Tackling Triple Nickel

Author Alan Richter
Published
February 01,2011 - 11:15am

When it comes to milling aerospace parts from Ti5553, difficulty is in the eye of the beholder.

CTEplus video illustrations

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Milling or otherwise machining the titanium alloy Ti5Al5V5Mo3Cr, or Ti5553, is something like having a bad first date, but a better second one. You get an unpleasant first impression of someone who later exhibits the opposite characteristics after you get to know them better. That’s a good thing, because the alloy has become an important workpiece material for critical aerospace structural components, such as landing gear.

As a result, new cutting tools and machining strategies are being developed for Ti5553. For example, John Palmer, U.K.-based global aerospace manager for ATI Stellram, LaVergne, Tenn., said the toolmaker developed its 5230VS chevron-style milling cutter when “looking at different ways to machine this challenging material.” However, by applying the correctly designed cutting tool with the proper milling strategy, toolpath and coolant supply in a rigid machine tool, “it becomes very easy to machine Ti5553,” he added. “You just have to think a bit more before you start to machine.”

Others interviewed for this article concurred. “Today, triple 5-3 has demonstrated to be very predictable with machining consistency similar Ti6Al4V,” said Michael Standridge, aerospace industry specialist for Sandvik Coromant Co., Fair Lawn, N.J. “The variances in the two materials create the need for different cutting data to be used to obtain similar tool life. Once you have your parameters set properly, triple 5-3 is relatively easy to machine.”

 

Courtesy of ATI Stellram

ATI Stellram offers a variety of milling cutters for machining titanium and other difficult-to-cut materials.

The main problem part manufacturers have when initially milling titanium is trying to transfer the knowledge gained from machining other metals, according to Gary Churchill, technical director for metal distributor Titanium Metal Supply Inc., Poway, Calif. “Then they sort of short circuit,” he said.

He recalled one shop that complained about hard and soft areas in large titanium forgings, which Churchill suspected was not the case. He asked if the customer had previously machined titanium. The response? “No, but we machine aluminum all the time.” After providing recommended speeds and feeds for titanium, the perceived problem of poor-quality material disappeared.

This article focuses on the properties of the workpiece material, design of the cutting tool system—including substrate, geometries and coatings—and appropriate coolant application to effect-ively mill Ti5553 to produce aerospace structural components. [Editor’s note: machine tool design for cutting titanium was covered in the September 2010 article “Ti Machines.”]

Made to be Difficult

Nicknamed “triple nickel,” Ti5553 is considered more difficult to machine than the more common Ti6Al4V because of its higher alloying elements of molybdenum, vanadium and chromium. “These higher content percentages make Ti5553 more challenging to machine, so your tool substrate, geometry and coating need to be right,” said Nick Trott, a technical sales manager for M.A. Ford Europe Ltd., Derby, U.K., a division of Davenport, Iowa-based M.A. Ford Manufacturing Co. Inc., which manufactures the tools. “Cutting parameters and milling strategy also play a big part in machining this alloy.”

Trott added that the alloying elements that make Ti5553 challenging to machine enhance the properties desired for high-load aerospace structural components, such as landing gear.

Palmer explained that heat treatments penetrate further into a Ti5553 part than one made of Ti64, increasing the linear load section strength in Ti5553 up to about 6 " compared with about 2 " for Ti64.

Aerospace part designers welcome the material’s ability to reduce part weight while not sacrificing strength, but chromium and molybdenum make the material “more aggressive [to machine],” Palmer said. “You must have a tool material capable of withstanding that type of cutting aggression.”

Balanced Cutting

Tools designed to cut other titanium alloys are also suitable for machining Ti5553, but not at the same speed. Some machinists reduce the cutting speed for Ti5553 to half of what is appropriate for Ti64, noted T.J. Long, engineering manager for indexable milling systems at Kennametal Inc., Latrobe, Pa.

Not all applications require cutting speeds that conservative, but triple nickel isn’t going to be cut faster. For example, when taking a 0.050 " axial DOC with a high-feed mill, 140 to 160 sfm is appropriate for milling Ti64 but 100 to 120 sfm is the range for Ti5553, according to Terry Carrington, aerospace industry product manager for Iscar Metals Inc., Arlington, Texas. 

Standridge concurred. “You must run slower to achieve a balance of tool life and tool security,” he said.

Balance is also the operative word when developing a substrate for tools designed to cut titanium. Such a substrate must have a balanced mixture of hard particles to resist heat and tough particles to absorb shock, Standridge emphasized.

That’s typically achieved, in part, by sintering submicron-grain carbide for hardness with 10 to 12 percent cobalt binder content for toughness, Trott noted.

For its chevron-style mill, Stellram selected its X500 carbide grade because it withstands crack propagation caused by heat or vibration through the workpiece and stays intact in the worst machining conditions, according to Palmer. He noted that the grade can rough Ti5553 at a “relatively high” surface speed of 80 to 150 sfm.

For finishing, Palmer added that ultrafine micrograin substrates offer the opportunity to produce sharp cutting edge profiles, precision ground or honed to provide a polished profile, rather than a standard ground surface found on tools for general milling. “If you have coarse-grain carbide and produce a sharp cutting edge, the edge is going to look sawtooth rather than razorlike,” he said.

Carbide isn’t the only tool material suitable for milling Ti5553. Cobalt and P/M HSS tools are often applied to cut titanium, particularly on older, less rigid machines. Still, Palmer feels the productivity gains carbide tools provide make HSS tools part of history. That’s because HSS traditionally run at 20 to 30 sfm and carbide tools can finish Ti5553 at up to 300 sfm, he noted.

Geometries at Work

Effectively milling titanium requires positive geometry to shear the material, reducing cutting forces, pressure and generated heat. “The positive cutting action is not just found in the edge line, but in the axial and radial rakes in the tool body tip seat,” Standridge said.

As previously noted, tools for milling titanium require sharp cutting edges. That’s because titanium exhibits a low modulus of elasticity, which leads to a “springiness” characteristic whereby titanium parts may move under the force of the cutting edge and then spring back, according to Iscar Metals.

Creating that sharp edge without negatively impacting the tool material and edge strength can be a challenge. Although an as-pressed insert has a stronger cutting edge than an insert with a ground edge, titanium typically needs to be cut with an edge that’s ground sharp, according to Carrington.

Beyond BLAST milling drawing.tif

Courtesy of Kennametal

Kennametal Beyond Blast inserts channel coolant through the insert to the tool/workpiece interface to provide efficient coolant delivery, lubricity and heat transfer (see below).

thru_insert_cooling revised.tif

Courtesy of Kennametal

An example of how Beyond Blast through-insert cooling works on a specific workpiece.

“Anytime you put a grinding wheel on a piece of carbide, you have reduced the integrity of that cutting edge—period,” he said. “You can’t put heat on a piece of carbide without changing its physical structure a little. It pulls binder out of that carbide.”

An edge that’s too sharp, however, can be easily damaged. “Typically, the primary failure mode in titanium machining is microchipping,” Long said, “which tends to progress to macrochipping.” To combat that, a light hone on the edge is suitable for stable machining environments and a T-land edge preparation may be required for less-stable conditions, he noted.

Sharpness is also a concern at the end of a tool. To protect that corner, or end edge, M.A. Ford offers a range of corner radii, Trott noted. “If you gash a tool with a sharp corner, it would not have the strength to tackle Ti5553 or any other titanium alloy for that matter,” he said.

Chip Shape

One common tool geometry isn’t always required when milling titanium. “Our titanium endmills don’t have chipformers or chipbreakers,” Trott said. “We don’t find them necessary.” 

Because titanium “doesn’t particularly like to be machined, you have to make the cutting edges as sweet cutting as possible, but you also have to make them resilient to withstand wear and loading, which we believe we’ve achieved with our titanium endmills,” Trott said. “Once you have an endmill designed for the job, with the correct parameters and milling strategy, Ti5553 becomes pretty easy to machine.”

Once a chip is formed, it must be evacuated to avoid recutting it. To effectively export the chip from the cutting zone, ATI Stellram designed its titanium milling tools with a round flute to match the shape of the chip instead of the traditional V-type flute for standard cutters. 

Palmer noted that the critical geometric feature is sufficient clearance on a tool’s cutting edge. Without it, built-up edge appears on the flank rake, which creates friction with the workpiece surface and additional heat. Eventually, a piece of the built-up metal deposits itself on the workpiece via friction welding, damaging the cutting edge and galling the workpiece.

“When machining steel, you talk about BUE on the front primary rake,” Palmer explained. “With titanium, yes, it does build up a slight deposit on the cutting edge, but the big danger is the buildup of material on the flank surface on the relief of the cutting edge.”

As with other materials, vibration must be minimized when milling titanium alloys. One way to achieve that is with variable, or differential, pitch, noted Long. Changing flute spacing can break up vibration-generating harmonics.

Long added that increasing the number of teeth on a cutter boosts productivity because the chip load and cutting speed are limited when milling titanium. But increasing the number of teeth reduces the ability to provide differential pitch and may cause chip packing. “You have to weigh the benefit of a higher number of teeth against having a coarser pitch tool with a differential pitch and/or a more open chip gash,” he said.

Although a rigid machine is extremely important when machining titanium, Long noted that plunge milling and high-feed cutters can boost productivity on less-rigid conventional machines.

A variable-helix tool also helps reduce vibration by altering the repetitive cutting cycle that induces harmonics. Standridge explained different helix angles induce different cutting forces in the material, with a high helix tending to grab or pull the workpiece into the cutting tool and a smaller helix tending to have the opposite effect, pushing more on the material. Toolmakers design various helix angles in combination with sharp, microscale edge lines to create an effective balance between shear effect and edge stability. It’s advantageous, for example, when finishing thin walls. “You need to balance the cutting forces with a sharp cutting action to minimize heat generation in the material for the best overall stability in the component feature,” he said.

Keep It Cool

Titanium is a poor heat conductor, so only about 25 percent of the heat in the cutting zone—which can be 1,000° C or hotter—is transferred to the chip. Compared to steel, only one third of the heat is transferred. Therefore, coatings such as aluminum titanium nitride, titanium carbonitride and titanium silicon nitride—generally in a multilayer configuration—create a heat barrier to extend tool life without altering edge sharpness by being too thick.

According to Ravi Iyer, senior engineer in Kennametal’s Product Engineering Group, a physical vapor deposition coating is desirable for milling titanium. He noted that a PVD coating is more effective than a CVD one in a wet environment, which is highly recommended because of titanium’s low thermal conductivity and to minimize the risk of igniting titanium chips—a fire that’s difficult to extinguish. 

In addition, a PVD coating exhibits a compressive residual stress state, which is well suited to the thermo-mechanical load cycling caused as the milling inserts enter and exit the cut, Iyer explained.

Titanium can be milled with uncoated tools, but modern coating processes have overcome any issues end users had with coatings, according to Iscar’s Carrington. “If you’re running an uncoated product today, by and large you’re leaving money on the table,” he said.

In addition to providing a heat barrier, a coating provides lubricity. “Titanium is starving for lubrication, and anything a little slicker is going to perform better,” Carrington said, noting that he feels TiCN provides more slickness than TiAlN.

Coolant also provides the lubricity titanium craves, but only if the coolant concentration is high enough. According to Carrington, machining titanium is significantly more effective when a 14 percent coolant concentration is applied, which compares with a 5 to 7 percent concentration when machining multiple materials and 9 to 11 percent for high-temperature alloys.

Carrington recalled how he helped achieve acceptable cutting speed and tool life for one customer’s titanium milling application, but on the next product run received complaints about the endmills. After examining the tools to make sure they weren’t ground incorrectly and reducing cutting speed by 30 percent, tool life was still about half of what it was initially. Convinced the tools were the problem, the customer switched suppliers only to find that the original endmill outperformed the others he tried even at the reduced parameters for the original. On the third product run, the customer revealed the problem was eliminated. “I said, ‘OK, I give. What’s the deal?’ He said, ‘We changed the coolant and percentage.’ Same part, same machine, same cutting tool, same cutting parameters—dramatically different results,” Carrington said.

Under Pressure

How coolant is directed to the tool/workpiece interface is also critical. External coolant systems still might be the primary method, but high-pressure/increased flow-rate, through-the-tool coolant systems are becoming more popular. “More than any other material, the use of high-pressure coolant is very beneficial when cutting heat-resistant superalloys like titanium,” Standridge said. “High-pressure coolant directed properly into the cutting zone creates a separation point, so there’s less contact time between the titanium chips, the workpiece material and the cutting edge.” That reduces edge line wear.

 

Courtesy of Sandvik Coromant

The challenge in milling is to deliver the proper flow rate to achieve high-pressure levels because a multiple-flute mill has numerous coolant holes, which creates more area for the coolant to fill. To address that, Sandvik Coromant developed a jet nozzle system in which threaded nozzles are positioned in coolant holes to restrict flow at the cutting edge and increase pressure, similar to placing your thumb over a garden hose.

The challenge in milling is to deliver the proper flow rate to achieve high-pressure levels because a multiple-flute mill has numerous coolant holes, which creates more area for the coolant to fill, Standridge noted. To address that, Sandvik Coromant developed a jet nozzle system in which threaded nozzles are positioned in coolant holes to restrict flow at the cutting edge and increase pressure, similar to placing your thumb over a garden hose, he explained. (See video illustration above.)

The goal is to achieve a coolant pressure of about 1,000 psi, or 70 bar, but an adequate flow rate is also important, Standridge noted. “We typically like to see pumps with a minimum flow rate of 15 gpm.”

Although high-pressure coolant’s quenching action helps cool the cutting zone, it also assists in mechanically forming and evacuating chips. To enhance those coolant functions, Kennametal introduced Beyond Blast tools with through-coolant inserts to channel fluid directly to the milling cutter/ workpiece interface. (See illustrations above.)

“Coolant forces are getting under the chip and directing it away from the cut, whereas the coolant is coming from outside the cutting zone and pushing the chip back into the cut in a traditional coolant system,” said George Coulston, Kennametal’s vice president of Innovation Ventures. “Beyond Blast pushes chips out of the cutting zone.”

Extending tool life is one of the main benefits of reducing cutting zone temperature. “By having more efficient coolant delivery, we can lower the temperature at the tool/workpiece interface,” Coulston said. “Carbide’s strength deteriorates as the temperature increases.”

Effectively milling Ti5553 and other titanium alloys requires a systematic approach in which using a rigid machine, programming toolpaths to reduce the cutting force and securely holding workpieces—among other elements—must be considered collectively. Having the proper cutting tool alone won’t guarantee success. Nonetheless, the tool is what meets the metal and strips the chip from its parent. And having the right tool goes a long way in preventing an application from being truly challenging. CTE

Richter1.tifAbout the Author: Alan Richter is editor of CTE. He joined the publication in 2000. Contact him at (847) 714-0175 or alanr@jwr.com.

 

 

 

Vacuum heat treating titanium for airframe applications 

Aeronautical engineers continually search for new and optimal materials to achieve specific design requirements throughout an airframe. Many considerations impact the structural design of an aircraft, such as the complexity of the load distribution through a redundant structure, the large number of intricate systems required and the operating environment of the airframe. All are primarily governed by weight savings. Thus, the optimal materials are composites, such as carbon fiber-reinforced plastic, and titanium alloys.

Composites, which excel at handling tension, greatly reduce maintenance due to fatigue compared to aluminum. When engineers analyzed loading and environmental factors, aluminum was determined to be a poor choice. Titanium is also a low-maintenance, high-strength material. In the new Boeing 787 Dreamliner, about 15 percent of the total airframe is titanium.

Within the 787 structure, most of the heat-treatable titanium is used in landing gear, structural fittings, floor structures, extrusions and nacelles.

While Ti64 has been the workhorse titanium alloy in aerospace applications, several flight-critical parts in the 787 aircraft are made of the Ti5Al5V5Mo3Cr beta alloy. Compared to Ti64, Ti5553 exhibits an enhanced hardening ability, and higher strength, fracture toughness and cycle-fatigue behavior. As a result, parts made of Ti5553 forgings are used in highly loaded locations, such as flap tracks, pylons, sides of body chords and landing gear.

Without the proper heat treatment of this exotic material, specific critical metallurgical properties could not be attained. Heat treatment of Ti64 typically involves a solution treatment above the beta transus temperature (the lowest temperature at which a 100 percent beta phase can exist), followed by fast cooling (water quenching). The part is then age hardened at a prescribed temperature for a period of time. This treatment works well for most wrought products. However, aeronautical engineers must design for improved “buy-to-fly” ratios—the mass of material required to machine a part compared to the mass of the original part.

Because Ti64 distorts excessively during water quenching, an alloy is needed that can be “control cooled” to manage distortion. That alloy is Ti5553. The BASCA (beta anneal slow cool age) heat treatment of Ti5553 produces superior ultimate properties and better buy-to-fly ratios than Ti64 by enabling engineers to design near-net-shape parts.

Engineers design parts according to a variety of heat treat specifications. Because there is no universal specification, contradiction and nebulous statements exist within specification documents. The following are summaries of the top five heat treating problems.

1. Heat treat specifications do not describe critical cooling rates for quenching per units of measure (degrees F or C). The exact cooling rate of “water,” “oil” or “air” is unclear because cooling rates of liquid quenchants vary based on the media’s immersion temperature, agitation and heat-exchange rate.

2. Titanium heat treat specifications assume subsequent thermal processing will produce an oxygen-rich layer of alpha case. This is not necessarily true with today’s vacuum technology. There are many methods to help minimize formation of an alpha case in vacuum.

3. Severe problems can arise when titanium alloys contact hydrogen-rich environments. Typically, hydrogen pickup occurs during metal manufacturing and processing, especially at noninert, elevated temperatures. Deep-vacuum processing along with elevated temperature depletes this hydrogen down to single-digit-part-per-million levels. Specifications must state a maximum allowable hydrogen ppm level to help eliminate the risk of hydrogen embrittlement and material failure.

4. Specifications do not acknowledge the use of graphite for fixturing in vacuum. Graphite fixturing enables an engineer to design more precise near-net-shaped components. Graphite’s attributes include excellent heat transfer characteristics and coefficient of thermal expansion, which mimics titanium’s CTE and remains dimensionally stable and strong at a high temperature. Also, graphite can be machined to hold tight tolerances during thermal cycling.

5. Specifications do not clearly define and differentiate between work and control thermocouples. All values must be recorded and reported on only the workpiece thermocouples.

Boeing and Airbus project the global airplane fleet will double by 2029, and the use of composites and titanium in airframes will continue to grow. Currently, Boeing’s and Airbus’ combined titanium consumption is 60 million lbs. per year.

To thermally process titanium, metallurgists must interact with Boeing, AMEC and SAE committees to discover and remedy specification shortfalls. These actions will improve thermal processing and enhance safety in military and commercial aircraft.

—Robert Hill Jr., president of Solar Atmospheres, Hermitage, Pa. Contact him at (866) 982-0660.

Contributors

ATI Stellram 
(615) 641-4200
www.atistellram.com

Iscar Metals Inc.
(817) 258-3200
www.iscarmetals.com

Kennametal Inc.
(800) 446-7738
www.kennametal.com

M.A. Ford Manufacturing Co. Inc.
(800) 553-8024
www.maford.com

Sandvik Coromant Co.
(800) SANDVIK
www.sandvik.coromant.com/us

Solar Atmospheres
(866) 982-0660
www.solaratm.com

Titanium Metal Supply Inc.
(888) 748-8510
www.titaniummetalsupply.com

Related Glossary Terms

  • alloys

    alloys

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

  • built-up edge ( BUE)

    built-up edge ( BUE)

    1. Permanently damaging a metal by heating to cause either incipient melting or intergranular oxidation. 2. In grinding, getting the workpiece hot enough to cause discoloration or to change the microstructure by tempering or hardening.

  • built-up edge ( BUE)2

    built-up edge ( BUE)

    1. Permanently damaging a metal by heating to cause either incipient melting or intergranular oxidation. 2. In grinding, getting the workpiece hot enough to cause discoloration or to change the microstructure by tempering or hardening.

  • chemical vapor deposition ( CVD)

    chemical vapor deposition ( CVD)

    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.

  • clearance

    clearance

    Space provided behind a tool’s land or relief to prevent rubbing and subsequent premature deterioration of the tool. See land; relief.

  • composites

    composites

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

  • coolant

    coolant

    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.

  • cutting force

    cutting force

    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.

  • cutting speed

    cutting speed

    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).

  • edge preparation

    edge preparation

    Conditioning of the cutting edge, such as a honing or chamfering, to make it stronger and less susceptible to chipping. A chamfer is a bevel on the tool’s cutting edge; the angle is measured from the cutting face downward and generally varies from 25° to 45°. Honing is the process of rounding or blunting the cutting edge with abrasives, either manually or mechanically.

  • embrittlement

    embrittlement

    Reduction in the normal ductility of a metal due to a physical or chemical change. Examples include blue brittleness, hydrogen embrittlement and temper brittleness.

  • endmill

    endmill

    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.

  • fatigue

    fatigue

    Phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.

  • fracture toughness

    fracture toughness

    Critical value (KIC) of stress intensity. A material property.

  • galling

    galling

    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.

  • gang cutting ( milling)

    gang cutting ( milling)

    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

  • grinding

    grinding

    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.

  • grinding wheel

    grinding wheel

    Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.

  • hardening

    hardening

    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.

  • hardness

    hardness

    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.

  • high-speed steels ( HSS)

    high-speed steels ( HSS)

    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.

  • lubricity

    lubricity

    Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.

  • milling

    milling

    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • milling cutter

    milling cutter

    Loosely, any milling tool. Horizontal cutters take the form of plain milling cutters, plain spiral-tooth cutters, helical cutters, side-milling cutters, staggered-tooth side-milling cutters, facemilling cutters, angular cutters, double-angle cutters, convex and concave form-milling cutters, straddle-sprocket cutters, spur-gear cutters, corner-rounding cutters and slitting saws. Vertical cutters use shank-mounted cutting tools, including endmills, T-slot cutters, Woodruff keyseat cutters and dovetail cutters; these may also be used on horizontal mills. See milling.

  • milling machine ( mill)

    milling machine ( mill)

    Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

  • modulus of elasticity

    modulus of elasticity

    Measure of rigidity or stiffness of a metal, defined as a ratio of stress, below the proportional limit, to the corresponding strain. Also known as Young’s modulus.

  • physical vapor deposition ( PVD)

    physical vapor deposition ( PVD)

    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.

  • physical vapor deposition ( PVD)2

    physical vapor deposition ( PVD)

    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.

  • pitch

    pitch

    1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.

  • plunge milling

    plunge milling

    Highly productive method of metal removal in which an axial machining operation is performed in a single tool sequence. The tool makes a series of overlapping, drill-like plunges to remove part of a cylindrical plug of material one after another. Because of the increased rigidity of a Z-axis move, the tool can cover a large cross-section of material.

  • quenching

    quenching

    Rapid cooling of the workpiece with an air, gas, liquid or solid medium. When applicable, more specific terms should be used to identify the quenching medium, the process and the cooling rate.

  • rake

    rake

    Angle of inclination between the face of the cutting tool and the workpiece. If the face of the tool lies in a plane through the axis of the workpiece, the tool is said to have a neutral, or zero, rake. If the inclination of the tool face makes the cutting edge more acute than when the rake angle is zero, the rake is positive. If the inclination of the tool face makes the cutting edge less acute or more blunt than when the rake angle is zero, the rake is negative.

  • relief

    relief

    Space provided behind the cutting edges to prevent rubbing. Sometimes called primary relief. Secondary relief provides additional space behind primary relief. Relief on end teeth is axial relief; relief on side teeth is peripheral relief.

  • residual stress

    residual stress

    Stress present in a body that is free of external forces or thermal gradients.

  • sintering

    sintering

    Bonding of adjacent surfaces in a mass of particles by molecular or atomic attraction on heating at high temperatures below the melting temperature of any constituent in the material. Sintering strengthens and increases the density of a powder mass and recrystallizes powder metals.

  • superalloys

    superalloys

    Tough, difficult-to-machine alloys; includes Hastelloy, Inconel and Monel. Many are nickel-base metals.

  • titanium aluminum nitride ( TiAlN)

    titanium aluminum nitride ( TiAlN)

    Often used as a tool coating. AlTiN indicates the aluminum content is greater than the titanium. See coated tools.

  • titanium carbonitride ( TiCN)

    titanium carbonitride ( TiCN)

    Often used as a tool coating. See coated tools.

  • titanium carbonitride ( TiCN)2

    titanium carbonitride ( TiCN)

    Often used as a tool coating. See coated tools.

  • titanium nitride ( TiN)

    titanium nitride ( TiN)

    Added to titanium-carbide tooling to permit machining of hard metals at high speeds. Also used as a tool coating. See coated tools.

  • toolpath( cutter path)

    toolpath( cutter path)

    2-D or 3-D path generated by program code or a CAM system and followed by tool when machining a part.

Author

Editor-at-large

Alan holds a bachelor’s degree in journalism from Southern Illinois University Carbondale. Including his 20 years at CTE, Alan has more than 30 years of trade journalism experience.