Optimization of marble machining with vacuum brazed diamond-coated milling tools

Natural stone blocks, slabs, and plates can be processed using various tools in CNC (Computer Numerical Control) systems to create decorative products. The aim of this study is to determine the wear mechanism of diamond coated milling tools produced by vacuum brazing method, which provides efficient and long-lasting use, with optimum parameters. For this purpose, six different metamorphic-origin marbles, commonly used in decorative products, were processed on a CNC machine. The study consists of two stages: preliminary experiments and main experiments. In the preliminary experiments, based on previous studies and experiences, the processing parameters were selected as follows: a cutting depth of 1 mm, feed rates of 1500, 2250, and 3000 mm/min, and spindle speeds of 6000, 9000, and 12000 rpm. The milling experiments were carried out as a full cutting operation, i.e. with the entire tool entering the material and removing chips with a cutting width equal to the tool diameter. Tools with different diamond grain sizes, coded as T301, T405, and T602, were used in the experiments. These tools were evaluated based on specific energy (SE), surface roughness (Ra), normal force (Fz), and radial force (Fr) criteria. According to the results of the preliminary experiments, the T602-coded tool demonstrated superior performance compared to the other tools. Therefore, the main experiments were conducted using T602 (50/60 mesh) coded tools. Additionally, since continuous cutting was performed instead of full slotting in the applications, an appropriate cutting width was determined for marbles with coarse and fine grain sizes. Based on surface roughness measurements, a cutting width corresponding to 40% of the tool diameter, or 2.4 mm, was found to be suitable. Based on the findings from the preliminary experiments, a full factorial experimental design was applied, and a total of 16 experimental models were created. During milling, forces occurring on the cutting axes, specific energy, displacement, noise levels, and post-milling surface roughness and grain size analyses were conducted. To optimize the experimental data, the Response Surface Methodology (RSM) was applied using the Stat-Ease 360 software. In optimizing milling performance, parameters such as surface roughness (Ra), which determines the final surface quality, normal force (Fz), which significantly affects surface roughness, and specific energy (SE) and material removal rate (MRR), which are critical for energy efficiency and productivity, were considered. The optimal processing parameters for each type of marble were determined individually. Based on these parameters, wear tests were conducted at intervals of 1, 3, 5, and 7 m² to examine the wear behavior of the diamond milling tools in detail. During the wear tests, the wear types on the tool surface were analyzed in detail using SEM and Dino-Lite digital microscopy. In general, the observed wear types included edge/corner wear, fracture, and chipping. The wear rate of the tools ranged between 30-48% for fine-grained marbles, while it was determined to be between 54-61% for coarse-grained marbles. The most abrasive effect was observed in Marmara White marble, with a wear rate of 54%. Additionally, optimal processing parameters (12000 rpm spindle speed and 3000 mm/min feed rate), which could serve as a reference for all marbles, were determined, and the wear behavior of Afyon Gray marble was compared under these parameters. According to the results, a 48% higher wear rate was observed under the reference parameters. In conclusion, it was found that the characteristic properties of marbles are critical not only for determining the optimal processing parameters but also for processing efficiency and tool life. The study revealed that diamond-coated tools produced using the vacuum brazing method provide lower wear rates and higher machining performance compared to traditional methods.