A year ago, we stated machining titanium had become “just another day at the office”. But although it might be routine, disagreements remain about the best machine configuration. Some excellent old ideas are often overlooked. And at least one potentially game-changing technology is just now being introduced commercially.
A Guideways Debate
Everyone agrees that effective titanium machining requires a stable machine base, and that there’s no substitute for mass when it comes to vibration dampening. But what do you give up in speed for greater mass? And what’s the best way to move that mass? Like everything else about machine tools, there is no “right” answer and every choice is a compromise. Still, the compromises are not what they used to be.
Let’s focus on the biggest challenge both in volume and material: aerospace parts. As Robb Hudson, CEO, Mitsui Seiki USA Inc. (Franklin Lakes, NJ), explained, “To this day, aerospace parts still have a very high buy-to-fly ratio. You buy a tremendous amount of raw material and end up flying only a very small portion. To maximize structural integrity, you start with a very large billet of material and cut the workpiece component out of the heart of that material.” That points to a big machine with a big column and a big spindle. Many would add that further optimizing the rigidity and vibration dampening of that configuration requires boxways. Others would decry that solution as too slow and clumsy in contouring.
According to Doug Rizzo, training coordinator at Doosan Machine Tools America (Pine Brook, NJ), an improved understanding of the dynamics of ways, friction and ballscrews, along with better controls, have made even large boxway machines “more athletic.” For example, Rizzo said, “Fanuc controls have servos tuned to the machine and its motion dynamics. Even the most basic controls have some form of look-ahead. You’re no longer battling stick-slip or friction the same way. Way lube systems work better now. Turcite coatings are better. The grind on the ways of our machine is like a mirror. Ballscrews look like they’re chrome plated now, the grind is so good.”
Rizzo pointed to a 45,000 lb (20.4 t) Doosan NHM 6300 on which he drilled through 4″ (102 mm) of Ti6Al4V with a 2¼” (57.2 mm) Kennametal carbide indexable drill “about as fast as I was drilling cold rolled steel a couple of years ago” (i.e. at 20–25 ipm [508–635 mm/min] at about 2200 RPM). Yet the machine has a rapid traverse of 787 ipm (20 m/min).
On the other hand, while Ray Buxton, general manager, Mazak Corp. Canada, (Cambridge, ON) agreed that “a heavy cast iron machine is always going to give you the best characteristics to reduce vibration,” he disagreed that it needed to have boxways. “I’ve been using linear roller ways on very large machine tools for over 20 years extremely successfully and without the stick-slip characteristics you get from boxways. Your ability to corner much faster and take advantage of control capabilities is much better with linear rollerways.” That’s especially true if you need five-axis contouring capability or greater flexibility in general, which is Buxton’s main argument for linear rollerways.
While Doosan and others offer both box and linear rollerway machines, Okuma America Inc. (Charlotte, NC) now goes a step further and offers a hybrid approach, according to Machine Center Product Specialist Errol Burrell. “It has a box style above and linear roller guides below, giving you rigidity combined with dynamic speed on the X, Y, and Z axes,” he said. Burrell also explained that in addition to vibration dampening, solid cast iron construction also helps maintain thermal stability, which further contributes to good results when machining materials like titanium.
Battle of the Spindles
Another point of agreement is that although Ti6Al4V can be machined at relatively high speeds, the best approach for most titanium alloys, especially harder varieties like Ti5553, is to attack it at low RPMs combined with high feed rates using state-of-the-art cutters. Mitsui Seiki calls this approach “low-frequency machining” and it achieves material removal rates of 30–40 in.3/min (492–655 cm3/min) in titanium Ti6Al4V. (Here “frequency” refers to the number of times per minute that a tooth on a milling cutter contacts the part.)
“RPM depends on the milling cutter,” explained Hudson. “If we’re talking about a Harvi cutter from Kennametal with a long cutter body that looks like a porcupine, you could be running as low as 125–150 rpm, but you’re just ripping material. Instead of producing tiny chips, you’re ripping off silver-dollar-sized chips.”
Doing this requires optimal spindle clamping technology, which Hudson said is the new KM4 spindle interface from Kennametal. Clamping force and the bending moment force on a KM4 spindle is about three to four times higher than a standard CAT 50 Big Plus spindle. It also has much greater clamping force than HSK 100 or 125.
“The stronger you can make the interface between the spindle and the tool, the more rigid it is and the more it acts like one integral piece, as opposed to two independent pieces,” said Hudson. “So when you’re doing face milling or slot milling [with] a lot of tangential load on the spindle face, you want high clamping pressure, so the bending moment between the tool and the spindle needs to be very high.”
Rizzo said Doosan has machines with HSK spindles but he recommends dual-contact BIG Plus CAT 50 spindles for high-torque applications. Dale Mickelson, specialist–die/mold for Methods Machine Tools Inc. (Sudbury, MA) agreed. “The BIG Plus spindles on our Yasda machines offer four times the tool life as standard CAT and two times the tool life as HSK, yet most aerospace companies are using standard CAT 50 machines to cut titanium. BIG Plus also gives you a longer reach, because it has both face and taper contact, which also gives you better support against vibration. You can run heavier chip loads with better tool life and finishes.”
The Yasda spindle also has an advantage in cutting titanium, according to Mickelson. Yasda’s preload self-adjusting system is a unique mechanical design inside the spindle that enables self-adjustment preload. It provides high preload when using bigger cutters at lower speeds and low preload when using smaller cutters at higher speeds.
Mazak’s Buxton said most aerospace customers in Canada “have gone to the HSK-style toolholder. We’ve seen KM4 and put it on machines but have not seen a big push towards it. You do need face and taper contact, but we don’t see a need to change to newer configurations like KM4.”
Burrell said, “We feel the HSK, Capto, and BIG Plus interfaces are sufficient. Dual contact is always a good thing. The jury’s out on KM.”
Buxton added that Mazak’s approach recently has been to “develop machines with higher spindle speeds while maintaining torque. This gives you the flexibility you need in most job shops, because they don’t get to machine Ti-5553 or any other grade of titanium all the time. They have to be able to do a broader spectrum. With higher speeds and high torque at lower speeds we’ve given them the best of both worlds.” It should also be noted that even the high-torque Doosan NHM 6300 mentioned earlier runs up to 8000 rpm.
In another example, John Force Racing Inc. (Brownsburg, IN) machines its dragster clutch assemblies (flywheel, pressure plate, and pressure plate cover) out of billet Ti6Al4V using a Hurco VMX 6030i Performance series vertical mill. Nic Barnes, shop manager, said the machine combines the necessary rigidity with “very smooth rapid travel at 1348 ipm (34.2 m/min) in X and Y and 900 ipm (22.9 m/min) in Z. We tap at 2000 rpm and there’s no hesitation when it gets to the bottom like on other machines. Everything is fluid. We previously had to use a tapping head to prevent tap breakage but we have no such issues with the Hurco.”
Those rapid travels and spindle speeds up to 12,000 rpm make the machine well-suited to various applications, which is important since the shop machines titanium only about 15% of the time.
Tuning is Key
Mitsui Seiki’s Hudson added another important note: “For low-frequency machining, not only is the spindle configuration important but also the machine has to be tuned to function properly at those low frequencies. We’ve done tap testing on all our machine models, including tapping the machines in various states of cut during low-frequency machining. Many companies tap test their machines with everything centered; the spindle in the middle of the travel, the table in the middle of the travel. But workpieces often force you to machine on the far reaches of machine travel, so you also have to tap test out there and understand where the machine’s excitation points are when you’re machining at certain frequencies.”
Mitsui Seiki has worked with Boeing to tune its machines and, in some cases, redesign elements, including castings, for low-frequency cutting. Otherwise, an excitation point in a linear axis can become a harmonic that begins to amplify itself and travel throughout the entire machine tool and even into the workpiece. “That causes an undesirable vibration that’s going to lead not only to premature cutting tool failure but premature component failure in the machine tool,” said Hudson.
Loving the New Cutting Tools
Sources interviewed for this article raved about new cutting tools and their ability to cut titanium. In particular, Mickelson of Methods Machine Tools said “the last year has seen the introduction of more silicon and chrome-type tool coatings. These tools have a sharper edge than aluminum coatings, which round the edge. You actually have to run heavier chip loads with these new coatings to avoid vibration due to the sharper edge. Silicon also has higher lubricity than other coatings, reducing the tendency of titanium to create built-up edge on the tool. Plus, today’s carbides have a heavier micro-grain, so you can run both higher surface footages and heavier chip loads. That sometimes demands a higher RPM spindle.”
Barnes of John Force Racing started with four and five-flute tools but has since learned to use six and seven-flute end mills for higher material removal around the outside of the part and five-flute end mills for smaller pockets. Finally, besides giants like Kennametal and Sandvik, smaller players like Fraisa are said to make outstanding tools for titanium.
Software & Control
It might seem surprising in 2018, but Mickelson and others said modern CAM software is underutilized. “Trochoidal machining and dynamic milling will deliver nine times more tool life than slotting,” he said. Okuma’s Burrell said Vericut’s Force software “optimizes your cycle times without even doing a cut in titanium. You input all the attributes of the machine, horsepower, drives, etc., and the software calculates the actual force needed to cut the part and how the machine will actually behave. It’s entirely virtual, on a PC outside the machine, yet nearly 100% accurate. This goes a long way to strategizing how you would approach a titanium project.”
He noted that operators ordinarily program a part, set speeds and feeds, then back off about 30%, working back up in a process that can take a couple of weeks. “With Force from Vericut, you’re getting the maximum performance from the machine out of the gate,” said Burrell. “This is very interesting, especially for titanium, because it is definitely one of those materials where you have to start conservatively.”
Most modern CNCs have adaptive control, which modifies the feed rate based on spindle load. Third-party software to perform this function is another option. Mickelson said the TMAC system from Caron Engineering (Wells, ME) “uses the actual horsepower of the machine and has adaptive cutting which increases and decreases the feed rate based on the horsepower the machine is using during the cut. For example, when the cutter gets worn, it starts using more horsepower and the software lowers the feed rate to keep the operation within the parameters you set. A CAM package wouldn’t know the tool is worn, or even broken, and would keep pushing the machine.”
Mazak offers both its own programming suite and a new SmoothX control. “We’ve shown that even 15% improvements in cycle times in titanium are not unusual just by upgrading to the new control,” said Buxton. “Our SmoothX control and its ability to drive through corners so much faster is a competitive advantage, because many titanium parts we see today require machines with five axes and contouring capability is absolutely key. If the machine no longer bogs down in the corners, you also get increased cutter life.”
‘Hot Knife Through Butter’
Alternative machining methods are also worth a look, according to Burrell: “There are some nasty materials out there now that cause you to look outside normal machining. We’re looking at things like ultrasonic machining, which is like applying a jackhammer motion but at about 33,000 Hz. The tool—normally a diamond cutter—hammers away at the material, shattering it, at the same time it’s rotating and cutting it. It works on materials that are 70 Rockwell and higher but has no use at all on anything softer.”
Hudson said ultrasonic milling is fine for light work but not for the heavy titanium machining required in most aerospace applications. For that he pointed to Blue Arc, which uses a high-speed beam of electrons to erode and remove metal. A GE report last year said Blue Arc “can cut titanium like a hot knife slicing through butter.”
“We’ve been working on this with GE for the last four to five years and we’re ready to launch it commercially,” explained Hudson. “Blue Arc machines not only titanium but nickel alloys and other hard-to-machine materials. It doesn’t really care what material you’re cutting. It’s an electrical discharge machining process, yet you get much higher material removal rates than you can possibly get with EDM or even ECM (electro chemical machining).”
Hudson noted that material removal rates are similar to what Mitsui Seiki achieves with low-frequency machining, but because it’s a noncontact process the perishable tooling costs are 75–90% lower than conventional machining. Surprisingly enough, it also consumes less power because the process doesn’t draw a heavy load on the spindle motor or any of the axes because there is practically no resistance.
Blue Arc is currently aimed at roughing and semifinishing, so Mitsui Seiki’s machine is a hybrid that performs both Blue Arc and conventional machining in one platform. First, the Blue Arc process roughs the part using high-amperage, low-voltage electrical energy. Then a dedicated robot removes the Blue Arc head and replaces it with a conventional multiple-point cutting tool for finishing. “It’s potentially a very disruptive technology,” said Hudson.