RU2768103C2 - System for electrochemical abrasive grinding - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: invention relates to machine building and can be used in electrochemical machining of parts with grinding wheels. System for electrochemical grinding of part comprises working table, abrasive wheel consisting of abrasive grains bonded by current-conducting binder, connected to negative pole of current source, and nozzle for supply of working liquid - electrolyte, into interelectrode space. Current source has steeply falling current-voltage characteristic and is a generator of unipolar current pulses. Working table is connected to positive pole of current source through electronic switch made with possibility to switch current from working table into short-circuited circuit and consisting of two high-speed transistors, one of which is configured to connect current source to short-circuited circuit, and second - to working table. Controller is connected to current source to control electronic switch and current source, which in its turn is connected to analogue-to-digital converter installed with possibility to measure voltage between working table and abrasive wheel.

EFFECT: improvement of processing quality and reduction of tool wear with increase of electrochemical anode current share.

1 cl, 2 dwg, 1 ex

Description

The invention relates to mechanical engineering and can be used in the electrochemical processing of parts with grinding wheels.

Known flap grinding wheel for electrochemical grinding of difficult alloys and steels, where the petals of the flap grinding wheel is made in the form of a metal mesh coated with abrasive material (patent RU 2103123, IPC V23N 5/06, publ. 27.01.1998).

The disadvantage of the known device is the increased consumption of the grinding wheel and low grinding performance.

Known abrasive wheel for electrochemical grinding, consisting of abrasive grains bonded together, containing conductive inserts located perpendicular to the axis of rotation of the abrasive wheel, which are placed between the abrasive inserts, where the conductive inserts are connected to the negative electrode of the DC source (patent RU 144707, IPC B23H 5/10, published on August 27, 2014).

Known grinding wheel, rigidly fixed between two disks of conductive material connected to the negative pole of a DC source. The grinding wheel has the ability to contact the surface of the workpiece connected to the positive pole of the DC source, and the discs of conductive material are located at a distance from the surface of the workpiece, forming a gap that can be filled with electrolyte from the fitting (patent RU 2522503, IPC V23N 5/ 08, published on July 20, 2014).

The closest is a system for electrochemical grinding of parts (author's certificate SU 1590237, publ. 09/07/1990), containing a desktop for mounting a workpiece on it, an abrasive wheel for electrochemical grinding, consisting of abrasive grains bonded with a conductive bond, mounted on desktop with the ability to move relative to it and connected to the negative pole of the current source, and a nozzle for supplying the working fluid - electrolyte into the interelectrode gap between the abrasive wheel and the workpiece.

The disadvantages of the known methods and devices are as follows.

During processing, the conductive abrasive wheel comes into contact with the workpiece, which inevitably leads to the breakdown of the interelectrode gap (IEG) and the formation of an electric arc. Since the arc has low electrical resistance, and the current source, as a rule, limits the current in the short term, the voltage across the MEP drops sharply. This, in turn, leads to a decrease in the anode current flowing through the electrolyte.

The anode current has a positive effect on the process of electrochemical abrasive grinding (ECAS):

- removes burrs and rounds sharp edges;

- reduces efforts on the circle due to the additional removal mechanism;

- removes the hardened layer formed by abrasive grains.

The electric arc is an ionized channel with high electrical conductivity, and its burning is also facilitated by the fact that at the point of arcing on the anode and cathode, the temperature reaches thousands of degrees, which enhances the electron emission. Therefore, as the circle rotates, the arc does not move to the place of the smallest interelectrode gap, but gradually stretches: its ends “stick” to the starting points. After reaching a sufficient length, the arc breaks, the voltage on the MEC returns to its original values, and the anode current increases again.

The burning of an electric arc has a negative impact on the ECAS process:

- leads to increased tool wear;

- forms areas of thermal influence on the workpiece;

- reduces the positive effects of the anode current.

Known methods and devices do not include special measures to limit the arc current. At the same time, with an increase in the processing area (for example, with simultaneous processing of several parts), the current in the arc increases, which leads to its longer burning, and, accordingly, to a deterioration in the processing performance.

The technical problem solved by the invention is to improve the quality of workpiece processing and reduce tool wear.

EFFECT: increased share of electrochemical (anodic) current.

The problem is solved, and the technical result is achieved by a system for electrochemical grinding of a part, containing a desktop for mounting the workpiece on it, an abrasive wheel for electrochemical grinding, consisting of abrasive grains bonded with a conductive bond, installed on the desktop with the ability to move relative to it and connected to the negative pole of the current source, and a nozzle for supplying the working fluid - electrolyte into the interelectrode gap between the abrasive wheel and the workpiece. Unlike the prototype, the current source has a steeply falling current-voltage characteristic and is a generator of unipolar pulses, and the desktop is connected to the positive pole of the current source through an electronic key, made with the ability to switch the current from the desktop into a short-circuited circuit and consisting of two high-speed transistors, one of which is configured to connect a current source to a short-circuited circuit, and the second to the desktop, and a controller is connected to the current source to control the electronic key and the current source, which in turn is connected to an analog-to-digital converter installed with the ability to measure voltage between working table and abrasive wheel.

The technical result is achieved as follows.

The pulsed current source has a steeply falling current-voltage characteristic. During an electrical breakdown in such a source, the electric current does not increase, which leads to a drop in the power dissipated in the interelectrode gap and a decrease in the negative effects associated with arcing.

When the wheel approaches the workpiece, the interelectrode gap decreases, which leads to a drop in its electrical resistance and an increase in the anode (electrochemical current).

When using pulsed current, the electric arc is interrupted every time the pulse duration ends. In addition, the development of the arc also takes time, that is, after the start of the impulse, it does not occur immediately. Both of these factors lead to an increase in the proportion of the anode current and to a corresponding improvement in the ECAS parameters.

In addition, the system controls detect the occurrence of an electrical breakdown of the MEC and turn off the supply of electric current to the gap until the next pulse by using a high-speed switch that switches the current from the interelectrode gap to a short-circuited circuit. This leads to an even greater increase in the proportion of the anode current.

The essence of the invention is illustrated by drawings, where:

in fig. 1 shows a functional diagram of the proposed system;

in fig. 2 - oscillograms of current and voltage.

Figure 1 shows:

1 - desktop

2 - workpiece

3 - abrasive wheel with conductive bond

4 - electrolyte supply nozzle

5 - current source with a steeply falling current-voltage characteristic

6 - electronic key

7 - analog-to-digital converter (ADC)

8 - current source controller

(a) and (b) - high-speed transistors that make up electronic key 6. Transistor (a) connects current source 5 to a short-circuited circuit, and transistor (b) - to the desktop

S - direction of rotation of the abrasive grinding wheel

P - direction of the electrolyte jet

F - direction of movement of the desktop

In FIG. 2 marked:

t _i - normal pulse duration; t _p - pause duration; t _c - corrected pulse duration; t _o - shortening of the pulse length; t _s - the duration of the arc.

The device works as follows.

On the desktop (1) connected to the positive pole of the current source

(5) with a steeply falling current-voltage characteristic through an electronic key

(6), workpiece (2) is installed. A welding transformer or an inverter can be used as a current source with a steeply falling current-voltage characteristic. A rotating abrasive wheel (3) with a conductive bond is connected to the negative pole of the current source (5). The working table (1) and the abrasive wheel (3) can move relative to each other in different directions (F) to ensure the working feed. With the help of a nozzle (4) between the circle (3) and the workpiece (2), an electrolyte jet is supplied. The electronic key (6) consists of 2 high-speed transistors (a) and (b), one of which (a) connects the current source to a short-circuited circuit and the second (b) - to the desktop. The controller (8) of the current source (5) is connected to an analog-to-digital converter (7), which measures the voltage between the working table (1) and the abrasive wheel (3). The controller controls the electronic key (6) and the current source (5).

The current source (5) on the instructions of the controller (8) generates current pulses with a large front, while in the electronic key (6) the transistor (a) is open, and the transistor (b) is closed, thus, the current source is connected to a short-circuited circuit. After the current reaches the set value, in the key (6) the transistor (a) closes, and the transistor (b) opens, and the source current switches to the desktop (1).

After the end of the set pulse duration, the electronic key (6) switches the current back to the short-circuited circuit. Thus, current pulses with a short edge are applied to the desktop (1).

The ADC (7) continuously measures the voltage at the interelectrode gap (IEG) between the working table (1) and the circle (3) and transmits this data to the controller (8), which controls the current source (5) to provide the specified voltage of the IEG. Thus, during the pulse duration, the current source (5) is a current source, and for a long period of a plurality of pulses, it is a voltage source.

If no breakdown occurs within a given pulse duration, then the normal pulse duration t _i is formed (the first waveform in Fig. 2).

Since the current source (5) has a steeply falling volt-ampere characteristic, during the electrical breakdown of the MEP, the voltage drops, and the current does not change (the second oscillogram in Fig. 2). This limits the power of the electric arc and allows the controller (8) to determine the moment of occurrence of the MEP breakdown. Then the controller (8) using the key (6) switches the current into a short-circuited circuit, forming the corrected pulse duration t _c . In this case, the arc burning time t _s determined by the speed of the ADC (7), the controller (8) and the key (6), is reduced by the value t _o .

An example of electrochemical abrasive grinding using the inventive system.

The described device for the process of electrochemical abrasive grinding is implemented in the machine EAG400.

The current source based on the inverter converter is installed in a separate electrical cabinet.

The electronic key and ADC are mounted directly on the mechanical structure of the machine.

The following electrical parameters of the processing mode were recorded:

- pulse duration t _i =2 ms;

- pause duration t _p =2 ms;

- voltage U=8 V;

- impulse current I=50 A;

- system response time to a short circuit (arc burning time) t _s =0.025 ms.

Thus, the processing control system provides the specified voltage due to feedback by changing the current level of the current source, calculates and sets the current level of the current source to provide the specified voltage in the pulse in the pause between pulses, detects the moment of breakdown of the interelectrode gap and turns off the current before the end pulse duration.

This leads to an increase in the proportion of anode current and an improvement in the quality of workpiece processing.

Claims (1)

A system for electrochemical grinding of a part, comprising a work table for mounting a workpiece on it, an abrasive wheel for electrochemical grinding, consisting of abrasive grains bonded with a conductive bond, mounted on the work table with the possibility of moving relative to it and connected to the negative pole of the current source, and a nozzle for supply of the working fluid - electrolyte into the interelectrode gap between the abrasive wheel and the workpiece, characterized in that the current source has a steeply falling current-voltage characteristic and is a generator of unipolar pulses, and the work table is connected to the positive pole of the current source through an electronic key, made with the ability to switch current from the desktop into a short-circuited circuit and consisting of two high-speed transistors, one of which is configured to connect a current source to a short-circuited circuit, and the second - to the desktop, and to the source t At the same time, a controller is connected to control the electronic key and the current source, which, in turn, is connected to an analog-to-digital converter installed with the ability to measure the voltage between the work table and the abrasive wheel.

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