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The machining temperature changes not only affect equipment operation and machining performance but can also reflect on product quality and component status during processing.

For example, the measurement of temperature in the cutting zone has high correlation with machining quality. In general, cutting temperature gets higher along with the increase of cutting speed, feed rates, and depths due to the frictional heat generated on the cutting tool‐workpiece interface. This increased temperature can soften the workpiece so that material can be removed from the workpiece easily; however, higher temperature might also accelerate tool wears.

Thermal couple is the most commonly used sensor to measure the temperature of target objects by directly converting heat into electricity through thermoelectric effect, which creates the temperature‐dependent voltage when the temperature difference exists between two different semiconductors inside the sensor.

Once the equipment or specific components are determined to be the monitoring targets, the measurement can be completed by measuring temperatures around the installation place. As shown in Figure 2.8, one patch‐type thermal couple directly contacts with the surface of the metal shell that surrounds the machinery spindle to monitor the temperature changes. Temperature information here reflects the operation conditions.

Figure 2.8 Installation of thermal couple.

When the quality of oil or grease for the rolling‐element bearings deteriorates, insufficient lubricant quantity and viscosity may increase operation temperature and cause bearing or the spindle malfunctions. Temperature information can be a very wide‐range of temperature up to thousand degrees at a low sampling rate compared to other sensing techniques. Thus, more storage space and de‐noising methods for filtering the signals are not necessary.

However, sensing distance is one challenge that has to be taken into consideration. Figure 2.9 indicates that the linear distance between the thermal couple and the spindle can also lead to the inaccuracy of the obtained temperature.

Figure 2.9 Distance between a thermal couple and spindle.

Another challenge is that sensing the real temperature of semi‐product directly during machining is rather difficult than sensing it on the equipment or components since the installation on the real machining zone is impractical. For example, the temperature of the melted plastic affects the final quality of injection molding processes the most. However, in general, only the injection mold or nozzle temperature can be accurately obtained instead of the real temperature from the melted plastic. In this case, indirect optical temperature sensor is probably the only solution to this issue, yet the cost is relatively high and it is hard to be fixed inside the machine or the mold.