1 Proposed issue
Electrolytic grinding was first used to grind carbides. Due to the large investment in equipment and the complex maintenance of the machine tool, its development speed is relatively slow. However, since this method has a good leveling ability, grinding force and grinding heat are very small, there are no burns and cracks on the surface, so there are great advantages in some fields. For this reason, both domestically and internationally have been working to improve the quality of its processing. In recent years, the newer method is to use pulse power for electrolytic grinding, which has improved the processing effect more effectively. We have also conducted preliminary studies in this area. We will introduce and discuss some of them in the following.
2 Measures to Improve the Quality of Electrolytic Grinding
2.1 Key Factors Affecting Electrolytic Grinding Quality Over the years, many studies have been conducted at home and abroad in order to improve the quality of electrolytic grinding. Past research was mainly confined to considering two main parameters—electrode voltage and feed rate—that is, from the electrolysis and mechanical action directions, and agreed that achieving a reasonable match between the two is to increase electrolytic grinding. The key to processing quality. The relevant data also quantified the matching ratio between the two, assuming that 90% of the materials were dissolved by electrolysis, and 10% of the materials were mechanically removed to best match. In fact, we also found in the experiment that if the electrolysis is too strong, the mechanical leveling of the grinding wheel is significantly weakened, the surface roughness value is increased, and the workpiece is not bright; otherwise, if the electrolysis is too weak, the mechanical action is too strong. The electrolytic grinding is similar to ordinary mechanical grinding, scratches appear on the surface, and the surface roughness value also increases.
2.2 Measures for Adjusting Electrolytic and Mechanical Interactions 1. Reduce Mechanical Feeding In order to precisely adjust the matching of electrolysis and mechanical action, it is necessary to start with electrolysis and machinery. The adjustment of the mechanical action It is important to be able to achieve micro-feeds, the amount of each feed is very small, so that the mechanical action can achieve precise regulation.
2 The regulation of the electrolysis of the pulsed power supply was previously achieved by adjusting the voltage. However, the inter-electrode voltage should not be too small, because from the anodic polarization curve of the metal (curve 2 in Fig. 1), the anode potential must reach a certain value in order for the anodic electrolytic dissolution to occur. A variety of metals have a minimum dissolved voltage in a specific electrolyte, and the inter-electrode voltage is only greater than this value, the metal can be dissolved, and the electrolysis needs less electrolysis in precision electrolytic processing, which creates a conflict. In this case, a pulsed power supply can be used, and the electrolysis time can be shortened by reducing the duty cycle of the pulsed power supply in order to achieve a micro adjustment of the electrolysis.
Figure 1 Electrode polarization curve
In addition, the pulse electrolysis grinding has a perturbation effect on the electrolyte, so that the electrolysis product is easily eliminated, the electrolyte renewal is accelerated, and the electrolyte flow field is more uniform, thereby improving the processing effect.
3 Current-regulated electrolysis and electrolytic grinding Power sources used in the past have been using voltage-regulated currents, but in the electrolytic grinding process, it is more reasonable to use current regulation directly. Because, according to Faraday's law: M=ηKIt
Where M is the weight of dissolved or precipitated material on the electrode;
K - Electrochemical equivalent of the weight of the electrolyte;
I - processing current during electrolysis;
η - current efficiency;
t - electrolysis time;
It can be seen that the magnitude of the electrolytic removal is directly proportional to the current I, which is not directly related to the voltage. In the processing system, the inter-electrode voltage U and the machining current I have the following indirect relationships:
U=U1+IR+U2
In the formula, U1 and U2 are the polarization potentials of the cathode and the anode, respectively. If voltage regulation is used, although the inter-electrode voltage U can be made constant, during the electrolytic processing, fluctuations in the inter-electrode resistance R are unavoidable due to the generation and elimination of bubbles and electrolytic products and the changes in the inter-electrode gap, and the current will accordingly change. This makes the amount of electrolysis impossible to control precisely, and this problem does not exist when using current regulation. Although the inter-electrode voltage fluctuates, it does not affect the electrolysis removal, as long as the voltage used is higher than the anodic dissolution voltage of the workpiece metal.
2.3 The explanation of the leveling mechanism of the workpiece surface by the constant current source The mechanism of the leveling of electrolytic grinding is that under the action of the workpiece, the surface is oxidized to form an anode passivation film, and the resistance of the film is large, which hinders the electrolysis. The passivation film at the high point is constantly scraped off by the grinding wheel, exposing the new metal to be electrolyzed, so that the metal at the high point dissolves faster. The low point is due to slower protection and dissolution of the passivation film, and the workpiece is constantly leveled by the presence of poor removal speeds at the microscopic high and low points. When using an adjustable constant-current power supply, the external circuit current is controlled to be constant, rather than constant current density throughout the processing area. On the contrary, the nature of the high-passivation film scraped by the grinding wheel does not change, so there is still a difference in removal speed at the microscopic high and low points, and the workpiece can be continuously leveled.
3 Development of Adjustable Constant Current Pulse Power Supply
3.1 Overall Design The object of this article is Ф 2mm hole, the tool used is Ф1.8mm × 2mm electrolytic grinding head. Since the relative area of ​​the processing area is small and the required current is not large, the overall composition of the power supply is shown in FIG. 2 . Where RL is the load resistance. The entire power supply can select DC output and pulse output through the shift switch.
Figure 2 power supply schematic
3.2 current source design The current source converts the full-wave rectified voltage Ui into a direct current Io and outputs it to the load RL. The circuit is implemented using a CW317 chip. The principle is shown in Figure 3. Since both ends of the current source cannot be opened, a bypass resistor R is connected across the load, and the current is adjusted by the adjustable resistor R2. The maximum output current of the power supply can exceed 1A, which can meet the needs of small hole electrolytic grinding.
Figure 3 adjustable constant current power supply schematic
3.3 The design of the main vibration level The main level of the pulse power supply in this paper is realized by the 555 integrated circuit. The principle is shown in Figure 4. In this circuit, the pulse width and pulse can be continuously adjusted by adjusting the potentiometers R2 and R3; changing the value of the capacitor C1 can change the pulse width and the adjustment range between the pulses. The pulse width and pulse width of the circuit can be adjusted between 70μs and 3.36ms, and the pulse period is adjusted between 140μs and 6.72ms.
Figure 4 Principle diagram of the main level of the pulse power supply
4 Pulse Electrolytic Grinding Process Test
4.1 Effect of Pulse Electrolytic Grinding on Size Removal The machining test of Ф2mm holes was performed under different electrical parameters. During the test, the amount of removal is measured at regular intervals (the reciprocating movement of the electrolytic grinding head once in the axial direction), and the average value of 5 points is taken. The instrument used is the DGC-8ZG inductive sensor and the DGS-6B digital inductance micrometer of the Central Plains Instrument Factory. The minimum resolution of this instrument is 0.01m and the measurement error is ±0.05μm. The measurement results are shown in Figure 5. Other experimental conditions were as follows: the electrolyte was a NaNO3 and NaNO2 based composite solution; the radial single step feed amount was 1 [mu]m; the electropolishing head reciprocation speed was 30 mm/min; the pulse width was 0.25 ms; the pulse interval was 0.25 ms, 0.5 ms, and 0.75 ms; Peak current density 0.9 A/cm2, 1.4 A/cm2, 1.9 A/cm2, 2.4 A/cm2.
Figure 5 Dimensional Accuracy in Pulse Electrolytic Grinding
The test results show that at different current densities, the workpiece removal during pulse electrolytic grinding is smaller than that during DC electrolytic grinding, ie pulsed electrolytic grinding has higher dimensional controllability and higher machining accuracy. In addition, when the electrolysis amount (the product of the electrolysis current I and the electrolysis time t) is the same, the removal amount during the pulse electropolishing is larger than the removal amount during the DC electropolishing. This is due to the intermittent electrolytic action during pulsed electrolytic grinding, which creates a perturbation of the electrolyte and makes it easier to eliminate electrolytic products. In addition, because the electrolytic solution has more time to be updated during pulse electrolytic grinding, the flow field characteristics between the poles are improved, so that some components can form valence oxides to dissolve.
4.2 Effect of Pulse Electrolytic Grinding on Surface Roughness The previous mechanism analysis shows that pulsed electrolytic grinding can obtain better surface quality. This article has conducted a test on this, and the test results are shown in Figure 6. The test conditions are as follows: the electrolyte is a NaNO3 and NaNO2 based composite solution, the radial single step feed amount is 1 [mu]m, the electropolishing head reciprocation speed is 30 mm/min, the pulse width is 0.25 ms, and the pulse duty ratio is 1:2, 1:3, 1:4, peak current density 0.9 A/cm2, 1.4 A/cm2, 1.9 A/cm2, 2.4 A/cm2.
Figure 6 Effect of electrolytic grinding on surface roughness
The test results show that pulsed electrolytic grinding has higher machining quality than DC electrolytic grinding. In addition, the duty cycle increases during pulsed electrolytic grinding and the surface quality increases accordingly. This is due to the fact that the heat of the process zone during the pulse interval, the gas evolution, and the electrolytic product are sufficiently eliminated, and the electrolyte has more time to renew, making the flow field and temperature field of the electrolyte more uniform.
5 Conclusion
The key to electro-grinding to obtain high processing quality lies in the reasonable matching of electrolysis and mechanical action. In this paper, a newly developed adjustable constant-current pulse power supply was used for electrolytic grinding test, which proved that pulsed electrolytic grinding has better processing quality than DC electrolytic grinding.
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