Machining equipment Manufacturers of XXL machines rely on versatile concepts

Editor: Rüdiger Kroh

There is new movement in the portal milling machine market. Although centres are still designed to produce as many chips as possible for decades to come, this is no longer enough. In precision, flexibility and cost-effectiveness, the heavyweights are getting nearer and nearer to the small and medium-sized processing units.

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Portal milling installations with fully hydrostatic bearings are the specialists in precise processing of large parts. They run, as a rule, for 20 to 30 years trouble-free.
Portal milling installations with fully hydrostatic bearings are the specialists in precise processing of large parts. They run, as a rule, for 20 to 30 years trouble-free.
(Photo: Waldrich Siegen)

The current trend point to portal milling machines with universal applicability that cover interdisciplinary application areas. Christian Zierhut, member of the executive board of F. Zimmermann, Denkendorf, Germany, said he is convinced of this. Last year, the supplier of portal milling machines presented its FZ 33 compact. With 2.5m in the X axis, 3m in the Y and 1.25m in the Z, this machine is said to provide generous working space, while requiring only a comparatively small footprint. With this centre, Zimmermann said it intends to address the needs of as many users as possible in the automobile and aerospace industries as well as in tool and mould construction.

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With its modular concept, this dynamic monoblock-structured portal milling centre covers a broad application spectrum and can be integrated flexibly into existing infrastructures. With the help of high-resolution measurement systems, the patented milling head technology reportedly enables fast and precise positioning. In addition to these technical aspects, installation is said to offer economic advantages. too. The efficiency effects, for example, resulting from assembly due to the modular construction, the standardisation of component groups and the easy supply of stock parts, are passed on directly to the customer. “Users can process a broad spectrum of components in this compact installation and profit from an extremely attractive price/performance ratio and a short delivery time,” Zierhut explained. In addition, the universal and flexible usability of the FZ 33 compact becomes apparent through increased investment security and used-price stability, the manager added.

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This modular idea is also available in the Asca portal machines from Schiess. “By using standardised component groups, we can deliver high-quality, individual production solutions promptly to our customers,” Marc Springstein, sales manager at Schiess Tech, said. The foundation of the Asca series is reportedly the system-building set, cost-effective configuration and short delivery times. With this concept, Schiess has responded to the fact that the time window for realising installation solutions is being constantly reduced.

The milling portal Ascamill is flexible and comes optionally with a table that is 2, 2.5 or 3m wide and 4-10m which takes loads up to 40 tonnes. Because the axes are equipped with absolute measurement systems, the time-consuming work of homing is eliminated, the company noted.

With its tradition-rich Vertimaster series, in contrast, Schiess continues to rely on made-to-measure orders. No wonder, as these CNC controlled portal milling and turning centres are, according to the firm, among the largest machine tools in the world. If required, their face plates can be extended to up to 18m by means of a centring star.

These giants are used to produce impellers for water turbines, housings for gas and steam turbines and pressure vessels, and heat exchangers and reactors for the chemical process industry, with an individual configuration each time. “We develop Vertimaster installations explicitly designed for the specified process solution. Geometry and performance of each machine are tailored to the customer’s production tasks,” Springstein explained. Here, any desired combination of turning, boring and milling, as well as grinding and drilling out, is possible. Schiess recently presented new developments and modifications in the field of milling heads and tool guides. In addition, it has also improved operating comfort. In the future, it will be possible, for example, to collect all relevant information at the input terminal via a traffic light system and to have a one-glance overview of other optical signals.

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.

Large centres close in on the efficiency of small units

Germany’s Hermle has extended its machine programme. Although the large Hermle machines with modified gantry construction, such as the C60 U MT, with workpiece dimensions of 1m x 1m x 1m and weights up to 2.5 tonnes, do not belong directly to the segment of portal machines, it is nevertheless evident here as well that the concept of raised efficiency, already proven with smaller machines, is being transferred increasingly to large processing centres. These include 5-axis simultaneous processes or combined milling and turning or automated machine loading.

This article, by Editor Rüdiger Kroh, originally appeared in German in ETMM’s sister magazine MM Maschinenmarkt.