October 2012 / Volume 64 / Issue 10|
Overcome a weighty issue
By CTE Staff
A twin-spindle vertical machining center always boosts productivity compared to a single-spindle VMC except when the twin-spindle machine is down for avoidable repair work. Benton Harbor, Mich.-headquartered Whirlpool Corp. experienced unintended downtime after purchasing two Chiron twin-spindle VMCs to machine zinc Classic Gearcase cover plates for KitchenAid-brand mixers and applying the same tools the company previously used in single-spindle VMCs.
The tools, which machined four different part features, consisted of a reamer for finishing a 30.048mm-dia. bearing bore and a step tool for machining the 132.2mm-dia. part face, a 98.42mm-dia. bearing bore and a 81.0015mm-dia. clearance bore. The step tool had one insert per cutting diameter on an adjustable cartridge.
However, the 17.6-lb. step tool was too heavy for the twin-spindle machines, which have a tool-weight capacity of about 15.5 lbs., noted Christopher A. Cornelius, senior project engineer, KitchenAid Div. “We experienced a high level of broken toolchanger arms,” he said. “The arm is a multiple-piece design that Chiron refers to as a ‘basket.’ ”
Images courtesy of Mapal
Although the company incurred costs replacing the broken baskets, Cornelius emphasized that machine downtime was the main cost culprit. Because the smaller diameter bearing bore had to be concentric to the other bores, there were occasional step-depth and offset, or true-position, issues with the bores, he added.
After being informed about the toolchanger-arm problem, Chiron suggested contacting toolmaker Mapal Inc., Port Huron, Mich., which has worked with the machine tool builder on numerous projects over the years, noted Mapal OEM Manager Bryant Riddle. He suggested a way to meet the tool-weight requirement while also improving the alignment of the two bearing bores (because they would be machined at the same time) and reduce cycle time. A second boring tool would be added to the step tool design in place of the reamer, and the two tools would be combined in one module-style tool.
“The module system allows for absolute alignment of the tool to the spindle,” he said, adding that the replaceable head repeats within a 0.002µm runout of the tool body. With the same tool doing all the machining, “there is no quality issue with step depth or concentricity.”
The module configuration enabled Mapal to design the step tool as two pieces that connect together and to hollow out the tool, reducing the weight to 12.5 lbs. when assembled. The new tool has two inserts per cutting diameter, which are only adjustable for diameter, instead of one insert per diameter that is adjustable for length and diameter.
“All insert pockets were milled in the proper location for lengths,” Riddle said about the new tool. “This eliminates the need for the tool setter to make any length adjustments on the tool. All step depths produced are within 0.040µm of nominal location.”
Cornelius noted the machining parameters remained the same per insert, thereby doubling the feed rate after the tool switch. In addition, having one tool perform all the machining operations instead of two tools eliminated a tool change, reducing cycle time from 93.0 seconds per four parts to 84.07 seconds—a 9.6 percent productivity improvement. (The parts are held in one fixture with four nests per table side per machine.) Part capacity increased from 1,163 pieces per shift to 1,288 pieces, or 281,250 additional parts per year without overtime. As a result of the capacity increase, Whirlpool will postpone buying a new machine tool for 1 year, when it will be needed to meet increased demand.
In addition, the module-style tool improved part quality. Cornelius pointed out that the large bearing bore diameter went from a Cpk of 1.07 to 2.05, the small bearing bore diameter went from a Cpk of 1.09 to 2.05 and the face location went from a Cpk of 1.19 to 1.99.
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