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High-volume manufacturers of cutting tools, dental instruments, solenoid valves and other precision products must have a reliable source of bar material in various sizes, finishes and metallurgical compositions to meet their processing requirements. In some cases, because of the end use application, or where machining operations are performed using Swiss-type screw machines with pull-through chucks, specialty metal bars with very precise tolerances on diameter are specified. These bars are produced in lengths up to 14 feet using centerless grinders to control size and finish in what amounts to a continuous process. Although the market for precision ground barstock has grown, suppliers have been hampered in their ability to consistently produce the material with the required tolerances. This factor recently led Böhler-Uddeholm (South Boston, Virginia), a supplier of high speed steels and powder metallurgy produced tool steels, to upgrade its centerless grinding capabilities.
“We have four older Cincinnati grinders capable of meeting the standard industry tolerances of ±0.001 to 0.002 inch on diameter for high speed tool steel in sizes from 1/8 to 3/4 inch diameter,” explains Trevor Biggs, Böhler-Uddeholm’s manufacturing manager. “But we could not achieve the necessary tolerances for precision-ground bars on a repeatable basis without investing in new technology.”
Böhler-Uddeholm executives formed a team that included several machine operators and maintenance personnel to evaluate new centerless grinding equipment. They selected a Viking Super Series centerless grinder from Landis Cincinnati (headquartered in Waynesboro, Pennsylvania) to add to their in-house grinding capabilities.
The Viking is arranged to automatically grind barstock using a thru-feed grinding cycle. The machine is fitted with an automatic bar loading and unloading system. The automation, which includes an infeed table and outfeed table, enables the automatic operation of the grinder by supporting and feeding bars as they are ground. The automation tables and the grinder have been engineered to grind barstock from 0.100 to 1.125 inch (2.54 to 28.58 mm) in diameter and from 3 to 16 feet (0.9 to 4.9 m) in length. The Viking centerless grinder uses a thru-feed work rest to support a bar as it is ground.
A Zumbach 2000 post-process laser gage system is used to control the automatic operation of the centerless grinder. An air blow-off ring is included on the outfeed table to clean bars as they exit the grinder, before they enter the laser gage head.
Rollers on the infeed and outfeed tables, which turn at different rates, are powered by variable speed motors capable of feeding the barstock at speeds up to 40 feet (12.2 m) per minute. The outfeed table rollers turn faster to unload the bars at the same rate or a faster rate than the rate at which they are ground. These separate rates are programmed in the automation control.
In the automatic mode, the table rollers laterally adjust to compensate for wheel wear and wheel dressing. This compensation is controlled by signals from the Zumbach 2000 post-process gage.
The infeed table, which incorporates a cradle and elevators, loads and feeds the barstock into the Viking centerless grinder. It also supports each bar as it passes through the grinder. Straps in the cradle unit raise the bars up to the elevators, which then raise the bars one at a time to the top of the infeed chute. The infeed chute functions as a magazine-type loading platform for the bars before they are released, one bar at a time, from an escapement to the infeed rollers.
A carriage for the roller subassembly allows the rollers to be retracted from the grinder during setup. The outfeed table removes barstock as it exits the grinder. Fingers in the outfeed escapement lift the ground bars from the outfeed rollers and allow the bars to roll onto the outfeed chute and then to the outfeed cradle to be unloaded from the table.
“Using the Viking centerless grinder in our precision grinding process, we have been able to produce tolerances as tight as +0.0003 inch, -0.0002 inch on diameter over a 10 to 12 foot long bar,” Mr. Biggs says. “We were hopeful that we would be able to find a machine that could handle our complete bar diameter range, from 0.1 up to 1 1/8 inch diameter. The Viking grinder gives us that capability.”
According to Mr. Biggs, the ability of the Viking grinder to compensate quickly and finitely is essential to maintaining precision tolerances on long parts through a virtually continuous process. Landis engineers attribute this to the Viking’s sub-micron control resolution,
2 1/2 inch diameter ballscrew, dual-axis slide drives, linear scale feedback and AC brushless servomotors combined with a rigid, nodular cast iron and epoxy granite base that provides 3,000,000 pounds/inch of static stiffness. Both of the grinder’s wheels use “Twin-Grip” mounting to eliminate orbiting and resist wheel-separating pressures under heavy loads. This arrangement is said to provide maximum rigidity between the spindles, a critical element for producing consistent part geometry. The Viking’s PC-based control speeds and simplifies setup and use, while the dual processors provide the ability to create or edit programs, or even use third-party software for SPC or other management reporting programs while the machine is in cycle.
After conducting test grinds on a Viking centerless machine at Landis’ headquarters, company engineers recommended tooling Böhler-Uddeholm’s new machine with an 18-inch diameter by 10-inch wide grinding wheel, up from the standard 16- by 8-inch wheel. A 14-inch diameter regulating wheel was also specified over the standard 12-inch reg-wheel. “The larger diameter wheel gives them 30 percent more square inches of abrasive to reduce the stock removal per revolution of the part and increase grinding wheel life,” says Landis’ Greg Payne, manager of centerless grinding products. “We also recommended a resinoid bond grinding wheel instead of a conventional virtified bond abrasive in order to provide some resilience in the abrasive,” says Mr. Payne. “When these long rods go through the grinder, they do have some runout in them. Occasionally, a severely out-of-round bar is loaded that would tend to beat a standard vitrified-ceramic type bond wheel to death. The resinoid bond abrasive helps resist the effects of bar whipping. Part-touching details such as the rollers on the infeed tables are hardened as well.”
Mr. Biggs acknowledges that supplying straight bars to the grinder is one of the company’s biggest challenges in attaining tight tolerances. The company now uses both an in-line drawing machine and rotary-type straighteners for processing the long bars prior to grinding. “Our objective is to determine what we can do to both guarantee straightness and to monitor it,” he says. “As we progress in our efforts to keep the bars straight, I think we will be able to hold even tighter tolerances using the Viking grinder.”