## 2 Materials and Methods
### Planning and Implementation of Wear Measurements
#### 2.2.4. Close-Range Photogrammetry  
To obtain a dense point cloud and three-dimensional model of the worn double disc cutter, a close-range photogrammetry technique was used. First, a plastic background plate with 375 printed targets was created, with the targets or markers serving as georeferencing points. The plate, which measured 100 \(\times\) 60 cm, was flat and firm enough not to deform, i.e., all points were in the same plane. It also stood on a metal base, which ensured the verticality of the plate. The markers were placed in a grid with equal spacing in the x and y directions, as shown in Figure 7. The coordinates of the markers were then calculated based on an arbitrarily chosen marker representing the starting point of the plane coordinate system (z = 0) and the fact that the markers were 40 mm apart in both the x- and y-axes. Figure 5: Measuring the diameter with a manual caliper. (**a**) Labeled profiles on the upper cutter ring (top); (**b**) the method of manually measuring the diameter with a caliper. Note that the white dots on the surface of the disc cutter were used for close-range photogrammetry. Secondly, a special turntable was built to rotate a double disc cutter around its vertical axis. A base was used to place the double disc cutter in a stable position directly above the center of the turntable. This ensured a constant distance between the camera and the photogrammetric object at all angles of rotation around the vertical axis. After the cutter was placed on the base, it was covered with a dense network of 162 numbered points (markers) that were drawn over the entire surface. The markers serve as control points and tie points to achieve the highest possible accuracy in the photogrammetric reconstruction. In addition, a 360\({}^{\circ}\) measuring circle (protractor) was placed on the upper part of the disc cutter to mark 18 sectors with an angle of 20\({}^{\circ}\). At each sector boundary, nine points were marked at approximately the same locations in the vertical direction, as shown in Figure 8.  
Figure 6: Weighing the worn disc cutter using a digital hanging scale. Figure 7: A background plate with targets used for close-range photogrammetry of the worn double disc cutter. The upper (top) and lower (bottom) parts of the double disc cutter were photographed separately using an NEX-5 camera (Sony Co., Minato City, Tokyo, Japan) equipped with an E-mount 16 mm f/2.8 lens, which has a resolution of 4592 \(\times\) 2576 pixels and a pixel size of 5.48 \(\times\) 5.48 \(\upmu\)m. Camera calibration was carefully performed using a checkerboard pattern to ensure the precise determination of internal parameters such as the focal length, optical center, and distortion coefficients of the lens. Photography was carried out in such a way that 11 photos were taken in each shooting position as the camera was mounted on a tripod with a special slider that allowed the camera to be moved horizontally and in the millimeter range. In this way, a total of 198 photos were taken for each side of the disc cutter (the upper and lower parts). To achieve the precise rotation of the disc around its vertical axis, a laser beam was used, as shown schematically in Figure 9.  
The photos were then processed using Agisoft Metashape 1.6.3 software. To improve accuracy and resolution while maintaining finer detail, ultra-high-quality and aggressive filtering techniques were used during processing. This ensured that only the most accurate and relevant points were retained. The top and bottom point clouds of the disc cutter were created separately and then merged into a final dense, high-resolution cloud. The entire process, from image alignment to the creation of a dense point cloud and final model, took 1 h and 47 min. The photogrammetric analysis was performed on a high-performance PC equipped with 64 GB RAM, an i9-10900F 2.8 GHz processor, and an NVIDIA Quadro RTX 5000 GPU graphics card with 16 GB GDDR6 memory. Figure 8: Marker points on the disc cutter’s surface.