|Photo courtesy of Nordex |
One wind turbine, depending on its size, is comprised of 10 to 25 tons of iron castings that range in weight from less than 100 lb to as much as 80,000 lb, states the American Wind Energy Association (AWEA). Typical wind turbine hubs, for instance, can weigh in at 36,000 lb and measure 15 ft in diameter.
And if part size and weight aren’t enough of a challenge for machine shops, these large wind turbine components often require hundreds of hours of precision machining that includes boring, milling, drilling, tapping and turning. Add to that required tolerances as tight, if not tighter, than those of aerospace parts.
When it comes to processing massive wind turbine components, shops primarily use horizontal boring mills, vertical turning centers, horizontal or universal machining centers, and horizontal turning centers, said Rich Curless, chief technical officer at MAG Industrial Automation Systems. But no matter what type of machine is used, those with multitasking capabilities – combining dissimilar operations such as milling, turning, drilling, tapping, perimeter scalloping, boring, hobbing and grinding – eliminate error stack up from part transfers and provide one-stop processing for lean manufacturing of turbine parts.
Many wind turbine component shops process such parts as gear case assemblies, planetary carriers and hubs on horizontal boring mills. These machines, with contouring heads and programmable boring bars, can reduce the number of tools, and tool changes, required to complete parts.
One tool can bore multiple diameters and produce complex part geometries. A contouring head combined with a programmable boring bar can perform as many as nine to ten different operations – atypical for a boring mill. Operations include threading, grooving, turning, contouring, taper turning and more, internally and externally, and rightangle heads and precision rotary tables add capabilities for five-sided part processing in single setups.
Vertical turning centers, when running wind turbine bearing rings and planetaries, normally apply stationary tools on the ends of movable rams, but automatically interchangeable powered heads also allow the machines to do drilling, milling, tapping, and similar operations with live tooling, again, all in one setup.
The addition of full-contouring, variable positioning C-axis tables make it possible to drill or mill turn features anywhere on a workpiece. And turret heads provide productivity on parts that don’t require a lot of reach into them.
As their name implies, universal machining centers can approach parts from a horizontal or vertical orientation. These machines, designed for large, complex-geometry parts, easily handle five-axis/five-sided machining. And with automatically interchangeable heads, the machines deliver fast processing and reduce work in process.
Spindle rams are mounted on crossrails, so the machines can have a rail or moving-bed design. A rail design allows for multiple workzones under the spindles, so operators can load and unload parts while the machine continues working.
In another machining concept from MAG Industrial Automation Systems, torque tables in the workzone of a universal machining center add vertical turning work with fixed tooling capabilities to the machines’ list of operations.
To increase machining flexibility for wind turbine components, StarragHeckert Inc., gives shops the choice of a horizontal main spindle with AC motor or motor spindle, along with an a extendable NC quill and a swiveling horizontal/vertical head for five-sided machining with its HEC machining centers, part of the machine tool builder’s Athletic Series. The machines thus can be matched ideally to wind customer requirements.
According to Curless, horizontal machining centers are a mainstay of prismatic parts manufacturing, and the machines are growing in size, speed and multitasking capability to meet needs in wind energy manufacturing. He indicated that his company’s twin-pallet machines for wind parts typically have table sizes of 1,250 mm by 1,600 mm and handle workpiece weights up to 15,400 lb. The twin pallets and quick work exchanges ensure high rates of production, while robust cast-iron construction and oversized ballscrews provide muscle and speed for processing large parts in volume.
In a study conducted by Heller Machine Tools, the machine tool OEM compared four approaches for machining a gray iron wind turbine rotor hub. The approaches involved a plain table horizontal boring mill, a rotary table horizontal boring mill, a rotary table horizontal boring mill with table changer, and a horizontal CNC machining center. Assumptions included three shifts at 70 percent efficiency, 1,200 units per year, and that all machines use the same tooling, machining feedrates and cutting speeds.
What Heller found was that, of the horizontal boring mills, the machine with the table changer offered the best combination of handling and machining times. However, a shop would need at least three such machines to match the desired level of production, and it would take four plain table machines to get the annual production done. Also, the machines would require five operations and part handling time between each operation, while the spindle stood idle, to complete the work on a rotor hub.
“The large-capacity CNC horizontal machining center delivered the shortest machining cycle time by far and nearly zero part-handling time, owing to the fact that part handling is done outside the machine’s work envelope, on the automatic pallet changer and while the spindle is working. And only two CNC machining centers are required to accomplish the rotor hub production needed. Machining center cycle times are one-third less than the horizontal boring mill, and setup times are less than 12.5 percent of the time required for the plain table horizontal boring mill. Spindle cutting time is over 84 percent of the overall cycle time with the CNC machining center, as compared with only 45 percent with the plain table horizontal boring mill, 64 percent with the machine with a table changer,” explained Vincent Trampus of Heller Machine Tools.
Some machine designs for wind turbine parts combine the advantages of different types of machines. For example, the You Ji HMC-1000BP from Absolute Machine Tools Inc. merges the benefits of a horizontal machining center with those of a boring mill. The heavy-duty cutting, high-accuracy machine tips the scales at 77,000 lb and features a moving saddle design with a box-in-box structure that reduces moving mass versus a moving column design. It allows for faster accelerations and rapid rates, reported Steve Ortner, president of Absolute Machine Tools.
|Main components of a wind turbine nacelle. |
Another system that marries the benefits of two machine types for processing wind-turbine components is the Matsuura CUBLEX from Methods Machine Tools Inc. That mill-turn machine joins the capabilities of 5-axis cutting with those of a vertical or horizontal turning lathe.
According to Scott McIver, chairman and vice president of product development at Methods Machine Tools Inc., Matsuura’s 5-axis and turn-mill technology is well suited to complex wind turbine gear sets and knuckles, which can be manufactured in just two operations to eliminate the need for a turning center, and make more accurate parts. There’s also room for as many as 500 tools, which could essentially eliminate the need for a tool room.
“We see 5-axis becoming larger, and the addition of mill turning products in conjunction with 5-axis machining centers growing in size and popularity in complex, lower-volume work that is critical to wind turbines and power generation,” said McIver.
Dr. K. Hiramoto, a vice president of engineering at Mori Seiki, would agree. He said that shops doing wind turbine components not only need 5-axis capability in their machines, but also high torque and large work envelopes.
For wind turbine parts, Mori Seiki will introduce its NMB3650/36100 double-column vertical machine. Five-axis capability comes from the machine’s A,C-axis universal head, while a 3.5-meter work envelope – 3,600 mm between columns – provides room for large parts. The machine also will feature Mori Seiki’s Zero Chip technology that uses a vacuum within the machine spindle to collect and evacuate chips at the cutting tool.
Hiramoto said that double-column machines are often considered slow. However, he indicated that the NMB3650/36100 feeds at 30 meters per minute and sports a chip-to-chip toolchange time of 12 sec.
Doosan Infracore has also introduced a big five-face, doublecolumn bridge mill for wind turbine part machining. A widerthan- usual machine passage width accommodates a 106-in.-wide by 397.6-in.-long table and a load capacity exceeding seven tons.
With a 50-hp spindle, the machine face mills stainless steel at a rate of 35.7 ipm, and cast iron at 44.3 ipm. The machine’s table moves along four guideways that equalize load distribution, while a roller-bearing pack assembly beneath the table and a slide bearing further absorb vibration.