SW: High-productivity makes it big

12/15/2008 - Oliver Hagenlocher

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High-efficiency production with multi-spindle machining centers from SW has always been associated with small and medium-size workpieces. Those who had to 5-axis  machine workpieces with edge lengths above 500 mm had to be satisfied with single-spindle automatics. This was something that affected the job shop and the subcontractor for the automotive industry in particular, as these companies attach great value to the high degree of reliability and productivity for which the machines from Waldmössingen are known. SW has now closed the gap in their sales programme.

Innovation as a continuation of the tried and tested
The high productivity rating of the existing horizontal machines from SW has a great deal to do with the arrangement of twin swivel-trunnions with two fixture plates on rotary axes. One of these plates carries two or four workpieces and is located in the machining area, whereas the other one is used by the operator to load and unload the components.  Cycle time-concurrent workhandling minimises idle times, but it also limits the size of the workpieces that can be machined. The contours of the workpieces have to fit in with the indexing radius of swivel and rotary axes. To be able to machine larger workpieces the whole arrangement of twin swivel-axes and the rotary axes with their fixture plates would have to be disproportionately larger.
Wolfgang Armleder, head of development at SW, describes the demands made on such a design development as follows: “We want to give our customers a compact machine, as the footprint of the production tool has become a decisive criterion of procurement today.” Reiner Fries, senior executive at SW, adds: „Innovation should not be seen as an end in itself. It orientates itself on market demands. We have taken this as a valid reason to build on the tried and tested and to implement a practise-oriented combination with innovative elements.“

An additional linear axis completes the successful balancing act
The specialists at SW have come up with a particularly clever concept. Important machine modules and assemblies have been taken from the existing, highly dynamic, tried and tested BA W06 series. The innovative element of the BA W06-2W is the additional W-axis opposite the Z-axis. This auxiliary axis has a stroke of 1000 mm and serves to transfer the workpieces from the loading point to the extended machining area. The axis features a single swivel-trunnion with an integrated fixture plate that holds the workpieces during the 4-axis machining cycle. The plate accommodates fixtures and workpieces of up to 750 kg in weight. In line with the modular design of the BA W06 series a planetary fixture plate with two further rotary axes can be added for the 5-axis machining of the workpiece. This has a number of advantages, as Armleder explains: “A single swivel axis with one fixture plate can be accommodated without changing the machine size, whilst it also provides the space needed for the larger swing diameter required to swivel the clamped workpieces. This is the new concept’s most impressive feature. Without making the machine bigger, we have opened up a considerably larger machining area. It makes it possible to machine workpieces with a swing diameter of 800 mm with no problem.“

The application range is extended
The use of a W-axis has other advantages too, as it also enlarges the machining area in Z-direction, by 225 mm. This allows, for example, for deep-hole drills to be used that can drill holes up to 550 mm deep. Reiner Fries, senior executive at SW, explains that – depending on the application – large components can now be complete-machined on the BA W06-2W without the need to transfer them to a deep-hole drilling machine. This applies, in particular, to the automotive components targeted, such as oil sumps and gearbox casings. With this machine, manufacturers will considerably reduce throughput times and profit from lower costs and greater flexibility.

Simple, low-cost link-up
For loading and unloading of the workpieces the W-axis moves out of the machining area and to the front of the machine in 1.2 seconds, with the linear axis carrying the partition wall that separates loading and machining area. This design eliminates the need for additional moving parts or drives, thus reducing costs and ensuring the reliability of the operation. In the load/unload station the workpieces can be deposited or removed from either above or below. This makes the interfacing of machines much simpler. Where workpieces are moved on a conveyor belt that runs at right angles to the machine, a twin-gripper lifts them into the clamping position. When loading from above, a gantry with double-gripper removes and inserts two workpieces at the same time. It is also possible to have a system that combines top and bottom loading/removal. For instance, the raw-parts are brought in on a conveyor, loaded, and then machined on three sides. The gantry loader then picks them up, turns them over and reinserts them to have the other 3 sides machined. The finish-machined components are then deposited in the workpiece storage sector of the conveyor belt below. A particular advantage of this concept is that it removes the need for multiple – and therefore cost-intensive – workpiece clamping fixtures. The machine only changes the components in the two clamping fixtures on the fixture plate. The conveyor belt can be equipped with the simplest of workpiece carriers. In batch production, this considerably reduces the cost and effort spent on loading the BA W06-2W. It also offers much greater flexibility in the changeover to a different component or to another variant in the same component family.

Long machining times put workhandling times into perspective
What is lost with the new design is one advantage of the existing SW machines, namely cycle time-concurrent loading and unloading. Herr Fries is ready for the argument: “It is clear that some people will argue that the new design is going to lead to the loss of the high degree of productivity our machines are known for. That is not going to be the case. When we took a closer look at the range of components this machine is aiming at, we came to the conclusion that such large workpieces – oil sumps, clutch housings, etc. – were already subject to actual machining times of at least 5 and often more than 10 minutes, so that the relatively short changeover time to be added to the loading and unloading times only made a difference of a few percent.”

Building on tried and trusted components
From the outside there is very little difference between the new BA W06-2W and the other machines of the tried and tested BA W06 series. That has very valid reasons. The specialists at SW have retained a number of reliable, well-engineered components.  These include the machining unit with linear motors that is built into the back of the SW monobloc and the two Z-axes for parallel and serial machining with workpiece-length offset. Travel in X- and Y-axis is 600 mm. The motor spindles are arranged at a distance of 600 mm and travel 500 mm in Z-direction. The linear motors accelerate the axes at up to 20 m/s² to a top speed of 100 m/min. The high dynamic rigidity allows slip-stick values of up to 1000 m/s³. This results in the shortest possible positioning times even where the trajectory is small. A power supply of 35 kW and a torque of 80 Nm amply cover the machining with large HSK-63 tools. The two synchronous motor spindles need just 0.7s to reach a speed of 17,500 rpm. The fixture plate can be equipped with additional rotary tables as a fifth axis. These also feature direct-driven torque motors. It allows for the five-sided or – with interpolation – the 5-axis machining of workpieces in a single setup. Every spindle can be supplied from a store of 84 – optionally 144 – tools held in the pick-up magazine. The chip-to-chip time for tool changes is approx. 2.75s.