Call It High-Productivity Machining
Cutting is fast, even if spindle speeds are not super high
By James R. Koelsch
MTU Aero Engines (Munich, Germany) had long wanted the same kinds of returns that manufacturers of aluminum airframe components have gotten from high-speed machines for almost a decade. The problem was that it produces rotor disks for jet engines from high-temperature ductile alloys like Inconel. Not only did the spindles on these machines lack the power and robustness necessary to cut the alloys, but the rotary axes on the five-axis machines necessary for this precision work simply would not be able to keep up with such spindles, even if they existed.
MTU, and other companies like it, simply could not risk using high-speed technology. Even the slightest uncontrolled structural changes in the material could detrimentally affect operating safety in the air. They and their customers had to eat the cost of lumbering along at the feed rates allowed by the 5000-rpm spindles and the multiple setups that were customary for this kind of work.
A new generation of machines, however, has altered the situation in a radical way. Their high-performance spindles are much more robust than those used on older equipment, and their rotary drives deploy the same magnetic motion technology used by linear motors. The result is twofold: a migration of high-speed technology into the cutting of difficult-to-machine aerospace alloys, and incremental improvements to high-speed machining in aluminum and other more speed-friendly materials.
Shops machining titanium and high-temperature alloys are no longer stuck with machining at only 5000 rpm. MTU, for example, now operates a 24-kW, 15,000-rpm universal milling machine from Mikron Corp. (Holliston, MA), a subsidiary of the Agie Charmilles Group. Because the five-axis machine also drives both the table and swiveling head directly with the rotary version of “linear motor” technology, the rotary axes can keep up with the spindle and the linear axes.
“With direct-drive technology, you’ve eliminated the wormgears and all the limitations of mechanical transmission,” says Mal Sudhakar, vice president, Mikron. “Rotational speeds were maybe 20 or 30 rpm for a mechanical table. Today, they’re 250 or 360 rpm for the direct drive system. Their acceleration rates are about 10 times higher, too. So, this technology has big implications for impellers, blisks, and blades, because now you can machine in a true five-axis simultaneous mode at high speed.”
If the magnetic drives boost performance so much, you might ask, why didn’t Mikron use linear motors on the linear axes? “You can get up to 2-g acceleration and feed rates of at least 1600 ipm [40 m/min] with ballscrews,” he answers. “And that is plenty fast for the mid-size machines that we make, which typically have up to 40" [1-m] of X-axis travel.” He points out that the cost of ballscrews is typically about 30% that of linear motors. Thus, travels really need to be longer to justify the expense of linear motor technology.
As it is now, the machine gives MTU both the flexibility of a five-axis machine and the rigidity and speed previously reserved for three-axis machines. The spindle swivels more than 135°, while the round table rotates 360° and can be hydraulically clamped. Mikron equipped MTU’s machine with a number of options that the shop needs to machine turbine and compressor disks, such as the continuous process monitoring of spindle speed and cutting fluid pressure and volume.
On its five-axis Mega machining centers, Cincinnati Machine (Cincinnati) also applies ballscrews, but in tandem to actuate each of the linear axes. Because the machine has six ballscrews instead of three, it is responsive, accelerating at 2 g. “Your parasitic time goes down substantially,” says Mike Zambenedetti, senior applications engineer. “Also, you don’t get any rocking in the machine. It’s much more solid cutting.”
Besides keying on rigidity, the designers paid special attention to heat dissipation. “We don’t want to grow one side more than the other,” says Zambenedetti. “That would introduce twisting and skewing.” The dual-ballscrew design helps to manage hot chips, one source of heat, by opening the center of the machine and allowing them to fall through the machine to conveyors beneath it. To dissipate the heat generated by the extra load that high acceleration places on the motors, the designers added insulation between the motors and the bed, and directed the cutting fluid to flow over the bed.
Because spindle speeds must be much slower when machining titanium than those used when cutting aluminum, 600 rpm usually being considered ultrahigh speed, Dan Cooper, senior applications engineer at Cincinnati, prefers to call the process high-metal-removal machining. “A general removal rate might be 6–8 in.³/min [98.3–131.1 cm³/min] in titanium,” he says. “We’re shooting for three times that. We’ve made some cuts recently removing 24 in.³/min [393.3 cm³/min] in a roughing application.” For cutting speeds, the goal is around 600 fpm (183 m/min), which is about five times the customary rate now used.
Besides enhancing rigidity, the open center created by dual ballscrews has another advantage: They allow placing the workpiece closer to the machine’s center of gravity. Builders such as Mori Seiki USA Inc. (Irving, TX) are using this driven at the center of gravity (DCG) technology to reduce the cantilever effect inside the machine, thereby controlling a source of vibration.
The result is better surface finishes at higher feed rates, according to John Goes, vice president, operations, Ellison Machinery Co. Northwest, (Kent, WA), one of Mori Seiki’s distributors. The technology also improves the aesthetics of corner radii, minimizing the need to tune the servos for faster deceleration before changes in direction, and greater acceleration afterward.
Goes reports that a growing number of aerospace manufacturers are recognizing the importance of stability within system superstructures for getting the greatest returns on their investments in high-speed machines. “DCG technology provides this stability, thereby minimizing the residual vibration that ultimately travels to the tool tip and degrades the tool,” he says. “This phenomenon is magnified in the alloys that are being specified with increasing frequency in aerospace applications.” Consequently, DCG technology pays dividends by freeing users to boost their feed rates without worrying about any degradation to surface finish, tool life, or both.
The increase in productivity can be quite marked—so much so in some applications that machine loading can become an issue. To alleviate the problem, Goes recommends linear pallet pools (LPPs), a system of pallets delivered by a rail-guided vehicle. “With the right choice of LPP, a manufacturer can reap the true benefit of high-speed machining, optimum spindle utilization,” he says.
Aluminum has not been neglected, despite the emphasis today on transferring the benefits of high-speed machining to difficult-to-cut alloys. Mikron, for example, is developing a spindle capable of higher metal removal rates than its 40-kW, 42,000-rpm spindle. “We’re working on a 75-kW spindle capable of 30,000 rpm,” says Sudhakar. The enabling technologies are permanent-magnet motors with higher power densities and larger-diameter cylindrical roller bearings made of materials capable of withstanding the higher forces encountered.
Because Cincinnati Machine builds larger machines for cutting parts from slabs of aluminum, its engineers regularly deploy linear motors, but in the opposite way from Mikron. Among the most recent examples is the HyperMach profiler that they developed for machining ribs, bulkheads, spars, and other monolithic airframe structures. A linear motor drives each of the linear axes, but an innovative application of ballscrews drives the rotary axis.
Although the original prototype shown at the International Machine Tool show four years ago had an A-C gimbal head driven by a “linear motor,” an A-B head driven by ballscrews seems to suit the parts that typically go on profilers. These parts tend to be flatter, and their angles tend to be almost vertical, causing the rotary axes to be perpendicular to the spindle’s axis of rotation. With A-C heads, on the other hand, the C axis points in the same direction as the spindle’s axis and can require some very rapid changes in angular velocity at crossover points. Consequently, the A-C gimbal head tends to be better-suited for machines cutting deep-profiles that require the spindle to be more horizontal, and not pointing in the same direction as the C axis.
The switch in heads also reduces weight, an important goal in the design. The machine’s designers began with a clean sheet and used finite-element analysis, looking to minimize weight, provide stiffness where it was needed, and make the machine as responsive as possible. “The key really is the high acceleration rates for moving in and out of small corners rapidly, slowing down and getting up to speed again,” says Mark Kohring, senior project engineer.
In fact, because the strategy is to boost productivity by eliminating parasitic times due to ramping up and down as the machine maneuvers along the toolpath, spindle speeds are not in the ultrahigh-speed range. Cincinnati usually plans to fit this profiler with 18,000 or 24,000-rpm spindles, depending upon whether the toolholders are A100 or A63 HSK. “Just because you have higher spindle speeds, it doesn’t mean you can use them,” notes Dan Cooper, senior applications engineer. “You can run out of speed and power.”
For Kohring, the technological development that makes linear motors practical is five-axis spline interpolation in the CNC. “Before this generation of controls, we had to be careful about applying ‘acc-and-dec’ and step velocities, because the programs tend to be divided into discrete spans,” he explains. “Five-axis spline interpolation fits a smooth curve through those spans in real time and produces a very smooth surface.” Not only are processing rates high, but users also need not generate and post-process polynomial output as they must do for NURBS.
The goal of the design was to make a single-spindle HyperMach at least as productive as a three-spindle profiler for a similar capital investment. Users would profit from the new technology through the lower operating costs resulting from the need for only one set of fixtures and tools. The domestic aerospace suppliers that ordered the two HyperMach machines currently being built expect even greater returns, because both companies have ordered twin-spindle versions of the machine. Cincinnati claims that each machine more than doubles the throughput of their counterparts in the previous generation of three-spindle, five-axis profilers.
Feed rates might be the limiting parameter for cutting large aluminum structures, but they are less important than spindle speeds for small, precise parts, such as those that go into satellites and other spacecraft. “If you’re working on a part that might be only a half-inch cube and need to maintain a 1-µm tolerance with a 0.003" [0.08 mm] end mill, you’re not going to be feeding a 50,000 or 70,000-rpm spindle very fast,” says Tom Dolan at Mitsui Seiki (USA) Inc. (Franklin Lakes, NJ).
Spindle speeds this high are necessary to hold tolerances, prevent deflection and distortion, and produce fine finishes. The specifications are necessarily severe because designers miniaturize the components to save space and weight, and require the devices to function properly in severe conditions. Because of the tolerances and finishes, Dolan likens the work to mold making. “You have to be able to control the tool tip as effectively as you can,” he says. “Often, you’re working with tools that can be only a couple of thousandths in diam. So you need to run them quite fast to get the required surface speed.”
The emphasis on spindle speed does not rule out linear motors, however. Although extremely precise ballscrews are often sufficient to deliver the required positioning accuracy, Mitsui Seiki also uses linear motor technology to eliminate the deficiencies and mechanical noise inherent in ballscrew drives. “By going that route, we’re able to produce complex, often tiny workpieces, with quite a great degree of speed and precision,” says Dolan.
As important as drives are to precision, the design of the machine tool must be fundamentally correct. “If your machine tool is not geometrically sound, then you have to think twice about whether you can actually move that tool tip into space accurately,” says Dolan. “The axis must be perfectly square, the guideways perfectly parallel, and the spindle parallel to the axis advancing it toward the work.”
So choosing the right machine is the first step in getting good results from high-speed machining. And that requires evaluating the entire system comprising the machine, CNC, tools, and fixtures. “Machine specifications alone cannot determine the effectiveness of a machine’s design,” notes Goes at Ellison. “Rapid traverse rates, maximum programmable feed rates, and acceleration-deceleration times are only indicators of a machine’s capabilities.” In other words, individual specifications like spindle speed are important but only part of a mix of parameters in what’s called high-productivity machining.
A Secret for Faster Cutting
Engineers at Boeing Co.’s Renton, WA, facility learned one of the best-kept secrets about squeezing more from high-speed machines. Without investing much, they could correct the productivity problems that a three-axis CNC mill was having while producing shims for fitting skins to airframes. They just had to turn on and tune some features in the CNC.
“CNCs already have a lot of capability not being taken advantage of,” says Bill Griffith, CNC product manager, GE Fanuc Automation NA Inc. (Charlottesville, VA). Most builders engage few of these features on their standard lines because most users don’t need them. And the few that are turned on are typically set for average use.
Well, the average settings weren’t cutting the aluminum shims at the expected speeds. “They wanted to cut at 300 ipm [7.6 m/min using a commanded 200-ipm or 5.1 m/min feed] with a 150% feed-rate override, but couldn’t get the accuracy that they needed,” recalls Gene Moss, the applications engineer from GE Fanuc who helped Boeing’s engineers troubleshoot the problem. During a reciprocating motion resembling hem stitching, “the machine was not completing the entire move to the end point. So the operator had to reduce the override to 80%.”
Not only did the operator have to slow the machine so it could hold the required tolerances, but the rapid reciprocation also stressed the mechanical components to the point where the shop had to replace ballscrews. “Because they were cutting aggressively, the acceleration was pretty harsh, causing the machine to make a banging sound when it reversed direction,” says Moss.
After evaluating the machine and application, Moss found that the application didn’t need the advanced high-speed features that die and mold applications typically require. Rather, he could see that three simple features on the GE Fanuc 18i-MA CNC would give the machine the sought-after performance.
The first was a new acceleration profile. Moss replaced the original exponential profile with a bell-shaped one to ramp the feed down as the tool approaches a turn and up again as it comes out. Because the new profile does this faster than did the exponential one, it not only shortened the time to make each maneuver but also smoothed the motion, limiting the shock to the machine. “So they’ve had fewer mechanical problems and repairs,” reports Moss.
The other two features improved the machine’s ability to boost accuracy around corners by 70% and in circular motion by 50%—both at higher speeds. “Velocity feed-forward follows the commanded toolpath more faithfully by giving you a higher position-loop gain,” says Moss. “The gain reduces the error between the command and where the tool actually is at any moment in time.” The last feature, Automatic Corner Deceleration, reduces corner rounding by slowing the motion to speeds that ensure the machine can reach the end points during the hem-stitching operation.
So the machine now runs at the desired 300 ipm and 150% feed-rate override, and holds the required accuracy. The result is 53% shorter cycle times.
This article was first published in the March 2006 edition of Manufacturing Engineering magazine.