Advanced Materials Find Their Groove
CBN, PCD, and ceramics are growing in use for the toughest machining jobs
By Jim Lorincz
Advanced cutting-tool materials such as cubic boron nitride (CBN), polycrystalline diamond (PCD), and ceramics are following a path well-traveled by other materials to gain acceptance by end users. The reason is the overall value that they can deliver, if properly selected and matched to the right applications, in spite of "sticker shock" that may be attached to CBN and PCD.
These cutting tools are reshaping the way in which hardened metals and some superalloys are being processed in industries as diverse as high-volume automotive, quality-conscious tool and mold, and precision-critical aerospace and medical device, among others.
"We see the same thing for CBN that we saw some 50 years ago for carbide and 30 years ago for coated carbide," says Henrik Sandqvist, product manager, Seco Tools Inc. (Warren, MI). "The materials first appeared in their uncoated state, and then a way was found to bind coatings to the substrate to protect the insert from heat and increase wear resistance. The coatings function primarily as a barrier to heat that would destroy the insert, and are based mostly on titanium nitride, both for hard turning and grey castiron machining," Sandqvist explains.
The pattern of acceptance for CBN is no less true for PCD and ceramic cutting tools. Manufacturers are looking for productivity improvements that result from eliminating processes, multiple setups, and the handling that may be required for workpieces within additional setups. The ideal metric for cost evaluation is through a complete cost-per-part study.
Turning hardened metals, for example, is gaining wider acceptance because it eliminates grinding and the steps necessary to produce parts by machining in the pre-hardened state, heat treating, and then subsequent finish grinding.
Applications for hard-turning steel and alloy steel, and machining powder metal (PM) parts, cast iron for cylinder blocks, crankcases, and crankshafts harder than RC 40–45, for example, are well-suited to CBN inserts. Case-hardened workpieces or parts with welds are efficiently machined by ceramic tools.
Technology trends in today's machine tools and cutting tools are paving the way for greater acceptance of the materials.
- Modern machine tools, CNC lathes and machining centers offer both the rigidity, horsepower muscle, and speed required to machine hard metals, difficult-to-machine materials such as composites and nonferrous metals, and exotic alloys such as Inconel, Monel, and Stellite.
- A variety of new coatings, especially titanium nitrides, expands the application range of CBN cutting tools for reliable and consistent performance in machining steels hardened to RC 45 or harder, with longer tool life.
- Wiper geometries and multiplecorner ceramic and CBN inserts can generate excellent results in a variety of applications.
Ceramic, CBN, and PCD cutting- tool materials each have special capabilities based on their physical composition that lend themselves to certain applications:
- Cubic boron nitride (CBN) is a manmade material that is second only to diamond in hardness. CBN is stable under high temperatures to 1000° C that are encountered when machining hardened ferrous and alloy steel materials.
- Polycrystalline diamond (PCD), made by sintering diamond particles with a metal matrix under high temperatures and pressures, can be tipped onto carbide substrates and/or in some cases EDM'ed to create cutting tools for machining nonferrous metals, as well as plastics, composites, and other highly abrasive materials.
- Ceramics include various materials formed at high temperature from silicon, alumina, and zironia that— due to hardness, thermal stability, and resistance to wear—make excellent cutting tools. Silicon nitride, for example is used in high-speed machining of cast iron; and when coated with titanium nitride can also be a good choice for machining hardened steel.
The process of selecting the right cutting-tool material depends on the combination of the hardness of the metal, the type of cutting being done—continuous, light interrupted, or heavy interrupted—and the machining process being used, whether single-point turning or milling.
For turning hardened metals ranging to the low RC 40 level, coated carbides work fine. Metals harder than RC 45 can be machined with cermets, ceramics, and from mid RC 50 to mid RC 60 with CBNs. Milling for semifinishing and finishing can be done with new, tougher versions of CBN and ceramics.
A survey of recent product introductions by cutting tool manufacturers turns up an array of cutting tools with multicorners, wiper geometries, and a variety of coatings and other significant advances in cutting tool technology.
"In automated production, the consistency of performance that CBN can deliver is especially important, for example in machining cast irons and powder metal parts for automotive," says Chris Wills, product specialist, Mitsubishi Materials USA (Irvine, CA) "CBN can be used for both milling and turning, although the larger segment of the business is in turning."
Mitsubishi Material's newest product introductions feature coatings and multiple corner versions. Its MBC020 coated CBN grade is intended for both continuous and interrupted cutting of hardened steel parts such as shafts and gears in the automotive industry.
The inserts are available in single-sided single-tip, singlesided multiple-tip, and double-sided multiple-tip versions.
Mitsubishi Carbide evaluates the potential of CBN insert use on an application basis. "We do a Virtual Tool Report in which we measure the existing level of current shop performance for drilling, milling, turning, etc. and then we'll put our product into the matrix and show the value of performance and actual cost of manufacturing using our CBN product," Wills says.
Wills believes that education about tool performance; showing the value of a tool with the true manufacturing cost, not only the purchase cost, has to take place at the accounting and purchasing levels, not just at the shop level. This will show the true tooling cost and ultimately the savings the customer will see in their manufacturing.
Mattias Goetze, application specialist, Walter USA Inc. (Waukesha, WI) says that CBN is especially advantageous in applications such as hard-turning steel and alloy steel in mold and die and automotive, as well as machining powder metal parts, cast-iron cylinder blocks, crankcase, and crankshaft workpieces. "The benefits of CBN include high tool life, less downtime, higher cutting speeds, improved cycle times, and better surface finish," he says. Walter is introducing a new line of multiple-corner inserts for finish-turning hardened steel and grey cast iron.
It isn't difficult to see why hard turning is increasing in popularity. "Grinding machines are expensive. CNC lathes are versatile. They can be programmed with tool path, and don't require a preshaped grinding wheel," says Seco Tool's Sandqvist. "Also, machining is done dry, a distinct environmental benefit."
CBN use is growing "for everything that goes into automotive gears, shafts, and all the kinds of parts that are case-hardened or carburized. Components that have to be cleaned up after heat treatment are responsible for CBN growth," he says.
In addition, milling with CBN is becoming more widely known. "CBN is an excellent material for cast-iron machining, both for turning and milling," says Sandqvist."More and more people are realizing that you can mill with CBN, something that wasn't done five or ten years ago, because there are tougher inserts and special toolholders exist. I would prefer if milling cutters had axial adjustments so that inserts can be adjusted within a couple of tenths to each other. CBN milling is much more sensitive to having the inserts leveled out than milling with carbide tools."
Seco Tools has introduced two coated CBN grades and a new wiper-design insert. The two grades are CBN400C for long life in finishing grey cast iron, and CBN160C for finishing case-hardened steels. Seco's Helix wiper geometries are designed for high-feed machining of case-hardened steels where standard wipers cannot be used due to vibrations caused by a weak setup or lower radial-cutting forces.
These wiper geometries enable the operator to obtain a smooth surface by lessening the effect of the feed pattern which is created in conventional turning on the workpiece. Helix wipers are available in brazed-format CBN050C, a coated, low-CBN grade. Low-content CBN inserts cover the whole range of turning operations from continuous light interrupted to heavy interrupted, and feeds to 300 m/min.
Although CBN is a mature material, Rich Maton of Sumitomo Electric Carbide Inc. (Mt. Prospect, IL) points to coatings and a new chipbreaker as important developments, especially for applications including transmission components and engine-block cylinders. Typical automotive workpieces for turning include axle shafts, ring and pinion gears, different types of yokes and driveshafts, bearing caps, and the like.
"We have different grades for milling cast iron and hardened steel materials. With a typical milling application in cast iron you're looking at 2500–5000 fpm (762-1524 m/min) with CBN versus 1000–1500 fpm (305-457 m/min) for carbide," Maton says.
"We offer three different ceramic-based coatings: TiN coating is the BNC80 grade for low surface roughness. With the TiN coating being the smoothest coating, we're looking for a 16 µin. (0.406 µm) rms. The TiAlN BNC200 works in a variety of materials and is by far the most popular grade, because it can be run both in interrupted cuts and in continuous cuts for finishing and roughing. The TiCN BNC150 is for high temperatures, and can run to 300 m/min in hardened steels."
Sumitomo's newest grade is the BNC300 PcBN substrate combined with a TiAlN ceramic gold coating for longer tool life when turning hardened steel with interrupted cuts. All corners are brazed individually for strength. For light interrupted cuts, BNC150 PcBN has a TiCN ceramic coating for improved wear resistance. Tool life is said to be extended by as much as 40% compared with other PcBN grades.
Taking stock of exactly what you are trying to accomplish is as important as cost, especially when CBN is compared with carbide or cermet.
Darrell Johnson, product manager, Valenite (Madison Heights, MI) suggests carefully evaluating shop processes and machining objectives. "Determine the end purpose of the process, surface finish desired, and hardened material that is to be removed. Have overhangs as short as possible and pick feed rates that are controllable, ranging from 0.003 to 0.006 ipr [0.08–0.15 mm/rev]," he advises.
"You have to approach hard-part turning a little differently." Johnson explains: "You can't run the part too slow or you can fracture the insert or lose cutting efficiencies. Typically, you can start on the high side, about two-thirds of the maximum speed for the material and then check tool life. You can always back off," he says.
Valenite's ValEDGE hard-part turning system is intended for chining steels and alloys hardened to RC 45–62 levels and includes its full range of ceramic and CBN inserts plus the ValTURN ProGRIP family of toolholders, which feature a spring-loaded clamp design that securely grips the insert in two directions, downward and to the back pocket wall for secure, consistent clamping.
"The CBN we use is effective for really tight tolerances, when you're replacing some grinding operations with CBN, or when you have to hold a tight dimensional tolerance or surface finish," Johnson says.
"We recently introduced the VPC225 coated multitip CBN for roughing and finishing operations and two ceramics, the VPZ205 coated micro-grain tool for finishing with moderate dimensional and finish needs and the VPZ215 coated ceramic for roughing operations where tolerance and surfaces are less critical."
Typical applications for CBN include induction-hardened shafts, alloy steel gears and some bearing steels, among other automotive parts. "For ceramics, the application range is a little different because we look for applications where we have to remove all of the hardened material on the surface. Many end users weld different components and they can't weld the hardened steel. For these applications, you have to cut through a hardened surface that may be RC 58 to the softer metal beneath," Johnson says.
Machining centers with the necessary speed and horsepower are making it possible for a line of specialized end mills with indexable ceramic inserts as small as 3/8" (9.52-mm) diam to extend the capability of machining hardened materials, especially in the mold and die industry. "Without the rpms and today's improved linear-feed controllers, it wouldn't be possible to run ceramics in that small an end mill," explains William Greenleaf, marketing manager, Greenleaf Corp. (Saegertown, PA).
"And of course ceramic end mills go all the way up to 12" [304.8-mm] diam, When you get that big in machining hardened steels and exotic alloys, you need big horsepower," explains Dale Hill, Greenleaf application engineer.
"For hardened materials in the mold and die industry, manufacturers can eliminate much of the machining that they would do in the soft state with the need to either finish grind or EDM after hardening. In many cases, they can go right to the heat-treat process, bringing that block of steel out in the hardened state and using ceramic tooling to machine it and eliminate setup and tear down. These steels are in the RC 50–55 range, outside of the typical carbide application area," Greenleaf explains. "We aren't eliminating rotating tools like the solid carbide end mills, but we're looking at high metal removal with the use of ceramics compared with small-diam carbide end mills.
Greenleaf is releasing its first high-feed mills engineered for WG-300 ceramic inserts. With this new high feed mill, face milling of hardened materials at feed rates up to 200 ipm [5 m/min] is possible. We saw an opportunity to expand the applications for our WG-300 whisker-reinforced ceramic composite cutting tools with a high-feed mill," Greenleaf says.
Greenleaf's second-generation WG-600 ceramic is designed for finish turning high-strength alloy materials and hardened steels. "WG-600 was developed to extend tool life in applications where it's able to finish the cut without interruption that requires stopping and blending in the cut after indexing the tool. Semifinishing steel rolls after heat treating to mid-level hardness can be done without stopping. If you stop half way through the process, for example, you can leave a blending-in mark that can introduce stress and, in turn, the possibility of cracking in the final heat treat stage," adds Hill.
Sandvik Coromant Co. (Fair Lawn, NJ) has introduced two new CBN grades: CB7025 for unstable interrupted cutting and CB7015 for stable conditions with continuous cuts and long engagements.
"We moved the braze back and away from the heat in the cutting zone. The Safe-Lok multi-corner design of the CBN tip allows for both sides of the negative insert to be used. This is especially important in high-volume automotive applications such as CV joints or gears with high wear requirements," explains Jack Lynch, product manager.
Sandvik Coromant has also married CBN with wiper technology for high-volume applications replacing grinding. "The wiper geometries allow for increased feed rates and shorter cycle times, while still providing an excellent surface finish," Lynch says.
Sandvik Coromant optimizes its hard-part turning with two different wiper designs. The WH geometry is best suited for a single-pass strategy utilizing one toolholder and the shortest possible cycle times. For a two-pass strategy involving two tools, the WG wiper is recommended for semiroughing, then the WH geometry for finishing. Tool changes are reduced and component quality, as measured by tolerances and dimensions, is increased.
Kyocera Industrial Ceramics Corp. (Mountain Home, NC) offers carbide, ceramic, cermets, CBN, and PCD cutting tools for automotive, medical, aerospace, and steel-roll industries, among others. It continues to expand its lineup of CBN products. "A mainstay of our CBN lineup has been the KBN900 coated solid insert for roughing hardened steel and cast iron where medium to heavy shock is encountered," says Todd McCallister, applications engineer.
Kyocera has introduced new coated CBN grades that include the KBN25M and KBN10M with special coating and KBN25C and KBN10C with TiCN coating. The special coating reduces oxidation wear at temperatures equal to or greater than 1000°C, along with imparting high hardness for high-speed hard-part turning, either in continuous, light, or heavy interrupted cutting.
High-speed applications include continuous to light interruption cutting of tool steel with KBN10C and light to medium interruption cutting with KBN10M. KBN25C is intended for continuous to light interruption cutting of alloy steel and KBN25M for light to heavy interruption cutting with KBN25M for alloy and tool steel. "CBN is especially suitable for machining hardened metals with interrupted cuts," explains McCallister. Ceramics can't take the thermal or mechanical shock and are best used in continuous cutting operations," he says.
Emuge Corp. (West Boylston, MA) offers a hard-milling end-mill line with PCD and CBN end mills. "Both PCD and CBN end mills are extremely durable, providing up to 50 times more tool life than conventional carbide end mills," says Stephen Jean, milling products manager. "PCD is an efficient tool for machining highly abrasive, nonferrous materials, including graphite, high-silicon aluminum alloys, fiber-reinforced synthetics, and copper alloys. CBN provides a cutting edge for end mills that machines hard steels to RC 66 as well as hard castings," Jean adds.
This article was first published in the September 2007 edition of Manufacturing Engineering magazine.