Cutting tool materials based on aluminum oxide and silicon nitride. Ceramic tools can withstand higher cutting speeds than cemented carbide tools when machining hardened steels, cast irons and high-temperature alloys.

“Even if the cutting tool is expensive, you can put a hard coating on it and you will get a much better performance out of it,” Derflinger said. “That is why in the future more and more tools are going to have specialty coatings.”

In industries like automotive that require strong, lightweight materials, parts are also made of aluminum-silicon alloys. However, the higher the silicon content, the more abrasive the material.

Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.

Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.

The ideal coating would have a very hard, protective surface that simultaneously maintains the sharp cutting edges. Image courtesy of Oerlikon Balzers

Angle between the side-cutting edge and the projected side of the tool shank or holder, which leads the cutting tool into the workpiece.

“An often-overlooked component of ID work is accounting for the axial, radial and tangential cutting forces produced when turning,” said Mike Csizmar, regional sales manager at Horn USA Inc., Franklin, Tenn. “This is also true on external operations, but due to the increased L:D ratios associated with boring, these forces become more pronounced, affecting dimensional qualities. If you have a choice, axial (Z-axis) cutting force is preferred. A rule of thumb is that DOC should be equal to or greater than the tool nose radius, thus generating greater axial force. This provides the ability to control chatter, diameter and taper in a more efficient manner, and allows you to get the most out of your cutting tool.”

When drilling, a force that is directed axially—along the direction of machining. The magnitude of an axial force rises with the drill’s diameter and the chisel edge’s width. Axial force is also known as thrust. When turning and boring, the term “feed force” is commonly used instead of “axial force.” See cutting force.

When it comes to machining very abrasive materials, uncoated carbide tools experience accelerated wear. To increase tool life, high-performance coatings provide a vital protective barrier. He said the ideal coating would have a very hard, protective surface that simultaneously maintains the sharp cutting edges that enable clean, precise cuts while boosting productivity.

“The problem with the carbon and graphite fibers is that they are very high strength and extremely abrasive,” said Volker Derflinger, senior manager at Balzers, Liechtenstein-based Oerlikon Balzers, which has produced coatings for components and tools for more than 30 years. “For cutting tools to withstand heavy wear, it needs a specialty coating with a very high resistance to abrasion.”

The solution, experts agree, is to apply the shortest tool possible relative to tool diameter, preferably no greater than a 4:1 length-to-diameter (L:D) ratio. Boring bars should be on center or, in some cases, a few tenths (0.0003") above center to allow for deflection. And use a boring insert with a 0° lead angle whenever possible, so cutting forces are directed opposite the direction of cut.

The Balinit Hard Carbon coating has high hardness—40 to 50 gigapascals—making it appropriate for applications that require enhanced wear protection. In addition, the thin, smooth application helps maintain sharp cutting edges. For example, at a Malaysian manufacturer producing aluminum hard disk drive baseplates, a coated carbide endmill exhibited less abrasive wear and produced 95% more parts with 55% lower production costs than an uncoated tool.

Let’s start with cutting tools. Don Graham, manager of education and technical services for Seco Tools LLC, Troy, Mich., said that if the setup is fairly rigid and the correct cutting parameters can be achieved, indexable PCBN inserts are generally the best bet for hard turning.

“Alternatively, when the machine architecture allows, a combined configuration of turning and grinding can become very attractive for mid-volume requirements, an architecture that EMAG machines inherently provide.”

CNC grinders are routinely called upon to produce part roundness of 1μm (0.00004"), maintain diametral tolerances of ±2.5μm (0.0001") and impart surface finishes as fine as 8 rms or finer. Sheehy said the only way a CNC lathe can compete in this arena is if it is designed from the ground up for hard turning.

Graham said the company’s TH carbide can successfully turn materials up to 65 HRC and, unlike PCBN, is available in a range of geometries and chipbreaker configurations. And carbide is less prone to breakage in some applications—a casehardened shaft, for example, where it’s possible that softer material might be encountered, which would quickly dissolve the cutting edge and spell near-certain doom for the PCBN insert.

The PACVD process allows the diamond coating to be applied at thicknesses from 6µm to 12μm, enabling customization to suit the application.

Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

One coating type increasingly being utilized in these industries is strong, nonhazardous PVD. PVD describes a variety of vacuum deposition methods that can deposit thin coatings. The process typically coats tools and components at relatively low temperatures from 150° to 500° C, which avoids altering the fundamental material properties.

Some might wonder about the difference between hard turning the inside of a part (boring) and its outside. Most agree that boring is generally more difficult than OD turning, regardless of material hardness. That’s because boring bars are less rigid than other turning tools, creating problems with chatter and tool deflection. Because quarters are often tight in a bored hole, chip evacuation can be a challenge, leading to coolant starvation and workpiece galling. In addition, achieving sufficient surface speeds becomes increasingly difficult for small part features, such as bores, and PCBN and ceramic inserts require high cutting speeds.

Ceramics should be run dry in hardened materials, Kohler said. He recommends Greenleaf’s WG-600 silicon-nitride grade—a CVD-coated version of the company’s whisker-reinforced WG-300—as a good starting point for most hardened steels, as well as its new XSYTIN-1, a phase-toughened ceramic grade designed specifically for high-performance roughing and interrupted cuts.

Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.

Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.

High-temperature (1,000° C or higher), atmosphere-controlled process in which a chemical reaction is induced for the purpose of depositing a coating 2µm to 12µm thick on a tool’s surface. See coated tools; PVD, physical vapor deposition.

Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.

Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

Tool-coating process performed at low temperature (500° C), compared to chemical vapor deposition (1,000° C). Employs electric field to generate necessary heat for depositing coating on a tool’s surface. See CVD, chemical vapor deposition.

Opinions vary on the definition of hard turning. Some industry experts say it’s the single-edge cutting of hardened steels from 58 to 68 HRC, while others suggest hard turning begins at 45 HRC and includes hardened irons and superalloys. All, however, agree it presents difficulties but is quite manageable provided the right cutting tools, machine and process parameters are used.

Engagement of a tool’s cutting edge with a workpiece generates a cutting force. Such a cutting force combines tangential, feed and radial forces, which can be measured by a dynamometer. Of the three cutting force components, tangential force is the greatest. Tangential force generates torque and accounts for more than 95 percent of the machining power. See dynamometer.

He said standard PVD-applied metal-doped carbon coatings have a hardness of up to about 15 GPa whereas “diamondlike” carbon coatings range from 20 to 50 GPa. In comparison, a diamond coating reaches a hardness of 80 to 100 GPa.

Turning with ceramic inserts is usually done dry. Here the toolholder is mounted face up, driving cutting forces down into the machine’s load-bearing surfaces. Image courtesy Greenleaf.

Distance between the bottom of the cut and the uncut surface of the workpiece, measured in a direction at right angles to the machined surface of the workpiece.

Finally, when machining ceramics, which typically occurs in the dental industry, PACVD diamond coatings can substantially boost production and extend tool life while imparting fine surface finishes. As an example, when machining a zirconium-oxide workpiece for a dental application, a microscale PACVD diamond-coated ballnose endmill produced about 900 finished parts compared with about 100 parts for an uncoated tool.

“Within any cutting process, the coating is constantly being removed,” Derflinger said. “The thicker the coating, the longer it takes to wear it off. Once you are into the carbide, the wear is accelerated further. So a thicker coating normally gives a longer tool life, which then lowers manufacturing costs.”

Some manufacturers may be inclined to apply uncoated carbide cutting tools or tools with traditional coatings because of familiarity with such methods. However, those who take advantage of the superior capabilities of high-performance PVD and diamond PACVD coatings will improve part quality and lower production costs, improving the bottom line.

Some machine builders, Hardinge included, offer turning and grinding machines, making them trusted advisers on which process is most suitable for a given part. Another is EMAG LLC USA, Farmington Hills, Mich., which also offers machines that grind and hard-turn. The company’s director of sales, Kirk Stewart, agreed that hard turning offers many opportunities for improvement in productivity and part quality, and proper machine design is critical to success.

Because machine tools repeatedly remove material, frequently at high spindle speeds, to shape a workpiece, carbide cutting tools are often applied instead of HSS ones to retain a sharp cutting edge and extend tool life. However, when machining highly abrasive materials, such as carbon fiber-reinforced polymer, glass fiber-reinforced plastic, graphite, aluminum alloys or ceramics, even carbide tools can rapidly wear.

Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.

Cutting tool materials based on aluminum oxide and silicon nitride. Ceramic tools can withstand higher cutting speeds than cemented carbide tools when machining hardened steels, cast irons and high-temperature alloys.

When carbon content in composites or silicon content in aluminum alloys becomes too high, cutting tools typically require a diamond coating to minimize wear. Traditionally, PCD-coated cutting tools have been utilized in such instances. PCD is a composite of diamond particles sintered together with a metallic binder. Diamond is the hardest and therefore most abrasion-resistant material.

When it comes to machining aluminum alloys, including those with silicon concentrations of 17% or higher and ceramic particles, Nano’s diamond coating can replace more expensive PCD tools. For instance, in an application where a Duralcan composite workpiece composed of ceramic particle-reinforced aluminum materials was drilled, a PACVD diamond-coated cutting tool drilled 20 times more holes compared with even diamond­like carbon coatings.

“With aluminum-silicon alloys, there are very hard silicon particles embedded in the aluminum,” Derflinger said. “When you have to cut the material, the silicon content is extremely abrasive and can rip up the carbide tool. Even tooling with typical protective hard coatings can degrade very quickly.”

Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).

Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

Among the PVD options are several carbon-based coatings that provide a unique combination of extreme surface hardness and low friction coefficient properties. One example, Balinit Hard Carbon by Oerlikon Balzers, is suitable for machining nonferrous materials, including aluminum alloys with up to 12% silicon content.

Balinit Hard Carbon by Oerlikon Balzers is deposited on tools to machine nonferrous materials, including aluminum alloys with up to 12% silicon content. Image courtesy of Oerlikon Balzers

Grinding operation in which the workpiece is rotated around a fixed axis while the grinding wheel is fed into the outside surface in controlled relation to the axis of rotation. The workpiece is usually cylindrical, but it may be tapered or curvilinear in profile. See centerless grinding; grinding.

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Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.

“As a cost-effective, high-performance alternative, specialized PACVD-based diamond coatings can increase the service life of the tool,” Derflinger said.

Del Williams is a technical writer in Torrance, California. He writes about health, business, technology and education and has a Master of Arts in English from California State University, Dominguez Hills.

As a cutting tool material, PCD has good wear resistance but lacks chemical stability at high temperatures and dissolves easily in iron. So PCD tools are usually limited to materials like high-silicon aluminum, metal-matrix composites and CFRP. In addition, PCD tools are geometrically limited in structure and may be too rough or unrefined for optimal machining of the wide range of nonferrous materials. Finally, the initial cost of PCD cutting tools can be quite high.

“Hardinge Super-Precision lathes offer 0.1μm programmable resolution,” he said. “Axial errors are mapped and compensated for electronically. All mating surfaces within the machine are hand-scraped, the linear guide ways and ballscrews are oversized, and the base of the machine is filled with composite polymer for vibration damping. Not only does this produce the accuracy and rigidity needed to replace many grinding operations, it also increases tool life during hard turning by up to 30 percent.”

In a growing number of industries, including automotive and aerospace, manufacturers continue to place more emphasis on design and weight reduction. Designers subsequently increasingly use composite fiber-reinforced plastics in many parts, but these composites are exceedingly rough on cutting tools.

Kip Hanson is a contributing editor for Cutting Tool Engineering magazine. Contact him by phone at (520) 548-7328 or via e-mail at kip@kahmco.net.

Groove or other tool geometry that breaks chips into small fragments as they come off the workpiece. Designed to prevent chips from becoming so long that they are difficult to control, catch in turning parts and cause safety problems.

Horn added to its Supermini line for boring workpieces as hard as 66 HRC without the use of PCBN. Image courtesy Horn USA Inc., Nico Sauermann.

“Hard turning offers lower machine investment, reduced setup and tool inventory, fewer operations, faster cycle times and greater process flexibility,” he said. “Unfortunately, many shops can use it only for semifinishing of parts prior to grinding, primarily because the majority of CNC lathes are unable to achieve the extreme tolerances and form accuracy produced by cylindrical grinding machines.”

Cutting tool material consisting of polycrystalline cubic boron nitride with a metallic or ceramic binder. PCBN is available either as a tip brazed to a carbide insert carrier or as a solid insert. Primarily used for cutting hardened ferrous alloys.

Aluminum containing specified quantities of alloying elements added to obtain the necessary mechanical and physical properties. Aluminum alloys are divided into two categories: wrought compositions and casting compositions. Some compositions may contain up to 10 alloying elements, but only one or two are the main alloying elements, such as copper, manganese, silicon, magnesium, zinc or tin.

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Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

Hard turning is used to finish a variety of parts, such as bearing journals and races, brake drums and rotors, cylinder bore liners, gears, pinions and splines—or to semifinish those same components prior to grinding. Properly applied, it achieves an accuracy best measured in microns and, in many cases, is faster and more cost-effective than cylindrical grinding.

Indexable ceramics are another option. Jack Kohler, applications engineer for Greenleaf Corp., Saegertown, Pa., said the cost of ceramic cutting tools falls somewhere between PCBN and carbide, and offers equivalent or better performance in some applications. “Ceramic does quite well in the 50-HRC to 65-HRC range,” he said. “Cutting speeds would be comparable to PCBN. Figure around 700 sfm on a 55-HRC A-2 tool steel, for example, with only slightly lower tool life.”

Tom Sheehy, applications engineering manager for Hardinge Inc., Elmira N.Y., said hard turning can be performed on virtually any lathe and provides many benefits.

Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.

The combination of coating hardness and a low friction coefficient can also dramatically improve production even when dry machining. In an application machining CFRP and thermoplastics, for instance, a Balinit Hard Carbon-coated countersink produced 180% more parts than an uncoated tool. In another case, a coated carbide endmill doubled the parts produced when dry machining compared with an uncoated tool using lubricant.

“The Hard Carbon coating works on CFRP and GFRP but only when the fiber content is on the lower side,” Derflinger said. “The more fiber content, the more abrasive the material is, and then you need an even harder coating.”

Materials composed of different elements, with one element normally embedded in another, held together by a compatible binder.

Despite these capabilities, some parts are not suitable for hard turning. Bearing seals, for example, often call for ground surfaces, which eliminate the possibility of fluid escaping through what is essentially a microscopic thread-wide channel produced by single-point turning. And the “white zone” created when material softens and subsequently rehardens during turning (and grinding to a lesser extent) may cause premature component failure. Sheehy said both of these situations can be minimized with the right tooling and a few process adjustments.

Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

“A cutting speed of 600 sfm is a good starting point for PCBN,” he said. “We recommend a double-sided round insert where possible, carefully rotating it as the tool wears. Depending on depth of cut, this might provide 10 to 20 uses per side. Some shops are scared off by the relatively high price of these inserts, however. In these cases, we’d likely suggest a PCBN-tipped insert —or even one of our new superhard carbide grades.”

Condition whereby excessive friction between high spots results in localized welding with subsequent spalling and further roughening of the rubbing surface(s) of one or both of two mating parts.

The Nano coating also works well with abrasive CFRP and GFRP materials. In one example, approximately 380 holes were drilled with a PACVD diamond-coated tool when drilling workpieces made of CFRP and aluminum compared with about 60 holes with an uncoated tool.

For cutting tools to withstand heavy wear, a specialty coating with a very high resistance to abrasion is needed. Image courtesy of Oerlikon Balzers

Cutting tool material consisting of natural or synthetic diamond crystals bonded together under high pressure at elevated temperatures. PCD is available as a tip brazed to a carbide insert carrier. Used for machining nonferrous alloys and nonmetallic materials at high cutting speeds.

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Balinit Diamond Micro and Nano coatings are examples of PACVD-based diamond coatings formulated specifically for the needs of a range of highly abrasive, nonferrous materials. While both are well suited to machine GFRP, CFRP and ceramics, Micro’s formulation is ideal for graphite.

As an alternative, plasma-assisted chemical vapor deposition can be used to apply crystalline diamond structures in varying thickness and roughness. This can be highly advantageous for machining CFRP, GFRP, graphite, nonferrous materials and ceramics. The diamond coating extends tool life while improving cutting quality and surface finish. With the PACVD process, a carbide cutting tool is sequentially coated by two different gases in a heated vacuum container assisted by plasma. The alternating cycles that built the atomic layer on the surface and the number of cycles thus control the thickness of the final coating.

Kohler said he’s seeing increased use of ceramic in high-temperature alloys, such as Inconel 718 and Hastelloy, although he warns shops to steer clear of titanium, as this presents a fire risk because titanium chips can burst into flames at the high cutting speeds common with ceramics. Regardless of the metal being cut, ceramic inserts usually come with a slight hone, land or combination of the two at the cutting edge to prevent chipping and increase strength.

Tool that cuts a sloped depression at the top of a hole to permit a screw head or other object to rest flush with the surface of the workpiece.

Process of increasing the surface hardness of a part. It is accomplished by heating a piece of steel to a temperature within or above its critical range and then cooling (or quenching) it rapidly. In any heat-treatment operation, the rate of heating is important. Heat flows from the exterior to the interior of steel at a definite rate. If the steel is heated too quickly, the outside becomes hotter than the inside and the desired uniform structure cannot be obtained. If a piece is irregular in shape, a slow heating rate is essential to prevent warping and cracking. The heavier the section, the longer the heating time must be to achieve uniform results. Even after the correct temperature has been reached, the piece should be held at the temperature for a sufficient period of time to permit its thickest section to attain a uniform temperature. See workhardening.

Initially, carbide is also less expensive, although Graham pointed out that cost per edge favors PCBN. “PCBN might cost 10 to 25 times more than carbide, but you’re also going to get 50 to 100 times the tool life. Certainly, for high production, PCBN is the way to go.”

Ability of the tool to withstand stresses that cause it to wear during cutting; an attribute linked to alloy composition, base material, thermal conditions, type of tooling and operation and other variables.

Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

In these cases, further hardening carbide cutters with specialty coatings can significantly improve wear resistance and service life. For extremely expensive cutting tools, this not only reduces costs but shortens cycle times. These coatings come in a variety of types, from physical vapor deposition coatings to proprietary diamond coatings.

“A significant benefit of changing from grinding to hard turning is the reduction in capital investment,” Stewart said. “Grinding, however, does have its place and is, in some instances, a faster process when multiple features are ground simultaneously—a situation that is ideal for high-volume applications. Thus, in a high-volume environment, when there are only one or two features that require finishing, hard turning might be the better process for overall capital investment.