The Science of pCBN
Development of new grades spurs wider application
By Jim Destefani
Polycrystalline cubic born nitride (pCBN) cutting tools have been around for decades, since materials scientists first synthesized pCBN using a process similar to that developed to produce industrial diamonds. pCBN is second only to diamond in hardness; unlike diamond tools, however, pCBN tooling can be used to machine ferrous alloys--even steels hardened to RC 60 and higher.
Development of application-specific grades--and improved knowledge of how the materials will behave--is helping to expand the application range of pCBN tools, which now includes turning, boring, and milling of hardened steels, cast irons, superalloys, and materials such as hardfacing coatings. Here's a look at the current state of pCBN materials and application science.
New pCBN grades run the gamut from low-content (50 - 60% CBN) materials to inserts containing 80% or more of CBN crystals. According to Bill Shaw, technical specialist, Kyocera Industrial Ceramics (Mountain Home, NC), application based on CBN content may be somewhat counterintuitive.
"Typically, higher-content pCBN grades [80% CBN and above] are used for machining powder metal [P/M] materials and cast irons," he explains. "Continuous cutting applications in hardened steels generally use lower-content grades, which sometimes confuses users. The lower-content grades seem to hold size and surface finishes better than harder grades in hard machining applications."
Kyocera's latest grades feature a more uniform CBN grain size and homogeneous microstructure that Shaw says result in improved toughness. Scheduled to be available in the US by mid-year, grades KBN510 and KBN525 "can handle a bit more shock and interruptions than other grades" and are intended for hardened steel applications, he says. "Another new grade to be introduced later will have a higher CBN content and will be aimed at cast iron applications."
Although familiar black uncoated tools have been the standard in pCBN, coated grades are now becoming more common, according to product manager Darrell Johnson of Valenite Inc. (Madison Heights, MI). "Coatings have a couple of functions," he says. "They help operators spot worn corners, but they can also provide some tool life improvement and allow higher speeds, depending on the work material."
Johnson says TiAlN type coatings in particular may help improve tool life in hard part machining applications. "Coatings are mostly PVD, because CVD temperatures can cause delamination of pCBN segments from the backing as well as problems with the braze," he adds.
According to Shaw, TiN coatings can improve lubricity, heat dissipation, and chip flow. "A bonus is, a gold-colored insert makes it easier for operators to distinguish a worn edge," he says. "If the operator's in doubt, the tendency is to err on the side of caution and assume the worst. So TiN can help get more corners in use and reduce tooling costs."
Another advance is solid pCBN grades, which can replace carbide tools with brazed pCBN tips in some applications, according to Henrik Sandqvist, product manager, Seco Carboloy (Warren, MI). Solid pCBN inserts may cost more than brazed tools up-front, but they can pay for themselves in longer tool life and more parts per insert, he says.
"I would consider using a solid insert if I have a process that's running well and in control with tipped inserts, if I know tool life, and if I'm sure my operators can handle multiple cutting edges," Sandqvist says. "You need to make sure you're going to use all four, six, or eight available edges to take advantage of a lower cost per edge."
Carboloy manufactures several solid pCBN grades, including CBN200, a general-purpose grade for gray cast iron, roughing of hardened steel, and hardened P/M materials. The solid grades feature holes, which facilitate reliable seating of the insert in the toolholder or milling cutter body.
Insert geometry is starting to play a part in improving the productivity of pCBN tools as well. The hardness of the pCBN segment makes production of geometry on that portion of the tool difficult, but some Kyocera grades are offered with a chipbreaker molded into the carbide substrate that carries the brazed pCBN segment. "Molding a chipbreaker in the carbide behind the pCBN seems to improve chip formation by allowing the chip to roll off the edge better, resulting in improved chip control and better part surface finishes in some applications," Shaw explains. "Examples include case-hardened materials where there's a gradient in hardness and the material would tend to become more ductile or stringier as you go." The company's BB chipbreaker geometry is available in three variations depending on the size of pCBN segment on the insert corner.
On the application front, hard turning is a solid niche for pCBN tooling, according to Sandqvist. "Hard turning is generally regarded as machining of parts with hardness above about RC 45," he says. "Finish hard turning in automotive applications is growing. For every grinding machine that's due for replacement, engineers are beginning to look at hard turning as a possible option."
Sandqvist says tooling costs for hard turning can be much lower than those for grinding. "And, for many parts, usually no further finishing is needed. Some parts, such as bearings, may require a secondary finishing operation, but you need that anyway even if you grind them," he adds.
Shaw echoes those comments. "It's more economical to run a turning machine than it is a grinder," he says, "so more people are considering single-point machining of hardened parts. There will always be applications that will always require grinding. You can achieve good microfinishes with a single-point tool, but whatever you do you're always going to have tool marks."
According to Johnson, the machine and setup are crucial to success in hard turning. "The more rigid the setup as far as fixturing and machine, the more likely you are to have success," he says. "Simple things like tool overhang or center overhang, or how you fixture the part, can have a big impact. The idea is to keep everything as rigid as possible and keep overhangs as short as possible. A particular problem is chatter, and that usually results from harmonics in the system. So, anything that can minimize deflection is beneficial."
"CBN milling is also very interesting," says Carboloy's Sandqvist. He reports more application in the automotive industry, in, for example, face milling of cast iron transmission casings and similar components.
But pCBN milling is also growing in the mold and die industry, according to Markus Heuwinkel, manager of product application, Walter USA Inc. (Waukesha, WI). "Finishing using pCBN can provide very good surface finish and tool life in machining of molds that are heat treated after rough machining," he says.
"Most of the cutting is done before the part is hardened, so we're talking about a maximum depth of cut of 0.040 - 0.060" [1 - 1.5 mm] for semifinishing and much less for finishing," Heuwinkel explains. "Minimum workpiece hardness for pCBN milling would be about RC 54 - 56. In general, the harder the material, the better the tool life of the pCBN.
Heuwinkel says high-speed machining, either with pCBN or carbide, can minimize or eliminate bench finishing. Walter's focus for pCBN milling is on button, ballnose, and octagonal inserts in the company's WCB 50 grade, he adds.
Regardless of the specific application, pCBN milling requires a solid machine and setup to minimize vibration and chatter. Heuwinkel recommends an HSK connection or other system that offers simultaneous contact with both spindle face and taper. Workholding also must be solid to minimize vibration and chatter. And, according to Sandqvist, development of solid pCBN inserts with holes enables more solid seating of the insert in the cutter body.
A Selection of pCBN Products
Here's a quick look at some of the current crop of pCBN tooling products from a variety of manufacturers. For more information, circle the appropriate number(s) in the reader service box at the end of the main article.
CBN end mills from Emuge Corp. (Northborough, MA) are said to provide tool life up to 50 X longer than carbide tools in tough applications. The tools can machine hardened steels to RC 66, as well as cast steels, and nickel and cobalt-base alloys. pCBN end mills are available from stock in ballnose, torus, and flat-end designs, in inch sizes from 3/16 to 1/2" and metric sizes from 4 to 12 mm.
Aimed at machining of cast irons and powder metal (P/M) alloys, BN700 pCBN inserts from Sumitomo Electric Carbide (Mt. Prospect, IL) have a high pCBN content that results in a very hard edge and very good wear-resistance. The grade is available in multicornered geometries with numbered edges for ease of indexing. All inserts are manufactured with precision-ground tolerances, and users can select wiper, honed, or no edge preparations.
The ValEdge hard part turning system from Valenite Inc. (Madison Heights, MI) combines pCBN insert materials and toolholder technologies to deliver high-performance machining of steels hardened to RC 45 - 62. pCBN tools include multicornered, coated VPC225 for roughing and finishing operations. Inserts incorporate both a T-land corner break width and angle as well as a light hone finish, and are available in approximately 110 available insert styles, sizes, and geometries.
Two pCBN grades from Iscar Metals (Arlington, TX) are aimed at machining of hardened steels and cast irons. IB55 is said to be well suited to semifinishing and finishing of hardened steels, even under interrupted cutting conditions. For machining gray cast iron and cast irons with hardness greater than RC 45 as well as heat-resistant alloys, the company offers grade IB85. All IB55 and IB85 inserts have a single-tipped working corner. Available edge preparations include T-lands and hones.
Coated pCBN inserts from Kennametal Inc. (Latrobe, PA) are said to offer up to double the tool life of uncoated grades when machining hardened materials. Grade KB9640 features a CVD aluminum oxide coating, and is designed for roughing to semifinishing of hardened steels and cast irons. It is a high-content pCBN grade available in round, square, triangle, and 80, 55, and 35º diamond shapes. Featuring a PVD titanium aluminum nitride coating, grade KB5625 is designed for finish turning of hardened steels with moderate to heavy interruptions.
WCB 30 and WCB 50 from Walter USA (Waukesha, WI) are uncoated pCBN grades aimed at turning and milling of cast irons and hard materials. For turning, inserts are available with negative geometry in ANSI and ISO C, D, S, T, and W styles or with positive geometry in C, D, and T styles. Milling tools include CPR button cutters for copy milling, ballnose cutters for finishing, and octagonal cutters with a single pCBN cutting edge.
ExoCarb Max pCBN end mills from OSG Tap & Die Inc. (Glendale Heights, IL) are designed for finishing of steels hardened to RC 50 - 65. The ballnose tools have a unique radius said to provide high rigidity and extended tool life with very fine workpiece surface finishes that minimize secondary polishing. The mills' ball end has a 5-µm radius tolerance, and their nose geometry allows cutting with the center of the ball. The company also manufactures corner radius pCBN end mills in one, two, or four-flute configurations.
Secomax pCBN100 from Seco-Carboloy is said to be the first solid pCBN grade for finish machining of hardened steels, and the first with a pin lock hole. With a fine grain structure and low (50%) pCBN content, the grade can handle light continuous cuts or moderately interrupted cutting of hardened steels, hardfacing alloys, and sintered steels. Plunging is possible using the full cutting edge length of the insert, which can be used with standard M, P, and D-style toolholders. For applications such as hardened steel finishing, W and T-style insert geometries are available in standard edge preps as well as a wiper cutting edge.
Mitsubishi Materials USA Corp. (Hoffman Estates, IL) offers a variety of coated and uncoated pCBN grades for high-speed cutting of cast irons, hardened steels, and P/M materials. An example is MB8025, aimed at continuous or interrupted cutting of hardened steels. Produced using a sintering process that is said to improve resistance to both fracture and wear, the grade also features pCBN inserts brazed directly to the substrate for improved brazing strength. A variety of insert geometries and edge preparations, including a wiper geometry, are available.
Kyocera Industrial Ceramics (Mountain Home, NC) produces several CBN grades for varying applications. KBN10B is aimed at finishing of hard cast irons and heat treated steels at depths of cut to 0.020" (0.5 mm). For hardened steel machining, KBN25B has a submicron grain size and binder formulated for improved toughness and chemical stability. KBN65B is designed to provide long tool life in high-speed machining of cast irons, while KBN900 is a solid pCBN grade with a PVD coating for rough turning of hardened materials and high-speed cast iron machining. New grades KBN510 and KBN525 are due for release this spring.
This article was first published in the January 2005 edition of Manufacturing Engineering magazine.