Ten to 15 years ago, machining titanium was a difficult, nasty job. Many shops didn’t want to do it at all, while others did so gingerly and slowly. But vast growth in aerospace and medical applications has led to much better ways of machining Ti. This article focuses on these changes, what challenges remain, and prospects for the future.
Continuous process development has made machining titanium almost routine. “Ten years ago, there was very little information about titanium’s machining characteristics,” said Brian List, applications engineer for machine builder Makino Inc. (Mason, OH). “Many manufacturers were cutting blind and making parts through trial and error. Using hard metal techniques with tools made for aluminum on multipurpose machines created a process that was costly, required constant monitoring, and produced unpredictable tool life.” Today, however, many shops have mastered the process, using tools designed for titanium and other difficult-to-cut materials on machines that provide the required stability and torque, he said.
Jan Andersson, global manager, TechTeam and marketing for Greenleaf Corp. (Saegertown, PA), agreed that major improvements have been made. Newer grades of titanium are still challenging. “Ten to 15 years ago, two grades of titanium—Ti-17 and Ti 6-4—were machined and not much else,” he said. “Today, more difficult titanium grades, such as 10-2-3 and 555.3, are being machined as titanium components are replacing those previously made from stainless. Ti-6-4 and Ti-17 are relatively forgiving; you can go straight into the part and don’t have to think too much about toolpaths. If you do that with the new titanium alloys, you’ll see a significant reduction in tool life and an unpredictable process.”
While still not a slam dunk, machining titanium has become routine, according to Mark Terryberry, applications engineer, Haas Automation Inc. (Oxnard, CA). “With today’s tool coatings, we get tool life not possible before,” he said. “In the past, we would be changing end mills all the time—now we can make it through entire parts and even extend tool life or bump up surface footage and run our parts faster.”
He noted that aluminum titanium nitride (AlTin) coatings are extremely hard, extending tool life and helping dissipate heat—critical when machining Ti, a poor heat conductor. “The geniuses making these end mills have grinds so perfect and geometries so well-engineered that we can cut hard materials like titanium or even Inconel without chipping or breaking tools.” Also, adaptive toolpaths from CAM software developers provide steady, productive cutting. “All you hear is a constant drone at a constant rpm,” said Terryberry. “It’s a beautiful sound.”
Dave Campbell has also seen changes in how titanium is machined. He is president of Heartland Enterprises Ltd. (Fredericksburg, TX), a machine shop that turns large-diameter parts on heavy-duty lathes. “We turn a lot of titanium for aerospace applications using robust, high-torque, rigid tooling and a lot of coolant to dissipate the heat,” said Campbell. “We like to use top-of the-line Okuma 45s and V100s; they can take a beating and they last a long time.”
What has changed is the shop being able to take deeper, more aggressive depths of cut. “We were pretty timid when we first got into titanium, but through trial and error we’ve upped our game—and the machines are higher-torque and more stable.” The shop uses custom toolholders and Campbell noted that using DE vibration bars from Sandvik Coromant’s Silent tools line provides added rigidity.
Incremental changes like these are the key to improving Ti machining, said Marc Kinnemann, manager, technical center and training, Mitsubishi Materials U.S.A. Corp. (Schaumburg, IL). “You can’t be terribly efficient by increasing surface speed, so cutting tool manufacturers made end mills with as many cutting edges as possible to increase feed rate,” he said. “Machine builders realized they needed stiffer spindles in different rpm ranges, and all those changes have added up to a better process.”
So much progress has been that many shops are machining lights out and don’t give it a second thought, according to Mike Kerscher, national applications engineering manager for machine builder Mazak Corp. (Florence, KY). “Ten years ago, the act of cutting titanium unmanned would be practically unheard of, not to mention extremely risky,” said Kerscher. “Now we have customers machining the material lights out on a regular basis.” Unmanned titanium operations are not for everyone, he cautioned. They require very predictable processes, and having different machines in an automated cell also helps.
Another change in Ti machining is additive manufacturing (AM). “The most dramatic change in the last decade has been the introduction of AM,” said Greg Hyatt, senior vice president and chief technical officer at DMG Mori Advanced Solutions Inc., DMG Mori Seiki USA (Hoffman Estates, IL). “With AM, every Ti component can be deposited near-net.” As a result, high-torque is no longer required as rough milling is eliminated, leaving only finishing operations for conventional machines. Without high-torque spindles, the need for machine mass and rigidity is reduced.
A hybrid machining strategy is effective, according to Hyatt. “Our in-machine software for chatter reduction is even more effective with the finishing tools than for roughing, making this solution perfect for Ti machining in combination with AM,” Hyatt said. “Our ultrasonic-assisted milling solutions were limited to finish operations, but when AM eliminates the roughing, ultrasonic can be applied to most of the remaining milling ops.”
High-Torque Machine Tools
While AM has a place at the table, conventional machining is still the predominant Ti strategy. “While AM will help us reach the goal of near-net shapes, it will be some time before the process is perfected, and we continue to improve high-torque solutions,” said Jeff Wallace, engineering manager for DMG Mori USA.
Makino has several machines specifically for titanium, including the T4, T2 and most recently the T1. Brian List noted that his company’s 1000 Nm high-torque spindles, with HSK 125 or HSK 100 adapters supported by large casting structures, can drive cutters smoothly through titanium. “Machining in the horizontal position with high-pressure, high-volume coolant ensures that chips are cleared and cutters are cool. Compact five-axis construction allows roughing operations that interpolate offset from the final finished shape of the part.” This reduces or eliminates semi-finish passes and increases tool life and finishing speed by eliminating steps and extra material left by traditional roughing.
Mazak’s Super High-Torque Mega 8800, an 800-mm HMC, is designed for titanium and other tough-to-machine materials. Also, the Mazak HCN 6800, a 630-mm HMC, is available with a hard-metal package that includes a high-torque spindle, vibration-dampening bed casting and an extra heavy-duty column. “We also can include additional trucks [roller bearing packs],” said Kerscher. “Instead of four trucks, two on each side of the column, we use six trucks to add stability.”
One issue is that Ti shops may want to cut other materials, such as aluminum, at higher speeds. Also, they may want to rough Ti on four-axis machines, then finish cut on five-axis machines. “We can facilitate that with our Palletech automated pallet delivery system, which allows combinations of horizontal machines and five-axis machines within the same production cell,” said Kerscher.
Haas Automation’s Terryberry agreed that large, high-torque machines aren’t always needed for titanium machining. “It all depends on part size,” he said. “If you are machining large parts using end mills more than 1″ [25.4 mm] in diameter and need a high MRR [material removal rate], you need a high-torque, 50-taper machine, such as our VF-6 through VF-12 VMCs, for slot cutting and other operations. But if you are doing detailed parts with small inside corners using end mills less than 1” in diameter, you don’t need extra torque.
“For example, the standard inline-drive spindle on our 40-taper VF series machines runs at 8100 rpm, and our torque doesn’t start to taper off until we’re past 3000 rpm,” he continued. “You’re typically not going to run a half-inch end mill at more than 200 or 300 sfm [61 or 91.4 m/min] in titanium, which is below 2500 rpm; that’s still well within our wide torque band. The great tool coatings available allow us to go as high as 350 sfm [107 m/min], but at an rpm that keeps us in the meat of our torque curves.” He noted that the Haas CM-1, a compact 20-taper machine with a 30,000-rpm spindle and available five-axis rotary table, is widely used to machine titanium dental implants and small titanium medical parts.
When machining titanium, training is crucial to understanding cutting forces, chip thickness, and radial engagement, according to Makino’s List. “We need to identify the red line—a maximum surface speed at which titanium can be machined. As long as the cutting tool stays under this ‘red line’ it will continue to machine properly.” However, that red line is increasing. “High-flow, high-pressure coolant systems have moved the red line to higher speeds. This higher speed, combined with the large axial depths of cut possible on a stiff machine construction, are producing productivity rates never before possible,” said List.
New Tool Geometries
Changes in cutting tool design are widely credited with more productive Ti machining. “We’re all running the same sfm and using chip loads between 1 thou’ and 5 thou’ per tooth,” said Terryberry of Haas. “The only thing we can play with is our radial engagement and the number of tool flutes. There are now end mills with 13 or more flutes, which allows increased feed rates. And instead of 10% radial engagement, some shops are as high as 30% radial engagement.” He noted that multiflute tools tend to pull out of conventional toolholders, so more secure options are required.
Tools specifically designed to cut titanium and other challenging materials are an important part of a machinist’s arsenal. “Our TurboForm [TF] Chip Form, with high positive geometry, is effective in shearing titanium and reducing heat in the cut zone,” said Jim Wyant, applications engineer/project development, for Greenleaf. “It works well in titanium, Inconel and other heat-resistant alloys.”
Greenleaf’s Andersson added that milling and turning titanium requires different strategies. “For milling, you need lower engagement to get chip thinning. You can tolerate high temperatures but they have to be even.” Turning is focused on tool entry and exit, he said. “You don’t want a 0° lead angle where you are smacking straight into the part, going from no force to full force on the cutting edge and the reverse on exiting the part. You want to feed in and out with a 45° lead angle or use chamfers.”
For cutting titanium and high-temperature alloys, Mitsubishi Materials’ VQ Smart Miracle endmills have an (Al,Cr) N coating developed with “Hyper Arc Activation” technology. The addition of chromium makes the coating ideal for titanium. “It incorporates features like a variable angle helix in the cutting edge to prevent chatter,” said Kinnemann. “The gullet shape, in conjunction with a smooth, ZERO µ surface, helps curl and form the chip more efficiently. Through-coolant holes target the cutting edge.” He noted that the company’s iMX exchangeable head endmill series, also for titanium and other alloys, offers a wide range of geometries with 4, 6 and 12-flute options.
To successfully tap titanium alloys, speed recommendations are critical and will result in tap failure and/or shortened tap life if not followed, according to Mark Hatch, product director at Emuge Corp. (West Boylston, MA). “We recommend a tapping speed of 10–13 sfm [3–4 m/min], both turning into the tapped hole and exiting out of the tapped hole.”
Emuge has two taps for cutting threads in titanium. Through-hole applications use the Emuge Rekord C-Ti series tap, with an 8–10° left-hand spiral flute designed for right-hand cutting in through holes. It has a 4-5 thread chamfer as well as face rake and relief characteristics. Blind-hole applications use the Emuge Rekord D-Ti, with a 10-15° right-hand spiral flute designed for right-hand threads. It has a 2-3 thread chamfer in addition to face rake and relief characteristics.
“Our C-Ti and D-Ti taps have new High Relief Geometry (HRG), which increases space between the friction surfaces for enhanced lubrication and reduced torque load in both forward and reverse direction,” said Hatch. “HRG counteracts the high compressive forces produced by the extreme elastic memory of titanium.”
While machining titanium is easier than it was 10 years ago, challenges remain, according to DMG Mori’s Wallace. “As titanium alloys become more commonplace, and even though we look at it as the ‘aluminum’ of the 21st Century, the ability to master this amazing material remains elusive,” he said. “With the introduction of beta phase titanium, technologies such as additive, ultrasonic machining and cryogenics are poised to make the next major breakthroughs.”
At IMTS 2016, Mazak demonstrated its full five-axis Variaxis i-800T Multi-Tasking Machine that can be equipped with a cryogenic through-tool cooling system from 5ME, which delivers super-cold liquid nitrogen to the cutting edge. The system ensures that nitrogen does not become a gas before having the opportunity to cool the tool. “This keeps the cutting edges at a consistent temperature so they last longer and you can push the tool harder,” said Kerscher.
Finally, greater adoption of available technology can make a major impact. “I still see many shops using 15-year-old methods to machine Ti,” said Terryberry. “They’re stepping down with a small tool, taking an 1/8th of an inch, which is very hard on the tool. The same 3 or 4 mm of the tool gets worn away quickly. Contrast that with shops using the full length of the end mill with adaptive toolpaths. They are stepping over radially instead of axially and taking the entire DOC in one shot, so they are taking many fewer passes. They’re making the part in six hours instead of 16.”