Overcoming the Interrupted-Cut Blues

Oct. 13, 2009
Seco TM4000 Duratomic turning grade insert for stainless steel with MF4 chipbreaker. Interrupted cuts can be tough on tools. The pounding tools take during interrupted cutting frequently leads to severe ...
Seco TM4000 Duratomic turning grade insert for stainless steel with MF4 chipbreaker.

Interrupted cuts can be tough on tools. The pounding tools take during interrupted cutting frequently leads to severe edge wear that cuts short tool life. However, by carefully analyzing the tooling wear pattern and making the appropriate grade and geometry changes, the ravages of interrupted cutting can be overcome.

For example, Main Manufacturing in Grand Blanc, Mich. – a family-owned business that produces hydraulic flanges and components – was experiencing short tool life and reduced productivity due to interrrupted cuts when machining high-pressure flanges from 304 stainless steel. “The severe interrupted cut was definitely affecting tool wear,” said Main Manufacturing’s plant supervisor Terry Bettinger. “We were only able to achieve about 1.5 parts per insert due to edge chippage.”

The flange starts out as a 2.25-in. by 1.5-in. rectangular plate, which is rough-faced down to a hub in the center of the part. The company was taking three passes at 0.100 in. each and a final pass at a depth of cut of 0.050 in. The job was running on a Doosan 240 CNC lathe at 300 sfm and a feedrate of 0.01 ipr.

Main Manufacturing had been a long time user of tools from Seco Tools. Coincidentally, Seco had recently launched its new TM4000 Duratomic turning grades for stainless steel and was in search of a demanding interrupted-cut application. Seco’s sales and application specialists Joel Henige and Todd Miller asked if they could give the new grade a try. Seco Duratomic coatings rely on an alteration in the crystal structure at the atomic level, and have consistently provided both improved toughness and wear-resistance. In lab tests, the TM4000 construction had shown to improve resistance to crater wear and edge breakdown substantially.

“We initially decided to run the TM4000 using the same parameters as a competitor’s tool,” said Seco’s Miller. “But, to everyone’s surprise, TM4000 performed no better than the competitive insert.”

“We knew that TM4000 was capable of high machining parameters,” said Henige. “So we upped the cutting speed to 650 sfm using the new MF4 chipbreaker, which is designed to provide consistent chip formation and breakage in stainless steel turning and other ISO materials.”

The main advantage of MF4 is its positive cutting angle, which significantly reduces cutting forces to allow for higher cutting speeds and increased productivity. With the MF4 and the enhanced cutting data, the team was able to triple tool life and complete three parts.

“However, this was far from the result that we had anticipated,” Miller said. “We examined the inserts under our magnifying loupe, but couldn’t see any particular failure mode.” Then Miller and Henige decided to take a good hard look under the microscope, which provides 60 times the optical power of the loupe. This closer inspection revealed that there was an edge-chipping problem. The chip flow was eroding the back of the land on the MF4. By the wear pattern, they also noted that a larger nose radius would help increase edge strength. As a result of this analysis, they recommended that Main Manufacturing increase the feedrate to 0.016 ipr using a 0.0468-in. radius. Because they didn’t see any other heat-related problems, they left the cutting speed at 650 sfm.

“We were pleased,” said Main Manufacturing’s machinist Craig Bendle. “Productivity doubled relative to our original setup, and tool life jumped to seven parts per corner, which was seven times better than we were getting.”

However, Henige and Miller weren’t done with their analysis. Another pass under the microscope revealed that the failure was now being caused by thermal cracking. “Large differences in temperature between the cutting edge and the insert can cause cracks perpendicular to the cutting edge,” explained Henige. “Interrupted cutting applications such as this one tend to generate high heat when in the cut, causing these temperature fluctuations.”

Since the presence of coolant, which was water in this case, can increase the thermal cracking problem, the team decided to run the application dry to see what would happen. This change doubled tool life yet again to achieve 15 parts per corner, and now the failure mode was simply due to normal abrasive flank wear.

“We went from removing 3.6 to 12.483 cu-in./minute,” Bettinger said. “With our in-cut time reduced by one half, our part output increased from 17 pieces per hour to 54 pieces per hour. The improvements were beyond our expectations.”

The result was that Main Manufacturing saved 85 percent in cost as well as reducing machining time by 52 percent. “But even more exciting,” said Main Manufacturing general manager Bob Mackey, “is that we can take what we’ve learned from this experience and apply TM4000 to hundreds, maybe even thousands, of other similar jobs that we run.”

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