Ti machines

Courtesy of Makino

A titanium workpiece is machined in a Makino T4 5-axis HMC, which includes an active damping system.

Designing machine tools for cutting titanium—especially 5553.

Titanium comes in a variety of flavors. Those alloys provide a higher strength-to-weight ratio than aluminum and good corrosion resistance when paired with carbon fiber-reinforced plastic, making them ideal for replacing aluminum parts in aircraft to increase fuel efficiency. 

By weight, existing aircraft, such as the Boeing 737, primarily utilize aluminum and specialty steels, but next-generation aircraft, such as Boeing’s 787, will use 15 percent titanium, 20 percent aluminum and 50 percent composites, as well as 10 percent steel and 5 percent other materials, according to machine tool builder Mitsui Seiki (U.S.A.) Inc., Franklin Lakes, N.J. Therefore, the requirement for more titanium machining is the next challenge for machine tool builders.

How pervasive is use of titanium alloys becoming in aerospace and other applications? “We call titanium the aluminum of the 21st century,” said Jeff Wallace, manager of the Machining Technology Laboratory, DMG/Mori Seiki USA Inc., Hoffman Estates, Ill. “It seems to be making its way into everything we touch, even consumer products but predominantly aerospace.”

“There are several different grades of titanium in the 787, depending on where and what the requirement is for the material,” said Scott Walker, president of Mitsui Seiki.

That includes the well-established Ti6Al4V grade and the newer Ti5Al5V5Mo3Cr, or triple nickel, which is 20 to 35 percent stronger than Ti6Al4V. That means 5553 is more desirable for reducing weight while still meeting the needs of stressful, high-tensile aerospace applications, such as landing gear, support and floor beams, wing spars and critical structural components.

It also means more power is needed produce a chip when machining 5553. According to Walker, 5553 is about four times more difficult to machine than Ti6Al4V in terms of tool life, stock-removal rate and the required resiliency of the machine structure to push a cutting tool through the metal. Because of its reduced machinability and the corresponding higher cost of a finished part, Boeing reduced the amount of 5553 parts in the 787 by 5,000 lbs., to 21,000 lbs., and is considering further reductions, he noted.

Machine Design

Walker pointed out that the first requirement is to design a machine tool for low-frequency machining, primarily milling. That’s because end users typically cut the more difficult-to-machine titanium grade at low rpm while hogging as much material as possible to boost productivity and profitability. How low is low? Mitsui Seiki, for example, stated a 1 "-dia., 4-flute endmill would run at about 90 rpm, and conducted tests machining 5553 at 67 rpm with a 3½ "-dia., 6-flute cutter. The low spindle speed produces a low-frequency vibration in the machine structure, potentially exciting the structure’s natural frequency in low-frequency ranges and causing chatter.

Machine builders perform tap testing to determine a machine’s natural frequencies by tapping structural portions of the machine tool and measuring the structure’s excitation frequencies. “Any kind of mechanical system has natural frequencies at which it tends to vibrate,” explained Mark Larson, manager of titanium process development for Makino Inc., Mason, Ohio. “Different machines and different spindles have different points that are natural frequencies. Those are the areas you want to stay away from.”

Courtesy of Mitsui Seiki

Mitsui Seiki is building “heavy-metal” machines capable of handling bending moments at the tool/taper interface up to 35,000 in.-lbs. and able to remove 16 in.3/min. of Ti5553 on 5-axis trunnion platforms.

For low-frequency machine tool designs, the machine must have a structural design to cut at a frequency below 350 Hz, especially at 20, 95 and 320 Hz, Walker noted. That’s because the slow hitting of each cutting edge sends a shock wave through the machine, and the repetitive hitting when low-speed milling will set a frequency that’s transmitted through the machine, he explained. If that frequency is 20, 95 or 320 Hz, the cutter will chatter because the machine structure is excited. Minimizing or preventing that from occurring traditionally involves making machine beds thicker, columns more robust and mating members stronger using gibs and enhanced interfaces to keep the motion members securely fastened.

Paul Schroeder, senior product specialist for MAG Industrial Automation Systems, Fond du Lac, Wis., added that a machine for cutting titanium starts with a stable structure, including heavy-duty ways with a wide way spread and large (80mm-dia.) ballscrews. To provide stiffness and counteract detrimental harmonics, the major components for MAG’s 1250 series horizontal machining centers, for example, are cast ductile iron, including the X- and Z-axis beds and column, and the 1,250mm × 1,600mm pallets, headstock and rotary table housing are gray cast iron. 

“On top of that, we performed dynamic analysis of the machine to verify structural integrity,” Schroeder said.

Damping Vibration

On its NH series HMCs for difficult-to-machine materials, such as titanium and nickel-base superalloys, DMG/Mori Seiki’s Wallace noted that the spindle has a passive damping system to change spindle frequency as needed during heavy cutting. In addition, an active damping system is in development.

Larson said Makino’s “active damping changes the amount of friction in the ways and therefore absorbs more or less of the vibration, depending on how things are changing dynamically with the machine. The machine quickly adjusts to vibration picked up by the monitoring system.”

Walker emphasized that a builder should only add dampening after designing the machine structure for low frequency and identifying and reducing the amplitudes of frequency peaks during the tap test. “Dampening on a structurally designed and tuned machine will perform six to eight times better than if you were to take dampening and put it on a standard, general-purpose machine,” he said.

That machine structure and assembled components must be highly resilient, Walker added. Resilient, high tensile-strength materials can provide the required rigidity, but these machines cost more to build than a general-purpose machine. “If you visualize the machine continually bending while taking heavy cutting loads over its life, expensive, resilient machines will provide 75,000 hours of machine tool life,” he explained. “General-purpose machines will not perform at these high productivity rates or last as long.”

In addition to providing machines for roughing titanium built with traveling columns and tables on box guide ways, Wallace emphasized that DMG/Mori Seiki focuses on linear guide-way machines for high-speed machining with light DOCs. 

“We can feed them two, three, four times faster than an old box guide-way machine,” he said, adding that he hasn’t seen a clear preference for hogging or high-speed machines from the company’s customer base.

Courtesy of MAG

MAG’s 1250 series for 5-axis horizontal machining of titanium features an 180,000-position, A-axis tilt spindle with 856 ft.-lbs. (1,161 Nm) of continuous torque.

Others feel is it more efficient to perform heavy cutting with a heavy machine, where one pass removes the titanium instead of repetitive passes. “You can get productivity benefits from going very quickly and taking light cuts, but you’re also looking at a machine that’s going to have a lot of wear,” Larson said, noting that tool and part geometry, such as corners, limit how fast a high-speed machine can go. 

Torque and Coolant

When taking heavy cuts in titanium, the spindle motor requires high torque because of the power required to produce a chip. The torque requirement depends on part size, tool diameter, how many cutting edges are engaged, DOC and the metal-removal rate, but having at least 1,000 Nm (738 ft.-lbs.) of continuous torque is desirable, according to Schroeder. “Aerospace people tend to look at the continuous rating because when they start cutting parts, they cut for extended periods of time,” he said. The new MAG 1250 series HMC, for example, offers a variety of spindles with ratings as high as 2,600 Nm.

Courtesy of DMG/Mori Seiki

Mori Seiki designed its NH8000 DCG horizontal machining center for the rigidity required when machining titanium in part using dual ballscrews and box-in-box construction. 

A machine with less torque can cut 5553, but less effectively. According to Walker, a machine with 800 ft.-lbs. of torque can remove about 1.2 in.3/min. with a 1¼ "-dia., 4-flute carbide endmill taking a ¾ " DOC, whereas a 2,000 ft.-lb. machine can remove 9 in.3/min. because the user can increase the radial DOC.

Because titanium doesn’t conduct heat well, not much heat enters a titanium chip once machining creates it. Compared to about 85 percent of the heat generated during cutting entering the chips of some metals, “in triple nickel, you’re lucky to get 20 to 25 percent of the heat into the chip,” Walker said. He added that the heat, which can be up to 3,200° F, is focused at the cutting edge—a recipe for short tool life.

Courtesy of Mitsui Seiki

Magellan Aerospace produced this titanium component on a Mitsui Seiki HU80A 5-axis machining center.

Therefore, plenty of coolant must reach the tool/workpiece interface to effectively machine titanium by cooling the interface, providing lubricity and clearing the chips. As one of the five ADVANTiGE technologies for reportedly quadrupling productivity and doubling tool life when machining titanium (see Look-Ahead in the August issue), Makino’s T series machines deliver 1,000-psi and 52.8 gpm of coolant flow through the spindle and about 50 gpm of flow through the nozzle on the tool perimeter. “We’re dousing it with a lot of coolant to blast away the chips and heat,” Larson said.

On the turning side, Wallace said DMG/Mori Seiki is experimenting with technology “that is showing a lot of promise” to deliver ultrahigh-pressure coolant at up to 10,000 psi. However, the spindle’s rotary couplings need to be redesigned to handle that level of pressure, Wallace noted. “It has been retrofitted on machines but you blow components up,” he said.

Interface Issues

The cutting tool/spindle taper interface is also critical when designing a titanium cutting machine. Although a variety of tool/taper interfaces exist, not all are suitable for cutting titanium. According to Wallace, HSK is the most rigid for milling and Capto is the most rigid for turning. In addition, he favors shrink-fit toolholders when suitable.

The selected tool/taper interface must withstand the large bending moments created by cutting forces, according to Walker. Mitsui Seiki stated that its 4- and 5-axis “heavy-metal” machining centers for difficult-to-cut materials are capable of handling bending moments at the tool/taper interface of up to 35,000 in.-lbs. 

When shopping for a machine to cut titanium, Walker recommends that end users first find the best performing cutting tool when machining triple nickel on a general-purpose HMC, knowing tool life will be short and productivity low. “Then go to the builder and say, ‘with this cutter and these speeds and feeds, go deeper and deeper radially in Z until reaching the maximum metal-removal rate,’ ” he said. “That will determine who has the most robust machine designed for high stock removal in the heavy-metal arena.”

That arena is projected to stay busy as manufacturers machine more titanium aerospace parts. “Between the Joint Strike Fighter, the Boeing 747 and 787 and the Airbus A320 and A340,” Walker said, “probably 600 horizontal 4- and 5-axis heavy-metal spindles are required to accommodate the current backlog between now and 2017.” CTE

About the Author: Alan Richter is editor of Cutting Tool Engineering, having joined the publication in 2000. Contact him at (847) 714-0175 or alanr@jwr.com.

Machinists have done the “impossible.” Although initially predicted to be “unmachinable,” titanium 5553 can be cut. However, it requires a different approach than machining the more common Ti6Al4V alloy, according to Steve Lovendahl, business operations specialist for The Boeing Co., Portland (Ore.) Machine Structures. “Our priority is developing processes for new materials for the industry,” Lovendahl said. To help achieve that when cutting Ti5553, the aircraft manufacturer partners with machine tool builders. “We’re looking for relationships where we both work to understand the priorities for the machine based on the best process to machine the part,” Lovendahl said.
The process begins by providing machine builders with part dimensions, primary operations needed to produce the part and machine requirement estimates, such as torque, horsepower and spindle speed. To avoid disclosing proprietary information, that information can be presented as basic numerical data on an Excel spreadsheet rather than details about the actual part and manufacturing process. “That spreadsheet doesn’t tell your process but it lets the machine tool builders know the expectations for their machines,” Lovendahl said. “We focus on what’s the best way to manufacture the part and then relay that information to the machine tool builders. They determine what the best machine is, not us.”
For one part, the process resulted in Boeing creating a flow line of five Mitsui Seiki machines that the builder modified slightly to meet defined expectations. In a flow line, only the workpiece moves from machine to machine while the fixtures remain static for each machine. Having the number of machines equal the number of setups minimizes non-value-added time, compared to having a single machine perform all the operations and removing an existing fixture and installing a new one. 
“One answer is not good for everything, but for certain products one-piece flow has tremendous advantages,” Lovendahl said. “We’re looking at the overall process and trying to avoid the tunnel vision of bearing down on just one particular aspect of a project.”
Boeing also shares cutting tool and machining data with machine builders. This allows them to create a stability lobe diagram for an operation and model the selected machine to ensure the operation can be effectively performed. (A stability lobe diagram summarizes cutting performance based on the cutting tool applied and shows, for example, which spindle speeds and axial DOCs provide stable machining and which ones cause chatter.)
That information could include a 2 "-dia., 8 "-long tool taking a ½ " radial DOC and 2 " axial DOC with a 3-ipm feed in a 5553 workpiece. “The builder could create a stability lobe diagram that would show whether the operation fits within the parameters of the machine,” Lovendahl said, adding that the diagram must be created early in the process before a machine commitment is made.
In addition, Boeing partners with toolmakers to develop process information that it shares with machine builders when selecting equipment. For one project, Boeing worked with Kennametal, documented the process and included that data in the machine request, which led to Boeing acquiring a Giddings & Lewis 410 machine from MAG “that exceeded our expectations,” Lovendahl said.
“We’re trying to get a better understanding among all the partners early in the process as to exactly what the task is and not necessarily what the solution is,” he added. “By getting the cutting tool companies, the machine tool builders and the end user to all work together, everybody profits.”
—A. Richter

Contributors

The Boeing Co.
(312) 544-2000
www.boeing.com

DMG/Mori Seiki USA Inc.
(847) 593-5400
www.dmgmoriseikiusa.com

MAG Industrial Automation Systems
(920) 921-9400
www.mag-ias.com

Makino Inc.
(513) 573-7200
www.makino.com

Mitsui Seiki (U.S.A.) Inc.
(201) 337-1300
www.mitsuiseiki.com