Speciality machining: Chapter 3: Wire-cutting by EDM

1, Chapter 3 is about EDM wire cutting processing, the time is 2017.04.14, the material is 2014, the first section elaborates on theProcessing PrincipleThe characteristics and application scope of the definition of the Soviet Union for the 1950s, the principle is to use a fine metal wire, like copper wire, molybdenum wire, the use of numerical control technology, can be processed in a variety of two-dimensional, three-dimensional surface. It is divided into fast walking wire and slow walking wire, fast walking wire is reciprocating high-speed walking wire, speed of 810m / S, which is unique to our country; slow walking wire is a one-way low-speed walking wire, speed of 0.2m / s, is the main foreign machine. Including fast-wire EDM wire-cutting machine tools, tuning fork fast-wire EDM wire-cutting machine tools, column fast-wire EDM wire-cutting machine tools, liquid-impregnated fast-wire, wire speed is 810m/s. There are also slow-wire EDM wire-cutting machine tools, vertical slow-wire EDM wire-cutting machine tools, workpiece automatic threading mechanism, horizontal slow-wire EDM wire-cutting machine tools, immersed in the liquid-type slow-wire, the walking speed of 0.001 - 0.2 m/s. 0.001 - 0.2m/s. It is characterised by the existence of common features with EDM forming processing, i.e. electric.

2, involving pressure, current waveform, processing mechanism, productivity, surface roughness and other process laws, as well as the machinability of materials. The difference is that the fine wire: small pulse width, small average current, for medium-finish processing, the use of positive water (water-based) working fluid, which does not burn and can be unmanned processing; there is an open-circuit electrolysis effect, roughness arc discharge is generally unstable. Electrode relative movement of a variety of discharge state coexistence, showing “loose contact” type of light pressure discharge phenomenon, there is an insulating film medium, through the friction to achieve the discharge, without the need for shaped electrodes, can save money, to achieve rapid, flexible processing, wire fine can be used for processing micro-fine shaped holes, narrow slits, complex shapes of the workpiece as well as for precious metal processing. The loss per unit length of the electrode is relatively small, and the impact on machining accuracy is equally small. The range of applications covers electrodes for EDM forming of moulds and dies, which have complex shapes and are made of high-hardness copper-tungsten or silver-tungsten, and are used for machining parts, e.g. for trial production of new products. The machine body (main unit) consists of the wire EDM machine, as well as the numerical control, the pulse power supply, and the working fluid circulation system. The second section is about EDM wire.

3, an apparatus for cutting processing, wherein the machine tool body part contains, a bed, a coordinate table, a wire walking mechanism, a taper cutting device, which has a guide wheel offset wire holder, a guide wheel oscillating wire holder, and a double coordinate axis linkage device; with respect to the pulse power source, the pulse power source used for wire cutting processing by EDM, and the pulse power source used for forming processing by EDM, is the same in principle, however, due to the By the processing surface roughness and the electrode wire allowed to carry current limitations, wire cutting processing pulse power supply pulse width is relatively narrow, 260 s, a single pulse energy, average current of 15 A, usually smaller. Therefore, wire EDM has been using positive polarity processing. (B), pulse power supply: transistor rectangular wave pulse power supply, (B), pulse power supply: 2. high-frequency grouping pulse power supply, (B) pulse power supply: 3. energy-saving pulse power supply, 4. low-speed wire walking wire cutting processing pulse power supply, wire speed is low, electrode products of the chip removal effect is not good, there are expensive equipment, shall have a higher productivity, to provide a narrow pulse width, the large peak.

4, for the value of the current (c) related to the working fluid circulation system: First, the fast walking wire cutting has an insulating effect, play a cooling effect, but also to achieve the chip removal function. Secondly, slow wire cutting with insulation, chip removal operations can be carried out to achieve the purpose of cooling, temperature control, to complete the conductivity control. Fast walking wire wire cutting machine tool has a working fluid circulation system, slow walking wire cutting machine tool also has a working fluid circulation system, the third section of the contents of the control system as well as programming focus, focus on the experiments, the fourth section is the factors affecting the wire cutting process indicators, a talk about the main process indicators of the wire cutting processing, cutting speed is the area of the cut out of the centre line of the centre line in millimetres per minute to measure, the highest cutting speed is not counted! Direction as well as cutting quality and other aspects of the fast walking wire speed range is 80 to 180 square millimetres per minute. Cutting efficiency is per ampere of current can reach 20 square millimetres per minute amperage, surface roughness, fast-walking wire is Ra5 to 2.5 microns, the best case is Ra1 microns, slow-walking wire is Ra1.25 microns, the best is Ra0.04 microns, as well as the amount of electrode wire loss, the fast-walking wire electrode wire diameter reduction is generally so.

5, dimensional accuracy, shape accuracy, positional accuracy of less than 0.01mm machining accuracy, fast-walking wire accuracy range is 0.01 to 0.02mm, slow-walking wire accuracy range is 0.002 to 0.005mm. two, the impact of electrical parameters, 1. Pulse width ti: increase the machining speed can improve the surface roughness, but it will deteriorate, the general ti is equal to the grouping of pulses in 260 seconds can be small to Pulse interval t0: reduce the average current will increase the cutting speed, too small will produce arc, resulting in wire breakage, generally take t0 is equal to (4 to 8) ti. punctuation order may be wrong, you can adjust according to the actual situation. 3. open-circuit voltage if you increase, then the machining gap will be increased, in this case the chip removal will be easy to cut the speed of speed is also fast, but there is a disadvantage in terms of stability, that is, the wire will vibrate, wire loss, the wire loss of the wire, the wire will vibrate, the wire will be damaged. If the peak discharge current is increased, the cutting speed will be faster, but it will become rough, and the wire loss will be bigger than that of the wire, and even the wire will be broken. Generally less than 40A, the average current is less than 5A, slow-walking wire is a very narrow pulse width, electrode wire thick and a time to use more than 100A, and even . Discharge waveform is a high-frequency grouping pulse, the process is good, the current waveform front rises slowly, the electrode wire will have loss.

6, in the case of a small amount of consumables but the pulse width is narrow, this time the need for a steep leading edge, the purpose is to ensure that the processing efficiency. Third, the impact of non-electrical parameters. 1, the electrode wire and its moving speed has an impact on the process indicators. Molybdenum wire, whose diameter ranges from wire diameter to millimetres. Apply tension, can reduce the vibration of the wire, and thus improve the accuracy and processing speed, to prevent tension fluctuations need to use constant tension device. Coarse wire, with a large processing speed, wire speed faster, conducive to the working fluid brought in, the products of galvanic corrosion can be quickly discharged, but too fast will lead to increased mechanical vibration, precision, processing speed and roughness deterioration. Walking wire speed of m per second is slow walking wire, the largest case is less than mm of brass wire for roughing, the smallest case is more than mm of molybdenum wire, tungsten wire. 2, the thickness of the workpiece and the material has an impact on the process indicators. When the thickness is thin, the working fluid is easy to enter, the chips are easy to discharge, which is conducive to the elimination of ionisation, and the machining process is more stable. When the thickness is too thin, the electrode wire will shake, which will adversely affect the precision and roughness. When the thickness is larger, the working fluid is difficult to enter, the processing is unstable, the electrode wire is not easy to shake, the precision is higher, and the roughness is better. In terms of cutting speed, it speeds up as the thickness increases and then decreases, and there are difficulties in flushing liquid and chip removal with maximum values.

7. material affected by the melting point, affected by the vaporisation point, affected by thermal conductivity and other factors, such as emulsion, for example, for copper stable, fast, for aluminium stable, fast, for quenched steel stable, fast; stainless steel stability is poor, speed is low, surface is poor, magnetic steel stability is poor, speed is low, surface is poor, unquenched high carbon steel stability is poor, speed is low, surface is poor; cemented carbide is more stable, speed is low, the surface roughness of the surface is small. 3. Preset feed speed has a great impact on the process indicators, its impact is reflected in the tracking of etching speed, but also reflected in the maintenance of machining gap constant in the optimal value, which in turn affects the machining accuracy as well as surface quality. If the speed is too fast, there will be frequent short-circuit conditions, resulting in slower cutting, surface roughness, slit burnt yellow, or even broken wire; if the speed is too slow, it will be biased open circuit, sometimes open and sometimes short-circuit, slit also burnt yellow. And in the best state, it is the current, the voltmeter needle stable and immobile, to ensure that the processing speed and surface quality. Fourth, a reasonable choice of electrical parameters. 1 when the requirements of high cutting speed, the need for high no-load voltage, short-circuit current, pulse width, but this is subject to the surface roughness, the constraints of the dissipation of ionisation. 2 when the requirements of the surface roughness is good, if the thickness of less than 80mm, select the grouping of the pulse wave power supply, the ability to reduce the individual pulse. 3 if the requirements of the electrode filament loss is small, the pulse front should rise slowly. 4 for the Thickness requirements, thin workpiece required parameters smaller, thicker workpiece required parameters to be enlarged appropriately. V. Reasonable adjustment of the frequency converter feeding method, if too large, too small, the processing state will be partial short-circuit, circuit breaker. Adjustment methods are as follows: First, through the oscilloscope for observation, will find that the processing wave is the most intense, while the other waves are quite light; Second, the use of voltmeter and ammeter in accordance with the observation and analysis of the processing state of the method, the pointer swing is the smallest; Third, based on the processing current and the ratio of the short-circuit current to regulate the processing current, the ammeter indicated by the average current compared to the short-circuit current, the short-circuit current is the pulse power supply Short-circuit current is the pulse power supply short-circuit current, when this ratio is in seventy per cent to eighty per cent, it will be close to the best state, processing will become stable, cutting speed is also the highest.