Latest Core Inserts Meet Near-Net-Shape Turning Needs

Cutting-tool makers now know more than ever about making general inserts for specific metalcutting applications. Here is some of what they've learned.

Article From: 4/1/1997 Modern Machine Shop

In the past decade, the cutting-tool industry has widely reengineered its standard offerings of turning insert carbide grades and chipgroove geometries for applications in steels, stainless steels, and cast iron. Abetted by new capabilities for computer-aided grade and groove design, this effort has largely succeeded in replacing an ever-expanding array of more specialized grades and grooves with rationalized core selections that provide good performance in the great majority of applications in the most commonly used manufacturing materials.

The development has yielded important benefits for tool users. In tandem with now widespread color-coded insert selection charts, the core grade/groove offerings have removed much of the guesswork and uncertainty from insert selection, making the process fast and easy for manufacturers with increasingly less time and expertise to devote to the task. Similarly, the compressed offerings allow quicker selection of a "first choice" insert for the job at hand — of no small value in an era of shorter lot runs and more frequent tooling changeovers, where profitability depends critically on minimizing downtime between jobs. The core offerings also help reduce insert inventory requirements, contributing to increasingly vital cost control.

Today, major cutting-tool manufacturers are continuing to refine their core grade/groove offerings. Besides further simplifying insert selection, these efforts are resulting in improved and more cost-effective tool performance in meeting machining requirements vital to today's manufacturing. Two of the most important manufacturing requirements are capabilities for near-net-shape machining and for higher speeds and feeds. According to Bob Goulding, manager for stationary products at Carboloy Inc. (Detroit, Michigan), these are among the principal drivers in grade/groove developments for turning, especially from the automotive side.

Near-Net-Shape Machining

One of the benefits of the new generation of grades and grooves is their improved suitability for machining near-net-shape forgings and castings, now used increasingly in a variety of industries. Two prominent examples are cold-formed automotive components, made from high strength, low carbon ductile steels, and stainless steel castings used in aerospace.

In addition, tightly controlled powder metal (PM) preforms, which sometimes have only 0.010 inch of stock to be removed, are now widely used in the electric power tool, lawn equipment, and appliance industries. PM preforms are also making rapid inroads in automotive—where some components may require machining of only one or two dimensions.

On a preformed automotive gear, for example, only a face and a bore may need to be machined to ensure that the part runs true. For many near-net-shape parts, typical processing includes cleaning up the part with a single finishing or semifinish cut, heat-treating, and finishing by hard turning or grinding.

Challenges

Although near-net-shape parts have less stock to be removed and, often from fewer surfaces, turning operations performed on them present a special challenge for effective chip control and good tool life. One challenge is posed by the light depth of cut. It usually produces a thinner chip that is easily bent, but lacks the rigidity for easy breaking.

Because much near-net-shape machining is performed at both higher feed rates and higher cutting speeds, control of higher heat generation is a challenge. The chip is produced in an area of the chipgroove—on the nose radius, or just past it—where the placement of a hard obstruction to curl the chip for breaking will result in the transfer of excessive heat and pressure to the insert, causing cratering.

Finally, cutting forces can be quite high in applications requiring a semi-roughing pass at relatively high feed rates. Typical finishing inserts will not do the job in these cases, because they don't supply enough edge strength to handle either the high feed rates, or the scale or other surface irregularities often left on the part by the casting or forming process.

Designing The Chipgroove

To meet these challenges, chipgroove geometries for near-net-shape machining must be designed to break chips at depths of cut that are normal for a finishing tool, but also provide the edge strength needed to handle higher feed rates and withstand part surface irregularities.

These performance characteristics are in fact identical to those designed into many reengineered chipbreakers as part of the effort to extend their versatility and create a core chipgroove offering. For example, grooves for fine and light- to medium-duty cuts in steel can be designed with varying land widths and angles that provide the required edge strength for effective chipbreaking over an application range that now includes smaller depths of cut and higher feeds. Added edge strength at higher feeds helps control depth-of-cut notching in both near-net-shape and stainless steel applications.

At the same time, the groove designs provide better tool life through their application range. One contributing factor is a positive entrance angle, which promotes freer cutting and reduces the chip temperature. Chip control geometry is another factor. For example, making use of a strategically placed "bump" on the insert nose, rather than a hard obstruction, may be used on the insert to curl the chip for breaking. The pressed-in bump helps limit chip contact with the insert to only two points—the entrance angle and the bump itself (the exit angle). In consequence, the transfer of heat and cutting forces into the insert is reduced, improving tool life. The two contact points are also widely spaced, so any diffusion craters caused by a chemical reaction of the chips with the carbide will not grow together.

Strengthening Exercise

To enable more versatile chipgrooves to handle both the higher feed rates and surface scale and irregularities common to near-net-shape machining, the cutting edge must be made stronger farther out from the nose—where shallow-cut chipbreaking takes place. Computer-aided design (CAD) makes it possible to design varying land widths and angles to create both more positive, and stronger negative, geometries at the points required.

This can be seen, for example, in Carboloy's M3 chipgroove geometry for light to medium cuts in steel and for near-net-shape machining in most materials. Here the land, starting close to the nose, is tilted a negative 2 degrees for a length of 0.004 inch, then a positive 5 degrees for a length of 0.008 inch. A cross-section cut of the nose would show the land tilting a positive 5 degrees into a sharper entrance angle with a hone on the end. Away from the nose, the tilt of the land becomes negative, then progressively more negative as the distance increases.

"With the trend to near-net-shape machining, the major challenge for our industry is no longer effective chipbreaking at inch depths of cut," says Mr. Goulding. "The market is forcing us now to tailor new tooling for smaller depths of cut."

Even with a diminishing range in depths of cut, discretion must still be used in choosing chipgroove geometry. The key selection factor is productivity rate. This is based on the feed rate, which, together with surface speed, has the greatest influence on spindle power consumption and insert edge wear.

"If productivity is your main concern, you'd want to use the general-purpose groove recommended for higher feeds of 0.020 to 0.030 ipr [inch per revolution]," says Mr. Goulding. "However, if surface finish is more important, the best choice is a light- to medium-duty groove in a range from 0.012 to 0.018 ipr, which has a highly positive entry angle and a very sharp edge."

Selecting The Right Core Grade

While one of two core insert geometries will provide desired results in most near-net-shape machining applications, the appropriate carbide grade will depend largely on the particular material machined. In the case of cold-formed automotive components made from high strength, low carbon ductile steels, Mr. Goulding recommends a highly wear-resistant ISO P10-type grade. Carboloy's is called TP100. "If it's a typical medium feed/high speed application, he continues, "I'd use a hard grade with aluminum oxide that gives effective resistance to cratering, which is the primary mode of insert failure in high speed machining of carbon steels."

If near-net-shape machining of the ductile steel components must be performed at slower speeds (1,000 sfm or less) and a fairly heavy feed rate (above 0.015 ipr), an ISO P20 cutting grade is most suitable, according to Mr. Goulding. "Here, you'd want a tough substrate with a hard coating," he says. "It will provide good wear resistance at slower speeds, along with resistance to cratering and edge notching."

Cutting Near-Net-Shape Stainless Steels

Today's core turning grades and grooves can also efficiently machine stainless steel, near-net-shape castings, prominently used in the aerospace industry. The typical alloy composition of these aerospace parts makes them moderately difficult to machine. Stainless materials are finding application in a wide range of industries and run the gamut from fairly easy-to-machine alloy steels to extremely difficult-to-machine high temperature alloys.

Mr. Goulding points out that aerospace castings, like stainless materials in general, are inherently gummy and sticky, which means that chip adhesion and edge buildup are always potential machining problems. "But," he says, "there are now core grades and grooves that handle these problems, even with parts that are moderately difficult to machine."

An ISO P30 grade is best suited for machining the near-net-shape aerospace stainless castings, Goulding believes. "Our own core P30 grade has a smooth titanium-nitride top coating to resist adhesion and built-up edge," he says, "and the cobalt-enriched substrate creates a tough cutting edge that can withstand the very high pressures needed to cut stainless at lower surface speeds."

The right chipgroove for the application, according to Mr. Goulding, would be modeled after his company's light- to medium-duty M3 groove. Several features of this geometry make it suitable for near-net-shape applications in general and also provide for improved performance in machining stainless steels.

For example, the reduction in chip contact points helps reduce chip adhesion, and the positive entrance angle decreases material pressure welding, or buildup, in the groove, preventing possible "pickout" of the coating and carbide materials. In addition, the positive entrance angle helps combat hardening of work surfaces, which is a particular problem with stainless steels if more than one pass must be taken. A result of such hardening is notching at the depth of cut—though this is minimized in the case of this chipgroove because of the stronger edge away from the nose.

Performance characteristics provided by available core turning grades and grooves offer reliable solutions for stainless steel machining in the easy to moderately difficult range. This is not yet the case, however, with the extremely difficult-to-machine stainless materials. "With the high nickel/chrome stainlesses, you typically have to machine much slower," Goulding notes. "You need an even stronger edge, to resist the extreme pressure needed to cut the material. If you have too sharp an edge, the insert will break."

To meet these needs, Mr. Goulding suggests a very strong ISO P40 cutting grade, which has a high-edge-strength chipgroove, and very tough single-sided geometries.

Cutting Near-Net-Shape PM Preforms

Near-net-shape machining of powder metal preforms presents a special problem because of composition. "You can measure the hardness of a pressed component at a spot on its surface and get a reading of 30 Rc," says Mr. Goulding. "But if you measure the hardness of individual particles, the reading might be 55 Rc."

In general, Mr. Goulding recommends use of cermet inserts for machining PM materials with a hardness reading below 35 Rc, and PCBN (polycrystalline cubic boron nitride) inserts for harder PMs. "With the cermets," says Mr. Goulding, "you'll also want to specify a core groove designed for fine machining, one that has a positive entry angle to help reduce cutting forces."

Another cutting grade, recently released by Carboloy, has shown success in machining powder metal preforms. This grade has both a highly wear-resistant coating and a deformation-resistant substrate. It was especially developed for machining high strength ductile iron, increasingly used for automotive utility components. It has also proved highly effective in machining high strength alloy steels and gray cast irons, at slower speeds. "For good results with powder metals," Mr. Goulding notes, "you need to combine this new grade—designated TX150—with a general purpose chipbreaker that gives you both a very strong edge and a positive entry angle for reduced cutting forces."

The new grade's broad applicability derives from two essential components of its design. One is a hard, deformation-resistant substrate, ideally suited for machining cast irons. The other is a multiple coating that contains a thick layer of highly wear-resistant titanium carbonitride. The coating layers are applied by medium-temperature chemical vapor deposition (MTCVD). The result, according to Mr. Goulding, is a free-cutting edge that preserves full CVD coating hardness with increased toughness. "Those are the characteristics you need for machining hard ferrous castings or machining gray cast irons at relatively slow surface speeds," he continues.

The right insert geometry for machining either ductile irons or gray cast irons, Mr. Goulding says, is in most cases a flattop. However, for some applications, depending on the operating speed, a suitable alternative might be a general-purpose chipgroove with its strong edge.

Faster Cutting Speeds And Feeds

Another major driver of turning-tool grade/groove developments is the continuing demand for higher machining speeds and feeds. "To a large extent," says Mr. Goulding, "the same grade and groove characteristics that lend themselves to better near-net-shape machining also allow higher cutting speeds and feeds."

On the carbide side, Goulding notes, several core grades offer the needed performance characteristics for higher speeds and feeds. These grades are manufactured with improved bonding control and with a low temperature coating technique that helps maintain full hardness and improved toughness in the insert cutting edge.

On the chipgroove side, the reengineered fine and light- to medium-duty core grooves are now capable of higher feed rates than previous chipgrooves used in the same application range. A typical roughing chipgroove should handle the more demanding operations at still higher feed rates, as well as interrupted cuts.

Mr. Goulding notes that the core grooves have been tailored for good performance at the speeds and feeds used in today's manufacturing, and that "there are more lathes in use with less than 20 horsepower than there are with more." He adds that the trend today is also toward smaller machines, pointing out that horsepower capabilities are becoming less significant as the increase in near-net-shape parts drives down depth-of-cut requirements.

"What today's CNC lathes do have," Mr. Goulding says, "is higher spindle speeds, better controls, and better drives that can handle the complex geometries or forms on a part at higher feed rates. Even a moderate horsepower machine can give you high speeds and feeds. What you need, in terms of tooling, is a chipgroove that can accommodate the high feeds and still break the chip at a light depth of cut. You also need a carbide grade that will withstand the high temperatures generated at the high cutting speeds."

Higher speeds, and to some extent feeds, are mostly made possible by the cutting grade, Mr. Goulding notes, but the cutting grades and chipgrooves work together to provide overall performance capabilities.

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