Discussion on "Multiple Processing Technology" of Wire-cutting Machine Tool

1. Measures to control the spatial shape and position change of the electrode wire

   In the wire EDM process, the electrode wire will inevitably have unpredictable spatial shape changes under the action of the discharge force, so that the discharge point lags behind the support point in the feeding direction. In order to control the spatial shape and position of the electrode wire, the following methods can be used: increase the tension of the electrode wire, and make the fulcrum as close to the upper and lower surfaces of the workpiece as possible. Since there is no tension control device for high-speed and medium-speed wire-feeding wire-cutting machine tools, wire-feeding wire-cutting EDM machines usually increase the tension of the wire electrode by appropriately increasing the pre-tightening force of the wire winding and tightening the wire electrode during the cutting process. Now some people also use the constant tension mechanism, although it has a certain effect, but due to the slow response speed of the constant tension mechanism, the tension change caused by the instantaneous interference of the wire feeding system is difficult to be eliminated by the constant tension mechanism in time, and the instantaneous shape and position of the electrode wire Changes are still difficult to control, and this kind of constant tension mechanism is more complicated and inconvenient to use, and is rarely used in production practice. With ruby wire stopper. This method can not only limit the deflection and jitter of the electrode wire, but also shorten the actual distance between the guide fulcrum and the workpiece surface, which has a significant effect on stabilizing the spatial position of the electrode wire. However, due to the serious wear and tear of rubies during processing, the service life is not long, and high-durability consumer goods abrasive guides are used. The device adopts a guide made of high wear-resistant polycrystalline diamond with a diameter difference of 0.02MM from the diameter of the electrode wire. After using the guiding device, the spatial shape and position change of the electrode wire is obviously limited, and the machining accuracy and the machining surface quality can be significantly improved. And polycrystalline diamond has high hardness, good wear resistance and long service life. In the case of cutting with small taper (<=3 degrees), a set of high wear-resistance guiding device still has a good control effect on the shape and position changes of the electrode wire after half a year of use, which is used for HS-WEDM by multiple cutting processes. in good condition.

   2. Transformation of high frequency pulse power supply

    The high-frequency pulse power supply used by the previous HS-WEDM was designed based on a one-time cutting process. It is necessary to obtain a high cutting speed and ensure that the quality of the machined surface is not too bad, that is, when the surface roughness of the machined surface is RA <= 2.5UM. Under the circumstance, there is a high cutting speed, the pulse width of the high-frequency power supply is in the range of 4-40US, and the pulse parameter variation range is small, and the multiple cutting is different. When the first cutting is performed, the cutting speed must be stable. More than 100MM square / MIN, and do not care about the roughness of the processed surface, the focus is on stable processing and low electrode loss. For the second and third trimming, it is hoped that a better surface quality can be obtained. To this end, the following transformations were carried out on the high-frequency pulse power supply: the pulse peak current was doubled, the single pulse discharge energy and pulse current rising rate were controlled, the processing speed and processing stability were greatly improved, and the wire diameter loss of the electrode wire was greatly improved. The control is less than 0.02MM after cutting 50000MM square. The second cutting should double the quality of the machined surface on the basis of the first time. Since there is still a large machining allowance at this moment, it is still necessary to pay attention to the cutting speed; the set pulse parameters can ensure the machined surface roughness RA In the range of 1.4-1.7UM. The third is the finishing of the machined surface, which requires setting up a micromachining loop to obtain the desired machined surface quality. For this reason, the pulse width is reduced to less than 1UM to ensure a certain energy output to ensure the speed of light trimming.

    3. Research on multiple cutting process

      3.1 The first cut

  The main task of the first cut is high-speed and stable cutting. The principles for selecting and using the relevant parameters are as follows:

(1) Pulse parameters: High-peak current and high-energy cutting should be selected, and the method of grouping pulses and pulse power supply to increase one by one should be used to control the rising rate of pulse current, so as to obtain better process effects.

(2) Compensation amount F of the center track of the electrode wire:

  F=δ+1/2ΦD+⊿+S

 In the formula: F is the compensation amount, MM;

 δ is the average power discharge gap at the first cut, MM;

 ΦD is the diameter of the electrode wire, MM;

 ⊿ is the machining allowance for the second cutting, MM;

   S is the finishing allowance, MM.

  In the case of high peak current machining, the discharge gap δ is about 0.02UM, and the finishing allowance S is very small, about 0.005UM; and the machining allowance ⊿ depends on the machined surface roughness after cutting. Under the conditions of our test and application, the surface roughness of the first cutting is generally controlled at RA<=3.5MM, and considering the influence of the reciprocating wire cutting stripe, ⊿≈2X(5X0.0035)=0.035MM. In this way, the compensation amount F should be between 0.05-0.06UM. If it is too large, it will affect the speed of the second cutting. If it is too small, it will be difficult to eliminate the reversing stripes left by the first cutting during the second cutting. .

(3) Wire-feeding method: the wire-winding length of the entire wire-storage drum is used for reciprocating wire-feeding, and the wire-feeding speed is 8M/S.

  3.2 The second cut The main task of the second cut is to trim light. The relevant parameters are selected as follows:

(1)Pulse parameters: To achieve the purpose of trimming, it is necessary to reduce the pulse discharge energy, but the discharge energy is too small, which will affect the speed of the second cutting. In the case of taking into account the quality of the machined surface and the cutting speed, the selected The pulse parameters should improve the processing quality by one level, that is, the surface quality of the second cutting should reach RA<=1.7UM. The method of reducing the pulse energy mainly depends on reducing the pulse width, and the pulse peak current should not be too small.

(2) Compensation amount F of the center track line of the electrode wire: Since the second cutting is refined, the discharge gap at this time is very small, only 0.005-0.007MM, and the machining allowance required for the third cutting is very small , only a few microns, the two add up to about 0.01MM. In this way, the compensation amount F at this time can be about 1/2ΦD+0.01MM.

(3) Wire feeding method: In order to achieve the purpose of trimming, it is usually realized by reducing the wire speed. Although reducing the wire speed can reduce the jitter of the electrode wire, it is still difficult to avoid the reciprocating cutting stripes. After adopting a short-range reciprocating wire cutting patent and limiting the feed speed, the reciprocating cutting stripe can be basically eliminated after the second cutting, and the surface roughness RA of the machined surface is in the range of 1.4-1.7UM.

 3.3 The third cutting The main task of the third cutting is finishing, in order to obtain the ideal processing surface quality.

(1) Pulse parameters: Micro-machining pulse parameters should be used, pulse width T0.2 <= 1UM, and corresponding countermeasures should be taken to overcome the influence of parasitic capacitance and parasitic inductance of the line, and ensure the discharge intensity during micro-machining.

(2) Compensation amount of the center track line of the electrode wire. Since the discharge gap at this time is very small, only about 0.003MM, the compensation amount F mainly depends on the diameter of the electrode wire. If the electrode wire is ΦD during fine repair, then F=1/2ΦD+0.003MM.

(3) Wire feeding method: Since the machining allowance left after the second cutting is very small (⊿<=0.005MM), it is a technical problem to ensure that the finishing can be evenly trimmed in the third cutting process. First of all, the stable operation of the electrode wire should be ensured. The previous practice is to reduce the wire speed to below 1M/S, which can greatly reduce the vibration of the electrode wire and obtain a good process effect, but often the phenomenon of unstable processing occurs, which is extremely susceptible to the degree of contamination of the working fluid and its viscosity. Influence, in severe cases, it can even make people feel unable to repair normally. Considering that the working fluid requires a relative movement speed between the electrode wire and the work, in the case of 6M/S, the ultra-short-range reciprocating wire travel method is adopted, so that the length of each reciprocating cutting is controlled within one-third of the electrode wire radius. And limit the maximum feed rate of its processing process, resulting in a good process effect. Using this method to cut three times by different operators on different WEDM machines, all can obtain the effect of RA<=1UM, and the processing surface is glossy without streaks