FIGURE 6-1 The surface black&white strips in Wire cut EDM
The high-speed wire-cut EDM machine adopts a reciprocating high-speed wire-cutting method.
The surface of the steel workpiece being processed will often have obvious black and white separated stripes (see Figure 6 – l). This phenomenon severely affects the Wire cut EDM processing quality and the further promotion and application of Wire cut EDM technology.
After careful observation and study, it can be found that the appearance of cutting strips is related to the movement of the EDM electrode wire, eg . molybdenum wire or brass wire electrode The strip is black at the entrance for EDM electrode and white at the exit for EDM electrode. This is due to differences in chip evacuation and cooling conditions.
When the electrode wire is moved from the top to the bottom, the working fluid is brought into the slit from the top by the wire, and the discharge outcome is taken out of the processing area by the wire from the bottom. At this time, the working fluid at the upper inlet is sufficient, the cooling conditions are good, the working fluid at the lower outlet is less, the cooling conditions are poor, but the chip removal conditions are better than the upper ones.
The EDM working fluid in the discharge area forms the high pressure gas instantaneously due to the high temperature, and rapidly spreads outwards, making it difficult to discharge the upper galvanic products. At the same time, carbon black and other substances generated by the discharge gather on the upper processing surface to make it appear black. In the lower part, the chip removal conditions are good, the working fluid is small, and the carbon black in the discharge product is also relatively small. Moreover, the discharge often occurs in the vapor, so the processed surface appears white.
In the same way, when the electrode wire is moved upward from the bottom, the lower part is black and the upper part is white. This way, black and white cross stripes are formed on the surface of the reciprocating wire-cut electric discharge machining using molybdenum cutting wire or brass wire electrode (where to buy molybdenum wire? and need molybdenum wire price? Please visit www.edm-taguti.com, or www.filter-supplier.com , China’s biggest molybdenum wire manufacturer and exports self developed and patented “cold-drawing” molybdenum cutting wire.)
This black and white striped pattern usually has a certain influence on the roughness Ra of the machined surface. Depending on the stability of different wire transport system, the white stripe embosses a few micrometers to tens of microns more than the black stripe. Therefore, the discharge gap at the entrance is larger than at the exit, resulting in a white stripe protruding from the black and white stripes.
Based on the above reasons, it is not difficult to understand that the slit by the EDM electrode wire(molybdenum wire or brass wire) is not a straight wall (see Fig. 6-2), instead, the entrance is wide and the exit is narrow. When the wire moves back and forth, the EDM wire inlet and outlet are different in high-low while in the same cutting surface, which will affect the wire cutting machining accuracy and surface roughness.
FIGURE 6-2 The form of EDM electrode wire (molybdenum wire or brass wire) splitting
- The slope formed by the electrode wire movement
- Side view of splitting
6.1.3 Methods and Effects of Limiting Black and White Cutting Stripes
According to the cause of the black and white stripes, it can be seen that the black and white stripes are unavoidable when the wire(molybdenum wire or brass wire) reciprocates. Production practice shows that the depth of the black and white stripes changes abnormal, some are very obvious. Concave -convex are a few tens of microns apart; while some are not obvious, and the convex -concave differences are only a few microns. Moreover, on the same wire cut EDM using molybdenum cutting wire or brass wire electrode, sometimes black and white stripes are obvious, and sometimes are not. Experienced personnel in operation can restrict the outcome of black and white stripes, and those who do not have experience in processing will be in good times and bad, and even work out some jagged surfaces, which is unacceptable to users.
In order to limit the generation of black and white cut stripes, people have spent a lot of energy to explore its laws, and summarized many effective methods during the production practices, including:
(I) adjusting the dynamic balance of the storage tube;
(2) Eliminate the axial turbulence and radial jump of the guide wheel;
(3) During the processing, tighten the filaments at certain time interval to reduce the amplitude of the electrode wire(molybdenum wire or brass wire);
(4) Adopting a follow-up control method that allows a small amount of short-circuit wave forms between the poles to suppress the vibration of the electrode filaments;
(5) Using a spiral nozzle, so the EDM working fluid is evenly sprayed along the axis of the electrode;
(6) Select a strong detersive saponification solution as a EDM molybdenum wire or brass wire cutting fluid.
The above methods all have a certain effect on limiting the black and white cut strips, especially when they are obvious. In other words, when your machine has obvious black and white cuts in processing, it should be checked in accordance with the above methods and take necessary measures. It is believed these methods can improve, which is precious experience people summed up in production.
Using the above methods, although black and white stripes can be limited and improved, they cannot be eliminated completely, and the cut surface will still show some round-trip cutting stripes, which will have an impact on the surface quality of EDM wire cutting and machining adopts molybdenum wire or brass wire. With the development of mold technology, users have requested to improve the quality of EDM wire cutting, eliminating black and white cutting stripes. For this reason, some people have used short aluminum wire of 20 m – 30 m to cut, the surface quality of the machining has been improved obviously, and the black-and-white banding is basically not visible. However, due to the diameter loss of the short aluminum wire, the limited service life, and many problems brought by the frequent commutation, this short aluminum wire processing method can not be widely adopted. Others people use the unidirectional moving discharge machining method in which the electrical discharge machining is performed only in one moving direction of the wire electrode while the other moving direction is not discharged. Although this method can eliminate the round-trip cutting stripes, the average cutting speed in one direction is only 1/2 of the original which is too low. Therefore, it cannot be applied in production practice.
Technical personnel of Shanghai Electronic Equipment Co., Ltd. developed a patented technology for ultra-short stroke program-controlled round-trip cutting to achieve what people expected the no stripe EDM wire cutting using molybdenum cutting wire or brass wire after studied the characteristics and problems of the above-mentioned EDM short-wire cutting and one-way EDM cutting process,. The so-called ultra-short stroke program controlled round-trip cutting technology refers to the ultra-short thread cutting process performed in the entire winding range of the storage tube. The entire process is controlled by the program controller.
The production practice shows that the EDM wire cutting process using multiple cuts is an effective way to solve the surface streaks in the Wire cut EDM process using molybdenum wire or brass wire and the key is still to substantially eliminate the machining surface streaks during the second cutting and trimming. See Section 4. 3 of this book for “Electrical Discharge Machining with Multiple Cutting Processes” (where to buy molybdenum wire? and need molybdenum wire price? Please visit www.edm-taguti.com, or www.filter-supplier.com , China’s biggest molybdenum wire manufacturer and exports self developed and patented “cold-drawing” molybdenum cutting wire.)
6.1.4 Ultra Short Round trip Wire Cutting Technology using molybdenum wire or brass wire electrode
- Control method
The wire (molybdenum wire or brass wire) feed control for the ultra-short round trip EDM cutting process adopts the control method of 5 retreats and 4 reversals. That is, during each round trip, wire only cuts for a short period of time, and only one tens of meters of the reversing electrode wire is moved, but the running time of the cable is slightly longer than the retreat time, thus making the spool move forward., so that the processed electrode wire gradually moves to one end of the storage tube, and then move forward after changing the direction, still maintaining the forward time slightly longer than the retreat time. By such cycle control, ultra-short traverse wire control is achieved in the storage tube as well as wire in the full length of the Jane WRS.
The back and forth rotation of the stock tube is controlled by a digital program controller. The forward rotation time and the reverse rotation time are set by the operator during processing according to the processing conditions and requirements. The original stroke switch of the spool is still reserved for stroke limit protection.
Due to the axial running speed of the wire is closely related to the electrode wire vibration and processing stability. Therefore, it is required that the operating speed of the electrode wire be adjustable within a relatively large range. That is, when cutting thin workpieces, due to good chip removal conditions, it should be as low as possible to reduce the operating speed of the electrode wire, in order to improve the processing stability and surface quality; and when cutting thick workpieces or ultra-thick workpieces, due to difficulty in chip removal, the axial running speed of the electrode wire should be increased. In other words, the threading speed of the electrode wire(molybdenum wire or brass wire) is also set by the operator according to the processing conditions and required tasks.
- Control Principle
The forward rotation and reverse rotation of the stock tube are controlled by the digital controller. The forward rotation time t. and the reverse rotation time nt must be set in advance. So, what is the basis for setting the length of time? According to the experience of cutting short-term wire, many people use the time required to move the electrode wire by 50m or 30m as a basis, and it it can be properly adjusted if the processing effect is not ideal, In fact, though this method of setting the control time according to the length of movement of the electrode wire can perform cutting, or it can have a certain effect in most occasions, once the processing conditions change, for example, the thickness of the processing is changed, or the size of the processing current changes, the effects thereof will be significantly different, and sometimes no stripe cutting cannot be achieved.
The rotation time of the storage drum or the length of movement of the EDM wire (molybdenum wire or brass wire) during one commutation is only a phenomenon and regularity, and it is not essential. According to the above-mentioned causes of black and white stripes, it can be seen that the cutting strips are caused by frequent commutation movements of the EDM molybdenum cutting wire or brass wire electrode, and the control basis should be found based on the cutting pattern shown in FIG. 6-2. For the convenience of discussion, we enlarged the profile of the EDM wire-cutting parts into the shape shown in Fig. 6-3 (a). There is a transitional arc between the micro-convexities, the radius r and the wire diameter d, and the single-sided discharge gap 8 are related, that is
Fig. 6-3 (b) control length in Short cut
As shown in Fig. 6-3 (b), if the wire is cut from 0 to 01 only once during the commutation time of the stock tube, the bumps will differ by h; if only 02 points are cut in one commutation, That is, the cutting length is shortened from the original (l/2) d to (ll 4) d, and the concave-convex difference h will be significantly reduced. That is:
From the above discussion, we can draw the following conclusions:
- When shuttle cutting is performed on an ultra-short stroke, the setting of the reel switch time is based on the length cut during each commutation.
- The quality of the surface processed by the Thousand-high-speed EDM wire(using molybdenum wireor brass wire) cutting is all up and down Ra = l μm so that the length of each reversing cut is controlled at(1/4) d is more reasonable.
(3) In the case of a constant cutting speed, when the thin workpiece is cut, the storage spool changes frequently, and the reversing time is very short. However, when cutting a thick workpiece, the permissible time for each reversing is long, when the cutting thickness is above 100mm. In fact, it is not necessary to use short-range control, and cutting with full-range electrode wire will not appear obvious black and white cutting stripes.
- Cutting speed and processing stability
Ultra-short trip EDM, WIRE cutting using molybdenum wire or brass wire in the original high-speed wire-wound wire EDM machine, people will be concerned about whether its cutting speed and processing stability can be guaranteed, if these two problems cannot be effectively resolved, in the production practice it’s promotion is still a Puzzle because:
- In the past, when the spool was reciprocated, each reversing process could be processed for about 15 seconds, and the reversing process would stop for about 2.5 seconds. In this way, during the entire machining process, the effective machining time is (1 5 / (15 +2.5)) xl00% = 85. 7%; and the ultra-short round-trip cutting is usually only processed for 5s per turn, if the power cut is still needed 2. 5s, then the effective processing time is only (5 / (5 + 2. 5)) X } () () o / o = 66. 7%, that is, the average EDM wire cuttingspeed is only 66.7% of the instantaneous EDM wire cutting
- During ultra-short distance cutting, the wire feeding system rotates frequently at high speed, which may easily cause the damage of the yarn feeding mechanism, or even the phenomenon of wire breakage, which seriously affects the stability and reliability of EDMwire cutting processing.
The way to solve the above problems is to increase the positive and negative conversion speed of the storage spool, minimize no current time of reversing, and take effective measures to ensure stable processing. Shanghai DALIANG Electronic Equipment Co., Ltd. carried out the following technical reforms:
Replaced the original AC motor with a servo-controlled motor with the smallest rotating inertia, and designed the program control circuit for the servo motor rotation time and speed;
@ Reducing the rotational inertia of the storage tube, reducing the wall thickness of the wire string from 2. 5mm to 3 mm to 1.5mm and strictly performing the dynamic balance test;
@ Replace the past gear transmission with a synchronous toothed belt, eliminating the impact damage at the time of commutation and improving the working reliability;
@ The time for the power supply to the spool to switch from the original 2.5s ~ 3s to less than ls, and do not use the open type of control, but the pulse output current in the direction of the storage wire is changing with the speed gradually changes. That is to say, during commutation, the storage wire transitions from forward rotation to reverse rotation, and the pulse output current decreases first with the decrease of the rotation speed, and is zero before and after the rotation speed zero crossing. After the reversal, the pulse current also follows, the increase to the maximum, as shown in Figure 6-4. In this way, the commutation time is greatly shortened due to the inertia reduction of the wire feeding mechanism. For example, the time for each commutation cut is still 5s, and the commutation cut-off is 0. 5s, the effective machining time is up to (5/(5+0.5)) xl00% = 90.9 %, close to the ordinary EDM high-speed wire(molybdenum wire or brass wire) walking. Machine the entire storage EDM molybdenum wire or brass wire electrode plus the effective ratio of T. Moreover, the output of the pulse current is gradually increased, which can help reduce wire breakage and wire loss of the EDM electrode molybdenum wire or brass wire, that is, it can also improve the stability of EDM wire cutting processing.
(where to buy molybdenum wire? and need molybdenum wire price? Please visit www.edm-taguti.com, or www.filter-supplier.com , China’s biggest molybdenum wire manufacturer and exports self developed and patented “cold-drawing” molybdenum cutting wire.)