A special sensor system, for compensating thermal growth in high speed machining spindles, also enables machine tool operators to change the spindle without having to re-calibrate.

With milling and grinding machines, loss of accuracy can be a real problem, especially if high accuracy is required for machining of very precise components. Unwanted thermal expansion of the spindle can occur if the CNC system does not actively compensate for these temperature increases. If the spindle expands by even a small amount, the machine tool head can cut too deep.

By working closely with machine builders and spindle manufacturers, Micro-Epsilon has developed a special sensor system, the SGS (Spindle Growth System), for solving this very issue, which also enables machine tool operators to change the spindle without having to re-calibrate the sensor and electronics.

The sensor, Micro-Epsilon's eddy current eddyNCDT sensor, is integrated into the spindle and accurately measures any Z-axis expansion of the spindle.

The system enables the active compensation of the axial spindle extension via a CNC machine tool.

The result is a much more accurate milling or grinding machine.

Up to now, any solutions to the problem of thermal expansion of spindles, have been externally-mounted ones, with the sensor mounted outside the spindle itself.

This is a compromise in measurement technique as the real thermal expansion error is inside the centre of the spindle.

But mounting a traditional sensor inside the spindle presents a problem: most manufacturers seem quite happy to throw away the spindle when it is deemed to have reached the end of its useful life.

However, after buying a replacement spindle, the engineers or maintenance team then have to be called in to re-calibrate the displacement sensor with the electronics controller, which can take considerable time.

Depending on the skill of the machine operator, this can take up to 15 min to set up.

If the sensor was integral to the spindle, it would be impossible to perform this set up procedure and so a lower accuracy external mount sensor has to be used.

Normally, with eddy current sensors, the engineer has to calibrate the spindle at fixed displacements, in order to 'teach' the sensor and electronics the shape of the displacement curve.

10 or 20 point calibration is usually required to get this kind of shape.

But Micro-Epsilon's integrated, non-contact SGS sensor has an embedded (EEPROM) chip that stores calibration data, so that re-calibration of the sensor is not necessary.

To overcome the multi-point linearisation of eddy current sensors, a simpler capacitive sensor has been selected by some machine builders, as this system does not require such a lengthy set up process.

However, the limitation of the capacitive principle means that the sensor must always be in a clean environment - something which is very difficult to achieve on a milling or grinding machine.

* Technical advantages - the SGS sensor system was originally developed for Steptec, a Swiss manufacturer of machine tool spindles.

The sensor is based on Micro-Epsilon's standard eddyNCDT sensor but has a special aluminium, more compact housing and is rectangular in shape rather than cylindrical.

Guenther Schallmoser, OEM sales manager at Micro-Epsilon in Germany, explained the technical advantages of the sensor system: 'Other sensor manufacturers can provide similar accuracies to our solution, but where we have a technical advantage is in the interchangeability of the sensor system with the spindle.

No re-calibration is necessary, which saves time and money for the customer.' He said: 'Normally, customers have been using a number of externally-mounted temperature sensors along the spindle, perhaps three to five, to try to measure the temperature difference across it.

They then try to compensate for the thermal expansion changes.

But this is inaccurate and it is much better to use a displacement sensor to measure the precise thermal expansion of the spindle.' The SGS measuring system uses the eddy current measurement principle.

The sensor is controlled by electronics with a digital signal processor (DSP).

All sensor-specific data is stored on the sensor itself, using an embedded EEPROM chip.

After changing the spindle, the system is ready to run without any calibration.

The sensor has very high temperature stability of +/-0.01% FSO/ deg C.

Resolution is less than 0.5 micron and the sensor controller measures just 175mm x 110mm x 45mm.

The controller housing is made from a heavy duty cast aluminium, which gives it excellent protection against electromagnetic noise and dirty environments (IP67).

Micro-Epsilon normally supplies its SGS system to spindle manufacturers, for integration, but can also supply direct to the machine builder if required.