Solutions for Difficult Machining
PCD and CBN tools offer a fine edge
By Jim Lorincz
The machining challenges for two of the most advanced concepts in cutting tool materials are pretty well known. Cubic boron nitride (CBN) tools of varying designs are being used to cut hardened ferrous metals with or without interrupted cuts, as well as welded and clad metals. Polycrystalline diamond tools (PCD) are being used extensively for milling nonferrous metals and composite materials, plastics, and extremely difficult-to-machine superalloys.
Selecting the right type of cutting tool material depends on a combination of factors: the hardness of the metal or type of material, the nature of cutting being done—continuous, light interrupted, or heavy interrupted—and the machining process being used, i.e. single-point turning or milling. Drilling, reaming, and parting are other processes that can benefit from correct application of the tooling technologies.
PCD wafers are produced by sintering diamond particles with a metal matrix binder under high temperature and pressure. PCD chips can be cut by EDM or laser and tipped onto carbide or steel substrates, and then ground to create cutting tools that are especially effective in machining nonferrous metals such as aluminum at high speed, as well as abrasive composite materials and plastics.
Cubic boron nitride (CBN or PCBN) is a man-made material that is second only to diamond in hardness and is stable under high temperatures such as those encountered when machining hardened ferrous and super alloy materials. CBN inserts, which are widely used for turning hard metals like cast iron, are being used for milling because of tougher grades and the development of special toolholders. Hard turning, as a substitute for grinding, has benefited from the advances in machine technology, especially in the rigidity and power of CNC turning centers.
Sandvik Coromant (Fair Lawn, NJ) is well positioned to continue the development of CBN and diamond-tipped metal cutting tooling, thanks to the acquisition of Diamond Innovations (DI; Columbus, OH) and its parent company, the Sandvik Tooling Group.
"The acquisition will allow for the development of new tools through the combination of Diamond Innovations' technological expertise in CBN and PCD and Sandvik Coromant's expertise and research in cemented carbide and the material interface," explains Rick Askin, president, Sandvik USA.
Diamond Innovations is well known as the pioneer in the development of synthetic diamonds for industrial applications. Synthetic diamonds are the building block materials of both PCD and CBN products. In addition to metalcutting tools, they are used for grinding, rock drilling, wire drawing, polishing, and other applications for which superhard materials are required.
"From a machining perspective, the carbide backing makes up about 95% of cutting tools with a PCD or CBN tip for the cutting edge. There are a number of attributes that affect the performance of the tool, Askin explains. "One is the performance of the PCD or CBN; another is the performance of the carbide; and still another is the interface between the PCD and the carbide. This gives us a great opportunity to develop diamond tools with our expertise in cemented carbide, diamond technology, and control of the interface between the diamond and the carbide."
"With the rapid growth in aerospace and energy, material demands have changed the way we develop new innovations and manage our catalog of over 25,000 products," says Doug Evans, grade development specialist, Sandvik Coromant. "The ability to control the manufacturing process from the creation of polycrystalline diamond and CBN material to the finished product allows us to provide our customers with products that provide true performance improvements."
Diamond Innovation's products cover a wide range of applications:
- Borazon CBN is used for grinding hardened tool, die, and alloy steel, as well as nickel and cobalt-based superalloys.
- BZN Compacts tool blanks and inserts are used for ferrous machining including pearlitic cast iron, Cr and Ni alloyed iron, hardened steel, powder metal, ard facing alloys, and superalloys.
- Compax diamond tool blanks (PCD) are available in various grades for fine finishing aluminum, plastics, precious metal, green ceramics, graphite, and graphite composites; for interrupted machining of ceramics; roughing of metal matrix carbides (MMC); and machining difficult-to-machine materials such as fiberglass, MMC, and wood laminates.
Laser processing of diamond-tipped tooling has added a new dimension with the introduction of 3-D chipbreakers and clean cutting edges on TiroWave PCD and CVD tooling from CIMtek Group (Elgin, IL). "Laser cutting enables us to put one of four chipbreakers on the insert," explains Brian Nowicki, a CIMtek partner. The series of chipbreakers can be used for jobs ranging from fine finishing to heavy roughing. "They are optimized to break chips to sixes and nines that you would normally get in steel. We can even break small, controllable chips in the most difficult-to-machine aluminums," Nowicki says.
TiroWave PCD tooling is said to have a tool life up to ten times longer than conventional types when machining aluminum alloys, and is well-suited for machining magnesium alloys, all nonferrous materials, composites, reinforced plastics, and materials with abrasive binders.
Seven-axis laser processing has also enabled Tiro Tool (Innsbruck, Austria) to produce CVD thick-film diamond tooling that is a material cut to shape and brazed like a PCD tip. "The difference is that the CVD is 99.9% pure diamond, as opposed to 90% with PCD. It is up to 1.2-mm thick, and a solid material that can be fabricated to a usable cutting tool form," says Nowicki. This new CVD material is said to be nearly 50% harder than PCD, and can only be fabricated using Tiro's laser technology. Already proven in Europe where it has shown tool life increases of up to 700% over PCD, the CVD tooling was introduced to the US at IMTS and is available in turning, milling, drilling, reaming, and countersinking tools, including those required for machining composites for the aerospace industry.
Laser processing of TiroWave tooling is said to produce cleaner surfaces that allow for smoother finishes, and to prevent cutting materials from adhering to the tool edge in both roughing and finishing operations. TiroWave is available in all ANSI and ISO styles, including 80°, 55°, 35°, square, triangular, and round inserts. The technology is also available in drills, end mills, and indexable face mills.
Iscar Metals Inc. (Arlington, TX) identifies the hardmaterials group as comprising hardened steel and chilled cast iron with hardness ranging from RC 55 to 68, materials typically in constant use by the auto industry for bearing production and for die and mold machining.
The difficulty of machining hard materials varies, depending on the parameter of hardness combined with its depth (in case-hardened applications), and the microstructure of the work material. Successful machining results from component rigidity and geometry, combining with the machine's rigidity and its vibration-damping characteristics.
Iscar's advanced cutting tool materials cover applications such as interrupted cutting (milling, parting, and drilling) and continuous-cutting operation (turning and threading) for hard materials, employing CBN, ceramics, and submicron substrates—often with proprietary coatings. Geometries and chipbreakers are the natural avenue for improving surface finish, extending tool life, and increasing productivity.
As a rule of thumb, Iscar says that if the hardness ranges between RC 50–68 and the depth of hardness is greater than the depth of material to be removed for ferrous materials, CBN is the best medium to use. Iscar offers five CBN grades to suit a wide range of machining requirements.
- High CBN content grades (IB90 and IB85) are recommended for high-speed cutting of cast iron and interrupted/rough cutting of hardened steel. There is a tradeoff between hardness and wear rate for toughness and ability to withstand shock loading from interrupted cuts.
- Low CBN content grades (IB55 and IB50) are more appropriate for semifinishing and finishing operations. The shape of the insert nose determines the surface finish achieved within the limits of machine and component rigidity. For example, the inserts are designed in a form of brazed tips of CBN, embedded on standard ISO inserts and available in triangular, rhombic, square, and round shapes, as well as on Iscar's GRIP inserts.
The fact that both PCD tooling and CBN tooling are more expensive by several factors is cause for careful analysis of the applications being considered. Untended high-volume operations, especially in the automotive industry where long tool life and limited downtime are desired, are typically good candidates.
Chris Wills, product specialist, Mitsubishi Materials USA (Irvine, CA) explains that the company's Virtual Tool Report can be used to evaluate the cost effectiveness of any insert, not just CBN. "Basically, it helps users evaluate the value of an insert, any insert. But for me it's especially helpful in proving the value of CBN in reducing the actual cost of manufacturing, which is more important than the initial cost of the insert."
Cost comparisons are being evaluated typically against ceramics or carbide tooling, says Wills. "Applications range from high-temp alloys and most hardened metals, including continuous and/or interrupted cutting in the automotive industry for cast-iron products, and increasingly for powder metal parts. For steels, new chipbreakers are needed, especially in automated environments such as bar feeder and gantry loads where surface blemishes caused by chips can damage the quality of the part," Wills explains.
Sumitomo Electric Carbide Inc. (Mount Prospect, IL) has extended its BNC series of polycrystalline cubic boron nitride (PCBN) grades for high-speed machining of steel alloys with hardness greater than RC 45, as well as for machining cast iron and exotic materials. The four ceramic-coated grades feature three types of edge treatments.
Although CBN is a mature material, Sumitomo's Rich Maton points to coatings and the importance of edge prep for PCBN applications, as well as the chipbreaker. Typical automotive workpieces for turning include axle shafts, ring and pinion gears, different types of drive shafts, bearing caps, and the like. Other applications include engine blocks and heads and die and mold work.
One of Sumitomo's new grades is the BNC300 PCBN substrate combined with a TiAlN ceramic gold coating for longer tool life when turning hardened steel with continuous and interrupted cuts. All corners are brazed individually for strength. "Edge prep is especially critical in PCBN applications," says Maton. "We've added three different edge preps, light, standard, and a heavy edge prep, depending on the BNC series selected."
The BNC series covers the range from high wear resistance for finishing and light interrupted cutting (BNC 100) to high fracture resistance for finishing applications, with a combination of both continuous and interrupted surfaces (BNC 300). BNC 200 is recommended for medium-speed machining of carburized, inductionhardened bearing steel, and BNC160 was developed for special applications with low surface roughness in the 16µin. [0.406µm] range."
In PCD tooling, Sumitomo has introduced its Sumidia DA 1000, which offers superior wear resistance and toughness. This grade is available in the NF-style wafer, which features the same cutting length and half the thickness of previous PCD wafers. The DA 1000 will be offered only in the economical tip, the thinner PCD wafer, as an economical tool for nonferrous machining for medical, aerospace, agricultural, and automotive parts, says Maton.
Valenite (Madison Heights, MI) offers the ValEDGE hard-part turning system for machining steels and alloys hardened to RC 45–62 levels that includes a full range of CBN and ceramic inserts. Darrell Johnson, application engineer, recommends edge preps tailored to the application. "Every situation is different. You have to try different edge preps with the same grade and same parameters and see if you improve tool life, reduce cratering, and even break chips, though there's only a very narrow opportunity in chipbreaking." Valenite offers its CBN-tipped products with a light hone with the choice of either the 0.005" (0.0.13 mm) x15° T-Land for light-duty cutting and reduced force, or 0.005" (0.13 mm) x25° for general applications.
Machining composite materials has joined machining aluminum as one of the fastest growing and most challenging processes for which PCD-tipped tools are used. Materials such as carbon fiber, graphite, green ceramic, fiberglass, as well as an even more difficult-to-machine material such as Kevlar, pose challenges to precision machining, says Jason Lindsey of Lach Diamond (Grand Rapids, MI).
Lach Diamond offers five perimeter-milling end mills with patented designs for cleanly cutting composites. Lach's Type Z end mill is a corncob-style tool for perimeter roughing that features PCD tips positioned at varying shear angles all the way down to the tip of the tool. It is available in both plunging and nonplunging styles.
For face-milling aluminum, Lach Diamond's dia-compact monoblock face and square angle-milling cutters can be installed on machining centers without time-consuming adjustment. The PCD diamond tip is brazed onto the steel tool body. There are through-coolant holes between every tooth, and teeth are rotary finish-ground in the same cutting plane.
One of the critical ingredients of machining with PCD-tipped tooling is to have a sharp cutting edge. "The basic production tools to accomplish this are EDM, brazing, and grinding," explains Robert Sullivan, materials manager, Ingersoll Cutting Tools (Rockford, IL). "PCD tools are very effective in high-volume machining of aluminum parts for such applications as automotive engine pads or transmission covers. The primary concern is whether it's economical, considering the tool change and downtime involved." Size range of Ingersollss face mills is from 3 to 15" (76–381 mm) in diam for applications such as engine block heads, transmission cases, and graphite machining for EDM.
Walter USA (Waukesha, WI) offers both PCD-tipped and CBN cutting tools. "The advantages of CBN in hard-part turning include higher tool life, lower cost per part, and reduced downtime," says Walter's Matthias Goetze. "I would say that about 90% of our PCD-tipped cutting tools are specials and that about 80% of our CBN cutting tools are standard products," Goetze says.
For the automotive industry, especially for machining low-silicon aluminum parts, Walter offers chipbreakers on the PCD for better chip control in machining parts such as ABS housings and components for the aerospace industry. The company also offers two different PCDs, one for aluminum with silicon content to 12%, and one for aluminum with more than 12% silicon content. In 2008, Walter established a regrinding and refurbishing service at its Waukesha facility to support its customers in retipping and regrinding PCD-tipped tooling.
Emuge Corp. (West Boylston, MA) offers an expanded line of hard milling end mills with both PCD and CBN cutting edges. The end mills are said to provide up to 50 times more tool life than conventional carbide end mills for machining a wide variety materials. These include highly abrasive, nonferrous materials, including graphite, high-silicon aluminum alloys, fiber-reinforced synthetics, and copper alloys. Tools feature a coolant-through design for evacuating dust and chips. Emuge's PCD and CBN end mills are available from stock in ball nose, torus, and flat- end designs in sizes from 4 to 12 mm.
Milling with CBN, too
Milling is a demanding operation, often involving a great deal of interrupted cutting. PCBN grades used have to be selected appropriately, according to Seco Tools Inc. (Troy, MI). Typical applications include grey cast iron for automotive blocks, heads, and casings, and some tool steels.
Here's how Seco sizes up the choices. The main properties of a PCBN insert—toughness, wear resistance, and hot hardness—are determined by grain size, amount of CBN, and binder. The main difference between a turning insert and a milling insert is the grain size. PCBN inserts for milling usually have a smaller grain size to increase toughness. At the same time, the grain size, the amount of CBN, and the binder must be balanced to maintain superior wear resistance and hot hardness. Seco's milling inserts are available for a number of face and square-shoulder milling cutters specifically designed for PCBN. These negative cutters use solid PCBN inserts ranging from 2 ½ to 8" (63.5–203 mm) in diam. Square and round inserts are used for face milling and triangular inserts for square shoulder applications. For applications where the tool pressure has to be kept low and negative cutters cannot be used, there are positive cutters with solid or full-faced positive inserts. Only one side of these inserts can be used. By using mostly solid inserts, the number of corners per insert can be maximized. In a finish face-milling application with a cutter using solid round PCBN inserts, at least 15 usable edges per side can be utilized. For square or near-square-shoulder applications, cutters for solid triangular or square inserts are available, giving the user six and eight corners per insert, respectively. There are three basic requirements. When milling with PCBN. it should always be done dry. PCBN is a brittle material. Using coolant will promote thermal cracking of the PCBN when the insert goes in and out of cut. Always use conventional milling. The impact load on the insert when it goes into cut will be reduced, and the risk of edge breakage minimized. Increase speed by 10–50% compared to turning operations to compensate for interruptions.
This article was first published in the January 2009 edition of Manufacturing Engineering magazine.