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3.2 Cutting conditions
A two-fluted O500 μm end-mill and SM45C plates were used for the cutting tests. Federate and cutting speed levels were chosen according to the tool manufacturer’s recommendations. The depth of cut was increased by 100 μm for each step. The cutting conditions used for this experiment are listed in Table 1.
3.3 Sensor
The sensor used in this experiment is the strain gauge sensor, it consists of dynamometer, metal strain plate, base, insulation protection film and lead-out part. After measuring, the resistance strain gage resistance is 300 ohms, working principle of resistance strain gage is adsorption of strain resistance and resistance changes with the mechanical deformation in the substrate material, commonly known as the resistance strain effect. The metal conductor resistance value by the following equation:
Type of ：ρ—— The resistivity of the metal conductor（Ω•cm2/m）
S—— Conductor cross-sectional area（cm2）
L——The length of the conductor（m）
The most commonly used sensors usually dynamometer is a metal foil strain gauge and a piezoelectric crystal. The design is based on the a ring dynamometer dynamometer. Its working principle is dynamometer dynamometer is elastic element, paste resistance strain piece inside and outside of the ring, and connected into three bridge as a sensor for measuring cutting force X, Y, Z three direction, in lathe turning, turning the work piece rotation transfer to the tool, and then transfer from the tool bar to the dynamometer, deformation force ring the resistance strain gauge stuck to its upper also deformation, resistance value of R changes (R ± R). When the strain tensile, resistance wire diameter became fine, resistance value increases (R+ Δ R), when the strain slice under the compressive deformation, the resistance wire diameter, resistance value smaller (R- Δ R), which is the proportion of output sigma.
The measurement system is based on the experimental nature of the building, then used a number of universities universal application of the octagonal ring dynamometer, octagonal ring force transducer is pided into a fixed part, variable part of the bomb, loaded knife part, and the three parts of a whole, elastic section includes upper and lower two octagonal ring, octagonal inner and outer rings have several resistance strain gauges. The resistance strain gauge is composed of sensors, basal and lead. The sensitive element is high resistance wire, when the resistance strain gauge stuck on the elastic body, the resistance value change with elastic strain.
4. Experimental results
4.1 Micro machining condition measurement
The required preliminary cutting experiments were first per-formed in order to investigate the machine tool performance and sensitivity of the sensors with increasing cutting speed, as shown in Fig. 4. From the results, the maximum amplitude of rotation frequency was generated at 70,000 rpm (1.166 Hz).This range can be regarded as the natural frequency of the main spindle. If the natural frequency approaches the rotation frequency, resonance can occur. Since the performance of a machine tool can be easily affected by the natural frequency, the reliability of the acceleration signal can deteriorate. The hall sensor signals become stable from 10,000 rpm. From the results of the preliminary cutting tests, it is recognized that the natural frequency range should be avoided in the cutting con-dictions for precise signal acquisition and a stable cutting process with micro machine tools.
4.2 Indirect cutting force measurement
For the experimental purpose, the main spindle speed (rpm) was changed from 5,000 rpm to 50,000 rpm, and the frequency of the signals was measured using hall sensors and an acceleration sensor. From the results of the experiments, it could be confirmed that the supplied current frequency, rotation frequency, and rpm have a linear relation as shown in Fig.5. Thus, indirect measurement of the main spindle speed (rpm) can be performed using the hall sensors and acceleration sensor.