Sawing In The 60s

April 1, 2004
Who says standard bandsaw blades can t cut 60-62 Rc hardened steel?

Who says standard bandsaw blades can t cut 60-62 Rc hardened steel?

Starrett s lab technicians cut a 1 1 2-in.-thick test piece from this 9-in.-diameter hardened mill roll in just 25 min, down from 67 min.

A Starrett triple-chip grind, one-pitch, carbide-tipped blade cuts into a 60-62 Rc hardened mill roll.

Ray Jack (left) and Bob Sisson, both of Starrett, stand next to the Behringer HBP 420A bandsaw that accommodated a carbide-tipped blade for sawing a customer s super-hardened mill roll.

Cutting steel hardened to Rockwell numbers in the 60s is typically a job for a diamond-grit-edge blade, not a conventional bandsaw blade. However, one sawblade company successfully cut a customer's 9-in.-diameter part hardened to 60-62 Rc using a standard carbide-tipped blade.

The one-pitch blade, from L. S. Starrett, sports a triple-chip grind, which means it has two different tooth grinds positioned alternately along the blade. One tooth, called the chisel, has a triangular surface that cuts both sides of the kerf and produces two chips. The other is the triple-chip tooth that forms a third chip. This tooth is thinner and several thousandths of an inch longer and cuts the bottom of the kerf.

"Triple-chip grind is done only for our carbide line," says Gene Ramsdell, a Starrett metallurgist. "Experience has shown us that the triple-chip performs better than a set-tooth blade in overall cutting, particularly in solid sections."

The blade successfully cut a super-hardened 9-in.-diameter, 7-ft-long mill roll belonging to a company that operates a four-high rolling mill. Torsional shear had caused the roll to fail, and to determine the shear's origin, the company needed sample sections of the roll.

To get this done, it engaged the services of Sawtech in Lawrence, Mass., a shop specializing in metal cutting. While no stranger to cutting difficult metals, Sawtech took on the job, but still had doubts.

"We knew we could use a carbide-tipped blade," states Carmen Grasso, vice president and co-owner of Sawtech. "But we had no idea how long the blade would last or how long the cut would take." Grasso, in turn, contacted Starrett, which agreed to test cut the roll as an R & D project.

Starrett lab technicians made test cuts using a 16-in.-diameter-capacity Behringer HBP 420A bandsaw at band velocities ranging from 65 to 100 sfm. Two of the cuts took 67 min each to produce 1 1 /2-in.-thick samples from the full 9-in. diameter.

The company analyzed and balanced several factors and dialed in the optimum feedrate and band velocity. The result was a cut made in only 25 min. "Most would think a blade might be shot after a workout like this," said Ray Jack, a sales coordinator at Starrett, "but the blade was in great shape, even after all these test cuts."

At first, Starrett technicians considered a grit-edge blade. "But these generally run at higher band speeds than toothed blades. And, due to workpiece dimensions and size limitations for bandsaws that accept grit-edge blades, Starrett decided the most effective method was a blade with teeth," explains Jack.

"Normally, we use 55-57 Rc as the changeover point from carbide blades to diamond grit," says Ramsdell. "However for diamond blades, the saw must have a precise downfeed, and diamond-grit bands wear quickly because they measure only 0.020-in. thick. Also, we always consider economics for our customers, and diamond is a last option due to its cost."

Starrett technicians also considered the hardness of the roll's surface. At first they were told its case-hardened chrome molly surface was 64 Rc, but testing revealed a 60-62 reading. In addition, its core hardness was 50 Rc,

but technicians safely pushed the carbide-tipped blade past accepted limits by properly balancing feedrates, band speed, and coolant flow.

But Jack cautions that the results of the Sawtech project might not work in similar situations. Shops should study all the variables before attempting this type of cut.

"We made five cuts on this high-hardness material without damaging the blade. If we were in production, results could vary," he says.

Besides providing Sawtech's customer with the samples it needed, sawing these rolls gave Starrett solid knowledge on a difficult application. "This was a job like no other," comments Ramsdell. "Even though our technical department receives exceptional requests on a daily basis, the results of one usually help with others."

According to Ramsdell, data gathered from a past experience sawing rods and shafting actually contributed to the success of the roll job. The parts were Thompson case-hardened or hard --chrome-plated with nitrided or carbon nitrided 1 /8-in. thick. For these parts, Star-rett used either grit-edge or carbide-tipped blades and a solid saw with an accurate feed system to avoid forcing brittle carbide tips into the hard part surfaces.

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