Machining the helical pelletizer rotors that are used to cut plastic into pellets was taking way too long at Great Lakes Industrial Knife Co. (www.glknife.com) in Akron, Ohio. The company manufactures high-end industrial machining knives in materials such as CPM 10V, powdered metals, stainless steels, tool steels, tungsten carbide and zirconia ceramics. It was using three-flute, indexable-insert end mills to cut 15 27-in.-long slots in the cylindrical rotors that are made from 17-4 stainless steel with an Rc-35 hardness.
Each slot required nine roughing and three finish passes, and, after machining two slots, the shop had to index or change cutter inserts. Great Lakes Industrial Knife switched to
WhisperKut solid-carbide, three-flute, AlTiN-coated cutters for roughing and solid-carbide, five-flute, TiAlN-coated Ultra-Five Series ones for finishing. The new tools are from Dura-Mill (www.duramill.com).
The WhisperKut tools rough rotor slots in three passes rather than six, and saves 10 hours of machining time per part. The Ultra-Five tools finish cut the slots and do not leave steps as the indexable-insert cutters did.
The helical pelletizer rotors are complex. Each of the 15 slots in the rotors are off the centerlines of the cylindrical rotors, and each slot is designed to hold two 13.5-in.-long carbide knives that are placed end to end and locked in place by a wedge and screws. Because of the wedge, one portion of the wall in the slot has a 12-degree angle. In addition, the slots are designed with a one-degree angle from one end to the other, requiring the cutters to take deeper passes at one end and ramp up to the other end. The angle is designed to ensure a consistent knife height along the rotor length.
Shane Mullins, a machinist at Great Lakes, rough cuts an entire rotor at 1,750 rpm feeding 10 ipm using the WhisperKut cutters. With the machine idling and the cutter running but not in the cut, the machine shows a spindlepower load of 7 percent. Once in the cut, the power load drops to 1 percent and does not exceed 4 percent during the rest of the cut. The indexable tooling that previously was used for the cut started cuts with the power load at 18 percent, and Mullins changed inserts when the power load reached 25 percent, usually after cutting two slots or making 12 passes.
Mullins typically tweaks speeds and feeds of his cutters to optimize efficiency, and he bases the speeds and feeds he uses on data supplied by Dura-Mill. "The WhisperKuts are extremely accurate, and as they enter the material you can hear them because it's an interrupted cut," he says. The tools run quiet when they are buried in the metal, he adds.
