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What Makes the Cut in Heavy-Duty Turning?

By SME Media Staff

Success in turning large, heavy parts depends on a shop’s tool, machine and process choices

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This 50,000-lb (22,680 kg) backup roll is being repaired in a turning operation using ceramic inserts.

Don’t make the mistake of thinking that turning large metal parts—like 10,000-lb (4,536 kg) work rolls, for example—is no different than turning smaller parts. Not only are there significant differences in the machine and tool characteristics needed in each case, but also in the process itself. So for shops that decide to take up turning of large parts, the first step is getting up to speed on what it takes to do the job successfully.

To begin with, moving and positioning parts that are 10,000 lb and up for turning can be a formidable task, requiring cables and cranes capable of moving such heavy objects. More important, however, is having people who know how to do the job right. When moving large parts, “momentum and inertia can actually pull a crane off the rails, so you must have trained people handling these parts,” said Denny Carpenter, a sales and service engineer at Greenleaf Corp., Saegertown, Pa., which manufactures inserts and toolholding systems.

Before turning work begins, shops must also decide whether a machine they are using to turn small and medium-size parts can cut it in large part turning. “Even if [a larger] part fits into the envelope of their machine, it doesn’t mean it’s going to be the appropriate machine for what they want to do,” noted Carpenter. “The machine has to be rigid enough for the kind of parts they’ll be working on, and it has to have appropriate torque and power.” If a machine doesn’t meet these requirements, Carpenter added, it won’t be able to make a cut that utilizes the full potential of the insert and toolholder.

To determine whether or not a machine is up to turning big parts, check the horsepower first, advises Ron Crane, national turning and threading product specialist at Iscar Metals Inc., Arlington, Texas, a maker of carbide manufacturing tools. According to Crane, the minimum requirement for heavy-duty turning is probably a 50 hp (37 kW) machine. “Anything less than that and there is a good chance that you’ll stall the machine or not be able to use the tool to its full capability,” he said.

Special Cooling Considerations

Besides choosing the right machine, shops embarking on large-part turning must be prepared for major differences in the machining process. For example, heavy-duty turning often requires longer cut times, said John Winter, eastern U.S. product manager for cutting tool manufacturer Sandvik Coromant, Fair Lawn, N.J. In addition, there are special cooling considerations for these applications. For turning new railway wheels, for instance, Sandvik Coromant offers toolholders with high-pressure nozzles both above and below the insert that direct coolant into the cutting zone.

In the vast majority of cases, however, large-part turning is done dry, according to Carpenter. One reason for this is that applying coolant in a conventional manner can be a messy business in these applications, which employ big, open machines for the most part.

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Suitable for railroad wheel turning, this large rectangle insert includes a heavy-duty chip breaker.

“If you’re spraying coolant on a spinning 5' (1.5 m) diameter body, it’s going to throw coolant everywhere if [the process] is not enclosed, and you’ve got a major health hazard,” Carpenter said.

Dry turning of large parts translates into “a lot of heat buildup” compared to smaller part turning, noted Sarang Garud, a product manager at cutting tool manufacturer Walter USA LLC, Waukesha, Wis. Garud added, however, that this doesn’t matter because the inserts used for heavy-duty turning are much thicker and made of tougher grades than standard inserts.

In addition to a tough cutting substrate, dry turning of large parts calls for coatings that can take the heat for the long cut times. According to Travis Coomer, insert product manager for cutting tool maker GWS Tool Group, Tavares, Fla., much has been done to upgrade coating technology so that it can handle higher cutting temperatures, including the addition of aluminum—and Al10 in particular—to coatings.

To keep heavy-duty turning dry but still provide cooling, Greenleaf recommends that shops replace conventional coolant with chilled air directed onto the inserts. In addition to cooling the inserts, the air will blow chips away in the desired direction, Carpenter explained. What’s more, he added, using chilled air in these cases eliminates the disposal costs and worries associated with conventional coolant use.

Tool and Holder Requirements

In general, Winter said, heavy-duty turning requires inserts with strong carbide substrates and thick coatings that can handle plenty of tool pressure. Even though they are larger than tools used to turn smaller parts, the tools needed for large-part turning are available off the shelf in most cases.

“At Walter, we have some newer technologies coming up that are special for heavy-duty and large-part applications, but for the most part standard ISO inserts will work,” Garud said.

For higher metal removal rates during heavy-duty turning, shops should look for big, robust inserts capable of producing greater depths of cut, according to Aaron-Michael Eller, product manager for ISO turning and advanced materials at toolmaker Seco Tools LLC, Troy, Mich. While the industry-standard insert size is ½" (12.7 mm) IC, Eller said, those planning to take up large-part turning need inserts from 5/8-1" IC (16-25.4 mm), along with a large roughing chipbreaker that will allow them to move inserts across the workpiece as quickly as possible.

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The Heliturn insert features a helical cutting edge.

One example of a large insert suitable for heavy-duty turning is the 22 mm-high version of Iscar’s Heliturn. Crane described this insert as “a big piece of carbide” capable of removing over an inch of material per pass.

Instead of a flat geometry, Heliturn inserts feature a helical cutting edge modeled after a solid-carbide end mill. According to Crane, Iscar estimates that the helical design reduces the horsepower draw roughly 15-20 percent during machining operations.

This, he said, allows shops to get a bit more out of their machines. “Instead of using up all of [a machine’s] 50 hp (37 kW), now they might only be using 35-40 hp (26-30 kW),” he said. Employing the saved horsepower in a different manner “could allow them to achieve a greater depth of cut to be more productive.”

In addition to producing greater cut depths, tools for heavy turning must be able to withstand long cut times. In these applications, “you can have from 45 to 125 minutes of continuous cut time because the components are so large,” Eller said. Therefore, he recommends a cutting material with a chemical vapor deposition (CVD) coating for high performance.

Another important consideration for heavy-duty turning is the cutting tool’s edge preparation. Insert edges must always be prepared for the particular cutting situation they will encounter. “If there are holes, scale or rust, we would use a different edge preparation than we would for a finished, clean cut,” Carpenter said. In all cases, he explained, the idea is to direct the force generated by the cutting action away from the edge of the insert and into its base, which is the strongest part. “Edge preps” are particularly important for heavy-duty turning because it subjects the cutting material to higher forces, he said.

Besides their cutting tools, shops that turn large parts must give careful consideration to the toolholders they use. In many cases, chips produced by heavy-duty turning are so large and hot that they will erode a standard top clamp, Garud noted. He recommends that shops have special top clamps made for these processes. Another option is to use a chipbreaker when turning large parts, he added.

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Turning of used railway wheels is done in sets.

To deal effectively with the large loads placed on inserts during heavy-duty turning, “it’s all about rigidity,” Coomer said. “So you need a positive locking mechanism in the tool to keep everything rigid.” To boost toolholding rigidity, he noted that many inserts come with extra dimples on the bottom to improve integration with the toolholder.

Another option designed to hold inserts firmly despite large machining loads is Iscar’s Dove IQ-Turn. This system features a V-shaped rib molded onto the outside of the insert, as well a pocket of the same shape milled into the toolholder. When the rib enters the corresponding pocket, the result is firm and rigid clamping that prevents the insert from being moved by cutting forces, Crane said.

Turning Tech Moves Forward

The most recent developments in cutting technologies for heavy-duty turning include advancements in insert design, tool materials and coatings. One design advance addresses the troublesome amount of vibration that can be caused by rough turning of large, heavy parts. This vibration is reduced by what Garud calls the “soft” cutting edge of inserts featuring Walter’s HU5 geometry.

In this case, Garud explained, a soft cutting edge is not the opposite of hard. Rather, it’s an edge that produces soft cutting action that reduces cutting forces, as well as horsepower requirements and work hardening. “A soft cutting edge that still has the ability to do heavy-duty machining is a unique technological feature,” he said.

The soft cutting action made possible by HU5 geometry is the result of a curved cutting edge and deep chip groove that produce low cutting forces even at high feed rates, according to Walter. The geometry is a good choice for roughing stainless steel, titanium and high-temperature alloys, Garud noted.

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Thanks to a curved cutting edge and deep chip groove, HU5 insert geometry produces low cutting forces even at high feed rates.

As for materials suitable for heavy-duty turning, Greenleaf’s newest ceramic insert grade, known as Xsytin, is designed for this and other tough machining jobs. Carpenter describes Xsytin as a “phase-toughened” material created by aligning molecules in such a way as to make the material extra strong. “We found a way to create the strongest ceramic cutting tool edge that has ever been made,” he said.

Greenleaf claims that Xsytin is engineered to machine more materials than any other ceramic grade. Designed to turn difficult-to-machine materials at extremely high feed rates, Xsytin is recommended by Greenleaf for roughing, interrupted cuts and removal of scale in processes involving a variety of materials, including irons, heat-resistant superalloys, steel alloys and stainless steels.

Many of the latest technological developments impacting tools for heavy-duty turning involve coatings. One such development is high-power impulse magnetron sputtering (HIPIMS), a method used for physical vapor deposition (PVD) of thin films based on magnetron sputter deposition. Among other things, HIPIMS coating are supposed to lengthen tool life. Coomer reports that his company has gotten good results when putting HIPIMS coatings on inserts used to turn railroad wheels.

At Seco, meanwhile, the introduction of Duratomic insert coating technology is the most significant development affecting the turning of large parts, according to Eller. The Duratomic CVD aluminum-oxide coating process manipulates coating components at an atomic level. By controlling the atomic structure of the coatings, Seco claims it can ensure that the best part of the insert is engaged in the cut. The benefits cited by the company include improved insert wear resistance and edge toughness.

In particular, Eller touts the TP0501 Duratomic insert grade as a good choice for turning large steel parts because of its stability during long cut times. In addition, the heat resistance of TP0501 allows the highest possible metal removal rates without the need for coolant, according to Seco Tools.

Lining Up the Crystals

Insert coating technology has also advanced at Sandvik Coromant. In conventional CVD alumina coatings, the company notes, the direction of crystal growth is random. When developing its Inveio product, however, the firm reports that its experts found a way to control growth so that all the crystals line up in the same direction, with the strongest part toward the top surface of the coating layer.

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In this turning operation, a CVD-coated carbide insert cuts a marine shaft made of a proprietary high-strength steel.

In addition to strengthening the coating and improving wear resistance, Sandvik Coromant claims Inveio’s crystal alignment leads heat away from the cutting zone, which helps keep the insert’s edge in shape for longer times in cut.

These examples show that, though heavy-duty turning poses special problems, major tool suppliers have plenty to offer shops trying to meet the challenges.

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