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Multi-Axis Machining: Beyond Mainstream

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

Five-axis machining has progressed from exotic to mainstream over the past few years. Assessing how that transition plays out in the broader job shop environment requires untangling the many machine tool options available that exceed five-axis configurations.

This mill-turn configuration, the QuickTECH i42/i60 Ultimate mill turn from Absolute Machine Tools, has 10 axes, with two turning spindles, two milling spindles (with simultaneous machining) and two B-axes. It has up to 39 total tools, 24 of which can be live tools with B-axis milling capability. (Provided by Absolute Machine Tools)

Jeff Wallace, general manager of national engineering for DMG Mori USA, Hoffman Estates, Ill., reported that there’s been a “huge paradigm shift” among job shop owners with respect to multi-axis machining. Whereas two to three years ago only about 30 percent of DMG Mori’s job shop customers (outside aerospace) were using multi-axis platforms effectively, “I’d say conservatively 50 to 60 percent of our job shops have embraced, or are in the process of embracing, multi-axis.”

Klaus Miller, vice president of sales at Absolute Machine Tools Inc., Lorain, Ohio, agreed that this is the trend, but cautioned that for smaller jobs, the transition is happening “one machine at a time,” not as a wholesale switch. “For the most part, that market is still three-axis machining centers and two-axis lathes,” he said.

The inhibiting factor is price, Miller explained. “A three-axis 32 by 20 vertical machining center is about $85,000. The largest size fifth-axis trunnion table that could fit on that machine is only about 15 inches in diameter, so your work envelope is limited. And that machine starts at about $200,000, minimum.” On the other hand, he said, job shop owners now understand that they can double the output per machine by doubling the up-front investment with a five-axis machine. Personnel shortages make this increasingly attractive, Miller added. It’s the basic tradeoff: Machining all sides of a workpiece on a three-axis requires multiple setups, while a five-axis can do it all in one setup.

As Jared Leick, machining center product group manager at Mazak Corp. in Florence, Ky., put it, “customers who use multitasking for multi-face machining are taking advantage of easier processing, less work-in-process and less material handling. Being able to machine any part in one set up can be a huge advantage and keep shops very competitive.”

Wallace observed that with a Siemens (or comparable) control, an operator can “literally put a part down on a table of a multi-axis machine within about five millimeters of where it needs to be, probe that part, and be finished with setup. All you have to do is find the center of that part. You no longer need qualified fixtures and dowels or plugs to perfectly center the part on the table.” It’s arguably easier to run multi-face jobs on a five-axis machine than a three-axis. And Kevin Bates, Mazak’s general manager for the Midwest region, added that “quick-change tooling, tool presetting systems, tool management and auto tool setting systems” have greatly reduced what used to be complicated setups on multi-tasking turning centers.

There is one more factor keeping three- and four-axis alive: Some parts simply don’t require multi-axis machining. If they are high-volume parts, they’re probably very price competitive. Perhaps you can be confident you won’t need to adapt the machine to other jobs.

The axis layout of the QuickTECH i42/i60 Ultimate mill turn shown on page 34. (Provided by Absolute Machine Tools)

As Kenzie Roaden, product manager for Mazak’s Advantech group, sees it, that’s a recipe for the simpler machines. “If you have to turn out a lot of those parts, you need multiple spindles. And it’s very difficult to justify the burden rate of the more costly equipment. The three- and four-axis vertical market is not on the endangered species list.”

Finally, Dr. Scott Smith, the group leader for intelligent machine tools at Oak Ridge National Laboratory (ORNL), working with the Department of Energy’s Manufacturing Demonstration Facility in Knoxville, Tenn., made a point we can all relate to: “NC machine tools, which were once rare and expensive, are now common—yet they did not completely supplant manual machine tools, which are also still common and useful.”

We’ve all been in highly sophisticated shops that still have an old manual Bridgeport or the like. So we’ll probably see two- and three-axis NC machines for decades as well.

3 + 2 Versus Full Five

There’s a consensus that roughly 70 percent of the “five-axis parts” being made today don’t require simultaneous interpolation of all five axes. Likewise, surface finish and accuracy requirements vary across these parts.

This turn-mill CTX Gamma 3000 TC from DMG Mori also enables six-sided complete machining. (Image provided by DMG Mori)

Bates said we can consider two different categories of five-axis machining: five-axis positioning and full five-axis. As such, Mazak and others offer five-axis machines with different degrees of precision and control, including “entry-level five-axis machines that are more five-axis positioning rather than full five.” Bates said five-axis machine prices in general have come down, and the entry level 3 + 2 machines are even more attractively priced.

If you’re just trying to eliminate multiple three-axis setups and the attendant manpower burden, a 3 + 2 machine—or five-axis positioning—is probably the way to go. But if you’re making a complex part that requires contouring capability with a fine surface finish, “you’ll need a full five-axis machine,” explained Bates.

In configurations in which one cutting tool at a time engages one part at a time—even if that engagement requires five-axis simultaneous interpolation—it’s routine for today’s CAD/CAM systems and controls. But while five-axis interpolation might be the ultimate in what you can do with one tool, we need not stop at one cutting head and one workpiece clamping.

Beyond Five-Axis

“There really isn’t a functional limit to how many axes can be simultaneously computer controlled,” Absolute’s Smith noted. Increasing the number of simultaneously controlled axes in motion increases the machine’s cost and complexity—in both programming and training—but it’s doable and adds possibilities. For example, he offered, “the MedUSA robotic additive manufacturing system at the Manufacturing Demonstration Facility at Oak Ridge National Laboratory has three robots, amounting to 18 NC axes, plus a rotary table, for a total of 19 positioning axes under simultaneous control.”

Joe Wilker, product manager for Mazak’s multitasking and hybrid machines, pointed to INTEGREX machines from Mazak. “With a second spindle on a lower turret, you’re looking at a minimum of nine axes. And we can put an additive gantry on top of that, adding another five axes. So that’s 14 axes. But am I using those all at the same time? No, it’s usually five-axis at one time.”

Why would you need such configurations? To use Mazak’s vernacular, to be “done in one,” even for a complex part that must be machined all over. That’s only possible if you can approach it from any angle, then reclamp it to expose the formerly hidden side.

It must be a common need, because Miller of Absolute reported that seven- to nine-axis Swiss-style lathes are the hot commodity in the job shop market. “Many of these shops are finding that the Swiss machines are far more efficient. They are light years faster than traditional fixed headstock turret lathes.” He added that these machines have long been used in medical manufacturing for tight-tolerance, small-diameter, long length-to-ratio parts, while job shops have only recently emerged as a leading buyer.

“One of the big things driving our Swiss machine sales so high now, or sales of any multi-axis mill-turn center,” Miller explained, “are situations where one operator loads a part into a lathe chuck, then takes it off and puts it into a mill fixture, then takes if off again and puts it into another lathe to do the second half of the lathe cycle. We are completely eliminating those three-machine cells by putting in one Swiss machine, and it’s dropping that part complete.”

Miller referenced one family-run shop in Northeast Ohio that lost two members to illness in the span of two months, causing a 25 percent drop in production. It put in a Swiss turn and “within three weeks was running unattended overnight. They could load eight bars overnight, and they were getting 36 parts per bar. So they got almost 300 parts for free, no labor involved. Those 300 parts would have taken them two and a half or three days to do. Instead, they did it in four or five hours, unattended.”

The most popular layout in the job shop market, Miller said, has a main spindle with guide bushing (Z and C axes) and a sub-spindle, which Absolute sells under the Nexturn line. For more complex parts requiring lots of angled cross cuts and milled features, Absolute offers the QuickTECH line. These mill-turn machines boast up to 10 axes, with two turning spindles, two milling spindles and two B-axes, with up to 39 total tools—24 of which can be live tools with B-axis milling capability.

As Mazak’s Wilker suggested, these complex machines still generally engage the part with, at most, five-axis interpolation at a time, though in a twin-spindle lathe you might be machining the front end of one part in the main spindle while machining the back end of another part in the sub-spindle. Wallace explained that one reason for this is the limitation of the system’s ability to track the tip of the tool. He said Siemens offers an automatic function for this called TRAORI, while FANUC’s is called Tool Center Point control. But CAD/CAM tops out at five-axis, unless you use some tricks to generate several toolpaths and then combine them with “‘pre’ post-processing to synchronize the files.”

One example is a pinch-milling process DMG Mori created that cuts a turbine blade contour between two opposing spindles simultaneously. It’s even more complex than it sounds, because the upper and lower tools follow different contouring paths, synchronized with the axis motions. And the tools are quite different, as one is roughing while the other is finishing. The approach required “11-axis output to drive two separate cutting tools with one channel and one command line,” explained Wallace. He contrasted this with the type of multi-turret lathes just discussed. Those are multi-channel machines, stretching beyond five-axis. “But they are usually separate channels that are synchronized, not one channel, single command line,” he said. Wallace credited DELCAM as the only vendor willing to take up the challenge.

The Impact of Hybrids

As mentioned earlier, some approaches to equipping a subtractive machine with additive manufacturing capability involve additional axes. Wilker said Mazak offers three approaches, two of which add axes. The simplest approach exchanges the cutting tool chuck with an additive head, thus there is no change to axis layout. A second approach adds a ram head separate from the milling head that comes down parallel to it, but is basically the same configuration. The third approach uses a multi-axis gantry to bring in a completely separate additive head that can move across the entire part. In all three approaches, Mazak uses directed energy deposition with a powder feed. It also offers the ram design with hot wire deposition instead of powder. “There are many benefits to using wire versus powder,” said Wilker. “From a safety standpoint, you don’t have powder floating through the air, so you don’t have to wear masks.”

He clarified that the machine never performs additive and subtractive operations at the same time. This speaks to Smith’s notion that the additive and subtractive processes are inherently contradictory in that “the deposition process generally likes for the workpiece to be hot. And machining operations generally prefer for the workpiece to be cool. So managing that aspect of it is an issue. I would also say that metal powders and coolant don’t mix very nicely. So I’d have to have a strategy in place to get the powder out of the coolant.”

The Mazak INTEGREX I-200S AM combines full five-axis machining with this separate AM head mounted on a five-axis gantry. The AM head moves to a storage area away from machined chips or coolant during subtractive machining. (Provided by Mazak)

What’s more, observed ORNL’s Smith, you’ve paid for two working heads, but they can only work one at a time. “So, instead of having those two things in the same machine, it might be better to have different machines close to each other and a way to rapidly move parts back and forth between them.” ORNL is working on such designs. And given the differences between additive build rates versus material removal rates, not to mention the variety of part geometries to consider, it’s easy to imagine cells that combine very different numbers of each machine type.

DMG Mori’s Wallace summed it up: “The person or people that will fully utilize 3D manufacturing capabilities are not even born yet. So, we don’t know where this is going.”

Operators Getting On Board

Despite the dizzying complexity of many of the machines we’ve discussed, the builders are uniformly positive about how operators are embracing the new controls and software. Roaden said the evolution of smartphones and tablets has made everyone, including the more senior personnel in the shop, comfortable with touch screens, and machine tool interfaces have only gotten better and more intuitive. And simulation has been a huge help, especially with the younger generation.

“Our conversational Mazatrol programming language paired with a built-in machine simulation allows entry-to-mid-level operators to ‘see it before they run it,’ ” said Roaden. “This visual simulation creates confidence and an understanding of the machine movements.”

Mazak’s Bates went so far as to say that the new generation spent little to no time with manual machines and learned largely on CAD/CAM systems and simulators. “They’re more comfortable right out of the gate because they grew up on computers and they trust them. ... Some of the veterans who run machines will do a simulation on a computer and still doubt that it could happen in the machine. They’ll single block through a program to prove it out. A customer recently told me none of his operators dry cycle a machine. They just hit cycle start, full rapids, and go at it. Scares the tar out of him!

“Essentially, we can teach anybody Mazatrol,” he continued. “The difficult part is learning the cutting characteristics of material, cutting tool technology, workholding. ...That’s more tribal knowledge, and it comes from experience.”

Absolute’s Miller said “wait codes” are the biggest learning curve with multi-axis mill-turn centers and Swiss machines. “In order for these multi-channel, multi-axis machines to be able to have two turrets work on the same spindle at the same time, or two channels work simultaneously together, you have to build wait commands at the appropriate places to prevent interference.”

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