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A solid structure and precise servocontrols ensure grinding consistency to 1 µm. |
It was not an easy decision for a German-based company to choose American roll grinders, but the investment doubled productivity and significantly improved product surface quality, says Gerd Klenner, plant manager of the Thyssen Krupp Nirosta(TKN) GmbH plant in Dillenburg. The company machines about 30,000 rolls annually, which are used in rolling mills that produce steel, aluminum, copper, brass, and paper, as well as in the printing and computer industries.
Producing the rolls pushes manufacturing capabilities to the limit. “We work with forces of up to 14 MegaNewtons,” explains Klenner, yet tolerances are exacting. Even measured Ra-micrometer values for surface quality do not indicate everything. Surface flaws might not even be noticeable until the customer puts the roll in production, he says. For instance, optical errors on the rolls may transfer through onto strip steel as visible shadows and marks, says Klenner.
The unfinished rolls are 12% chrome steel and have a hardness of 61 to 63 HRC. Each roll must be ground with a parabolic crown profile of 0.05 mm with respect to diameter, which requires a contour profile consistency of 1 µm. The SuperGrinder TT from Capco Machinery Systems, Roanoke, Va. (www.capcomachinery.com), fulfills these requirements through a combination of solid mechanics and highly efficient controls and drives.
The units in Dillenburg have a work envelope of 450 X 2,000 mm and a 1,000-kg weight capacity. The SuperGrinders rest on 50-ton reinforced-concrete foundations, which themselves lie on eight spring-damping elements. The machine, with a Meehanite cast-iron base, weighs about 26 tons. This gives the process a stable surrounding. A number of other measures further increase stiffness of the equipment. For instance, the wheel head on conventional roll grinders moves through a pivoting trunnion or eccentric bushings and levers to generate the contour profile. Capco developed a direct in-feed system that reduces the number of moving parts and dramatically improves machine stiffness, says Ed West, the company's CEO. The wheel head consists of a single, massive casting guided in and out via a digital absolute servomotor, he explains, which results in substantially higher-performance grinding.
The process demands fast and robust controls. The core of the Open Architecture Control OAC10 from Capco is a Typ3 osa CNC-control with integrated PLC from Bosch Rexroth, Hoffman Estates, Ill. (www.boschrexroth-us.com). With a block cycle time of 1 msec for eight axes, the Typ3 osa is said to be the fastest control in its class. It handles up to 64 axes in 12 independent processing channels and communicates via standard bus systems. Bosch Rexroth DIAX04 servodrives provide fast response and outputs to 240 Nm. “With that we have combined the two most powerful elements that are currently available,” emphasizes West.
The SuperGrinder's CNC control regulates18 servoaxes through two Sercos rings, as well as automatic loading and unloading, and several Bosch Rexroth hydraulic packages for the lubrication systems.
A conveyor transports rolls on pallets to a loader that feeds the grinder. A metrology system measures the roll diameter before grinding, as well as the final roll diameter, profile, taper, and TIR. When grinding completes, the loader removes and places the roll on a work pallet. Three servodriven automatic steady rests, with six total axes and six babbitt pads, control the roll counterpressure for micron-exact grinding. The changes in force of the steady rests are continuously updated through the drive servocurrent loop at 125µsec intervals, and the steady-rest axes automatically reposition as required to maintain maximum workpiece stiffness. A digital absolute servoaxis keeps the grinding wheel properly positioned.
The Capco SuperGrinders automatically load, grind, and unload with high accuracy and repeatability. Programs for different rolls are generated through the graphical user interface and can be called at the push of a button. A complete machining cycle takes just 12.5 min. This doubled the productivity in the roll shop, indicates Klenner, and the quality of the ground rolls improved. “The rolling mill now runs considerably smoother and our error rate decreased significantly,” he adds, “which results in a dramatically higher net profit for our Dillenburg works.”