The German machine tool group Index recently made a major investment in machining centres at its Deizisau factory for the production of the large prismatic elements that go into Index and Traub lathes; it places considerable importance on in-house manufacture to control quality, shorten lead times and avoid bottlenecks, especially for core components like the lathe bed.

Factory manager Joachim Fink said: “Prior to the recession, we were out-sourcing a lot of work. Costs were high, and there was a risk that delays in procurement could cause problems for the Index and Traub assembly lines. To maintain build rate, it is essential to deliver machine beds to the assembly hall precisely on time.

“Furthermore, it was foreseeable that new developments such as our C 100/C 200 turning-centre series and the new R 200 turn-mill centres would require new machine beds that could not be machined by external manufacturers. We had to expand our in-house capacity urgently.”

Gantry-type milling machines were on the short-list, as their design promised a high degree of rigidity and capability. However, the size and type of workpieces to be machined — complex components up to 4m long and more than 2m high -— prompted Index to favour horizontal machining.

High rigidity and stability for precision metal cutting in the lower µm range was essential. Dieter Grözinger, manager of parts processing in the Deizisau plant, says: “Our previous machine supplier could not offer us anything suitable, and our research led us to Burkhardt+ Weber, which specialises in large machines with pallet sizes up to 2,000 x 2,500mm.”

Index, which along with Traub and B+W is represented in the UK and Ireland by Gosport-based Geo Kingsbury Ltd (www.geokingsbury.com), opted for the B+W MCX 1400 machining centre with a 1,600 x 2,500mm pallet capable of supporting up to 11 tonnes.

The specification required by Index was exacting, even by B+W standards. One of the major criteria was accuracy, especially with respect to angularity and parallelism over long slideways. Less than 10µm deviation over 1m (in some cases 2m) was specified. High tooling standards were also demanded; for the large spindle heads, a quadruple clamping arrangement was selected to support the tools, each of which can weigh up to 75kg.

Thermal stability

B+W attaches great importance to the thermal stability of its machines, which are designed to be thermo-symmetrical and thermo-stable. The X- and Z-axis ways, as well as part of the column, are cast in polymer concrete to add mass and increase thermal inertia.

Moreover, the 52kW main-spindle drive with a torque of 3,500Nm is water-cooled, as are the B-axis drive for the machine table and the X- and Z-axis drives. The electrical cabinet incorporates an integral water cooling system so that the heat generated is not discharged into the factory (during the winter months, the extracted heat is routed to the staff facilities).

Mr Grözinger says the machine environment plays a significant role, with respect to both shopfloor conditions and the coolant and chip management systems. “We require constant temperatures in the machine. Hot chips must be flushed out of the working area as quickly as possible, before they can transfer their heat to the axes or to the component. This calls for high-pressure flushing for efficient chip disposal and clean coolant, without which optimum working conditions are impossible.”

Satisfied with the quality of the first installation, Index next bought a slightly smaller B+W MCX 1200 with 1,250 x 1,600mm pallets to replace a large 18-year-old machining centre. Particular attention was paid to the coolant and chip management equipment. Initially, separate systems were planned for each of the large machining centres.

However, detailed planning revealed that a central plant would be more effective in terms of size, filtration, flow capacity and costs, especially as other machines in the prismatic machining area could be connected as well.

Index also needed a high degree of flexibility and availability from the new MCX machines, and Mr Fink was adamant that the two machining centres had to be equipped with a flexible pallet system, as the manufacturing philosophy in Deizisau is based on minimally attended night shifts and continuous production through breaks.

Index opted for a solution from the Finnish company Fastems, as some years ago it had deployed a similar storage and retrieval system to automate three machining centres of a different make with a pallet size of 500 x 400mm.

To serve the two B+W machines, Fastems installed an MLS-XXHD space-saving two-level flexible manufacturing system with a 16-station store capable of accommodating eight 1,600 x 2,500mm pallets for the MCX 1400 and eight 1,250 x 1,600mm pallets for the MCX 1200. The FMS has two loading stations suitable for both types of pallet and provides up to 8hr of unmanned production.

Second phase

With the two B+W machines and FMS fully operational, in 2011 Mr Fink and Mr Grözinger went to the EMO show in Hannover, where they researched machines to replace ageing jig borers that were used to finish-machine headstocks, turret housings and slideways. Again, they wanted to separate part preparation and set-up from the machining cycles in a highly automated flexible machining cell.

Mr Fink said: “Although we visited all relevant machine tool manufacturers, there were no models available that met our requirements. Either a conventional machine was recommended, which did not meet our productivity goals, or the manufacturers had neglected top precision machining in pursuit of other design concepts.”

Many potential suppliers quickly distanced themselves from the discussions when they recognised that the demands from Index would not allow a standard machine solution. Mr Grözinger says: “Starting from the accuracy standard of a high-precision jig borer, we set our machining tolerance at less than 5µm over 1,600mm of axis travel. For angular error over 1m, we asked for a positioning accuracy of less than 3µm.

"We also wanted a self-supporting machine that could be installed on a concrete pad, without the need for a special foundation. In the end, few potential suppliers were left; of those, we chose Burkhardt+Weber again.”

The Index machine specification was so stringent that B+W requested a three-month planning phase to check the feasibility of the project. The main area under consideration was the thermal stability of the machine, which would later be designated MCX 900 µ. Each individual component to be produced on the machine was discussed in detail with Index to determine the best approach for avoiding thermal change.

Mineral filling

To maximise torsional and bending stiffness, as well as thermal capacity, the machine ways for the X axis and the Z axis, as well as the thermo-symmetrical column, were filled with a mineral casting material, equipped with integrated water cooling circuits and controlled with temperature sensors — as on the previous B+W machines.

Class G0 linear roller bearings were mounted on hand-scraped locating surfaces for optimal axial flatness and straightness; the time needed to complete the hand scraping alone was around 350hr.

The specified positioning accuracies were achieved using the highest-quality ground ballscrews, thermally isolated absolute linear scales, and liquid-cooled axis servo motors.

The worm-gear arrangement driving the rotary table was a specially manufactured unit. B+W engineers say they regard electronic accuracy compensation in the control as acceptable only for fine-tuning, maintaining that true precision is only achievable mechanically.

The newly designed spindle of the MCX 900 µ is not gear-driven, but powered by a precision-balanced hollow-shaft spindle motor with internal cooling. It is rated at 32kW and offers 100Nm of ‘low-end’ torque at 100% duty cycle.

A Fastems MLS-HD FMS was part of the project; the supplier was also involved in the vibration-damping exercise, as its multi-level store had to be mechanically isolated from the machine. The plan is to install two MCX 900 µ machines and three B+W MCX 900 T mill-turn centres with torque-motor tables to provide true vertical turning capability for the five-sided machining of components. The five machines will be linked by a Fastems FMS. All the additional machines are currently on order.