The paper presents the study of the influence of the process of magneto-abrasive machining (MAM) on the characteristics of the surface layers of flat surfaces of parts made of ferromagnetic material U9 by machining with end-type heads based on high-power permanent magnets that form a magneto-abrasive tool of the "brush" type. For estimation of the influence of the process of magneto-abrasive machining on the surface layer, the parameters of surface hardness were analyzed after the machining of test samples with different powders and under different modes. The degree of influence of the MAM on the surface, both in terms of the hardness of the surface layer and the deformation of samples due to compressive residual stresses arising as a result of machining, was studied. The estimation of the state of the surface layer was performed by the change in hardness after machining, the magnitude of the degree of hardening, to some extent, by the parameters of roughness. The control of changes in internal residual stresses formed in the surface layers of samples due to the interaction of powder particles with the surface during machining was carried out according to the degree of their deformation after MAM. It was found that due to MAM, internal compressive stresses of 30–100 MPa arise in the near-surface layer of the material of the samples, while the magnitude of the stresses varied in inverse dependency, that is, with an increase in the working gap, in the vast majority of cases, a decrease in the magnitude of the stresses was observed. It was shown that the depth of the hardened layer under different machining conditions is up to 200 µm or more, and the strengthening coefficient varies from 10 to 40%.
To ensure the durability and working ability of cutting tools, especially carbide, it is promising to form a rational microgeometry of working surfaces, in particular, the shape and geometry of cutting edges, as well as their hardening. For this, it is necessary to carry out additional machining of the tool after its sharpening operation. The magneto-abrasive machining method in conditions of large working gaps with a rational ratio of the impact and frictional interaction of the magneto-abrasive tool with the machined elements makes it possible to form microgeometry and strengthen both the cutting edges and the working surfaces. To ensure effective machining, it is necessary to determine the regularities of forming the microgeometry of the cutting edges with pre-made protective chamfers. The paper presents the results of a complex study of the magneto-abrasive machining process of carbide not sharpened inserts with protective chamfers on the cutting edges made at different angles. It was shown that the process of edge rounding is linearly related to the duration of magneto-abrasive machining. Moreover, the rate of increase in the size of the radius of cutting edges rounding with protective chamfers increases linearly with an increase in the angle of inclination of the chamfers, all other process conditions being equal. The physical description of the hardening mechanism of the surface layer of the working elements of the carbide cutting tool was proposed. It was shown that the hardening process is associated with the rational imposition of forces arising as a result of the frictional and impact interaction of the particles and the formations of the magneto-abrasive tool with the machined surfaces. It was found that the nature of the change in surface hardness obtained after magneto-abrasive machining on the protective chamfers has the wavy shape depending on the time of the process. The greatest increase in surface hardness takes place on chamfers made at angles of 10 ° and 20 ° and is explained by the value of the ratio between the forces of normal and tangential origin arising from the contact of the magneto-abrasive tool with the machined surfaces.
The method has been developed and systematic studies have been carried out to measure the drag forces, that exerted by a magneto-abrasive tool on machined cylindrical parts with a diameter of 16 mm and the height of 30 mm, made of para-, ferro- and diamagnetic materials, in an annular working zone with the diameter of 200 mm with the working height of 30 mm at machining speeds in the range of 1 - 3 m / s, magnetic induction in the working areas varying in the range of 0.2 - 0.25 T. It is shown that the drag forces significantly depend on the magnetic and rheological properties of the magneto-abrasive tool. During magneto-abrasive finishing (MAF), the drag force for non-magnetic parts is almost the same and does not depend on the machining speed, and with an increase in magnetic induction in the working areas, its linear growth takes place. The coefficient of magnification of drag force determined by the magnetic field, for powders with a particle size of 400/315 µm is 960 N/T, and for a size of 200/100 µm it is 720 N/T. It is shown that the drag forces during the MAF of ferromagnetic parts vary in the range from 160 to 220 N and are 1.2–1.45 times higher than for non-magnetic parts, which is due to the action of forces of magnetic origin, the features of the interaction of structural elements of MAT with machined surfaces and the formation of blocking up zones between the surface of the part and the pole pieces of the annular bath. The periodic change in the drag forces of the MAT of the workpiece was established near their average value, and at elevated MAF speeds of more than 2–2.5 m/s, instabilities were recorded associated with a periodic decrease in the oscillation amplitudes of the drag forces as a result of rearrangement, changes in the dimensions and volumes of the structural elements of the magneto-abrasive tool and the conditions of their interaction with the machined surfaces.
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