— Reprinted with permission from Metalforming Magazine, July 2001.
During progressive stamping, if a part sticks and the carrier strip continues to lift, things turn ugly. The carrier bends and buckles, and won't feed. The result is mis-hits, double hits, torn screws, busted dowels, broken forms and wiped-out trim stations.
Not a pretty scene, but one faced by automotive stamper Fisher Company, Troy, MI, just a short time ago. After it tried to run the dies, and managed to survive mis-hits and other problems, the firm invested in a sensor-retrofit package to sense misfeeds, part ejection, strip buckle and end-of-feed parting. But sensors only protected the dies. A long-term solution to stamping the parts, to avoid frequent downtime episodes, was a must.
Micro-polished form stations (center) on the bottom half of the 14-station progressive die enable the part (shown testing on the die) to consistently release from the die during progressive feeding. Without the fine polish, to 3-5 microns, the part would often stick, leading to mis-hits.
Heat Buildup from Tight-Radius Forming
“Back in 1998 when this job first came in, we had a new press and new tooling, and couldn't make any parts,” recalls Roger Kelley, tool room group leader at Fisher Corp. “We were in big trouble.”
The part in question belongs to a seat recliner that Fisher makes for its parent company, Fisher Dynamics. The part winds up in General Motors pickup trucks. The challenge: wrapping the material, a high-strength steel 0.090 in. thick, around a tight radius at the same point where the material flanges down.
“Wrapping the part like that”, says Kelley, “generates a lot of heat, particularly in the top portion of the die. Heat, combined with the restriction to material flow, causes the material to build up onto the tool and eventually gouge it. Coating the top portion of the die prevents heat buildup; micro-polishing the bottom die section prevents the part from hanging up on the die.” Four different dies are used to make the seat recliner parts-inners and outers, left and right sections. Parts measure 16-in. long and exit the 14-station progressive die two-off. Part volumes average 30,000 per week. The dies, 11-ft. long, run on a new Minster 1200-ton press with 14-ft.-deep by 6-ft.-wide bed. The press cycles at maximum, 50 strokes/min., on these parts as 22-in.-wide coil stock feeds it with an all-servo coil line, also from Minster. Since early in 1998 when the program began, Fisher has been through three sets of die sections.
“The first set of die sections, which we did not polish or coat,” recalls Kelley, “was quickly scrapped. The second set, once we decided on the polish and coat procedure, ran for two million hits, with periodic inspections and maintenance as required at 50,000-hit intervals. That set of tools went out for repolishing and recoating in November 2000, so we're now on our third set of tools.”
Fisher Corp. operates a 100,000-sq.-ft. stamping plant with 250 employees, forming seat-recliner mechanisms for parent company Fisher Dynamics. The firm has been forming this set of recliner mechanisms for GM trucks since 1998 at a rate of 30,000 per month. The tight radius and flange features of the parts require the firm to take steps to minimize heat buildup in the die, thus prolonging die life and ensuring good part release.
Micro-Polishing Solves Sticking Problem
Before the firm invested in sending the bottom portions of the tools out for polishing, they might run for 10,000 hits before the parts would begin to stick in the die. Then Kelley learned about the capabilities offered by Fred Enot, Enot Services Inc., South Lyon, MI (tel. 810/612-6930; e-mail, firstname.lastname@example.org.) Enot has been fine-tuning his metal-polishing skills for more than 25 years, and can polish dies so that you'd swear they were chrome-plated. I know—I paid Enot a visit to learn more about the work he performs for Fisher.
Polishing dies is meticulous work that requires Enot to remove any gouges, nicks or other blemishes, polish the entire surface to a 3 to 5 micron finish (Ra), or less than 0.0001 in., while maintaining critical tolerances on radiuses. Enot uses his vast experience and knowledge about tool-steel alloys to develop a finishing procedure in order to minimize the amount of time it takes to complete a project.
He begins by evaluating the material surface and its hardness. Rather than start finishing on a radius, where more material is likely to be removed with each pass of an abrasive tool, he begins his work on flat surfaces. “This allows me to learn how aggressive I can be—if I'm too aggressive, I make more work for myself, and add time to the job, by over-grinding and creating more gouges or swirls,” shares Enot.
Typically, the first step is to remove any relatively major grooves or nicks in the tool, perhaps with a disc sander or stone. “Every step in the process”, he says, “enhances the next step, until I finish up with diamond polishing and cotton buffing.”
With larger imperfections removed and the surfaces smooth and flat, Enot takes up his pneumatic profiler and works the die surfaces in small areas with dabs of diamondgrit polishing compound. He selects from a vast array of profiler tools—of steel bar or wood strip—to obtain the desired results. The profiler vibrates the small tool at speeds to 10,000 strokes/min. front to back while Enot moves it in precise side-to-side motion.
“Manually moving the tool side to side while the profiler moves it back and forth is key to avoiding any dips or unevenness to the surface. I create a criss-cross pattern with the profiler to keep everything flat,” he says. “This flatness (measured in surface-finish terms as Rz), combined with the surface finish (Ra) enables the die to effortlessly release the part during progressive stamping.
“And,” he adds, “polishing along the direction that the material flows as the part lifts off of the die is critical. That seems obvious, but I find it is often overlooked by metal finishers.”
Polishing Also Enhances the Die-Coating Process
A. Extrusion punches on the top half of the die take an IonBond coating that enables the punches to make 100,000 hits before they're replaced. Note the red coolant lines and wedge-shaped nozzle fixtures that Fisher retrofitted into the die to help keep the parts and dies cool during forming and ensure good part release during feeding.
B. Fisher sends the tools for the GM-truck seat recliners out to Fred Enot, Enot Services, for micro-polishing. Polished bottom dies go right into the press; polished top dies go to a shop in Indiana for in application of TD heat-resistant coating. Here, Enot demonstrates how he meticulously moves a profiler side to side while it vibrates a polishing tool back and forth over a dab of diamond compound, achieving a 3- to 5-micron finish on tool-steel die sections.
The key to any die-coating process is producing a smooth and flat surface finish before applying the coating, and here Enot helps, too. He polishes the top sections of the dies before Fisher sends them out for coating. The coating of choice by Fisher: the TD process.
TD (thermal diffusion) is a thermal reactive diffusion process that forms a hard and dense layer of vanadium carbide onto the tool steel. The layer, 0.0001 to 0.0005 in. thick, diffuses into and onto the steel surface, optimizing peel strength to ensure long life.
In the case of Fisher's tools, the coating lasted for two million hits, successfully preventing heat buildup in the tools. While relatively new to the TD process, Fisher is a long-time user of another tool-coating process from Ion-Bond, Inc., Rockaway, NJ. Kelley relies on the Ion-Bond coatings to help prolong the life of his more intricate tools, such as the extrude tools used on the seat-recliner stampings.
“Uncoated, these tools might last only a few thousand hits. Coated, we get up to 100,000 hits from them,” says Kelley.
Cooling and Lubrication Complete the Puzzle
As it developed and implemented the procedure required to polish and coat the dies used to stamp the GM-truck seat recliners, Fisher also retrofitted an array of lubricant and coolant lines and nozzles to the dies.
“We see the success of this project,” shares Kelley, “where we went from practically being unable to make any parts to running 50,000 or more without a hitch, as a three-step process: micro-polishing, coating and lubrication. When we added sensors to the die, which we now specify as standard equipment on every new die we bring in, we also switched lubes, settling on Tuf Draw (from Fuchs Lubricants Co., Harvey, IL).”
When we visited Fisher last spring, Kelley was preparing for an onslaught of new dies—130 dies on the board for 2001, for the firm's 15 presses.
“Believe me,” he says, “we're already looking for dies that can benefit from this combination of micro-polishing and coating. We do not want to scrap out any more die sections.”