Thrifty and precise

November 2014

Machining centers with high-precision roller rail systems built by Rexroth are incredibly accurate and are now faster than ever – even when enormous forces are encountered.

The Schwäbische Werkzeugmaschinen GmbH in Schramberg-Waldmössingen, Germany, supplies equipment for quick and precise machining of both steel and light alloy metals. Included here is a new, single-spindle processing center, the BA one6. The horizontal working spindle is designed for 4- and 5-axis machining of large workpieces made of light alloys. This unit’s application range is broad. OEMs and suppliers to the automotive industry – as well as aeronautic and aerospace companies and manufacturers of hydraulic and pneumatic components – utilize these systems. Demands in all the fields of employment are high. The bar is continuously being raised in specifications for precision, speed and ruggedness.

Right on track

The central elements in the machining center are the new RSHP units – high-precision roller rail systems built by Rexroth. And they have a lot to do. When the tool, up to 300 millimeters in diameter and traveling at 130 meters per minute, makes contact with large aluminum workpieces, extremely high forces arise – and they do so suddenly. When processing, it is essential that high dynamics and precision be achieved when guiding the advancing axes. The RSHPs ensure that the axis is kept right on track and that all the tools are exactly positioned. With dependable acceleration rates and speeds, they support the superb dynamics of the linear motors. They also satisfy specifications in terms of stiffness. This is important because, at peak advancing forces as high as 27 kilonewtons, the strong magnetic fields in the linear motors exert more than 54 kilonewtons on the guides.

Gentle and fast

Rexroth has also achieved considerably less pulsation thanks to a newly developed inlet zone. It is convex so that the cylindrical roller is not placed under load suddenly. Instead, the load gradually increases. Even at maximum loading, this makes for a gentle transition. When compared with the previous version, the RSHP reduces height fluctuations as well as rolling and pitching of the runner block by as much as 66 percent. This improves the quality of the surfaces on the workpieces. And in spite of all this precision, the BA one6 is fast. With a generous working area measuring 600 millimeters along the X axis, 650 millimeters at the Y axis for the gantry, and 500 millimeters for the Z axis, this is no easy task. But at these long distances, the linear motors can make full use of their dynamics and – in conjunction with the RSHP – reduce cycle times considerably. “We achieve chip-to-chip of less than 2.5 seconds,” explains development manager Wolfgang Armleder of Schwäbische Werkzeugmaschinen GmbH. An additional dual-swivel bracket makes possible loading and unloading in parallel to the working times and reduces cycle times by up to 40 percent.

Higher load rates for longer service

The RSHP runner blocks ensure flexibility even during assembly, since they can be combined as desired with profiled rails of the same size. For Armleder this was a decisive factor in the selection of the components. The lubrication intervals could be extended by about 20 percent thanks to a new concept for the RSHP. This is a further benefit in everyday manufacturing operations. In mass production the new BA one6 units are normally in use 20 hours a day. Here the service lives of the components are a central consideration. With 20 percent higher static and ten percent higher dynamic load rates, the RSHP satisfies these requirements.