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Abstract: IN this study, a useful indirect cutting force measurement method using an acceleration sensor and current hall sensors is proposed. A series of experiments was performed on a precise micro machining stage with a high-speed spindle. A three-axis acceleration sensor was installed on the spindle head, and current hall sensors were connected to the motor current inlet cables. From the results obtained, the correlations of the tool teeth rotation and current amplitude showed a linearity of 92.0% precision for hall sensor signals, and 98.0% pre-casino for acceleration sensor signals. Even though the results using the acceleration sensor showed better linearity than those of the cur-rent hall sensors, the signals can be easily affected by chattering, spindle vibration, and other external disturbances. From this perspective, the current hall sensor can provide more robust results. 40971
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Keywords: Micro end-milling; indirect cutting force measurement; Frequency analysis; Sensor system
1. Introduction
Recently, with the increasing demands for precise micro component production, the importance of micro machining processes has been increasing in a number of fields, such as in the medical instrumentation, aerospace engineering, and com-putter industries. However, it is very difficult to observe the machining process as compared to the macro machining process due to its low material removal rate (MRR), very small tool size, high spindle speed, and low sensor signal levels, among others. While a micro tool dynamometer can solve these problems, its application is limited due to cost concerns and issues related to sensitivity, robustness, and work piece size. To overcome such problems, several studies using multi-sensors (e.g., AE, acceleration, displacement, and vision sensors) [1-3] and sensors for the main spindle and/or measurement of the feed motor currents have been reported [4-5]. The present study proposes a useful indirect cutting force measurement method based on a frequency domain analysis using an acceleration sensor and current hall sensors. A series of experiments was performed on a precise micro machining stage. Measured. Signals were analyzed at the frequency domain after FFT (Fast Fourier Transform), and the results were compared with the cutting force components measured using a micro tool dynamometer. The calculated correlations between the sensor sig-nails and the measured cutting forces showed a linearity of 98.0% and 94.5% precision for the acceleration sensor and hall sensors, respectively. It could thus be verified that the proposed indirect cutting force measurement method provides a useful method to monitor micro end-milling processes.
2. System Works
2.1Measurement method
An internal permanent magnet is connected with a rotational axis in a three-phase AC induction motor or Surface Permanent Magnet Motor (SPMM). It rotates by synchronizing with the supplied alternative currents. In the case of SPMM, the internal permanent magnet corresponds to the rotation frequency of the motor. The number of rotations of a three-phase AC servo motor (n) is related to the frequency (f) of the supplied current and the number of poles of the motor (p) according to the following equations:
Thus, if the number of poles of an AC servo motor is fixed, the feeding speed can be controlled by changing the frequency of the supplied current. The torque of a three-phase induction motor can be obtained by the magnetic flux ( Φ ) and current ( I ). The magnetic flux increases with an increase in current, rand magnetization occurs in a specific range of the magnetic flux. Therefore, the motor torque is related to the current ( I ). However, the voltage and current in the rotator of the induction motor are very difficult to measure because the rotator part is assembled in a set with an electric circuit. Therefore, indirect measurement of the current ( I ) is possible by measuring the current ( I ) in the stator of the induction motor us-sing hall sensors. The measured signals were analyzed at the frequency do-main after a Fast Fourier Transform, as shown in Fig. 1. For the measurement of induced vibrations in the machining process, an acceleration sensor was installed on the spindle housing. As shown in Fig. 2, the rotation (500 Hz) and the tooth frequency (1,000 Hz) were analyzed at a frequency domain of 30,000 rpm. The cutting force was correlated with the rotation frequency of the current signal and the tooth rotation frequency of the acceleration signal.