Too good to be true: at Sandvik Coromant in the town of Schmalkalden in eastern Germany, the measurement values from contact measurements on hobs were so good that a mechanical engineer actually started looking for errors. A search brought the problem to light: the hard hob inserts were ruining each ruby stylus. These days, the company's quality inspectors use a solid-diamond stylus that still shows no signs of wear after four years.
Mario Peter, a mechanical engineer in Sandvik's technology department, was growing increasingly suspicious each time a CoroMill 177 hob was measured. "Something wasn't right," he recalls. It took some time until they discovered exactly what, because at that time the tool for milling gears was still being developed. Moreover, the CoroMill hobs are customized units, and the measurement values on the inserts are different for each special tool. Yet even on parts with known errors, the measurement results were too good, prompting the mechanical engineer to go hunting for the cause. After extensive testing, it was clear that the hard plates for the cutting tools were creating grooves in the ruby-tipped stylus. Although invisible to the naked eye, these nevertheless affected measuring accuracy.
A CoroMill 176 hob during quality inspection. Thanks to the diamond stylus, the sharp edges of the inserts do not damage the stylus.
Mario Peter got a tip from ZEISS Industrial Metrology which made erroneous measurements a thing of the past: a ZEISS expert suggested that he try out a diamond stylus. With a price tag of 1,000 euros, Peter was hesitant at first – the stylus would have to last quite a while to justify the investment. The ruby styli were swapped out every month, but only cost 39 euros. "This switch has more than paid off," says Peter. The solid-diamond stylus used on the ZEISS MMZ-T is now four-years old and still shows no sign of visible wear, even though it is exposed to the same strains as the ruby styli.
"We're definitely also going to use diamond-coated styli in the future.""
Thanks to stringent quality requirements, customers achieve the gear quality stipulated in DIN 8 and DIN 9 with hobs from Sandvik Coromant. The company's customers require quality class B which, depending on the hob, means tolerances between two micrometers for gears in car gearboxes, and more than 10 micrometers for gears in massive wind turbines. However, many measurement values don't just meet the class B requirements, but also those for classes A or even AA. "This outcome is the result of using the diamond stylus," says Peter.