The prior art is familiar with machine tools where the rotary motion of the tool can be superimposed by an ultrasonic vibration of the tool, e.g. when a workpiece is machined by a tool.
EP 1 763 416 B1 describes in this connection a tool with a tool holder which, at a first end, has a tool holder support for adaptation to a rotary spindle nose and, at a second end opposite the first end, has a tool support, and with a tool head that can be inserted in the tool support, wherein the tool holder comprises a vibration motor.
In the case of such a machine tool, an ultrasonic generator in the tool holder, which produces the ultrasonic vibration of the tool, a vibrating body and the tool inserted in the tool holder provide a vibratory system which is excited by means of an electrical signal to mechanically oscillate, wherein the largest possible mechanical vibration amplitude is obtained when the vibratory system is stimulated with its resonance frequency.
When the tool is advanced into the workpiece, the vibratory system is dampened by the material of the workpiece and the friction between workpiece and tool, and the resonance frequency is shifted towards a somewhat lower resonance frequency. It is here known that the resonance frequency is further shifted with increasing damping.
This is quite often accompanied by the problem that there are marked fluctuations of the resonance frequency during processing. However, this simultaneously means that the damping capacity of the material that is being processed has changed.
This can be due to changes in the material inside the workpiece since in spite of an apparently homogeneous material of the workpiece an inhomogeneity (such as grain boundaries or inclusions of foreign material, etc.) can occur. On account of such changes in the material, it may be necessary to adjust the predetermined processing parameters, by means of which the tool processes the workpiece, to the respective material and its properties.
However, geometric changes in the workpiece, such as bores or recesses, are also a kind of inhomogeneity of the workpiece material, which optionally influences the predetermined processing parameters.
In order to determine when a change in the material occurs while the workpiece is processed, measurement systems are already known which utilize the principle of body-borne sound. Here, a corresponding body sound sensor is attached to the machine frame, usually at a considerable distance from the workpiece, said sensor measuring the arising vibrations and generating a measurement signal therefrom. A control unit assesses these measurement signals and can change the predetermined processing parameters during processing if measurement signal deviations are detected accordingly.
However, the drawback is that on account of the measurement sensor which is placed further away, the arising vibrations are not exclusively due to the change in the material inside the workpiece. These vibrations can be influenced or superimposed by vibrations from the environment. This can lead to wrong assessments of the measurement signal and, as a result, to a faulty adjustment of the processing parameters.
For this reason, it is important to detect, while the workpiece is processed, a change in the material as close as possible to the workpiece itself in order to avoid a superposition of external vibrations as far as possible.