Manufacturers of XXL machines rely on versatile concepts
Hydrostatic bearings and high power-motors
Likewise conceived for processing large parts is the Profimill series by Waldrich Siegen, designed for long service life and high precision. These machines are made with customer-specific passage widths starting at 4m and even exceeding 13m. They are built from high-quality cast components with hydrostatic bearings and have available motor powers up to 150kW and torques up to 16,000 Nm. That such a large power transfer is possible at all is due to the so-called Master Head concept, according to Andreas Bongardt, sales manager at Waldrich Siegen. In this machine, the vertical spindle unit reportedly forms a completely separate head unit which, when the sub-assembly is changed, is exchanged directly into the changer.
Because the main bearings of the vertical spindle unit are not permanently built into the feeder, they are not subject to compromises necessary because of lack of working space and can always have the dimensions required for performing at full power. Transverse beams and feeders are constructed as NC axes and can be traversed synchronously. In order to optimise the stiffness of the system, the circulating ball spindles for Z and W axis are pre-stressed and fitted with an additional axial bearing, said to raise the stiffness significantly in comparison to conventional spindle suspensions. In combination with a direct path measurement system, the Profimill series attains great accuracy, the firm added. Via an Invar rod in the feeder, it is furthermore possible to compensate for temperature-dependent changes of length in the Z axis. The full hydrostatics in all axes guarantee, even at high milling powers, decades of freedom from wear, the company said.
Market demands efficient units for complete processing
How blurry the lines between portal milling machines and conventional processing centres has become in the meantime is shown by the success of DMG Mori: over 900 portal machines have already been sold, the company reported. Currently, the programme comprises the DMU 210 P, DMU 340 P and DMU 600 P as well as the DMC 210 U and DMC 340 U, fitted with an automatic pallet changer.
The latest flagship from the DMU 600 P series was built to customer specifications with an extended range of features. It enables traversing paths of 6m (optionally up to 18.75m) in the X and 4.8m in the Y direction; the vertical portal passage width is 3.6m. Feed speeds up to 30m/min and accelerations up to 3 m/s² are said to provide high dynamics and short secondary times on long traversing paths.
A highlight of the machine are the three coupleable tables, which create a total processing length of 18m. Because the machine enables processing in alternating operation, secondary and tooling times are almost negligible. With the help of an adaptable indexing device, workpieces with diameters up to 3m and a length of 17m can be processed. Besides a rigid table bearing loads up to 40 tonnes, the DMU 600 P is also optionally available with a milling/turning table, which offers combined milling and turning processing in one clamping.
Said to be substantially smaller and very precise, the high-precision machine Dixi 270 is also available. “Until now, there was no machine in the world of these dimensions capable of precision to 35µm,” according to Tobias Steiner, CEO of DMG Mori subsidiary Dixi Machines. “Previously, potential users therefore had to go to exorbitant lengths to be able to produce parts of corresponding accuracy at all, although, conversely, it is precisely this work-saving that more than justifies the slightly higher price of the Dixi 270 compared with conventional processing centres.”
With the centre, it is possible to carry out preliminary work on heavy components of up to 3m diameter, 1.8m height and 12 tonnes of total weight in one clamping. Spindle powers are up to 1550Nm torque and, equally fast (at 60 m/min) and dynamic (with accelerations of up to 6 m/s2), to carry out final finishing. In Steiner’s view, this precise machine is suitable above all for processing motor or gear housings for aerospace, for engine blocks in motor sport, for spindle head assemblies and housing in machine tool construction, and for making high-quality injection moulding tools, DVD stencils, or prototypes.