## 3 Materials and Methods
### Experimental Setup  
The experimental setup was implemented on a milling machine to ensure easy and well controlled translational displacements (see Figure 17). The specimen was flattened on a plate, which was fixed to the (movable) base of the machine. The DIC camera was fixed to the upper frame of the machine so that it was fixed along any direction, and a light ring was positioned at the extremity of the camera lens to improve the local lightening and reduce the exposure time. A global shutter DMK 38 UX540 camera (The Imaging Source) was used, having a resolution of \(5320\times 4600\) (24.5 MP). The field of view was set to be approximately \(8\times 7\) mm\({}^{2}\), for a scaling factor of about \(1.549\) um/pxl. The vertical distance between the camera lens and the specimen was initially fixed to ensure the best focus at the hole's upper surface. The milling tool was initially brought into contact with the upper surface of the specimen. In order to avoid slight damage to the surface before the actual cutting, a calibrated thickness was placed between the milling tool and the specimen surface, while rotating the former for detection. This thickness was taken into account to finally set the column vertical position corresponding to the zero depth of the milling tool. This latter was then set to contact while not rotating and a depth indicator was introduced  
Figure 16: DIC procedure, definitions of reference (**a**) and shifted (**b**) subsets, and introduction of the displacement components with respect to the hole centre coordinate system. at this position, which was fixed to the vertically sliding column of the machine, leaving it fixed with respect to the milling tool's vertical position. At this status the depth indicator was zeroed, as represented in Figure 17. Despite the tip of the depth indicator being far from the milling tool, the rigidity of the machine frames ensured the same translational displacement between the indicator and the milling tool. After this zeroing, the cutting tool was initially brought up (along with the depth indicator), then the base was moved leftwards by a length equal to the distance between the camera and the milling tool axes. After taking the initial DIC picture without the hole, the base was again repositioned below the milling tool. The drilling operation was then started with accurate control of the drilled depth due to the depth indicator. The rotating speed of the milling tool was imposed at 200 rpm and the feed rate was 0.1 mm/s. These values were kept low to avoid local overheating and melting of the specimen material. After drilling, the base was again moved to the position that the specimen was below the camera, and the final DIC image (with the hole) could be obtained. Despite the depth being measured with a good accuracy, the diameter of the generated hole is only known as a nominal value. An accurate acquisition of the diameter is obviously required, as it is both the main parameter of the HDM and the scaling length of the DIC displacement measurement. After the drilling and the DIC acquisition (Figure 18a), an optical device was used to observe the drilled hole with a thin frame (Figure 18b). This latter can be translated and the frame movement was associated with a dial gate with a resolution of one hundredth of a millimetre (Figure 18c). This device was available with the RESTAN instrument supplied by SINT Technology Srl, Florence, Italy. By moving the frame with respect to the specimen, it is possible to align one of the two lines (horizontal or the vertical) with the lateral rim of the hole, and then the gaze translation to reach the other side can be easily measured. This can be repeated for the other line, allowing the average between the two diametral lengths to be finally calculated. The produced holes showed accurate roundness, confirming the correct applicability of the cutting parameters to the PLA. Figure 17: Picture of the experimental setup, including the milling tool, DIC camera, and depth indicator.