January 15, 2008
A company specializing in perforated sheet overcomes deburring issues with an automated system using nonwoven fiber brushes.
Hendrick Manufacturing Co. had to avoid a snag, and with automated deburring, it did. The 200-employee company has deep roots in the northeast Pennsylvania town of Carbondale, where company founder Eli Hendrick pioneered the craft of perforation back in 1876. The equipment has drastically changed since then, but the basic idea behind the perforating process hasn't. The machine punches sheet with tooling analogous to cluster tools on conventional punch presses, only these tools can be 60 inches wide and pierce thousands of small holes with every stroke. And, oh yes, these machines move at up to 600 strokes per minute (SPM). That's a lot of holes.
The company serves the gamut of industries requiring perforated sheet, one being the industrial laundry sector. One customer uses massive dryers that turn large barrels, with skins made of rolled, perforated stainless steel sheet that allows the air to circulate through hundreds of 1/4-in. holes. Those sheets come from Hendrick. The raw stock arrives on the dock as coils. The company uncoils it, perforates it, levels it, shears it, and then degreases it to remove the oils applied during the perforating process.
And then comes finishing.
Deburring for this application has remained critical; a collection of burrs could result in a laundry-load of torn clothing—and a very unhappy customer. For this reason Hendrick has gone to great lengths to deburr and grain the sheet surface. The application involves 0.125-in. stainless steel sheet, a relatively thick and hard material that makes deburring and graining—or attaining a flat, straight, unified metal grain—quite a challenge. The company's test to ensure a burr-free surface? Nylon stockings, of course. To test for a smooth finish, quality personnel sweep the finely woven material across the sheet; if stockings don't snag, most likely nothing else would either.
The company's legacy deburring equipment had one sanding head and two planetary belt heads. "This process required us to pass the sheet through the machine several times per side to get the finish we needed," said Robert Romanowski, engineering manager at Hendrick. "When you perforate, there's a natural way a burr is pushed down the bottom side. And then when the punch retracts, it can also leave a burr on the top side." The thick-gauge material provided additional challenges, he said. "With these heavier gauges, it's much more difficult to get inside those holes."
The company's goal was to achieve the required finish with only a single pass per side. And to get there, Hendrick, together with AM Machinery Sales, Warminster, Pa.—which supplied a Steelmaster deburring machine—and 3M's R&D center in Minneapolis, coordinated an engineering effort to eliminate those snags once and for all.
The automated system Hendrick tested involved a four-station Steelmaster flat-metal wet graining and deburring system. It uses a combination of belts and, rotating parallel to the conveyor carrying the sheet, two barrel brushes spanning the width of the machine.
In this application, conventional stainless steel barrel brushes didn't produce optimal results, mainly because of the heavy-gauge material commercial laundry customers demand. The brush bristles were able to reach inside the hole, but they couldn't successfully remove burrs or smooth the hole edges. The bristles themselves have no abrasives; their stiffness alone usually provides sufficient abrasion to remove burrs—but not on Hendrick's heavy-gauge sheet metal.
To overcome this problem, the company tested another kind of brush.
Instead of bristles, these brushes incorporate disks of 3M's Scotch-Brite® nonwoven fiber, similar to those kitchen scouring pads, but engineered for deburring and graining. Conventional deburring machine barrel brushes have bristles attached to a central axis positioned parallel to the workpiece. For these brushes, however, 14-in.-diameter disks of nonwoven fiber mount to that central axis. The more tightly packed these disks are, the harder the brush, resulting in a higher durometer rating.
Said Brian Janovec, technical services specialist at 3M, "For this application we opted for the medium density. [In 3M terms, a No. 5 cleaning brush.] These disks had to be able to dive into these holes slightly." That slight pliability allowed the brush to "give" enough to enter the holes, completely removing surface imperfections and rounding the edges, so that not even nylon stockings would catch. The brushes' pliability required a somewhat aggressive process with additional horsepower and pressure, but not so much as to mar the material.
After much testing, the parties involved chose 3M's Cut and Polish brush, with medium (about 100) grit. "This brush has more mineral than other types of web brushes," said Mark Sterner, 3M's business development manager. The mineral abrasive Sterner referred to is alumina oxide, significantly tougher than other minerals like silicon carbide.
"The silicon carbide can give you a nicer finish in some cases," said Sterner. But this application required aggressive finishing action that wouldn't be very kind to the relatively brittle silica carbide. During testing, the brushes with silica carbide actually fractured before they had a chance to remove any burrs. "True, the finish would be brighter," he added, "but you'd still have the burrs—and that's obviously a problem."
After technicians narrowed down the material choice, they next tested how these brushes should be applied to the perforated sheet for the best results. Due to the nature of automated deburring systems, these brushes "favored" one side of a hole more than the other. So to cover the hole's entire diameter, the application required two brushes moving in opposite directions, clockwise and counterclockwise, and oscillating left to right at high frequencies.
So how exactly do these barrel brushes favor one side of each quarter-inch hole more than the other? Imagine a close-up of each hole being conveyed under a nonoscillating barrel brush moving in the clockwise direction. To illustrate, visualize a line that runs the diameter of the hole; the top part is the leading edge, the bottom the trailing edge. As the brush bristles come down they first hit one side—the trailing edge—of that hole, then emerge from the hole's leading edge. The trailing edge—the one that takes the "first hit"—will always receive better coverage for deburring because that's where the brush's energy is focused most. A second barrel brush turning counterclockwise hits the leading edge first and, thus, ensures complete coverage.
"From here, the remaining challenge is where that line intersects the hole," explained Brett Mandes, vice president of AM Machinery Sales. That intersection point represents the left and right edges in between the leading and trailing edges. "Those two points where that horizontal line touches are not effectively deburred if you do not oscillate the barrel brush left to right."
This also illustrates why a brush is necessary for this application, he added. "You can't spin a belt in another direction; you can spin it only in the direction of the conveyor belt [motion]. Plus, the belt can't oscillate. It can only track slowly left to right, slow enough so it doesn't leave an S mark on the part."
The oscillating brushes, he said, also help attain a perfectly uniform, straight-line grain. "Without oscillation, you get a gap in the grain left to right. But when you oscillate left to right, you attain that perfect grain."
"Oscillation really helped us attain that fine finish," added 3M's Janovec. "Because you are working with the edges of these disks, they have a tendency to transfer their own grain pattern on the workpiece. The oscillation helps blend those grain patterns for a smooth finish."
What resulted was an automated four-head Steelmaster deburring machine 52 in. wide. The first head entails a hard, 55-durometer contact drum with a 120-grit belt (a 3M 777F). This processes the initial vertical burr, knocking the burr down flat. Note that the belt doesn't remove any burrs, just flattens them. The belts also remove some of the additional, small surface imperfections on the part itself. Next comes a 35-durometer drum with a 100-micron microfinishing abrasive belt. This soft and pliable drum starts to knock down the burrs into the holes. Again, the burrs are knocked down, but still are not removed from the sheet.
After two drums comes the first of the two barrel brushes, a 5A medium Cut and Polish cleaning web brush that turns clockwise and oscillates left to right. The fourth head has an identical brush moving counterclockwise. When head No. 3 moves to the left, head No. 4 moves to the right, oscillating in opposite directions for full scrubbing action. The part runs through the machine at about 9.8 feet per minute, and each side requires a single pass on the machine's 10-ft.-long conveyor.
Since Hendrick installed the new system last summer, "the company has been able to process a part in well under a minute [per side]," Mandes said.
"The best results come from equipment that offers variability in control," explained Janovec.
Brushes and belts may need to be customized or move in a specific fashion for optimal results, be it specific RPMs, drum hardness or rotation direction. For the job at Hendrick Manufacturing, these and other variables had to be tweaked to attain that perfect, smooth, nylon-stocking-friendly finish.
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