AIRCRAFT STRUCTURALS MACHINED 35 PER CENT FASTER

April 6, 2016

Bespoke Zimmermann portal milling machine features twin gantries, independent CNCs and 3-axis heads for up to 6-axis milling:

Kansas-based Triumph Structures – Wichita is a US subcontractor specialising in high-speed machining and sub-assembly of aluminium and titanium structural airframe components with wall thicknesses down to 0.020 inches. The company manufactures wing spars and stringers but also carries out a variety of production jobs on smaller sections such as bulkheads and landing gear, which creates a need for fast, reliable and versatile machine tools.

To expand on the capabilities of its existing 21 five-axis CNC machining centres for producing components up to 22 feet long and more than 20 other 3-axis and 4-axis machines on site, Triumph Structures – Wichita considered a variety of options. It decided on a bespoke machine from German manufacturer, F Zimmermann, whose UK-based sales agent, Geo Kingsbury, describes the project for the benefit of British and Irish manufacturers that may be interested.

The US subcontractor wanted a machine to serve multiple purposes, but primarily the machining of very long parts with volumetric compensation to manage material expansion as well as tool tip position over long cutting cycles, often lasting several days.

However, it was posited that the same machine might also be capable of running multiple smaller parts or operate along the entire machine bed with both heads working in tandem on the same part. The latter scenario would demand close attention to collision avoidance between the gantries, as well as consistency of machining at points on the component surface where the two machining heads intersect.

To meet these requirements, Triumph Structures – Wichita approached Zimmermann Inc in Novi, Michigan, a longstanding partner and supplier of portal machines, to request that it modify its FZ100 single-gantry machine to include a second gantry. Each would be equipped with an independent Siemens Sinumerik 840D sl control and a versatile M3 ABC fork milling head with three rotary CNC axes to provide simultaneous 6-axis cutting using both gantries.

Zimmerman controls

The ± 15 degree B-axis avoids the pole position problem of traditional 2-axis A/C rotary-tilt heads, whereby when A is at zero degrees it cannot move. The head is able to swivel, tilt and incline to any angle, providing constant feedrate capability, significantly reducing machining times and improving component surface finish, as cutter chatter on the surface of a component due to excessive C-axis rotation is eliminated.

In designing the final work envelope and machine structure, Zimmermann engineers determined that the best solution was a removable partition built at the midpoint of the machine bed, which would allow the production centre to operate as two machine tools in one. Without the partition, the machine bed would accept parts up to 80 feet long and process them using the twin heads working in tandem and monitored for collision avoidance by the two CNCs.

The machine was built over a period of eighteen months and parts were sent to Zimmermann to be tested prior to installation in Wichita. Owing to the volumetric compensation feature of the Siemens CNC, whereby execution of machining is based upon the actual tool tip position, the issue of the intersection where the twin heads meet was easily addressed and resolved. Surface integrity on the workpiece would be preserved, while operator and machine safety would remain paramount.

Harry Thurmond, president of Triumph Structures – Wichita commented, “The design of the Zimmermann head in combination with the look-ahead feature of the CNC provides significant speed advantages during typical aerospace structure machining.

“Over long runs, it can translate into cycle time reductions of 35 percent or more, especially as subsequent deburring and polishing are not required.

“We routinely achieve better than 125 RMS finish on inside pocket surfaces and up to a 32 RMS on the outside of the Series 7000 aluminium structurals we produce.”

On longer runs, he observes that the chilled coolant used in the Zimmermann is helpful in minimising thermal expansion of the material, a critical factor in long-run machining work here. As an added advantage, the machine produces workholding and fixturing devices. The machine is also equipped with probes so that it can be used as a CMM for in-process measurement of workpieces.

Another feature is DemTec composite fill in the machine base and side walls for enhanced stability and vibration damping. Two backlash-free, rack-and-pinion drives are sealed from contamination and guided on both sides. All axes have feed rates to 60 m/min and accelerations to 4 m/s2. Spindles are rated at 73kW / 95HP / 27,000 rpm and each head can access a 60-position toolchanger.

For communication of data, Triumph Structures – Wichita integrates the CNCs into their Ethernet network. Through a remote monitoring feature in the Sinumerik 840D SL CNC, Zimmermann can log all conditions on the machine in real-time.

Harry Thurmond concluded, “We have grown steadily over the last decades, since our incorporation of 5-axis work in the 1990s, and were ready to jump to a new level of competence for our customers, which represent the top players in both commercial and military aircraft.

“Our part length capability had been 22 feet and we were committed to expanding it to compete in the 40 to 80 feet range.

“As with all efficient aerospace structure machining, material removal rates have to be extremely high. We can start with a 5600lb billet that ends up weighing 100 lb.

“The Zimmermann dual-gantry machine addresses all of these requirements and is helping us to move our business forward.”

Main picture:The Zimmermann twin-gantry, portal milling machine built to suit the requirements of Triumph Structures in Wichita, Kansas.

Pictured in text above: An operator at one of the pair of Siemens Sinumerik 840D sl control systems.

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