Large Grind-Turn Machine Uses Renishaw Encoders
Eive Johansson, the company's VTG chief designer, said: 'Our largest machine used to accommodate parts up to 600mm in diameter, but the VTG4000 can handle diameters in excess of 4,000mm - 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 3um form deviation, yet, despite the considerable difference in relative size, the VTG4000 has been proven to achieve an exceptional form deviation of less than 1um, with feed resolution in 0.1um steps.
'At the heart of a Lidkoping machine are the linear slides. 'The combination of hydrostatic guideways, air seals and linear motors creates 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. Johansson saw the Renishaw Signum encoders at the EMO 2007 exhibition in Hannover, after which followed a period of rigorous testing by Lidkoping.
He continued: 'It was important that the linear encoders could be supplied in a continuous length of at least 4.5m 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,' said Johansson.
All Renishaw encoders feature an integral patented setup light-emitting diode (LED), which speeds installation and removes the need for complex external/separate setup equipment or oscilloscopes. 'With the scale attached and the read head approximately located, the indicator lights make it very easy to see how well the two elements are aligned and then make the final adjustments,' added Johansson. There are three different types of bearings in a wind turbine. The pitch bearings at the base of the blades are the largest, up to 4,000mm in many wind turbines, and probably the most important as they allow the blade pitch to be adjusted.
During operation, the pitch must be matched to the wind speed or the stress on the blades could cause them to collapse, so the reliability of high-quality bearings becomes a necessity. In addition, if the wind speed becomes too high, usually more than 25kph, it becomes necessary to create a stall condition to protect the blades. Also important are the main shaft bearings and the yaw bearings, which allow the turbine to swivel. The VTG4000 machine is designed so that the component does not need to be removed between machining operations, by performing both turning and grinding with two separate heads.
The two heads can be configured as required, usually grinding/turning or grinding/grinding. This means that the X axis of the machine, which carries the two heads on opposite sides of the component, must be 4.5m long so that both the grinding and turning tools can access the outside of the part. The VTG4000 is a solid machine, with some parts weighing 35 tonnes. It is, therefore, resistant to distortion and thermal variation, which is aided by closely controlled hydrostatic oil temperature and coolant temperature. Henrik Jonsson, Lidkoping plant manager, said: 'Hydrostatic guideways, which we've used since the 1970s, are used for all axes together with the linear motors.
'This combination is faster, more accurate and capable of far greater acceleration and deceleration. 'When you think that you can move the 25,000kg rotary axis with your finger, you realise how good the hydrostatic system is,' he added. Another important factor when producing such massive bearings is the thermal condition of the workpiece. During production, the part must acclimatise to the factory conditions for at least 48 hours before it is loaded onto the machine. Renishaw's LM10 magnetic encoder systems are fitted to the B axes of the grinding heads.
These encoders are exposed to the harsh conditions of the machine environment but, with a non-contact, non-optical design and sealing to IP68, cannot be affected by coolant or swarf. The LM10 allows up to 100m of travel and high-speed operation of up to 25m/s (4m/s at 1um resolution). It is available in digital or analogue output variants and in a range of customer-selectable resolutions. The machine also uses a Renishaw RMP60 radio signal transmission touch probe to accurately set the part in the machine's co-ordinate system. After the part is placed on the bed of the VTG4000, the RMP60 is loaded into the tool changer in the same way as a cutting tool and is used to find the exact position of key features, taking just a matter of seconds.
This data is used to update the co-ordinate system in-cycle, such that machining can start immediately with the knowledge of actual part position and dimensions. To automate another potentially lengthy manual task, the grinding wheels can also be exchanged using a tool changer. Although for many tasks a general grinding wheel can be used, it is often necessary to change to a dedicated face grinding wheel, angled wheel or customised profile, known as a 'dressed wheel'.
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