The surface quality of the wire-cut electric discharge machining is closely related to the servo feed control. If the servo feed control system has poor performance and improper operation, it often occurs with fast feeding, slow down, short-circuit back and forth, and even frequent broken wires ( molybdenum wire or brass wire ). At this point, the quality of the processed surface is definitely not good. In order to improve the quality of the wire-cutting surface, the operator should be familiar with the control principle of the wire-cut electric discharge machine, reasonably set the servo-feed control speed, and avoid short-circuit and open-circuit and even broken wires during the short-circuit. The specific process methods are:
(1) Reasonably set the servo feed rate. For the low speed wire cutting machine, the servo feed speed should be controlled in the state where there are few open-circuit waveforms between the anodes and no short-circuit waveform occurs. At this time, the average voltage and current during the inter-electrode machining are relatively stable; During processing, the feed rate of the low-speed wire cutting and the main feed should be appropriately slowed down, allowing 20% to 30% of empty carrier waveforms to appear.
For the high speed wire cutting machine using molybdenum cutting wire , due to the small tension of the electrode wire, the electrode wire jitter easily occurs. The tracking control method is usually adopted, that is, 10% to 20% of the inter-electrode short-circuit waveform is allowed to appear, and almost no empty carrier wave shape occurs. In this way, the cutting speed is reduced, but the instantaneous short-circuit can suppress the electrode wire jitter, which helps to obtain a good quality of the machined surface. When the tracking control is implemented, only the discharge voltage waveform and the inter-electrode short-circuit voltage waveform exist between the poles. Due to the existence of aluminum wire resistance between the two poles, the difference between the two is not large, the average voltage between the poles changes very little, the average current change is not large, so the ammeter and voltmeter pointer on the operation panel is in a stable state, can be regarded as tracking Good control. If an empty carrier wave appears, the average voltage and average current between the poles will change greatly. The current meter and voltage meter pointers on the operation panel will fluctuate greatly. The operator can conclude that the servo tracking control is poor.。
( 2 ) Initial cutting-in control method. In the initial cut-in, the flexible molybdenum wire or brass wire will not be damped, and it will cause severe to jitter due to the action of the discharge force. Operators generally worry about breaking the wire and reducing the servo feed rate. The result is not ideal, especially at high speed. In wire cutting, molybdenum wire breakage occurs from time to time.
The reasonable method is to reduce the jitter of the electrode wire during the initial cut-in, reduce the machining current during the cut-in (ie, increase the pulse interval), and decrease the discharge energy (pulse peak current) without reducing the servo feed speed in processing . After the wire is cut into the workpiece (3mm ~ 5mm cutting), if the machining is stable, the current meter and the voltmeter pointer are all stable, indicating that the servo feed speed is reasonable, and the pulse discharge energy and machining current can be gradually increased.
( 3 ) Servo feed unevenness from time to time. In the stable cutting process, sometimes there are feed control abnormalities, or difficult cutting phenomenon. This phenomenon is not a fault in the wire-drawing system or a non-conductive impurity in the workpiece material. As a result, the quality of the wire-cutting surface is destroyed.
When this happens, the operator must first carefully check whether the wire-drawing system is normal, whether there is loosening of the electrode, the guide wheel swaying and jumping, and poor contact of the electric block. After troubleshooting the molybdenum wire for EDM feed system, the internal uniformity of the workpiece material was analyzed again. If there is a non-conductive impurity inside the workpiece material, irregular cutting stripes will appear on the processed surface.。
5. Improve the smoothness of the molybdenum wire or brass wire movement
The surface quality of WEDM is closely related to the tension and smoothness of the electrode wire. Moderately increasing the preload and smoothness of the electrode will help to improve the surface quality of wire cutting.
(1) Increase the preload of the electrode wire (molybdenum cutting wire or brass wire) appropriately. In the allowable range of tensile strength of the electrode wire, moderately increasing the preload of the electrode wire can make the electrode wire straight, and reduce the jitter amplitude of the electrode wire, and improve the surface quality of the wire cutting processing. Increasing the pre-tightening force is relatively easy for a 1000-speed cnc molybdenum wire cut machine, as long as the tension of the wire tension motor is appropriately increased. However, it is more difficult for a high-speed thread cutting machine: tightening the electrode wire can increase the pre-tightening force of the electrode wire, but after processing for a period of time, the electrode wire will not only extend and extend, but also the wire diameter loss of the electrode wire. This can cause the wire to loosen. Therefore, operators need to be dealt with promptly in the process of processing.
(2) Eliminating the interference of electrode molybdenum wire movement and improving the smoothness of electrode wire movement. Operators are required to regularly check the wear conditions of the support parts of the wire transport system. It is found that the guide wheel has axial turbulence or radial runout; the guide wheel set must be replaced; the serious wear of the V-groove of the guide wheel can also cause the electrode wire to oscillate in the groove, and the guide wheel must be replaced in time; Block wear is serious, it will also directly affect the wire tension, or even broken wire phenomenon, the operator should also be promptly dealt with.
(3) Minimize the span between top and bottom support points. Prior to wire-cutting, firstly adjust the upper support point (Layers), the distance between the workpiece and the surface. In general, the upper and lower supporting points of the electrode molybdenum cutting wire or brass wire (upper and lower wire racks or guides) should be close to the upper and lower surfaces of the workpiece. The distance between the upper and lower surfaces of the workpiece and the upper and lower surfaces of the high speed wire feeder for molybdenum cutting wire is about 10mm.
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