I. Introduction
To obtain high-dimensional accuracy and fine surface roughness, real-time measurement of cutting forces is essential for tool condition and machining process monitoring [1]–[5]. Resistance strain gauge and piezoelectric-based dynamometers are well-known approaches for cutting force measurements. However, they have several limitations such as high cost, poor performances in harsh machining environments, and possible interferences with cutting dynamic [6], [7]. In addition, due to the size and weight, dynamometers often need modifying machine tool structure for installations, which may change the stiffness of the tooling system [8]. Therefore, some research and development efforts on sensory cutting tools were conducted for cutting force measurements [9]. For instance, a triaxial cutting force sensor based on the octagonal ring and strain gauges was developed and tested [10]. A smart turning tool was presented with four pieces of piezoelectric sensors as an array in the shank tool [11]. A turning tool with two pieces of piezoelectric films was proposed to measure cutting and feed force in real time [12]. Measuring transducers based on piezoceramic sensing element were designed for cutting force measurements in ultrahigh-speed metal processing [13]. A cutting-force measuring device was designed by using an optical fiber sensor to detect the displacement of the rear part on the shank tool [14]. In addition, Hall-effect current transducers were adopted to measure cutting forces in turning by detecting current signals in servomotors [15]. An instrumented cutting tool with surface acoustic wave (SAW) strain sensors on tool surfaces was investigated [16]–[18]. A self-sensing smart tool was presented using two stress cavity structures with piezoelectric films [19]. Few of the above methods and devices are suitable for multipoint and real-time cutting force measurements with low costs for the long term in harsh industrial environments.