End Mills Tackle Difficult Part While Reducing Machining Time

This machining business was faced with a part that tested its patience and tooling capabilities. A helical pelletizer rotor, which is used to cut plastic into small pellets, became especially problematic. Find out how this new tooling technology overcame these difficulties.

Case Study From: 6/21/2005 Modern Machine Shop

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Helical pelletizer rotor

The helical pelletizer rotor, which is used to cut plastic into small pellets, is being machined.

Great Lakes Industrial Knife Co. (Akron, Ohio) was founded in 1981. The company primarily manufactures high-end industrial machining knives in materials such as CPM10V, powdered metals, stainless steels, tool steels, tungsten carbide and zirconia ceramics. Chemical, pulp, paper, food processing, woodworking, packaging, rubber and metals are among the industries that the company serves. To produce the "wear-resistant" machine knives for which the company is known, it employs 35 people in a 35,000- square-foot facility, using an array of machine tools and specialty grinders.

"The knives that we produce can range in size from very small to as large as 185 inches by 15 inches by 6 inches," says Ron Kara, president and owner. "Typically though, if the knife is long, then it is also narrow and thin. If it is short, then it is usually thick and wide."

Shane Mullins, a machinist at the company, usually enjoys addressing machining challenges. However, he was recently faced with a part that tested his patience and tooling capabilities. A helical pelletizer rotor, which is used to cut plastic into small pellets, became especially problematic.

The cylindrical part is made from 17-4 stainless steel hardened to 35 Rc. It is approximately 42 inches by 6 inches in diameter. Solid carbide knives are inserted into 15 slots, which are 27 inches in length. These slots are then cut using a 1/2-inch diameter indexable-insert end mill with three flutes. Cutting parameters are 3,000 rpm, 9 ipm and 0.1-inch depth of cut (the C6 grade inserts have a TiAlN coating). About nine passes per 27-inch slot are required for roughing, with three passes for finishing. However, after machining two pockets, the inserts had to be indexed or changed out.

"The machining work on this part seemed to be a lengthy process," remarks Mr. Mullins.

Each slot holds two 13.5-inch carbide shears, which are placed end-to-end and fixed into position using a wedge and screws. (Each shear is comprised of several pieces placed next to each other in an unbroken series. They run from one side of the part to the other with no gaps in the blades.) Because of the wedge, one part of the slot's wall required a 12-degree angle, and all of the slots are off the center line of the cylinder.

The slot has a 1-degree angle from one end of the cylinder to the other, which, according to the company, necessitates a deeper slotting pass at one end. The floor has a ramp, so as the tool cuts, the cutter must gradually lift up out of the slot to give the bottom of the slot an uphill slope. This helps maintain a consistent knife height along the cylinder's length.

Mr. Mullins came upon an article in a trade journal about WhisperKut end mills from Dura-Mill, Inc. (Malta, New York). He noticed that the user was cutting the same material as was the case with the helical pelletizer. The company offered a free trial, so Mr. Mullins requested a 1/2-inch diameter, three-flute, AlTiN-coated WhisperKut.

"Using the new end mill, three passes per slot (compared to six) were required, and the total machining time saved was 10 hours per part," says Mr. Mullins. "This has given me the capability to put the part in, touch it off to find the part surfaces, press the "go" button on the CNC controller and let it run. I can now complete the part with one end mill."

After experiencing noticeable time savings, Mr. Mullins decided to order a 1/2-inch diameter, solid-carbide, five-flute, TiAlN-coated Ultra-Five series Dura-Mill cutter to complete the finishing work. The indexable cutters were producing steps in the finishing cut that were unacceptable. The finishing cutter solved this problem. According to the manufacturer, a WhisperKut can usually complete the roughing and finishing work. For this cylinder, however, the finishing cut on the slot required a specific corner radius that the roughing product didn't offer.

Using the roughing end mill, Mr. Mullins operated the part at 1,750 rpm at 10 ipm. He completed 45 passes at 27 inches in length at a 1/4-inch depth of cut by 1/2 inches wide (full cutter width). Mr. Mullins then performed 30 passes at 27 inches in length by 3/8-inch in depth by 0.2-inch in width.

Another unusual aspect of the cutting was the spindle horsepower used by the WhisperKut.

"When I started out, the machine was sitting idle with just the spindle running, without the tool being in the cut," explains Mr. Mullins. "The spindle showed 7 percent on the power load meter. As the tool approached the material and began cutting, the load meter went down to 1 percent. It didn't go above 4 percent during the entire cut. The indexable tool started out running at 18 percent, and I would change inserts when it got to about 25 percent, which made it through about two slots or 12 passes."

Mr. Mullins says he usually tweaks the speeds and feeds of each of his cutters to optimize efficiency. However, he reports that the speeds and feeds work so well with the end mill from Dura-Mill that he doesn't need to alter them.

"Judging from the wear on the tool, speeds and feeds could definitely be increased," he adds. "The end mill is quite accurate."

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