Seco-Carboloy CBN inserts increased Przisions-Brems-Scheiben's brakedisccutting speeds and extended tool life from 200 to 3,000 discs per cutting edge.
In its brakediscmachining operations, Prazisions-Brems-Scheiben discovered that CBN inserts must be kept clean and perform best in cutting nonalloy castings at least 12 days old.
While its ceramic inserts cut at nearly 4,000 sfm, Prazisions-Brems-Scheiben (PBS) GmbH of Lemgo, Germany, could not ignore the possibility of upping that speed to 6,550 sfm using CBN tooling. However, enthusiasm quickly faded for the manufacturer of cast-iron brake discs.
In initial tests conducted by cutting tool maker Seco-Carboloy, Detroit, CBN inserts increased brake-disc cutting speeds to 5,250 sfm, and tool life soared to 4,000 discs per cutting edge as compared with ceramic tooling that averaged 200 discs per edge. But these improvements were not consistent. Often, the CBN cutting edges would last for only 25 discs. Hans-Georg Nondorf, a Seco-Carboloy field engineer, determined the cause was timing.
Manufacturers often store cast iron castings for up to a year to improve machinability, but little is known about the actual aging process. For instance, at what stage in the process does optimum machinability with CBN inserts begin? In PBS's case, it was twelve days.
CBN tools lasted for 5 to 10 parts in test cutting one-day-old castings at PBS. However, tool life jumped to 3,000 parts when machining the same castings twelve days later. In addition, alloyed castings reduced tool life to an absolute minimum, even after a long aging process, so PBS confined its use of CBN to nonalloyed castings, which accounts for over a third of the company's production.
PBS manufactures brake discs from GG15, GG20 and GG25 cast iron in batches ranging from 500 to 8,000. The shop runs three shifts, producing about 1,500 discs per shift, and its 1.1-milliondisc annual output encompasses as many as 220 different disc types.
According to Hans Konrad, technical manager at PBS, absolute cleanliness is a necessity when working with CBN. Machine operators must thoroughly clean insert surfaces prior to reversing them to ensure proper seating in toolholders. Edge buildup, which results from extremely long durations of use, must be ground flat, or inserts could break after only 50 parts.
"The expense is justified for production runs of 1,500 parts or more," says Konrad, "which applies to about 30% of PBS total production. But the results within this scope are more than satisfactory." Each shift is now producing up to 20% more at 20% faster operating times.
Since CBN tool life is considerably longer than with ceramics, operators don't have to change inserts during their shifts. This time savings could further boost PBS productivity 25%, which alone would compensate for the additional costs incurred using CBN. www.carboloy.com