Electric discharge wire cutting can be divided into fast wire cutting and slow wire cutting according to cutting speed. As the surface roughness of the workpiece processed by fast wire cutting is generally within the range of Ra=1.25~2.5 microns, while slow wire cutting can reach Ra=0. 16 microns, and the fixed error, linear error, and dimensional error of the slow wire cutting machine are all fast and good, so the slow wire cutting machine has been widely used when processing high-precision parts.
For equal energy pulse current power supplies, research has shown that there are two important precursors of wire breakage: 1. The sudden increase in spark discharge frequency in a short period of time, due to the high discharge frequency, the local temperature of the electrode wire is too high, leading to wire breakage; 2. The normal spark probability decreases, while the abnormal spark probability gradually increases, which is also a precursor to wire breakage. Due to the increase in wire loss, the electrode wire becomes thinner and ultimately breaks. K. P. Rajurkar et al. pointed out that the sudden change in workpiece thickness during the machining process is one of the main causes of discharge concentration. Therefore, it is necessary to detect the changes in the thickness of the workpiece online, adjust the corresponding process parameters, control the feed rate and discharge frequency of the electrode wire, and obtain the optimal cutting speed under continuous wire cutting.
Due to the short duration of wire breakage precursor and the high real-time control of wire breakage prevention, the selection of control parameters is very important. In online cutting processing, pulse interval amplification can increase the time for the discharge gap to eliminate corrosion products, effectively improving the phenomenon of discharge concentration and greatly reducing the chance of wire breakage. Therefore, pulse interval becomes the preferred control parameter for anti breakage control.
1, Wire breakage factors related to electrode wires
1. Characteristics of electrode wire material
The electrode wire requires good discharge characteristics and high tensile strength. Therefore, brass with low zinc content (10%) is selected as the inner core, and brass with high zinc content is used as the coating electrode wire, which exactly meets the requirements of wire cutting. The electrode wire is made of branded molybdenum wire, which has high precision, high tensile strength, and good quality. Low temperature treatment of electrode wires is also one of the measures to reduce the probability of wire breakage. A company in Ohio, USA compared the electrode wire cooled at -2000C for 24 hours with the electrode wire that did not undergo low-temperature treatment, and found that the former had a 30% lower probability of wire breakage than the latter.
The carrying capacity of the electrode wire during the machining process is determined by the diameter of the electrode wire, so the diameter directly affects the wire breakage rate. Therefore, in the machining process, high-speed cutting electrode wires with appropriate diameter, smooth coating surface, no oxidation spots, or low-temperature treatment should be selected according to actual needs, in order to reduce wire breakage.
2. Silk tension and silk vibration
In low-speed wire EDM machining, maintaining as high and stable tension as possible under the strength limit of the electrode wire can ensure that the wire maintains the minimum hysteresis bending under the explosive force of discharge during rough machining without breaking. Appropriate tension can effectively reduce the vibration amplitude of the wire and keep it stable during the processing.
3. Electrode wire movement speed
Due to the small diameter of the electrode wire in wire cutting (usually 0.1-0.3mm), if the electrode wire moves too slowly, multiple discharges may occur at a certain point on the electrode wire, resulting in excessive erosion at this point. Under the action of wire tension and the explosive force of spark discharge, the wire is easily broken. So, under the condition that the wire allows for a certain number of continuous discharges, the wire speed should be adjusted according to the discharge frequency, taking into account the thickness of the workpiece. The discharge frequency and wire speed of rough machining and precision machining are different. If the diameter of the electrode wire is small, the workpiece is thick, rough machining is required, and the discharge frequency is high, then the wire speed is relatively faster. In actual processing, the electrode wire movement speed provided by the process database of the low-speed wire EDM cutting machine can be referred to.
4. Conductive block
Conductive blocks are often made of silver tungsten alloy, which has good conductivity and wear resistance. During the processing, the conductive block and the moving electrode wire remain in contact, leading to wear of the conductive block. The conductive blocks used in low-speed wire EDM cutting machines should be inspected in a timely manner, removed, and cleaned with cleaning solution to remove any dirt adhering to them. If they are severely worn, they can be replaced or replaced.
5. Waste silk processing
Low speed wire EDM machining is a unidirectional wire cutting process that generates a large amount of waste wire. If the waste wire is not removed in a timely manner, it is easy to generate additional capacitance between the electrodes, and it may directly conduct with the electrode wire in the processing area, resulting in concentrated energy release, causing wire breakage or even short circuit, and even inability to process normally. Therefore, when the waste wire falls, it should be removed in a timely manner. At present, high-end machine tools all have automatic waste wire processing devices. There are two ways to handle it: first, place the cutting device at the waste wire discharge port; The second is to install the wire breakage device on the processing head, and the cut waste wire is discharged through a punching pipe.
2, Wire breakage factors related to working fluid
At present, low-speed wire EDM cutting mostly uses pure water and distilled water, which are cheap and pollution-free. The use of working fluid has two main functions: insulation and cooling. Therefore, the working fluid should have good heat absorption, heat transfer, and heat dissipation functions.
3, Wire breakage factors related to servo control
The servo feed rate should maintain a certain relationship with the erosion rate of the workpiece, that is, maintain a certain value of machining clearance. Because when the servo speed exceeds the erosion speed, frequent short circuits will occur, while increasing the possibility of wire breakage; On the contrary, if the servo speed is too slow and the two poles are biased towards an open circuit, a short circuit will also occur during the machining process due to the open circuit, resulting in a decrease in cutting speed and an increase in surface roughness. So the servo should have uniform and stable feed, avoid crawling, small overshoot, high transmission stiffness, no obvious gaps in the transmission chain, and strong anti-interference ability. The frequency of discharge machining is very high, and the state of the discharge gap is constantly changing, requiring the feed adjustment system to quickly adjust according to the weak signal of the gap state. Therefore, the inertia requirements for the insensitive region, time constant, and movable parts throughout the entire process should be small, the amplification factor should be sufficient, and the transition process should be short.
