The words "catastrophic failure" do not roll easily off the tongue when you are the production manager for a major bearing manufacturer that supplies wheel bearings to high profile auto makers.
Yet, that is just what Syed Ahmed, preprocess plant manager for NTN Bearing's Hub Machining Division in Elgin, Illinois, faced when his machining operation had to be shut down to replace broken inserts, or even the toolholder itself. A series of interrupted cuts were too much for the WNMG SV315 grade inserts NTN had been using to rough and finish the faces and races of 1053 steel raw forgings for bearings. The cause of this was severe chatter problems when machining the four one-inch diameter air holes on the flange face.
Mike Otsuoka, manufacturing engineer, who wanted to machine 40 pieces per insert, would sometimes reach only 15 to 20 before failure occurred. The result was high scrap rate, additional time spent reworking the pieces, and costly, unproductive downtime.
Fortunately, NTN's Sandvik Coromant distributor suggested that the company switch to Sandvik's GC4035 inserts. "We ordered 100 pieces on a trial basis in the fall of 1993, and they worked really well," said Mr. Otsuoka. Since switching to the Sandvik inserts, NTN has seen tool life increase by approximately 25 percent. As a result, down time to replace broken inserts or tool- holders has been virtually eliminated. This has yielded a 20 percent improvement in the hourly production rate. Feed and speed rates have remained the same. Additionally, scrap rate is down an estimated 20 percent.
The raw forgings being machined are cylindrical in shape, roughly five inches in diameter and as long as three inches. Two turning operations are done to rough and finish the pieces to tolerance of 0.0039-inch. Depth of cut is 0.040- to 0.080-inch. Drilling and boring operations follow to create holes for the studs to which the auto wheel attaches. "We run a semi-unmanned machining operation, in which the insert is changed in 40 pieces," Mr. Ahmed explains.
"With the Sandvik inserts, the tool is still serviceable after machining 40 pieces. In some cases, you can't tell if the tool has been used or not." To make certain, Mr. Ahmed ordered TiN-coated versions of the Sandvik GC4035 inserts that would more readily show the wear, then tested them on the bearing job. "We've had great results," he states.
As a result, NTN is considering raising its set value of 40 pieces per insert.
NTN Bearing is more than happy with the toughness of Sandvik's GC4035 inserts, Mr. Ahmed concludes. They are now testing them on other applications involving severe interrupted cuts.