RU2507042C1 - Spark erosion machining of precision spherical surfaces - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to spark erosion machining of spherical surfaces. Said spark erosion machining is performed by revolving electrode fed along the axis crossing the revolving billet axis at the centre of spherical surface. Note here that tubular electrode composed of two aligned parts is used, each said part being divided into at least three equal segments distributed on circumference and axially shifting relative to each other. Note also that machining is effected with variation of area of electrode working end. For this, working end area is increased at preliminary machining by driving electrode both ends to working position. While in lapping, electrode working end area is decreased by relative axial shift of electrode parts. Now, machining is made by only one of said parts.

EFFECT: higher quality of surface machining.

4 cl, 5 dwg

Description

The alleged invention relates to the field of mechanical engineering and can be used in the development of technological processes and the design of technological equipment for electroerosive shaping of precision spherical surfaces in the form of spherical segments or ball belts.

A known method of forming a spherical surface using a tubular cylindrical tool. Processing is performed by three simple movements, which include rotational movements of the workpiece and the tool around its intersecting axes, and axial feed of the tool towards the workpiece. The axis of rotation of the tool is located at a given angle to the axis of rotation of the workpiece. The method is used in abrasive machining, mainly in the final finishing of spherical surfaces. There is also known a method of electrical discharge machining (EEO) of spherical surfaces, in which tubular cylindrical electrodes-tools (EI) are used, working with an inner or outer edge when processing convex or concave spherical surfaces, respectively [a.s. No. 442909 of the USSR, priority dated 07/10/1972, published 09/15/1974, B23P 1/00]. EEE is based on the removal of allowance due to metal erosion under the influence of successive electrical pulses arising in the interelectrode gap (MEI) - the space between the electrode-tool (EI) and the workpiece. In order to form the necessary MEP and to remove erosion products from the treatment zone, the EDM is usually carried out in a dielectric fluid medium into which the workpiece and EI are immersed. The use of tubular EI allows you to abandon immersion in the liquid and limit it to pumping through the EI cavity and the interelectrode gap.

EEE can be performed both in the rough, but high-performance roughing mode, and in the fine-tuning mode. The necessary processing mode is set by changing the power and repetition rate of electrical pulses. Due to the absence of mechanical forces during EEE, the error in sphere formation is determined only by deviations from kinematic accuracy, which, with careful adjustment of the technological system, can be reduced to arbitrarily small values. The disadvantage of this technical solution is the instability of the shape of the tool, the edge of which due to erosion of the EI loses its original shape, and the adjacent part of the EI section enters the processing, which leads to a deviation from the principle stated by the authors.

Closest to the proposed is the selected as a prototype method of electrical discharge machining of spherical surfaces with tubular EI, in which in the process of ER erosion it self-profiling occurs [Haldeev VN, Ivanov AA, Zavalishin Yu.K. Electroerosive shaping of precision shells of a spherical shape. - Sarov, 2011, pp. 15-18, Fig. 2.4]. The end face of the self-profiling electrode-tool in the process of erosion also takes a spherical shape, equidistant to the machined surface (figure 1). As a result, the working surface of EI reaches large sizes, which leads to a significant increase in productivity. This is especially important at the pre-processing stage, which consists in removing large stocks from the original workpiece. In addition to increasing the area of the working surface, in order to achieve high performance, an increased power of single electrical pulses is established, which negatively affects the quality of the processed surface. When finishing precision parts, high performance is also desirable, but the priority is to meet the requirements for shape, size and surface roughness. This problem is solved by reducing the power of single pulses. However, a change in the processing mode leads to a change in the MEA between the EA and the workpiece. As a result, the equidistant mutual position of the end face of the EI and the machined spherical surface achieved at the pre-treatment stage is violated. Closest to the surface of the workpiece is one of the edges of the edge EI, for example, the internal (figure 2), which for a certain time and conducts processing. The working surface of EI when processing the edge is very small, respectively, the processing performance is low. In this case, the correct sphere formation occurs only on a part of the surface covered by this edge; in the polar and equatorial parts of the workpiece, non-spherical elements P and E corresponding to the rest of the end face appear (Fig. 2). In the process of further processing, EI erosion occurs, a new end surface is formed, which is equidistant to the workpiece surface, but located already at a distance of the changed interelectrode gap. Full reprofiling over the entire cross section of the EI is achieved by removing from its end a certain amount of material AND, limited in cross section in figure 2 by a dashed line. To do this, a significant allowance must be removed from the surface of the workpiece, which leads to an increase in the complexity of processing, consumption of material and energy. It is possible to reduce the considered costs of re-profiling EI using thin-walled EI, but in this case, due to the small area of the end face, the pre-processing productivity will significantly decrease. A variant of application at the stage of preliminary processing of thick-walled EI, and at the final finishing of a replaceable, thin-walled EI, is possible, but this also does not lead to a solution of the problem, since when changing the EI it is established with an inevitable error requiring a long run-in, also associated with the removal of allowance and the costs considered.

The technical result of the proposed technical solution is to achieve high productivity of electroerosive shaping of precision spherical surfaces throughout the entire manufacturing cycle, including the stages of preliminary processing and final finishing, using a special tubular electrode tool.

The technical result is achieved by the fact that for machining precision spherical surfaces, a tubular-shaped electrode tool is used that performs a longitudinal feed along an axis intersecting with the axis of the rotating workpiece in the center of the spherical surface. The electrode-tool is formed of two coaxial parts, each of which is divided into an equal, not less than three, number of segments uniformly distributed around the circumference and provide the possibility of axial displacement of one part relative to the other. The processing is performed with the change in the end-face area of the electrode-tool, for which, at the preliminary processing stage, the end-face working area is increased by bringing both parts of the electrode-tool into working position, and at the stage of final refinement, the area of the end-face of the electrode-tool is reduced by relative axial displacement of the parts electrode-tool, after which the processing is carried out only by one of them.

The ratio between the width of the segments in each of the two parts of the electrode-tool is determined from the condition that the reprofiling of the segments of the part of the electrode-tool completing the final finishing of the spherical surface with a changed interelectrode gap is completed by the moment of removal from the surface of the allowance for its finishing.

A dielectric working fluid is pumped through the cavity of the tool electrode, providing its flow rate and pressure sufficient to fill the interelectrode gap along the entire perimeter of the tool electrode when one of its parts is withdrawn from the workpiece.

After electroerosive finishing, grinding the spherical surface is done, for which the part of the electrode-tool that was used for finishing is brought into direct contact with it and dispersed abrasive material is applied to the contacting surfaces.

Figure 1 shows a diagram of the electroerosive shaping of a spherical segment in the stage of complete self-profiling of the working surface of EI relative to the processed spherical surface;

figure 2 shows a diagram of the electrical discharge machining of the workpiece in the initial stage of finishing with a changed interelectrode gap;

figure 3 shows the end view of a universal EI for processing spherical surfaces, consisting of two parts with segments distributed around the circumference;

figure 4 shows a configuration diagram of universal EI for pre-treatment of a spherical surface with the simultaneous participation of both parts;

figure 5 shows a configuration diagram of a universal EI for finishing a spherical surface with one part, where:

1 - electrode tool (EI);

2 - blank;

3 - fixed part of EI;

4 - the moving part of the EI.

The process of obtaining a precision spherical surface from the original workpiece 2 is as follows. Both parts of EI 3 and 4 are brought into a position in which their segments form a common end and participate in work together, after which they pre-process the workpiece. It is completed by leaving a layer of material on the workpiece surface — an allowance for final finishing. The value of this allowance is taken to be sufficient to completely eliminate defects that have occurred on the surface during rough pretreatment, and at the same time, it is sufficient to completely reprofile the end face of the EI during subsequent refinement. To perform the final refinement, the movable part of the EI along its axis is shifted by a certain amount sufficient to ensure that only one part of the EI is involved in the surface refinement, for example, part 3. Since the area of the working surface of the segments of one part of the EI is significantly smaller than the total working area of EI, the duration of the reprofiling of the working surface due to a change in the interelectrode gap is reduced in the same proportion. To achieve the optimum ratio of surface quality and processing productivity when designing EI, such a ratio of areas in each of its parts is established that at the time of complete re-profiling of EI during finishing, the entire allowance is removed from the surface of the workpiece, sufficient to remove material defects and microroughness protrusions arising on pretreatment stages. The optimal ratio of the cross-sectional areas of the segments, which depends on many factors of the technological process, is determined experimentally when debugging the technological process on test blanks.

The amount of displacement in the axial direction of the movable part of EI 4. take such that it exceeds the amount of axial erosion of the segments of part 3 in the process of finishing the workpiece. But the removal of one of the parts of the EI from the workpiece increases the gap between the EI and the treated surface, which is accompanied by a drop in the pressure of the working fluid in the EI cavity. A negative consequence of this may be the incomplete filling of the interelectrode gap with the working fluid and a decrease in the EEO efficiency. In order to avoid this, the pressure and flow rate of the working fluid are consistent with the axial displacement of the moving part of the EI and such a combination of these parameters is adopted to ensure that the fluid filled the interelectrode gap along the entire perimeter of the working surface of the electrode-tool.

After finishing the moving part of the EI 4 is returned to the working position for joint pre-processing of the next workpiece.

The considered method can also be used in cases where the technical capabilities of EDM do not allow to achieve the highest surface quality, and abrasive finishing is used at the final stage of processing. Abrasive lapping is done with a fine-grained abrasive material, which is placed between EI and a spherical surface. For this, the segments of the mobile group are brought into contact with the workpiece, reducing the interelectrode gap to zero. When abrasive machining, mechanical loads arise, therefore, to maintain the best conditions for centering the electrode-tool, its segments should be evenly distributed around its circumference, and the number of segments in any part of the electrode-tool should not be less than three.

Claims (4)

1. The method of electroerosive processing of precision spherical surfaces using a rotating electrode-tool of tubular shape, performing a longitudinal feed along an axis intersecting with the axis of the rotating workpiece in the center of the spherical surface, characterized in that they use a tubular electrode-tool made of two coaxial parts, each of which is divided into equal, at least three, the number of segments uniformly distributed around the circumference, providing the possibility of axial displacement of one part of the relative is relatively different, and the processing is carried out with the change of the end-face area of the electrode-tool, which is involved in the processing, while at the preliminary processing stage, the end face working area is increased by bringing both parts of the electrode-tool into working position, and at the stage of final finishing, the area of the end-face of the electrode-tool is reduced by relative axial displacement of the parts of the electrode-tool, after which the processing is carried out only by one of them. 2. The method according to claim 1, characterized in that they use an electrode-tool, the ratio between the width of the segments in each of the two parts of which is determined from the condition that the reprofiling of the segments of the part of the electrode-tool, making the final finishing of the spherical surface with a changed interelectrode gap, is completed by the time of removal from the surface of the allowance for its finishing. 3. The method according to claim 1, characterized in that a dielectric working fluid is pumped through the cavity of the electrode-tool, providing its flow rate and pressure sufficient to fill the interelectrode gap along the entire perimeter of the electrode-tool when one of its parts is withdrawn from the workpiece. 4. The method according to claim 1, characterized in that after electroerosive lapping, a spherical surface is lapped, for which the part of the electrode-tool that was used for lapping is brought into direct contact with it and dispersed abrasive material is applied to the contacting surfaces.

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