HMCs Flex Their Muscles
Designs adapt to diverse machining needs
By Jim Lorincz
HMCs have proven themselves in manufacturing applications as diverse as machining large structural components for general engineering, precision parts for high-volume manufacturing, and specialized components for aircraft manufacturing.
HMCs are characterized by flexibility and adaptability well-suited for industries as diverse as automotive, aerospace, medical device, and moldmaking. Capabilities include:
- Ability to deliver high-volume production, as well as prototyping,
- Ability to run untended in cellular configurations with or without on-machine pallet changers and in combination with other machining processes such as CNC turning centers,
- Adaptability to fourth and fifth-axis machining capability using trunnions and rotary tables,
- Machines rigid enough to handle high-torque, low-rpm machining of the toughest metals, such as the titanium alloys favored by the aircraft industry for use with new, lighter-weight composites.
In contract-manufacturing applications, HMCs excel in producing precision parts, whether for prototyping or in production volumes. The new Kitchener plant of Eldorado Tool & Mfg Co. (Waterloo, ON, Canada), for example, has selected its Palletech system and Nexus HMCs from Mazak Corp. (Florence, KY) to manufacture prototypes and deliver large-volume production almost seamlessly.
The Mazak Palletech system enables Eldorado to provide just-in-time, low-to-medium lot manufacturing with minimum lead time. For repeat orders, the system eliminates setups and first-off inspection, minimizing inventory and maximizing the profit opportunity.
The Palletech manufacturing system consists of three HCN 6000 HMCs with 160-tool magazines, 40 pallet locations, two load/unload stations, and one stacker/transfer robot. There is also a stand-alone Mazak HCN 6000.
"We make one hundred production parts for off-road vehicles," explains Peter J. Harry, managing director. "That's one hundred setups. You can't make that many setups without something like Mazak's Palletech. If we didn't go for these flexible production systems, we would still be in business, but our growth would be extremely slow," Harry says.
Harry, who came to Canada as a toolmaker from Guyana in 1965 and founded Eldorado in 1974, after working for Raytheon and Budd Automotive, believes in always having capacity available for new business. "You must have capacity available at all times, because when the phone call comes through, it could be for a goldmine."
Eldorado manufactures a wide range of parts sizes and configurations, turning parts from 1/16 to 28" (1.58–711-mm) diam and in lengths from 1/16 to 40" (1.58–1016 mm), and milling parts from 1/16 to 24" (1.58–610 mm).
The importance of large-part machining is evident in the introduction by Haas Automation Inc. (Oxnard, CA) of the EC-630 HMC, its largest capacity HMC to date. The EC-630 features a 40 x 33 x 35" (1016 x 838 x 889-mm) work envelope, 50-taper geared-head spindle, dual-pallet changer with 630-mm pallets, and 50-pocket side-mount ATC. A 1° pallet indexer is built in, and a fourth axis is available.
The EC-630's geared head couples the motor directly to the spindle through a high-precision twospeed gearbox that provides 450 lbft (610 Nm) of torque for heavy material removal, and speeds to 6000 rpm for finish machining. The machine's two 630-mm pallets handle 2645 lb (1200-kg) of load; workpieces to 39.4" (1 m) in diam and height can be accommodated.
Hermle Machine Co. (Franklin, WI) has expanded its product line offering with the addition of the C 50 U Dynamic machining center for machining large and heavy workpieces. "We expect to see significant growth in the market for larger, high-value parts, and the C 50 U can meet that need with an automation-ready solution offering the quality and precision that Hermle is known for," says Kenneth Merk, executive vp.
The C 50 U Dynamic is designed for full simultaneous five-axis and/or five-sided machining of large automotive and aircraft parts or molds, a capability for which Hermle is known. The C 50 U, which is the largest in Hermle's C machine is already being used in Europe to machine engine blocks and other large parts.
Axis travels for the C 50 U Dynamic are 1000, 1100, and 700 mm (X, Y, Z). Workpieces with 1000-mm diam, 810-mm-height, and weighing 2000 kg can be handled. Models are available with NC swiveling rotary tables with clamping surface to 700 mm or 1150 mm diam. Both tables feature an integrated torque motor (C axis) and a tandem-drive motor (A axis). Rotational speed for both tables is 20 rpm for the A axis and 30 rpm for the C axis. Swivel range for both axes is +30/-115°.
Large-part machining with HMCs has been dominated by high-speed machining for the past couple of decades. That wasn't surprising, as the principal generator of business was found in an aircraft industry that needed aluminum profiles of every different size and shape. High-speed machining held the key to removing a lot of metal fast so that monolithic workpieces could be carved out of solid billets of metal.
The titanium grades that are now being utilized in the newly designed aircraft applications are different, very different. Contract manufacturers and OEMs here and around the world have taken the hint and begun to shape their capital equipment purchases to meet the needs of materials that have, at the very least, machining requirements that are directly opposite to those of aluminum.
Scott Walker, president, Mitsui Seiki USA Inc. (Franklin Lakes, NJ) explains: "Basically, the new titanium materials are utilizing new chemical compositions, creating new material grades that have never been machined before. The addition of materials like nickel makes the titanium stronger, tougher, and as a result more difficult to machine."
Walker explains: For example, the skin of newly designed planes will be carbon fiber materials that will be attached to a skeletal structure, and part of that skeletal structure will be composite materials instead of aluminum. The critical high-load areas will be high-strength titanium alloys, replacing the aluminum and stainless used in current aircraft. The plan is to make these components light, with much thinner wall thicknesses than in the past. The intent of the new aircraft is to reduce overall aircraft weight as much as 30% with the same flight-carrying capacity hence being much more fuel efficient and cost effective per mile flown.
"When you get into this type of material, this triple-nickel material, the objective in machining is to do just the opposite of what was being done in machining aluminum. Basically, you want to take a very deep depth of cut. You want to run at very low rpm, because when you start to cut this material, it becomes very hard, and to take a deep cut you need a low-rpm geared spindle. In other words, you need lots of torque at low rpm to be able to peel the material off. And you need machines that are able to cut at a much lower frequency," Walker says.
"High-speed machines and horizontals in general are not very conducive to low-frequency cutting. These new materials required rpm ranges that reside in the harmonic excitation ranges of conventional high-speed machine designs. So during low-rpm cutting, you can have chatter issues, which is not good for part accuracy or for cutting tools, which have a tendency to chip and break when the machine is chattering. If you try to increase the rpm to get above the harmonic excitation range then cutters wear out too fast. To slow an rpm and you cannot clear the chips and the cutters break. So the optimum cutting ranges fall right in the excitation ranges of conventional designed machines," Walker says.
"What is needed are tool steel boxway machines with high-torque spindles and robust structures that are designed to operate outside of the excitation ranges of low-rpm cutting," Walker says. "We have HMCs between 500-mm and 1500-mm pallet sizes that are designed with these criteria in mind, and all of our machine designs are being upgraded to be able to cut heavy metals. They have traditionally been used to machine Inconels, Waspaloys, and titaniums."
Selecting the right cutting tool is especially important. Manufacturers want to be able to use the most cost-effective cutters, and extend the cutter life of the cobalt-coated cutters that are favored.
Mitsui Seiki has been working with R&D teams at the key aircraft manufacturers. Currently there are over 16 new grades of titanium that have been tested. Says Walker: "Aircraft designers have selected the materials to fit exactly the loading characteristics of particular structures to meet very specific weight reduction issues. The key with these new materials is to establish new cutting data that matches the machine, tooling and process for the most cost-effective machining of these new materials. Perhaps in the future, process engineers can go to the Machinists Handbook and find this information, but today it's purely R&D development."
Greg Hyatt, vice president and chief technical officer for the Machining Technology Laboratory, Mori Seiki USA (Rolling Meadow, IL) points to the reversal in the aircraft industry, which is moving away from aluminum to composites and titanium. "Cutting these titaniums, especially the beta-phase alloys such as 5553, requires more torque and dynamic stiffness from the machine. These alloys are harder, tougher, and more abrasive than the 6Al4V typically machined. It must be machined at lower speeds and at lower rpm for the same-size cutters."
Hyatt points to the need for a more-robust structure for the HMC as critical to successfully machining these alloys. "We have found that the DCG design of our machines, such as the NMH 6300, is well-suited for heavy cutting applications of titanium. As you move up the Y axis in a traditional HMC, you get farther and farther off the ways and observe significant degradation in accuracy and metal removal rates. With the DCG design, once you get past the midpoint, cutting is getting closer to the upper way, improving stiffness rather than allowing it to degrade. The result is much less variation in static and dynamic stiffness through the work envelope."
Machine structure is all-important, Hyatt believes. "The cutting forces in machining these titanium alloys are too high for machines that weren't intended for the application. Even if a builder dropped an oversize spindle in a lightweight structure, it couldn't handle the power without chatter and dynamic instability. The design of the machine has to start with the spindle, because these high-torque spindles are physically large. The size of the motor is most closely correlated to the torque of the motor, not to the horsepower."
According to Hyatt, the design of the machine structure has to be such that the high-torque motor is packaged with a larger shaft and spindle bearing to be able to handle the application of all that torque, without allowing the machine to be driven into instability or chatter.
"We find the trunnion of the NMH particularly attractive, because it's a direct drive [DD] motor able to take the advantage of all the benefits of DCG design and improvements in the three linear axes. The point is that in a five-axis simultaneous cut, you are limited by the weakest (slowest) link in the system. The conventional gear-reduction rotary axis has become that slowest link and doesn't allow us to take advantage of the improvements in acceleration and velocity of the three linear axes which have been realized over the last decade. The low inertia and rapid acceleration of the DD trunnion overcomes this limitation and matches the performance of the linear axes in the DCG machines."
According to Ken Campshure, a Mega 1600 HMC just delivered by Giddings & Lewis (Fond du Lac, WI) to an aerospace manufacturer for machining titanium meets the critical requirements for high-torque, low-rpm machining, as well as overall stiffness. "The Mega 1600 machine was equipped the machine with a slightly different spindle, a 5000-rpm spindle with a larger spindle-bearing set and a slightly longer snout for longer reach into parts," Campshure says.
He describes the Mega 1600 as "a pretty classical design, featuring a bifurcated column. To provide more dynamic stiffness, Giddings & Lewis uses large linear ways with roller packs and ballscrew damping techniques." The Mega 1600 is available with or without pallet shuttle and with fixed or live spindle. Axis travels and workpiece load capacities vary accordingly: 7000-kg load with pallet shuttle and 15,000 kg without pallet shuttle.
"Cooperation between Giddings & Lewis and Cincinnati Machine is evidence of the growing synergies within the MAG organization. Cincinnati Machine has extensive experience working with the aerospace industry, machining titanium, and we bring large horizontal machining expertise to the table," Campshure concludes.
This article was first published in the July 2007 edition of Manufacturing Engineering magazine.