and accuracy of the measurement. These piezoelectric can be placed according to the direction of the Cartesian axes and measure the cut- ting force components in each one of them. Although these sensors are usually mounted on what is commonly referred to as a ‘triaxial force table’ (Figure 5b), there are also other solutions valid for other con gu- rations so that stresses can be measured on rotating parts and tools. In processes such as milling or grinding, the part can be clamped to the dynamometer. In contrast, in the case of turning, the dynamometer has to be placed between the tool and machine-tool. Another option is a more simpli ed system based on an electric power consumption meter (Figure 5a). Unlike other sensors, it is not housed into the working area of the machine but in the electrical panel of the machine. The signal can be directly obtained from an Ethernet cable or a Wi-Fi connection and the signal is directed to an acquisition card. Once the signal is ampli ed and ltered is nally stored in a computer. The power signal is directly related to cutting forces in machining ope- rations. Moreover, electric power consumption instabilities can be a vibration or malfunction indicator, so the signal can be easily used for modifying the process parameters accordingly in order to avoid defects. Other relevant example on process monitoring is measuring vibrations during machining processes. Vibrations can be recorded using accelerometers or even acoustic emission sensors. Depending on the process, the sensors could be limited to a determined direc- tion of vibration (uniaxial accelerometers) or prepared for measuring vibrations in any direction (triaxial accelerometers). Figure 5c shows a triaxial accelerometer connected as close as possible to the cutting zone in a machine-tool. Finally, there are other types of sensors that historically were com- monly used in the eld of research, but are becoming relevant in some industrial applications. For example, and focusing on the case of temperature sensors, thermographic cameras or pyrometers have been used in some research applications since 1990’s (Figure d and 6e). These systems have been traditionally very expensive for process monitoring and the provided information was dif cult to manage and store. However, during the last years, the price of these systems has dropped. In addition, the information of thermal images is relatively easy to process nowadays, so there are some processes that are intro- ducing thermal cameras to obtain in-process temperature elds and gradients. For example, some additive manufacturing machines are trying to implement these measurements in order to check each pro- cessed layer and introduce parameter modi cations for the next layer. A classical alternative for checking the temperature is based on ther- mocouples. These devices are cheap, reliable and easy to install. Its main drawback is the delay in the response, since present thermal inertia and it is necessary to give time to heat the thermocouple and read the signal. Thus, thermocouples are usually applied for stable temperature monitoring, such as lubricant temperature measurement or some machine critical points that need to be controlled. If the ther- mocouple detects a temperature variation, an alarm signal will be generated. INDUSTRY 4.0 Figure 5. a) Electric power monitoring (Schneider). b and c) Piezoelectric (Kistler). d) Thermal camera monitoring of LMD process. e) LMD monitoring by Pyrometer. >>53