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System senses expansion inside machine tool spindles
The German sensor manufacturer Micro-Epsilon has developed a displacement measurement system for that is small enough to fit inside the spindle of high-speed milling and grinding machines to measure thermal and centrifugal force extension of the spindle. These measurements can be output to a CNC system as correctional values, allowing the machine to compensate for any spindle positioning errors.
The eddyNCDT SGS 4701 spindle growth measurement system operates on a non-contact, wear-free eddy-current measuring principle. It is designed to work in harsh machining environments and is not affected by heat, dust, oil or coolants.
High-speed milling and grinding machines generate a lot of heat, which can affect tool accuracy, particularly when machining precise components. If the CNC system does not compensate for these temperature increases, unwanted thermal expansion of the spindle can occur, causing the machine tool head to cut too deep. The thermal extension of the spindle therefore needs to be compensated for, to keep the tool in a defined position at all times.
Until now, most attempts to solve this problem have relied on a sensor mounted outside the spindle. “This is a compromise as the real thermal expansion error is inside the centre of the spindle,” explains Chris Jones, managing director of Micro-Epsilon UK.
The new sensor and controller are small enough to be mounted inside the spindle, allowing measurements to be taken on its labyrinth ring, for example. They are factory-calibrated for either ferromagnetic or non-ferromagnetic objects, and can measure temperature as well as linear extension. The sensors offer a linearity of ±2µm and a resolution of 0.5µm.
