Making the Difficult Easy
Tools and technique double productivity in titanium and high-temperature alloys
By James R. Koelsch
Aerospace suppliers have learned firsthand why you should be careful about what you ask for. They wanted a revival of their industry and got programs like the Boeing 787 and Joint Strike Fighter—airplanes full of titanium and high-temperature alloys that are both expensive and difficult to machine. Because aircraft manufacturers are applying tremendous pressure on their supply chains to keep costs reasonable and competitive, the programs have the entire industry scrambling to boost metal removal rates to manage the ever-rising financial risk and reap greater returns from investments in technology.
For this reason, the entire supply chain has been re-evaluating cutting tool technology and technique. “You can’t live by today’s buzzwords,” says Michael Allawos, president of Mikana Mfg. Co. (San Dimas, CA), a 20-year-old shop specializing in difficult jobs. “We constantly get auction papers from bankrupt machine shops that were completely automated and roboticized.”
He has proven that advanced automation is not necessarily the path to success in the aerospace industry. His shop uses 40-taper VMCs to make parts from titanium, nickel-based alloys, and other exotic materials, yet regularly works with big aerospace companies like Lockheed and the Jet Propulsion Laboratories. Mikana is a partner in the Joint Strike Fighter (JSF) Program, and boasts about 70 part numbers in the landrovers on Mars.
“Most people say we’re nuts, because a 40-taper machine has to go between 30 and 40% slower than a 50-taper machine in titanium and Inconel,” he says. But for him, the move is part of a calculated strategy of investing more in know-how than in machinery. He creates cost efficiencies in other ways.
This is not to say that Allawos shuns technology. He doesn’t. As a partner in the JSF Program, for example, he has invested in the modern information technology necessary to establish an electronic link with Lockheed. “We’re hooked up to their vault, and can download prints and upload schedule information directly to it,” he says. “We tell the matrix when we’re going to be where we’re going to be.”
Rather embracing technology for technology’s sake, he invests in only the level of technology that he really needs. “Managing cash flow is more important now than it has been at any other time in history, because raw materials and cutting tools have gone up and are higher percentages of the cost of jobs,” he explains. “The material for a job might be 40–50% of the cost, rather than only 10–20%, so you have to lay out more cash for more time than you had to in the past before you get paid.”
Making matters worse is the underbidding by shops desperate for badly needed work. “Eventually, these people drop out of the business and you’re left with deflated prices,” says Allawos. “So you have to get even more creative to chase that dollar without falling into the same trap that other folks have made of making too many promises that you can’t keep.”
Being a good manager, for Allawos, means more than maintaining a good relationship with a bank. It means cultivating a culture of experimentation. One way that he has done so is to buy his 13 VMCs from local builders such as Haas Automation Inc. (Oxnard, CA).
Allawos encourages his machinists and engineers to experiment with cutting parameters to learn to push the capabilities of 40-taper machines in the shop, as long as they document the results. The typical experiment begins with trying a variety of cutting tools at the manufacturers’ speeds and feeds. The goal at this stage is to determine which tools perform best in the particular application.
The next step is to put the remaining tools through a number of trials at various speeds and feeds. “We find sometimes that we can go way past the recommended speeds and feeds, because they are too conservative,” says Allawos. “We end up burning up the cutting tools.” Especially in titanium, not taking a big enough bite works against you because the material work-hardens.
The latest success occurred on the muzzle of a gun for the F22 fighter. In a year-long study, engineers from Haas and experts at Mikana developed a toolpath and a set of speeds and feeds that allowed them not only to cut the Inconel 625 workpiece faster, but also to triple the life of the fine-pitch multiflute Hanita roughers. The uncoated 1" (25 mm) cobalt end mills engage the material in a full radial cut in 0.130 to 0.250" (3.3–6.4-mm) stepdowns (0.011" or 0.28 mm and chip load at 6 ipm or 152 mm/min). “And we’re not using any exotic cutting fluids, just plain soluble oil,” says Allawos.
On another job, this one in titanium, the team discovered that fewer setups did not produce the part in the shortest overall machining time. Although the initial goal was less setup, experimentation showed that a different technique that required more setups was actually faster. “Sometimes, side milling is faster than cutting with the heel of the cutter,” explains Allawos. “We stubbed everything up and went to more—but shorter—setups to get time out of the part.”
Cutting tool vendors are not surprised that aerospace shops are getting big returns from experimenting with their tools. “When you look at the economics of metalworking, the cost of cutting tools is not really a huge factor,” says Don Graham, turning product manager, Carboloy Inc. (Warren, MI). “It accounts for 2–5% of the final cost of a part, and perhaps slightly more for those made of high-temperature alloys. Yet the right tool used correctly can save anywhere from 30 to 80% of your machining costs, and even get parts out the door faster.”
Such savings have been becoming almost routine over the last five years, as the positive geometries developed for carbide inserts more than a decade ago move into the mainstream of the aerospace industry. “Sharp edges with a high-shear design help to reduce the pressure and heat that are generated during the machining process,” explains Don Battin, an applications engineer at Greenleaf Corp. (Saegertown, PA). The constant rate of technical improvement since these tools’ introduction, and the development of a greater number of stiffer and faster machines capable of using them, have generated impressive results in both turning and milling.
“Cutting speeds are typically about 100 or 120 fpm [30.5 or 36.6 m/min] for Inconel 718, which is in the middle of the difficulty scale,” offers Bruce Carter, aerospace business development specialist, Sandvik Coromant Co. (Fair Lawn, NJ). “You go as high as 200 fpm [61 m/min] in light finishing operations, if the cut isn’t very long, but you have to slow down for material that’s more difficult to cut.”
Results are equally impressive for titanium. “In recent years, the need to produce titanium parts quickly has prompted tool manufacturers to create coatings as well as carbide substrates that withstand increased surface temperatures,” says Battin at Greenleaf. “The resulting PVD and CVD coatings allow machining titanium above 400 fpm [122 m/min] in certain applications.”
Cutting fluid is a crucial element for milling titanium and high-temperature alloys with carbide, which is the opposite of the situation one encounters with conventional work materials. “For conventional materials with carbide tools, we typically recommend cutting dry to avoid the rapid thermal cycling that would cause cracking as the edge leaves and enters the cut,” says Sandvik Coromant’s Carter. “The amount of heat generated is so great in titanium and high-temperature alloys that another type of failure, such as flank wear or notching of the cutting edge, usually takes place before thermal cracking occurs. Without coolant, you don’t get much life from the insert.”
Tool life depends on delivering the fluid directly to the cutting zone. “You want as much pressure and flow as possible directed there,” says Carter. Conversely, coolant applied incorrectly is usually detrimental to the cutting process.” Through-the-tool delivery is the best way of directing the coolant to the right spot, so he recommends it whenever it’s possible.
To ensure that the cutting fluid is applied correctly, Carboloy’s French subsidiary has developed tools and nozzles for delivering coolant pressurized to 450 bars to the tool tip. The modular nozzles are machined and fitted together so they can deliver the coolant in the right direction without moving or leaking. “Directing the coolant at the tool chip interface does two things,” says Graham. “First, it helps to bend, break, and evacuate the chip from the cutting zone. Part of this is due to the cooling effect this technique provides. Second, it reduces the cutting forces.”
Ultimately, the mission of the new carbide substrates and coatings is to support the positive-edge geometries, and so make them capable of shearing aerospace alloys. When the right chipformer and coating on the carbide insert are applied properly, the part generally will cost less to produce, because it will need much less time on the machine. “This performance also has a lot to do with not having to re-cut a part due to insert-edge deterioration,” adds Battin. “In high-nickel alloys, smearing is a major problem during the finishing process. The higher cost of a precision-ground insert with the proper edge and coating is easily offset by increased tool life.”
As important as positive geometry is to productivity in titanium and high-temperature alloys, cutting-tool companies are exploiting geometry in other ways too. An example is the differential-pitch milling cutters that a number of vendors have developed to control vibration in “bungy” materials like titanium. “The spacing between flutes is uneven to help break up the harmonics,” explains Francois Gau, segment manager, global aerospace and defense industry, Kennametal Inc. (Latrobe, PA).
The differential pitch on a line of Kennametal’s solid carbide and indexable tools counteracts the vibration enough for a 1" (25.4-mm) diam end mill to cut titanium at 300 fpm (91 m/min) in some applications. Gau recommends the tools mostly for jobs such as structural components and casings that require more roughing than finishing. For blades, discs, and other components for inside jet engines, ball-nose end mills tend to be better suited.
Advances in materials science are the key to producing positive geometries that can withstand the extreme temperatures that develop when cutting difficult materials. Modern substrates contain both submicron grains of tungsten carbide and slightly higher concentrations of cobalt in the binder holding the grains together. The fine grains lower the substrate’s thermal conductivity, enhance its crack resistance, and most importantly increase its strength. Adding cobalt boosts toughness and fatigue resistance.
Although submicron grains and cobalt-enriched binders have been around for awhile, better manufacturing processes have made them more efficacious. For example, greater consistency in size of the granules in the powders leads to more consistent grains in the tool. Better control over the pressure, temperature, and concentration of the gases used during sintering also improve bonding and, therefore, toughness and strength. It also allows Valenite LLC (Madison Heights, MI) and a few other cutting tool manufacturers to decrease the outgassing of carbon that occurs during the sintering process. Less outgassing reduces the porosity in the final product, and inhibits tungsten from dissolving into the binder.
The latest generation of PVD coatings help cutting tools withstand the severe conditions in difficult-to-cut work materials in two ways. First, these coatings contain much higher concentrations of aluminum than their predecessors did, so much so that they are really aluminum titanium nitride (AlTiN) and not titanium aluminum nitride (TiAlN). The heat oxidizes the aluminum at the coating’s surface to create a very thin layer of aluminum oxide that is only atoms-to-nanometers thick. This layer performs two functions: It serves as a chemically inert barrier between the substrate and the chip, and it creates a thermal insulator that inhibits heat from flowing into the cutting edge.
“Historically, high temperatures and mechanical loads have softened the cutting tool materials to the point where they lose their strength and dissolve into the chip,” says Brian Hoefler, Valenite’s manager of product development. “The oxide layer delays the chemical interaction and prolongs cutting tool life. More importantly, it allows cutting at higher temperatures.”
He adds that the deposition of these coatings in several layers is the second reason that they contribute to tool life. A vendor might deposit as many as 100 of these layers, each containing a different composition of the constituent elements and a different orientation of the grains. Vendors have developed their own proprietary combinations that they think will perform best in various work materials.
Cutting tool companies also have been making incremental improvements to CVD coatings. Carboloy, for example, has refined its medium-temperature CVD process to deposit an aluminum oxide coating designed for turning high-temperature superalloys. This coating is thicker than PVD coatings, giving it a relatively generous layer of one of the best thermal insulators available. Not only does it protect the carbide substrate from the extreme heat generated while cutting these materials, but it also is tougher and wears longer.
Yet it is thinner and smoother than previous CVD aluminum oxide coatings. Being thin helps preserve the substrate’s sharp geometry and ability to cut freely by reducing the slight rounding of the edge that occurs during coating. “Because the coating is smoother, you get less metal buildup and, therefore, less chipping,” says Graham. “It used to be that if you tried to machine a superalloy with an aluminum-oxide-coated insert, you’d get quite a bit of buildup.”
The result is longer tool life and the ability to run at higher speeds. “Previously, people would turn Inconel 718 at 200 fpm,” says Graham. “Now we’re up to 300 to 400 fpm.”
Although coated carbides tend to be the norm for cutting titanium, they face stiff competition from ceramics in nickel-based high-temperature alloys such as Inconel 718, especially in roughing and semifinishing operations. In fact, ceramic has been the tool material of choice in these materials for the last 20 years, according to Greenleaf’s Battin. “Whisker-reinforced ceramic inserts routinely operate at 8–10× the speed of uncoated carbides,” he says. “Although the price of ceramic is three to four times that of carbide, it’s worth the expense because of the shorter cycle time and freed-up machine time. So machining these materials with carbide leaves a lot of potential productivity untapped.”
Cutting tool vendors are reporting that ceramic grades are cutting at speeds exceeding 1000 fpm (305 m/min) in some applications. “Speeds are phenomenal, and tool life is reasonable,” observes Gau at Kennametal. The speed advantage that ceramics have over carbide more than compensates for the shallower depths of cut that they usually require, and the shorter life that they sometimes have.
Although modern coatings on carbide tools have done much to close the productivity gap between carbide and ceramics, Battin notes that uncoated grades of ceramic tools still outperform coated carbide. Moreover, he reports that the coating that Greenleaf has put on its whisker-reinforced WG-600 ceramic inserts has preserved much of the lead that ceramics have had over carbide in some materials. “This is a developing technology that has a lot of potential,” he says.
Results are similar in turning and milling. “Historically, ceramics have been used more in turning high-temperature alloys such as Inconel and Waspalloy because most turning applications don’t have an interrupted cut that pounds on these hard, but brittle tools,” says Carter at Sandvik Coromant.
He credits the greater rigidity and higher spindle speeds of today's machine tools for the proliferation of milling applications. In milling inserts, whisker reinforcements grown into the ceramic give the material strength and toughness, much as steel wire reinforces concrete. “To mill successfully with ceramics, though, you need special techniques, such as large lead angles on the tools,” he says. “The best and most common way is to use round inserts to spread the chip load over a greater area. Round edges are much stronger than corners.”
He cautions, however, that rounded inserts are not suited to thin-wall parts, such as casings, and to some finishing applications. “The tradeoff for the added strength is that a round insert applies much more pressure in the cut,” he says. “Thin walls will want to deflect, chatter, or both.” At high speeds, round inserts sometimes can cause damage surface or induce stress. In these cases, he recommends rough milling with ceramics and finishing with carbide with some relatively sharp edges.
Deciding whether to use cutting fluids is not quite as straightforward for ceramics as it is for carbide. Although cutting fluid helps in some cases, it is detrimental in others. One reason is that ceramics withstand much higher temperatures than carbide. Consequently, cracking from the drastic changes in temperature as the edges on a milling cutter enter and exit a cut once again become problems while milling wet. In fact, putting coolant on a ceramic insert could make it more susceptible to thermal cycling than carbide would be.
The bottom line is that experimentation is as necessary in specifying cutting fluids as it is in specifying cutting tools. It’s the key to giving the big aerospace companies the cost reductions that they are demanding. “It’s not necessary to spend the big bucks and buy the fancy machines and software,” says Allawos at Mikana. “It can certainly help [to buy new technologies], but it does you little good if you can’t augment it with knowledge. It’s creativity that wins in this game.”
This article was first published in the March 2006 edition of Manufacturing Engineering magazine.