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Introduction

At the moment, milling is one of the most frequently used methods for producing surfaces by cutting, as it allows to obtain parts of almost any complexity. The main advantages of this method of processing are high performance, accuracy and quality of the processed surfaces.

Stages of increasing the efficiency of the milling process

Milling with side blades of end mills is widespread in general engineering and aircraft building. We give an example of parts manufactured at Rostvertol PJSC (Fig. 4). This is the fork with which the spar is attached to the rotor of the helicopter. The inner surface of the fork has high demands on the surface quality, since the accuracy and quality of the surface parameters affect the durability of the mount, and therefore the life of the helicopter. The data on the fatigue tests of the mount of the spar with the helicopter carrier system with the help of a fork, made in the conditions of PJSC “Rostvertol” show that reducing the wave height from 12 to 6 ?m makes it possible to increase the fatigue life 2 or more times. This determines the requirements for surface quality of similar products. Roughness also affects fatigue life.

The first stage is the choice of processing strategy. There are two possible options. The first option is associated with successive passes for processing the upper part of the workpiece, then the subsequent part.

The second stage – the choice of the machine. For milling a surface whose width is greater than the diameter of the cutter, it is advisable to choose a machine with five-axis contour control. This is due to the need to compensate for the error of the bending deformations of the cutter. In addition, to compensate for the dependence of the stiffness of the part along the trajectory of movement along the generatrix of the formed surface, it is necessary to ensure a constant ratio of force to the total reduced stiffness in the direction normal to the generatrix. To do this, as a possible processing strategy, software control of the speed of movement of the table in the direction of the generatrix is considered.

The third stage is the diagnosis of the kinematic perturbations of the machine in the unity of the estimation of the amplitude of the radial spindle beat and variations in the feed velocity of the table. To estimate the kinematic perturbations, you can use the technique described in [1, 2]. It is based on the processing of information from the feedback sensors on the position of the table, which are equipped with almost all CNC machines with loop control. The characteristics of kinematic perturbations, represented, for example, by autocorrelation functions, characterize the maximum achievable machining accuracy on a particular machine.

The fourth stage – the choice of tool. When choosing a tool, the following features should be taken into account.

The number of teeth (the step between the teeth of the cutter), taking into account the angle of inclination of the cutting blades, should ensure the continuity of the process without overlapping contacts. The diameter of the cutter is chosen on the basis of a compromise between the desire to increase it to ensure increased rigidity and the need to form the surface in the areas of its curvature .

It is necessary to ensure the geometric accuracy of the cutter blades. In the case described, the accuracy of the cutting edge radius should be two to three times higher than the required surface accuracy. It is necessary to take into account that during milling, in contrast to turning, the formed surface is directly determined by the whole geometry of each tooth along the axis of rotation of the tool.

When sharpening the cutter teeth, it is necessary to choose, if possible, the rake angle of the cutting wedge at least 12 ° -15 °. In this case, when the tool moves, the orientation of the force has a projection that almost coincides with the direction of movement of the cutter. Then in the areas of shaping there will be a minimum deviation of the tool from the ideal trajectory.

The angle of inclination of the cutter teeth must be chosen such that the axial component of the force, as noted earlier, has a direction toward the table on which the workpiece is fixed. In the case described, this is a milling cutter with the right orientation of the tilt of the teeth.

Recommended values of the geometrical parameters of the cutter during the processing of the material.

The fifth stage is the selection of technological regimes and control.

When machining a part whose rigidity parameters remain unchanged, the amount of feed per tooth when machining steel 40ХН2МА depends on the requirements for the quality of the formed surface. It has been shown above that the surface roughness indicator takes into account the kinematic characteristic, which increases with increasing feed rate; trajectory, depending on the forming attracting sets and independent processes in the cutting zone. As tool wear develops, the ratio between them changes. At the initial stage, the kinematic component has the main contribution. Then the influence of independent processes in the cutting hone significantly increases. The forming trajectory depends on the parameters of the dynamic system and the evolutionarily changing parameters of the dynamic coupling. In addition, it is necessary to take into account that when applying to a tooth less than 0.01-0.02 mm, an increase in roughness can be observed due to a possible jump of the tool movement trajectory. In this case, the waviness may increase even more. Therefore, strictly speaking, to ensure constant values of the height of asperities, it is necessary to change the feed rate when processing a single size of parts.

Findings

The proposed algorithm (steps) provides improved surface quality when milling with end mills. It includes the requirements for the tool, the choice of its geometry, technological modes and the parameters of the accuracy of the machine. In the general case, the problem of improving the microrelief of the surface being formed is solved using a method based on the use of information about acoustic noise measured by a microphone, followed by information processing. The proposed method allows in the cutting process to evaluate the limiting states of the process for obtaining information on tool replacement and changeover.

References

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  2. Hans, B. Kief. CNC for Industry / Hans B. Kief. – 2000. – Р. 198.
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