**2. Development and Testing of the Unique Hole-Drilling Device**

In the development of the unique drilling device, the authors used their own experiences obtained from using commercially produced devices SINT MTS 3000 (SINT Technology, Calenzano, Italy), SINT MTS 3000 Ring-Core (SINT Technology), and RS 200 (Vishay Precision Group, Malvern, PA, USA) not only in laboratory conditions but also by solving problems in technical practice [22–24]. The common feature of the aforementioned devices is adjusting the position of the milling cutter in the center of the strain gauge rosette. Positioning of milling cutter in RS200 and SINT MTS 3000 is performed manually using X-Y adjusting screws. In SINT MTS 3000 Ring-Core, semi-automatic positioning is undertaken because the center of the milled core is adjusted through a camera, which is not further used in the residual stresses quantification procedure. After drilling the hole or the core, its real position towards the center of the strain gauge rosette has to be determined. If eccentricity occurs, its value is entered into the evaluation software for the correction of the residual stresses calculation. Analysis of the eccentricity effect in the measurement of the hole-drilling residual stresses was carried out, e.g., by Ajovalasit [25], Beghini et al. [26] or Barsanti et al. [27]. As mentioned in the introduction, the topic of residual stresses determination using optical full-field methods in combination with the hole-drilling method is very up-to-date. Many authors devote themselves to develop their own drilling devices, where the optical system is mostly stationary, and the movement is undertaken by the milling cutter. There are two types of milling tool movements used—translational performed in the milling direction or a combination of translational and rotational movement.

In this paper, the design of such a drilling device is presented in which not only the milling tool moves to cut the hole, but also the optical system can be attached, used to determine displacement/strain fields, to the moving position (Figure 1). Achieving highly accurate positioning of the drilling device's mechanical parts (like 10−<sup>3</sup> mm) and desired repeatability of the measurements become crucial parameters required for the application of the above-mentioned methodology. Commercially produced drilling devices are also designed in such a way as to ensure the motion of their mechanical parts as accurately as possible.

In the design of the unique drilling device, the authors considered technical parameters of the optical systems used at the authors' workplace, namely LF/Z-2 reflective polariscope (Vishay Precision Group, Malvern, PA, USA), as well as single-camera (2D) and stereocamera (3D) digital image correlation system Q-400 (Dantec Dynamics A/S, Skovlunde, Denmark) (Figure 1).

**Figure 1.** The prototype of the unique drilling device with: (**a**) a single-camera (2D) Q-400 digital image correlation (DIC) system and polariscope LF/Z-2; (**b**) a stereo-camera (3D) Q-400 DIC system.
