With high precision milling and grinding machines, any loss of accuracy can be a real problem. Unwanted thermal expansion of the spindle, if no compensation is made, can lead to a machine tool head cutting too deeply into the component. Micro Epsilon is addressing this problem with its new Spindle Growth System (SGS) sensor, which is designed to allow operators to change the spindle without having to recalibrate the sensor and electronics.A Micro Epsilon eddy current sensor is integrated into the spindle to measure any Z-axis expansion, and this is fed back to the CNC system, which compensates for the movement. The sensor is based on the company's eddyNCDT sensor, and is contained within a compact rectangular aluminium housing rather than the usual cylindrical enclosureWith eddy current sensors, the spindle is normally calibrated at fixed displacements in order to 'teach' the sensor and electronics the shape of the displacement curve. A ten or 20-point calibration is usually required. However, the non-contact SGS sensor has an embedded EEPROM chip that stores calibration data, so no recalibration is necessary when changing the spindle (the sensor system being interchangeable with the spindle).The SGS sensor directly measures thermal expansion, in contrast to other compensating systems based on multi-point temperature measurement, which is inherently inaccurate. Capacitive sensors are sometimes used, and these require less set-up. However, they are very prone to fouling in dirty environments, which is difficult to avoid on a milling or grinding machine.