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Encoders for High-Speed Spindles in Precision Machining

Michael Martin, Product Management
Heidenhain in Traunreut, Germany

The trend continues in machine tool building toward increasingly compact machines with ever greater performance. The requirements are increasing both for productivity and for machining quality. This means that engineers strive for both a high stock removal rate in roughing operations as well as very high accuracy and flawless surface quality after finishing operations. Moreover, different and frequently changing operating conditions are confronted in day-to-day operations because part production usually involves small batch sizes. But the greater complexity of workpieces is also driving requirements on machine tools.

Figure 1. The new series of magnetic rotary encoders was conceived specially for mains spindles in machine tools.

Figure 2. The ERM 2400 series encoders consist of a scanning head and a circumferential-scale drum. The latter is available with smooth inside contour or with a keyway as anti-rotation element.

Figure 3. The scale drum of the ERM 2405 with keyway.

Figure 4. The encoder signal period is approximately 400 µm. Additionally, a reference mark is magnetized onto a separate track.

Figure 5. The increasing complexity of workpieces makes higher accuracy necessary on main spindles and position encoders.

Specializing in Machine Tools
To enable machine tools to work very efficiently and precisely, this has to be considered in the machine technique and its mechanical design. This is why direct drives are now becoming for common for feed axes. The benefits of direct drive technology are low wear, low maintenance and higher productivity. Direct drives have long been characterized by their compact design.

The benefits of direct drives also apply when they are used for spindles. Because there is no need for mechanical transmission elements, the drive chain is very rigid and backlash-free, making its movements highly dynamic and precise. They can therefore achieve higher performance and speeds than conventional drives. This is the only opportunity to exploit today’s cutting materials and enable a more economical cutting process. At low speeds, other positive effects of direct drive technology become apparent. The absence of mechanical transmission elements make it possible to attain higher accuracy by reducing hysteresis and elasticity error.

The increasing complexity of workpieces also intensifies the need for higher accuracy in spindles. It is not unusual for certain machining movements to be achieved only through the interaction of feed axes and the spindle. For example, when manufacturing a thread, a single-point tool needs to assume a defined angular attitude.

High Rigidity and its Results
The absence of mechanical transmission elements also means that the signal quality of the encoders has a stronger effect on positioning and drive performance. The velocity controller, for example, calculates nominal current values using position information directly from the encoder. These values are then processed to accelerate or decelerate the drive. Even small deviations of the position information result in unsteady drive performance and additional heat generation by the spindle. In some cases this increases running noise.

This increases the importance of the encoder and its signal quality. The size of the grating period greatly influences the signal quality and interpolation error. Measuring systems using coarse mechanical graduations, such as gears, are increasingly losing significance and being replaced by encoders with higher accuracy. While the optical encoders with their fine graduations are more suited for low shaft speeds, typically less than 2,000 rpm, the magnetic modular rotary encoders are ideal for spindles. The signal period of approximately 400 µm and the special process for applying the graduation enable the encoders to achieve accuracies and shaft speeds to fulfill increasingly challenging requirements for spindles.

For example, a drum outside diameter of 64.37 mm provides 512 signal periods per revolution. Gears with comparable dimensions, however, have a significantly smaller number of grating and signal periods. The interpolation error of the magnetic modular encoders remains significantly better than 1 percent of the signal period. This means that together with the smaller signal period, the encoder has a considerably lower, absolute interpolation error and therefore distinctly better output signals for calculating the motor’s nominal current. The result is greater speed stability.

Harsh Environments
The encoder in the machine tool is often exposed to heavy loads from cooling lubricants and chips. Sealing is made more difficult by the high spindle speeds on motors and relatively large diameters. An encoder is therefore required with low sensitivity to contamination. This is why the ERM magnetic modular encoders are particularly well suited with their rugged design. They can even operate under high humidity, heavy dust loads and in oily atmospheres.

For years, the stability of their signals under contamination has been making the ERM the preferred encoder for C axes on lathes. In addition, their large inside diameters eliminate constraints from the encoder in the machining of bar material. But main spindles in milling machines place even higher demands with respect to rotational speed and dimensions than in lathes. In most cases, previous magnetic modular encoders were not able to meet these requirements.

Summary
The continuously growing requirements for productivity and machining quality are driving a trend toward more direct drives on spindles. However, the lack of mechanical transmission elements increases the influence of the encoders and their signal quality on positioning and control loop performance. The available installation space near the main spindles remains very limited. For this reason, demand has been increasing for encoders that combine high signal quality with compact dimensions. Additionally, the encoders have to be designed for high shaft speeds.

For more information, please contact Heidenhain at www.heidenhain.com.

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