KMT Lidköping (www.kmtgroup.com/lidkoping/) has unveiled its new hard-turning VTG4000 (Vertical Turning Grinding) machine that uses a combination of Renishaw SiGNUM optical angle and linear encoders to achieve the required accuracy, and Renishaw magnetic encoders placed on the exposed axes of the cutting heads.

“Our largest machine used to accommodate parts up to 600 mm diameter, but the VTG4000 can handle diameters in excess of 4000 mm, the size of the largest wind turbine bearings. This is hard turning and grinding which can be very demanding, and the positioning accuracy is very important, with a direct effect on the quality of the finished bearings. A standard size machine, using ballscrews on the axes, will manage to maintain a 3 µm form deviation, yet despite the considerable difference in relative size, the VTG4000 has been proven to achieve an exceptional form deviation of less than 1 µm, with feed resolution in 0.1 µm steps,” said Eive Johansson, Lidköping’s VTG Chief Designer.

“At the heart of a Lidköping machine are the linear slides. The combination of hydrostatic guideways, air seals and linear motors create a stiff, accurate and maintenance-free system. To achieve dynamic stiffness we need high gain and the gain is linked to the quality of the encoder scales. It also makes a big difference that the angle encoders have the scale integrated directly onto the ring,” he added.

Mr. Johansson first saw the Renishaw SiGNUM encoders during the EMO 2007 exhibition in Hannover, and then followed a period of rigorous testing by the company. As he explained: “The choice of Renishaw was easy; we compared different scales assembled on our reference slide and bought the one with the best performance. It was important that the linear encoders could be supplied in a continuous length of at least 4.5 m and SiGNUM is the easiest way to achieve this. The Renishaw encoders also give more resistance to dirt. We have now fitted Renishaw encoders to all four linear slides and as expected, we have had no problems!”

“The rotary table has the same design principle, with hydrostatic radial and axial bearings, air seals and torque motors for the drive. The same analysis was done, and the Renishaw SiGNUM angle encoders were chosen,” he added.

The Renishaw encoders have an integral patented set-up LED that speeds installation and eliminates the need for separate set-up equipment/complex external, or oscilloscopes.

“One of the best features of the Renishaw encoders is how easy they are to set-up,” Mr. Johansson said. “With the scale attached and the readhead approximately located the indicator lights make it very easy to see how well the two elements are aligned and then make the final adjustments.”