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To reduce production costs and increase accuracy when producing complex parts requiring rotational as well as prismatic machining, Hermle introduced at EMO 2011 an 'MT' version of its largest 5-axis machining centre, the C 60 U, capable of turning components in the same clamping. Availability in the UK is through sole agent, Geo Kingsbury Machine Tools.
An unusual application was chosen at the exhibition to put the machine through its paces, namely fir-tree slot milling around the periphery of a 900 mm-diameter turbine fan disc using trochoidal machining strategies. It involved a tie-up between Hermle and another German firm, Iruba Innovations, which specialises in supplying turnkey manufacturing processes for the aerospace and power generation sectors.
A series of solid carbide fir-tree milling cutters were programmed to execute the trochoidal paths, whereby material is removed in repeated slices in a sequence of spiral paths, combining large axial cuts with small radial cutting depths. High spindle speeds and feed rates limit thermal and mechanical stresses on the cutting tool and workpiece, yet allow rapid metal removal for high productivity. The technique also greatly improves chip evacuation, which is traditionally a problem when cutting narrow slots in hard or tough materials.
To enable turning operations on the contour of the fan disc, the workpiece can be rotated at up to 450 rpm on the integral C-axis table, which is mounted on a ±130-degree swivelling A-axis trunnion moving at up to 15 rpm. The table has a 1,200 mm diameter clamping surface, sits within a 1,200 x 1,300 x 900 mm working envelope and can support workpieces weighing up to 1.5 tonnes.
Different spindle configurations with speeds from 12,000 rpm / 56 kW to 18,000 rpm / 35 kW are available. A 50-position (optionally 70-position) tool magazine is provided. Taper options are HSK-T, sizes 63 and 100, providing rigidity for static tooling during turning operations, for which purpose the spindle is locked. Unusually, turning can be performed not only when the table is horizontal or vertical, but also at any intermediate angle, allowing short toolholders to be used and opening up a wealth of new machining options.
The MT version is a derivative of the Hermle C 60 U machining centre and offers identical 5-axis prismatic metalcutting functionality, but is not simply an upgrade. Technological and safety requirements that apply to lathes have been taken into account and relevant safety standards have been adhered to, notably EN 12417 and EN 12478. Even the upper part of the machine enclosure and roof were considered when designing the guarding.
Another new safety feature is tool measuring and breakage monitoring. Milling tools are scanned by laser and turning tools are measured from two sides by 3D touch probes.
The construction of the 31-tonne C 60 U MT lends itself to the addition of high precision turning. A modified gantry positions all guideways above the working area. The Y-axis has a tandem drive for high machine dynamics and features a patented arrangement of three staggered guideways, one mid-way between and set back from the outer two, halving the Y-axis span. Production potential is high owing to 6 m/s2 acceleration to linear traverse rates of 50 m/min.
Bearings for the A-axis are set directly into the 17-tonne cast iron bed, promoting stable, vibration-free machining, particularly as the trunnion is driven from both sides by motors positioned on the axis centreline. The table, with integral balancing system, is driven by a high torque motor optimised for comparatively high speeds, making it possible to perform combinations of robust roughing and light finishing operations.
The machine can be operated in stand-alone mode or with an automatic pallet change system and can be further automated as required. It is controlled by either a Siemens S840D sl or Heidenhain TNC635 CNC system. The control panel can be adjusted by ±100 mm in height and the 19" screen tilts by up to 30 degrees.