Kansas-based Triumph Structures-Wichita (TSW) (
www.triumphgroup.com) is a US sub-contractor specialising in the high-speed machining and sub-assembly of aluminium and titanium structural airframe components with wall thicknesses down to 0.020in. The company manufactures wing spars and stringers; it also carries out a variety of production jobs on smaller sections, such as bulkheads and landing gear, so it needs fast, reliable and versatile machine tools.
To expand on the capabilities of its 21 existing five-axis CNC machining centres plus more than 20 other three-axis and four-axis machines, TSW considered a variety of options, before deciding on a bespoke machine from the German manufacturer F Zimmermann, which is represented in the UK by Gosport-based Geo Kingsbury Ltd (
www.geokingsbury.com).
The company 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. It was also reckoned that the same machine — a portal-type with twin gantries, independent CNCs and three-axis heads — could run multiple smaller parts or have both heads working in tandem on the same long 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).
TSW asked Zimmermann Inc in Novi, Michigan — a long-standing partner and supplier of portal machines — to modify an FZ100 single-gantry machine to include a second gantry. Each would be equipped with an independent Siemens Sinumerik 840D sl control and an M3 ABC fork-type milling head with three CNC rotary axes to provide simultaneous six-axis cutting (using both gantries). The ±15deg B axis avoids the problem associated with traditional two-axis A/C rotary-tilt heads, whereby the A axis cannot move when it is at 0deg. The M3 ABC head can swivel, tilt and incline to any angle, providing constant feed-rate 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.
When designing the machine, Zimmermann engineers determined that the best solution was a removable partition built at the mid-point of the machine bed, allowing it to operate as two machine tools in one. Without the partition, the machine bed would accept parts up to 80ft long.
The machine was built over a period of 18 months and parts were sent to Zimmermann for trials prior to installation in Wichita. Owing to the volumetric compensation feature of the Siemens CNC, whereby machining is based upon the actual tool-tip position, the issue of the intersection where the twin heads meet was easily addressed and resolved.
Harry Thurmond, president of TSW, said: “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, this can translate into cycle time reductions of 35% or more, especially as subsequent deburring and polishing are not required. We routinely achieve finishes better than 125RMS on inside pocket surfaces and up to 32RMS on the outside of the Series 7000 aluminium structurals we produce.
“On longer runs, the chilled coolant used in the Zimmermann helps to minimise thermal expansion of the material, a critical factor in long-run machining work here. The machine is also used to make work-holding and fixturing devices; and because it is equipped with probes, it can be used as a CMM for the in-process measurement of workpieces.”
Another feature of the machine is the DemTec composite fill in its base and side walls for enhanced stability and vibration damping. All axes have feed rates to 60m/min and accelerations to 4m/sec2. Spindles are rated at 73kW 27,000rev/min and each head can access a 60-position tool-changer. Furthermore, a remote monitoring feature in the Sinumerik 840D SL CNC allows Zimmermann to log all conditions on the machine in real time.
In conclusion, Mr Thurmond said: “We have grown steadily since our introduction of five-axis working in the 1990s, and were recently ready to jump to a new level of competence for our customers, which represent the top players in both commercial and military aircraft. Previously, we could accommodate parts up to 22ft long, but we were committed to expanding to compete in the 40-80ft range. As with all efficient aerospace structure machining, material
removal rates have to be extremely high. Wecan start with a 5,600lb billet that ends up as a part weighing just 100lb.”