## 2 Proposed Method
### Most Effective Actuation Strategy Selection  
In this section, the chosen actuation strategy is presented and motivated. Different actuation solutions were presented in literature for active boring bars. [PERSON] et al. [PERSON] et al. (2016) proposed a solution using magnetic actuators, mounted on a tailored system. Even if this kind of actuators are found to be effective, due to their capability of giving high loads, in the proposed solution the presence of the actuator, mounted on the external surface of the tool, reduces its overhang. In [PERSON] et al. (2017), a tool adaptor with an integrated active system is designed and numerically tested. The main drawback of this solution is the required actuation forces, significantly higher compared to other solutions due to inertia, since actuation is applied on a heavy and stiff region (holder). Therefore, in the most common applications [PERSON] et al. (2018); [PERSON] et al. (2019) (Figure 3), actuators are integrated in the tool body and actuation direction is parallel to the tool axis. In this scheme, motion normal to the tool axis is achieved by exploiting the moment (\(M_{act}\)) generated by applying the actuator forces (\(F_{act}\)) with a specific arm respect to the tool axis.  
Although the solution is the most feasible (applying the actuation force directly on the most flexible directions is complicated), it shows limitations due to the need of balancing two opposite  
Figure 3: Chosen actuation strategy.  
needs. On the one hand, the actuation effectiveness increases with the distance between actuator and tool axis, on the other hand this distance is limited by technological requirements.  
Since actuation system effectiveness is limited by the maximal radial dimensions, actuators with high energy density (i.e., force/volume ratio) and capable of providing an adequate actuation force in the required frequency range (e.g., 0-1 kHz) are required. Based on these requirements, piezoelectric actuators represent a preference choice. Two types of actuators may be employed: patches (or layer) or stack (or multilayer). Thanks to their flexible structure, the first ones can also be mounted on a curved surface and they do not required preload, their size is reduced and they do not require dedicated housing slots on the bar. However, their application in an active boring bar is very limited, since they cannot provide adequate actuation forces.  
On the contrary, multilayer actuators are able to generate higher loads and are suitable for dynamic applications, however, due to necessity to protect them from shear and tensile stresses, their integration on the boring bar is more complicated. Even if solutions with piezoelectric actuators were presented in literature [10], a systematic approach for active boring bar design that covers all the most crucial aspects has not yet been developed. In particular, the following open issues were highlighted:  
1. Solution for actuators integration and placing to optimize the system effectiveness considering the inertia of the system and electronics, two actuations directions and available space.
2. Solution for actuators housing protecting them shearing stress, positive tensile stress and from chip and debris.
3. A strategy that guarantees the dynamic and static equivalence between active boring bar and standard boring bar.  
Proposed approach supports active boring bar design considering all these aspects. In particular, the first goal was achieved and presented in Section 2.3. A proposed solution to protect actuators form shear and traction stress is presented in Section 2.4. The needs of protection system for actuators, while guarantee static and dynamic equivalence between standard and active boring bar are combined by means of the employment of protective cover properly designed, as presented in Section 2.5.