CAUSES OF DIFFERENT WAVES OF MACHINED SURFACES AFTER UP AND DOWN END-MILLING
Keywords:milling, self-oscillation, waviness, pitch, height, cutting surface
In modern industry, among the methods of surface treatment is widely used end-milling. Under certain conditions of its use there are self-oscillations, which significantly affect the quality of the treated surface. Various techniques are used to study this influence, in particular, the comparison of the basic fragments of the oscillogram obtained by cutting with the treated surface. It is established that it is formed by the first and last wave of self-oscillations during up and down milling, respectively. But depending on the direction of feed step and the height of the waviness on it have different meanings. Therefore, it is important to determine the reasons that lead to this result. This paper considers the features of each direction of supply that affect the formation of the treated surface. When superimposing fragments of the oscillogram obtained by up milling, it is seen that the deviation of the first wave of self-oscillations from the position of elastic equilibrium starts from the point of incision of the cutter in the part. Therefore, the pitch of the wave on the treated surface depends on the feed and the number of cuts required to cut the first protrusion on the cutting surface. The paper presents a formula for determining the length of the base of this speech. When using it, the discrepancy between the calculated wave step on the treated surface and the measured one does not exceed 4%. In the case of down milling, the last section with waviness on the cutting surface, which is cut and affects the shape of the treated surface, has a shorter base length than the opposite. This is the first reason why the pitch of the wave in the down milling is smaller than in the up. Also at formation of wave on the processed surface at down milling the feature in the form of shift on a phase of waves of self-oscillations on each following cut is observed. This increases the cutting time and the length of the cutting surface. Each subsequent forming depression is shifted towards the treated surface with a decrease in its deviation from the position of elastic equilibrium. However, the phase shift direction is opposite to the feed direction. This is the second reason why the pitch of the wave on the machined surface after the down milling is less than after the up. The calculation of the step of the wave of the treated surface after the down milling according to the results of the study of fragments of oscillograms shows that the error does not exceed 12% compared to the measured one.
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