Dual chucking saves damaged parts from the scrap heap.
The Harper dual-chuck method boosts productivity 25% while maintaining a 0.0005-in. or better tolerance on roll roundness and runout.
A quick-adjustment feature on the Rotech live chuck lets a Harper setup man true up a worn roll in just minutes.
Because damaged roll centerholes bog down the restoration process, Harper chucks the rolls by their journals rather than suspending them on centers.
Refurbishing a mechanical component can be more cost effective than scrapping it and buying a new one. But the job of reconditioning is not easy when the part weighs tons, tolerances are in tenths, and surfacequalityspecs range from 1 to 2 µ. These challenges, however, are manageable with the right kind of workholding. Just ask Lester Carter, the Charlotte, N.C., plant manager for Harper Corp. of America, a supplier and reconditioner of rolls for the flexographic printing industry.
Harper's roll-reconditioning operation is structured to handle ceramic-coated Anilox rolls, which are laser etched with ink-retention dimples on the surface. A typical roll might be reconditioned five to eight times in its life.
For the most part, rolls come in for reconditioning only because the surface is worn or damaged. Often, the original substrate is still within roundness and runoff specs, so the first step, done on a lathe, is to machine off the old coating and true up the steel substrate. Next follows a plasma stage, where the roll goes to a grinder for sandblasting. This roughens up the substrate just enough to create a good gripping surface for applying a new ceramic coating. The deposit runs 0.0060 to 0.0100 in. oversize to provide enough material for final grinding to size and superfinishing.
The last step, laser engraving, creates a pattern of millions of tiny dimples that distribute ink. Measuring only microns in depth, these dimples transfer ink onto food packaging, labels, and cartons.
At one time, Harper reconditioned its Anilox rolls with a honing process mounted onto a lathe, with the rolls suspended between centers. Today, however, it uses Ecotech grinders equipped with heavy-duty Rotech live chucks at both the workhead and tailstock. At the workhead end, the live chuck locks into the grinder's dead spindle, and the chuck is dogged to the grinder's faceplate to turn the part. With that arrangement, grinder operators load and true up a 1,000-lb roll for regrinding in a matter of minutes.
The three-jaw chucks, from Rotech Internationale, Porter, Tex., feature ceramic rolling elements and a "quick-true" system. This makes it easy for the operator to locate the roll on the grinder's centerline through simple touch-off indication. In addition, claims Rotech, the chucks are the first standard ones able to handle heavy workpieces—in Harper's case, weight capacities reach 30,000 lb—and still hold runout tolerances within 0.0005 in. Many reconditioned rolls even leave Harper round within 0.0001 in.
When good centers go bad
The drilled centerholes on roll journals take a beating when they are installed in a mill or printing plant. Millwrights typically use the centerholes to lift and swing a roll into place, then they hammer on the center to jog the roll into final location. With this sort of abuse, says Dale McMillian, president of Rotech, it's no wonder that so many centers are a wreck by the time they arrive at reconditioners like Harper.
The poor condition of the centers makes the restoration process time-consuming, skill-dependent, and iffy, remarks Keith Jordan, Harper production supervisor. "First, the journal is made of hardened steel, which requires grinding, not simple drilling," he comments. "Second, finding a true centerline on a damaged roll is tricky and uncertain. It takes a lot of experience, judgment, and finesse—and a little bit of luck. In addition, restoration work, with all its variables, places a costly diamond tool at risk."
Harper solved the "bad-centerhole" problem by bypassing it. Rather than spending time to restore the centers then suspend the rolls by those centers, it simply chucks the rolls by the journals. This method, says Carter, "is faster and gives a more accurate final result, provided you have the right chuck."
In fact, the dual-chucking approach increased Harper's throughput 25% and improved part quality and yield. "All reconditioned rolls leave our shop with roundness and runout in the fivetenths range," comments Carter. Dual chucking also lets the company recondition rolls that once would have been scrap. "Every roll we rescue saves the customer about half the cost of a new roll," he says. "So there's a lot at stake."
The customer cost varies with size and condition of the roll process. "And by every measure, the reconditioned roll is as good as new," says Carter.
"With the 50% saving versus a new roll, it's obvious that the more rolls we can rescue, the better it is for our customers. Our dual-chuck grinding method has brought back a lot of rolls that were headed for the scrap pile and saved our customers millions in aggregate."
Ecotech's Curtis Gibbs, who has worked with a lot of roll reconditioners, adds that the Harper dual-chuck method carries a good idea one step further. "Many shops use a Rotech live chuck at the tailstock but stick with the live center at the workhead end," says Gibbs. "That way, they need to recondition only one of the roll centers. Or alternatively, they put the least damaged end of the roll at the workhead and chuck the other end."
"Since we make both heavyduty live centers and live chucks, it doesn't matter to us which method the roll shop uses," adds Rotech's McMillian. "However, the Harper dual-chuck method eliminates one more variable in a very exacting process. And the roll is held during reconditioning much more like it is in actual use. After all, the rolls ride on their journals during operation in the printing press."