OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 does not occur as quickly as the decrease in cut thickness does at the end of the cut with a tooth, because elastic deformation of the MDTP system (machine-device-tool-part) does not allow it to be as sharply as theoretically expected. We believe that with an increase in the MDTP system rigidity, and, above all, with an increase in the end mill rigidity as the most flexible element, there will be a more rapid decrease in the forces at the end of the cutter tooth operation. A negative value of the sign of the force Px (Px = -120…-170 N) indicates the desire of the cutter to lift the workpiece, which is associated with a positive inclination angle of the cutter teeth of ω = 40º. An increase of the force Px in the negative direction within the range of numbers 2–5 (fig. 7) is associated with an increase in the cut thickness when turning the cutter. Moreover, a decrease in the range of numbers 5–6 is associated with a decrease in the cut thickness at the end of the cycle of cutting the allowance with the tooth in question. During further rotation of the cutter, the next cutter tooth begins to cut into the workpiece, so the cycle of changing the component forces is repeated (fig. 7, the range is between numbers 5 and 9). Down milling process When cutting a tooth into the workpiece (fig. 10, the range is between numbers 1 and 2), the cut thickness a quickly increases to the maximum value amax, so the component force Ph quickly increases to the maximum value. And Pv also quickly increases in the negative direction of the OY axis of the dynamometer (the cutter, with its rounded cutting edge and its rear surface, pushes the workpiece away from the axis of the cutter). Since the feed rate sm = 104 mm/min is very high, i.e. the cut thickness a is large, the force Py acting in the radial direction towards the cutter axis also includes a normal load acting from the rear surface of the tooth when cutting-in. Under the influence of forces Py and Pz, the component Pv continues to increase in a negative direction due to the high feed and movement of the cutter tooth to continue cutting. Since the cut thickness decreases during down milling, the undercutting of the metal under the cutting edge increases, but not as intensely as it does during up milling because of the presence of chips on the front surface of the cutter tooth. On the contrary, the Ph component at this time begins to decrease due to a decrease in the slice thickness and, accordingly, a decrease in the force Pz, and in addition, it rotates towards the OY axis, i.e., it acts more on the force Pv, and not on the Ph (fig. 10, the range is between numbers 2 and 3). Further rotation of the cutter leads to a further reduction in the cut thickness a to zero, which causes a decrease in the components Pz and Py, and therefore the components Ph and Pv (fig. 10, the range is between numbers 3 and 4). Due to the elastic deformation of the MDTP system, under the influence of force Pv, the cutter is repelled from the machined surface, which is why the depth of cut does not reach the specified value, and, in turn, this leads to a reduction in the time of milling with one tooth of the workpiece. In fig. 10 in the range between the numbers 4 and 5 the forces are equal to zero. As well as the change in force, Px during up milling, Px during down milling fluctuate up and down depending on the change in the cutting thickness a, and these changes are small. A negative value of the sign of the force Px indicates the tendency of the cutter to lift the workpiece, which is associated with a positive angle of of the cutter teeth inclination ω = 40º. During further rotation of the cutter, the next cutter tooth begins to cut into the workpiece, so the cycle of changing the component forces is repeated (fig. 10, the range is between numbers 5 and 6). However, Fig. 10. Graphs of force changes during milling when turning a sharp cutter for one revolution. Down milling with a 4-tooth cutter d = 8 mm, t = 4 mm, B = 2 mm, n = 500 rpm, sm = = 104 mm/min, specimen No.1 – carbide
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