Much of the HSM work is concentrated in mold and die work as in this operation using SGS cutting tools.

Much information on high-speed machining (HSM) involves aluminum, but what about the other metals? "We do so well with HSM of aluminum because the melting temperature of aluminum falls below the temperature at which many cutting tools start to have trouble," says NIST project manager, Rob Ivester. "For example, aluminum melts at 660ºC, but a carbide tool can withstand temperatures to around 1000ºC. However, iron melts at around 1535ºC and titanium at 1660ºC. So the problem is carbide tooling wears rapidly when machining steel or titanium alloys at high speeds."

Machine tool builders, software developers, and in particular, cutting-tool makers offer a spectrum of products for this market. In addition, many researchers are looking into some high-end machining problems. NIST is doing fundamental wear measurements for turning operations, particularly the temperatures of the cutting action, according to Ivester, "Temperature and impact are the big problems when it comes to cutting-tool progress. Heat, force magnitude, and impact do the damage along with cyclical pressure variations."

Unique problems abound when machining harder materials. Residual stress can be a concern. Both those initially in the workpiece and those induced by the cutting operations.

"A big part of our effort to improve cutting performance is to look at surface integrity," says Roger Grey, a grade development specialist at Sandvik Coromant (Fairlawn, NJ). "That is, what is the cutting action doing to the workpiece material? For example, we look at tool-geometry developments with respect to how the material is sheared from the workpiece. We must make sure only minimal stresses are introduced into the metal's surface that will cause adverse metallurgical events or introduce failure mechanisms. The deformation of grain structure adds residual stresses that can be a big problem, particularly in aerospace work.

"We do focus on the aerospace industry as the area with the strongest safety and production regulations. Once we place a product there, we can confidently expand its use into other areas," Grey concludes.

"Another issue is that with harder to machine materials, such as heat-resistant alloys, the tool spends more time in one location, compared to aluminum," says Sandvik Coromant business development manager Brian Davis. "Therefore, there is more heat generation and more pressure on the workpiece that might cause adverse deformation. This is very critical in complex or thin cross sections."

Another issue introduced with HSM is work hardening, particularly with thin-wall material. "We have known cases where a honeycomb part curled up like a potato chip when released from the fixturing, due to induced stresses," says NIST's Ivester.

Coolant for HSM operations is a controversial issue. Dry, mist, and flood cooling are all used. The problem is that, at present, there is no way to get coolant to the actual cutting surface, even with very high pressure, through-the-tool delivery systems. So the coolant in all cases has only peripheral influence on tool and workpiece temperature.

"For hard machining of RC 50 metals, which we call hard machining, we recommend air cooling to avoid thermal shock," explains David Kwon, engineering manager, OSG Tap & Die (Glendale Hts., IL). "Below that hardness we like it dry as well, except in gummy materials like stainless steel. When using small-diameter tools, a 1/32" [0.8 mm] diam end mill for example, we prefer mist cooling for best tool life and critical surface-finish accuracy."

Some, such as Dan Kellogg of Vega Tool Corp. (Schaumburg, IL), the exclusive North American Source for Hitachi tooling, state flatly that coolant is not practical. "It does carry heat away, but causes thermal damage to coatings."

"High-speed roughing and finishing is almost exclusively dry machining because you can't get coolant to the tip at those speeds," says Bret Hopkins, applications engineer, Makino (Auburn Hills, MI). "The only exceptions are gummy materials, like aluminum or some stainless. It builds up on the tool. In our experience, air blow increases tool life 10 to 20 times over the use of coolant.

"We recommend compressed air, or an oil mist in an air stream, to move the chips, not fluids that can cause thermal cracking of the tool coating.

Mist coolant is used sometimes when you need a very fine finish. It's used for the lubricant properties, not for the heat dissipation quality," Hopkins concludes.

"When it comes to coolants on ceramics, they are best run dry," says Sandvik Coromant's Davis, "In die-mold work, it's recommended to run dry to avoid thermal shock to the cutting tool. For applications in heat-resistant materials, such as titanium, heavy volumes of coolant are recommended to avoid chemical and abrasive wear at high speed. At the same time, you must accept the tendency for some thermal cracks. It is the least of two evils."

The cutting tool is the element of HSM that comes up for the most criticism from users, as well as the most development work by its manufacturers.

From the research side, NIST is looking at a new cutting tool material that has less chemical interaction with titanium. It's a coating that blends aluminum, magnesium, and boride. The investigation will concentrate on the heat transfer between tool and chip.

"We have a set of milling tools for high-speed machining work, chiefly molds and dies," explains Jeff Burton, executive VP of manufacturing, SGS Tools (Munroe Falls, OH). "Their design is the best combination of tool materials, geometry, and coating."

Hitachi stresses the "nanocoat," a single coating layer, in its tool designs.

For mold-grade steels up to RC 65 they offer the Power-Carb, a six-flute, flat-bottom tool available in sizes from 1/4 to 1/2" (6.4 - 13 mm). It's designed to run dry. For more complex contouring, the company offers the Turbo-Carb, a solid carbide, two-flute ball end mill in sizes from 1/32 to 3/4" (0.8 - 19 mm).

Both these tools are designed to operate with a chip load of less than 0.0005" (0.012 mm) with high-resolution motion control. Cuts are not heavy. "It's almost like grinding," explains Burton.

For roughing, SGS offers the Z-Carb for materials from low-carbon steel to metal around RC 50. The tool has an unequal helix and flute pitch spacing to minimize speed-limiting harmonics and cut vibrations.

"We have been rough milling nickel and cobalt-based alloys with ceramics using speeds in excess of 3500 fpm [1067 m/min] for some time," explains Kennametal's Marshall. "One of our best choices for high-strength materials has been our advanced Sialon, which is available in two grades. These grades run dry and have high thermal and shock resistance. We recommend using air for chip removal. These tools get a lot of use in aerospace applications where there are instances of them reducing cycle time from hours to minutes.

"One of our biggest problems to overcome is that our customers are unwilling to take advantage of the speed potential of this tool. For example, they may be used to cutting at 150 to 200 fpm [46 - 61 m/min] and they can cut at 3000 to 3500 fpm [914 - 1067 m/min] in 718 Inconel, but they are reluctant to try.

"When you run that fast, you plasticize material, which makes it easier to machine. Chip load is around 0.0035" [0.89 mm] per tooth with a 0.050 to 0.0100" [0.89 mm] DOC. Tool sizes range from 1 to 4" [25 - 102 mm] in diam. Power requirements for a 0.050" [1.3 mm] DOC are in the 20 - 30 hp [15 - 22.5 kW] range. Although tool life is around 6 to 9 minutes, it removes enough material to be an economically sound investment. We expect tool life to increase significantly with the next version of this design," Marshall concludes.

The latest tool designs from Hitachi Tools represent a change in philosophy from multilayer coatings to a single "nanocoat" about 0.001-µm thick. "This design gives longer tool life because it has a 75% lower coefficient of friction than TiAlNi and is three times harder," says Vega's Kellogg. "With this lubricity there is less heat, and less oxidation and wear, which is what eats through coatings. It can handle materials up to RC 80 and tool life can be increased 5 to 10 times."

"For our company, TiAlN is the most prominent coating," says OSG's Kwon. "It has a high heat resistance and works best in steels, titanium, and Inconel, as well as hard die steel. The coating is 15 layers thick, offering optimal tool life. We prepare the substrate in a unique way for greatest adhesiveness. It's only 3 to 5 µin. [0.08 - 0.13-mm] thick overall, which is not greater than a nanolayer coating.

"It's superior to multilayer coatings, which are limited in the number of layers possible using physical vapor deposition before they will no longer adhere. We have the Exocarb high-tech series of tools for harder materials, chiefly those used by our major customers, the die/mold operators," explains OSG's Kwon.

"One of our more successful HSM tools is the Exocarb ball nose cutter for both roughing and finishing. It has a two-flute ball nose with geometries that allow you to take 10% tool diam DOC in RC 40 and below. For maximum removal rates, we have the Exocarb MAX ball nose end mill with a spiral gash design that can go to a 20% tool diam DOC in many applications, while maintaining high speed and feed," says Kwon.

"Coatings may or may not be an advantage," says Sandvik's Grey. "For example, in aerospace work, you cannot use a coating that contains aluminum on titanium because of contamination problems. But generally, cutting tools used on all heat-resistant alloys use coatings."

"Mutilayer coatings on titanium are not a significant benefit," says Davis. "They are minimal. But it's difficult to say that every part and every material would benefit. The coating quickly wears off, and soon after the initial cut you are down to the substrate. So, we focus on the substrate material, since that is doing most of the cutting work."

Machine tools made specifically for HSM have some unique features. In evaluating these designs, notes Makino's Hopkins, today, when it comes to HSM of harder materials, machines can feed faster than tools can cut. Speeds of 2000 to 3000 fpm (610 - 914 m/min) are possible in aluminum, but with steel of RC 50, 400 to 450 fpm (122 - 137 m/min) are more common. You can achieve a chip load of 0.020 - 0.050" (0.5 - 1.3 mm) per tooth with aluminum, but 0.003 - 0.008" (0.8 - 0.2 mm) in hard steel is more the standard. Chip load is the driving force when it comes to machining harder materials.

"Machine tool design plays an important part, particularly rigidity. As the demand grows to work with ever-harder materials, any flexing or thermal growth can lead to inaccuracy in the toolpath.

"The spindle's operating range is also critical. The larger the rpm range of the spindle the better. For example, being able to rough at 1000 rpm and finish at 30,000 rpm with the same spindle is important. It's not good if you have to get up to 10,000 - 15,000 rpm before you get useful torque.

"High rpm is needed when using small-diameter tools. For example, in RC 50 metal, you don't need 20,000 rpm until you are less than 1/8" diam. You need these small diam tools to pick out fine part details. Chip load is still the key to good tool life, so often you need to run a small diam tool below its ideal rpm to maintain proper chip load. This is because you can't maintain high feedrate in small details. If you program for high rpm, say 30,000 - 40,000 rpm, and high feed rate, you get high rpm, but not the feedrate because of part geometry. This causes the tool to rub against the material instead of taking a cut. This rubs off the coating and shortens tool life," Hopkins concludes.

"Many of our machines are used on tool and die steels as well as exotics such as Inconel," explains Bob Burrows, VMC product manager, Haas Automation (Oxnard, CA). "There is a lot of contouring using a 30,000-rpm spindle. This type of cutting eliminates much of the hand polishing and bench work that was traditionally done. We don't offer specific machines for this work. Instead, we have basic machines and add specialized components to meet specific customer needs such as a linear scale, spindle chillers, and special-purpose software. In particular, we emphasize software that minimizes the need for operator skills.

"Because of the highly flexible design, the Haas machine tools can be configured for nearly any application or material simply by adding appropriate options and accessories. Each customer can tailor a Haas machine to meet particular needs. Options include 10,000, 12,000, 15,000 and 30,000-rpm spindles on 40-taper machines and 7500 and 10,000-rpm spindles on the 50-taper machines. The HSM control features look-ahead up to 80 blocks and a motion algorithm to provide contouring feed rates to 833 ipm (21 m/min) without path distortion," Burrows explains.

"One advantage of HSM on harder materials is that you can do a good job with an old machine as long as it has the necessary horsepower, feed rate, and rigidity," says Vega's Kellogg.

"When operators first started to look at high-speed machining of aluminum, the problem was 'appropriate machine tools,'" says Jonathan Saada, president, High Speed Corp. and representative for Hanita Cutting Tools (Thousand Oaks, CA), a company that specializes in aerospace machining of titanium, etc.

"We needed faster spindles, and rapid positioning capability, particularly controls, that could handle the rapid motion and complex shapes. The main change in tools was the move to longer-life solid carbide.

"With the tougher exotic materials, the situation is different: machine tools with slower, conventional spindles because the surface feeds are not that high. You don't need the high-spindle speed or rapid positioning capacity. But the tools are the issue, in particular, the carbide grade, geometry, and especially, the coating.

"We have seen titanium cutting speeds for finishing go from 60 fpm with conventional tooling to 500 fpm (18 - 152 m/min], and in super finishing, even to 1000 fpm (303 m/min). You don't need the sophisticated high-speed machines for milling titanium alloys. For example, with a 1/2" (13 mm) end mill at 500 fpm (152 m/min), 4000 rpm is all you need. Plus, because speeds are slower, you don't need sophisticated controls. The cutting tool and the cutter path become the key factors. High levels of heat are generated. To avoid friction it's important to use end mills with high, sharp helix angles. Among the best coatings we have found are TiAlN and AlTiN.

Simulation is a good way to determine how a cutting tool/machine tool combination will work before doing any actual cutting. Third Wave Co. (Minneapolis, MN) offers finite-element-based machining modeling software and engineering service.

"We are unique in that we don't just model the geometry as does a traditional CAM package," says company president Kerry Marusich. "We take material properties into account. Our programs simulate what the tool will do. It allows the user to do a cutting test on a computer. Inputs are the same for operating a machine tool: feeds, speed, tool type. But the output is more than you get from a shop test. It includes forces, temperatures, and stresses in the machine tool and workpiece."

Various tool materials can be simulated, before the tool exists. "We import the CAD model of the tool, then modify its features, including coatings and geometry," says Marusich. "These tests can pick up potential failure modes no one ever thought of."

One reason for the popularity of such software is the loss of experienced personnel. "New people know computers, but they don't have much shop savvy," states Marusich.

Haas Automation Inc Kennametal Inc Makino OSG Tap & Die Inc Sandvik Coromant Co SGS Tool Co

Third Wave Systems, Inc. Vega Tool Corporation Hanita Cutting Tools