Optical and magnetic encoders deliver position feedback on KMT Lidkoumlpingrsquos VTG4000 machine to achieve accuracy of 1 microm form deviation on 4000mm diameter workpieces

Large-Scale, High-Precision Turning/Grinding

Dec. 5, 2012
Linear motors for accuracy Turning/grinding in a single set-up Encoders for demanding applications

Blade pitch adjustment is one of the most critical functions for wind turbines, and the giant bearings in the adjustment mechanism must perform with the accuracy usually associated with small parts. Now, that degree of precision can be assured in the bearing manufacturing process with a vertical turn/grinding machine built by KMT Lidköping. It combines advanced motion systems with Renishaw’s SiGNUM optical and magnetic linear and angle encoders on critical axes to achieve form deviation of less than 1 µm on parts 4000 mm in diameter.

“This is hard turning and grinding, and is very demanding,” said Eive Johansson, KMT Lidköping’s chief designer. “Positioning accuracy is very important, with a direct effect on the quality of the finished bearings.” Prior to the VTG4000, the company’s largest machine could accommodate parts up to 600 mm (24 in) diameter, but the VTG4000 handles diameters in excess of 4000 mm (157 in), the size of the largest wind turbine bearings, while providing much greater accuracy.

Linear motors on the linear axes make the VTG4000 fast, and the rigidity of the hydrostatic guideways increases the accuracy. “A standard-sized machine, using ballscrews on the axes, maintains about a 3-µm form deviation,” said Johannson. “This machine maintains form deviation of less than 1 µm with feed resolution in 0.1 µm steps.”

Linear slides are at the core of Lidköping machines, and the combination of hydrostatic guideways, air seals, and linear motors creates a stiff and accurate, maintenance-free system. “To achieve dynamic stiffness we need high gain, which is linked to the quality of the encoder scales,” explained Johansson. “It also makes a big difference that the angle encoders have the scale integrated directly onto the ring.”

Design improves control of turbine blades

Pitch bearings allow wind turbine blades to optimize their angle according to wind speed, or to create a stall condition in high winds, so the reliability of these bearings is critical to efficient, safe operation.

The multi-tasking VTG4000 is designed to machine these large bearings in a single set up. It performs both turning and grinding with two separate heads that are configured as required, usually grinding/turning or grinding/grinding. The machine’s 4.5 m X-axis carries the two heads on opposite sides of the part, and allows both grinding and turning tools to access the outside of the part.

With some components that weigh over 35 metric tons (77,000 lb), the VTG4000 is very resistant to distortion and thermal variation, which is aided by closely controlled hydrostatic oil and coolant temperatures. “We’ve used hydrostatic guideways since the 1970s, and combining them with linear motors gives us a faster, more accurate machine capable of far greater acceleration and deceleration,” said plant manager Henrik Jonsson. “When you see that you can move the 25,000 kg rotary axis with your finger, you realize how good the hydrostatic system is.”

Johansson first saw the encoders that would end up on the VTG4000 at the EMO event in 2007, and Lidköping put the sensors through rigorous testing. “We compared different scales assembled on our reference slide and selected the one with the best performance,” he said.

“It was important that the linear encoders have a continuous length of at least 4.5 m, as well as high dirt resistance,” Johansson continued. ”SiGNUM fit these requirements the best. We have fitted them to all four linear slides, and have had no problems. Our rotary table has the same design principle, with hydrostatic radial and axial bearings, air seals and torque motors. We did the same analysis and chose the SiGNUM angle encoders.”

Renishaw’s LM10 magnetic angle encoders are fitted to the B-axes of the grinding heads. These encoders are exposed to the demanding conditions of the machine environment, but their non-contact, non-optical design, and sealing to IP68, protects them from the effects of coolant and swarf. The digital or analog output LM10 allows up to 100 m travel and high-speed operation of up to 25 m/s (4 m/s at 1 µm resolution).

All Renishaw encoders have an integral, patented set-up LED that speeds installation and removes the need for complex set-up equipment or oscilloscopes. “One of the best features of the Renishaw encoders is how easy they are to set up,” Johnasson noted. “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.”

To set the part in the machine’s work-coordinate system with accuracy, the VTG4000 uses a Renishaw RMP60 radio signal transmission touch probe. After a raw part is placed on the bed, the RMP60 is loaded into the tool changer and used to locate the exact position of key features in a matter of seconds. This data is used to update the coordinate system in-cycle for quick, accurate machining.

About the Author

Robert Brooks | Content Director

Robert Brooks has been a business-to-business reporter, writer, editor, and columnist for more than 20 years, specializing in the primary metal and basic manufacturing industries. His work has covered a wide range of topics, including process technology, resource development, material selection, product design, workforce development, and industrial market strategies, among others. Currently, he specializes in subjects related to metal component and product design, development, and manufacturing — including castings, forgings, machined parts, and fabrications.

Brooks is a graduate of Kenyon College (B.A. English, Political Science) and Emory University (M.A. English.)

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