Mirror Finish Helps Dies Release "Sticky" Partsby Brad
F. Kuvin, Editor 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 mishits 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.
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."
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 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 "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 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." |
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