There’s simply not enough reporting done on the shop floor to log the amount of productivity lost due to die issues,” says Bird. “Operators are doing the work that sensors could be doing, and managers don’t know what’s really going on—cams don’t return, misfeeds occur, parts stack up. Tracking and communicating downtime events can be used to justify a sensor program. It all starts with documentation.”

This die in use shows the installation of balluff sensors using wireless technology.
These photos of a die in use at Ultra Tool & Manufacturing show the installation of Balluff sensors using wireless technology. Top: A sensor remote receiver sends power to and receives sensor outputs from the adjoining die half. Middle: Sensor signals and 24 V DC travels through a ¾-in. air gap. Bottom: Remote sensor transmitter receives power from and transmits sensor signals to remote receiver on other die half.
Take the Stress Out of the Pressroom

What I have seen in too many pressrooms is an escalation in the amount of stress placed on those employees charged with meeting the rapidly climbing demands for production and quality, often while running tools that, as Stevens noted earlier, have not been run through a thorough tryout and troubleshooting process. An in-die sensor program can alleviate much of this stress and eliminate the finger pointing and chaos that inevitably results when a die breaks and a press shuts down.

“A lot of stampers have become familiar with basic part-ejection and misfeed sensor applications,” says Brian Tarbox, business development manager for sensor supplier Turck Inc., Minneapolis, MN. “Now, we’re seeing these shops look to do more with in-die sensors—using analog sensors, for example, to measure part features on the fly rather than during offline quality inspections. Here, sensors can trigger die adjustments for tool wear and variations in material thickness, eliminating the need to stop the press to make manual adjustments with shims.”

Well-versed in the use of analog sensors for in-die part measurement and on-the-fly tool adjustments is metalformer Ultra Tool & Manufacturing, Menomonee Falls, WI. It recently built a die with 32 sensors—27 digital and five analog sensors. The analog sensors measure three critical part features and relay that data while the press runs, at 60 strokes/min. The data is processed to trigger stepper motors that adjust the tooling to maintain tight, critical tolerances.

“Using this die to make agricultural parts, we were tasked with holding tolerances of ±0.004 in.,” says Ultra Tool’s sensor applications specialist Brad Schmit. “Implementing the analog sensors and adjustable die sections, we achieved tolerances of ±0.0015 in.

“If we had not installed analog sensors in this die, our operator would have had to pull parts periodically off of the line—one in every 50—for quality testing,” Schmit continues. “Whenever he would discover parts drifting out of tolerance—due to tool wear or inconsistent material properties—he’d have to shut off the press and shim the tool, sacrificing precious press time. Also, we might have just run 50 bad parts and if we were not segregating parts after every batch of 50, it’s possible or even likely that the customer would receive bad parts.”

Other Areas Where Sensors Have You Covered

Balluff’s Stevens notes another growing trend in the use of die sensors: to ensure accurate die positioning on the press bolster, particularly critical in quick-die-change applications.

“In addition to sensing for proper die location when changing dies,” Stevens notes, “for each new setup we can install sensors at the entry side of the die to check for accurate coil feeding. In the rush to change over a press line, stampers can rely on sensors to ensure proper setup.

“More than half of crashes and other unfortunate events that occur in dies during production that lead to die and press damage are caused by the tool design, the process or the setup,” continues Stevens. “Many crashes, for example, are caused by such relatively simple things such as insufficient spring force in the lifter pad, or a slug that’s not cleared out and catches the strip, causing it to buckle and double-over.”

A lot of these issues Stevens describes are caused by a rushed die-design and build process, as well as press setup. For example, an operator might pick up and load a die into a press with a forklift truck. It’s difficult to perfectly align each die every time it’s placed in the press, creating alignment issues with the feed line. This can cause misfeeds, improper part ejection and other issues that lead to crashes. All of these scenarios can easily and cost-effectively be avoided with the use of sensors.

Cost Justification a Challenge? Not Really

In addition to relying on feed sensors at the entry point of the strip and at the end of the tool, to ensure that the material has progressed as designed, Ultra Tool’s Schmit says that his firm also relies on stripper-height sensing. “We use sensors to look for slug pulls,” he says. “When we’re running cosmetic jobs, it’s a great advantage to be able to automatically shut off the press if we pull a slug.

“Investing in die sensing as much as we have,” adds Schmit, “is a cost easily justified. We can schedule production more consistently, and once the die goes into the press the operator can walk a confident that the press will run smoothly. And, if it doesn’t for some reason, we’ll be protected. In-die sensing allows us to take another step toward lights-out manufacturing.”

Ultra Tool & Manufacturing also takes advantage of a relatively new development in press controls, noted by Schmit as a perfect complement to in-die sensing and measurement: shift-register capability. This technology, offered by several control manufacturers, allows the press control to track any parts identified as bad by the sensors during the progression, and automatically segregate bad parts as they leave the press.

“Without this shift-register ability,” Schmit says, “we’d have to stop the press as soon as the sensor or the operator notes a bad part. The operator then has to single-stroke the press until the part exits the die onto the conveyor. But, thanks to these new controls, which we’ve been using for about a year now, bad parts automatically are diverted so that we don’t compromise the quality of our good-parts bin.”

New Technology: Increased Range and Wireless Technology

Other new sensor and control technology noted by Schmit as providing benefit to Ultra Tool is the continued development of smaller-diameter sensors with increased sensing ranges. As stampers more often look to retrofit existing dies with sensors, smaller models make it easier to install the sensors properly and safely so that they avoid being damaged during production. “Die space is als at a premium,” says Schmit.

“Smaller and longer-range sensors improve flexibility for the die designer,” adds Turck’s Tarbox. “Stampers now can get double to triple the sensing range for the same-diameter sensor—where a 4-mm sensor used to have a range of 0.5 mm on mild steel, it now has a range of 1.0 to 1.5 mm. And, for a required sensing range, stampers can employ smaller sensors, which generally have faster switching speeds and allow the press to run faster.”

Balluff’s Bird notes yet another technology development making waves in stamping shops: wireless communication.

“Wireless has made it to the pressroom,” says Bird. “This allows us to remove the umbilical between the die and press, particularly useful for stampers with small families of dies running repetitive parts. With wireless we can send power and signals to as many as 64 sensors through an air gap without a hardwired connection. Then stampers can change their dies out without having to manually disconnect and connect the sensor cables.”

Wireless sensor installation also makes an impact when running large dies split into two halves to make right- and left-hand parts.

“The first half of this type of die will likely perform all of the flat work such as piercing and punching,” explains Stevens. “The second half of the die—which is modular—then performs drawing, bending, cam piercing, etc. When the stamper exchanges this modular die half to make the left or right part, and since typically there is only a junction box on one half of the die, all of the sensor outputs must route from the second die half to the first half. In a rushed production setting, I have seen operators forget to unplug the umbilical between the dies and tear the cables out of the track.

“With wireless connectivity, everything is buried in the die shoe out of harm’s , and there’s no need to plug or unplug anything,” continues Stevens. “And, with capacity to 64 outputs, the stamper can ground some of the pins out to allow the press controller to automatically identify which die half has been installed. This quickens change-over and eliminates human error.”

Last but Not Least: Color Sensing

One last sensor development called on recently by Ultra Tool is color detection. The firm recently landed a contract to produce 6 million stampings per year that requires it to perform in-die assembly, which saved the customer more than 30 percent on the piece-part price, according to Schmit.

“The tool runs at 120 strokes/min., and we were changing coils every 2 hr., limiting productivity. Now, we run multiple coils welded together end to end by our steel supplier. But to make sure that we don’t stamp parts at or near the weld lines, our supplier paints a blue line to mark each coil-end weld and we employ a color sensor where the strip feeds into the die. This sensor signals the press control to route parts stamped from stock near that paint line into the scrap bin. We now run the line for more than 16 continuous hours without coil changes and without having to manually track scrapped parts.”

Now that’s making the most of precious press time. MF


See also: Balluff, Inc., Turck, Inc.

Related Enterprise Zones: Sensing/Electronics, Tool & Die

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