Electrical discharge machining uses pulsed spark discharge between the tool electrode and the workpiece electrode to generate instantaneous high temperature to erode the metal. Also known as electrical discharge machining, electroerosion machining, and electric pulse machining. Electric discharge machining is mainly used to process various high-hardness materials (such as cemented carbide and quenched steel, etc.) and complex-shaped molds and parts, as well as cutting, grooving and removing tools (such as drills and taps) that are broken in the workpiece hole. )wait.
EDM machine tools are usually divided into EDM forming machine tools, wire EDM machine tools and EDM grinding machine tools, as well as various special-purpose EDM machine tools, such as processing small holes and thread ring gauges. and EDM machine tools for special-shaped hole spinnerets, etc.
Electrical discharge forming machine tool
It is the main type of electric discharge machine tool. According to the structure of the machine tool, it is divided into gantry type, ram type, cantilever type, frame column type and desktop type. Spark forming machine tools can also be divided into ordinary, precision and high-precision spark forming machine tools according to processing accuracy.
EDM machine tools generally consist of a main body, a pulse power supply, an automatic control system, a working fluid circulation filtration system and fixture accessories. The machine tool body includes the bed, column, spindle head and workbench. Its main function is to support and fix the workpiece and tool electrode, and realize the feed movement of the tool electrode relative to the workpiece through the transmission mechanism. The function of pulse power supply is to provide energy for EDM processing. There are relaxation type, thyristor type, electron tube type, thyristor type and transistor type pulse power supply. The transistor type pulse power supply is the most widely used. The automatic control system consists of automatic regulators and adaptive control devices. The automatic regulator and its actuator are used to maintain a certain spark discharge gap during the EDM process to ensure that the machining process proceeds normally and stably. The adaptive control device mainly performs single-parameter or multi-parameter adaptive adjustment of various parameters of the gap state change to achieve the best processing state. The working fluid circulation filtration system is an indispensable part of EDM, and kerosene, transformer oil, etc. are generally used as working fluids. The working fluid circulation filtration system consists of liquid storage tanks, filters, pumps, control valves and other components. Filtration methods include media filtration, centrifugal filtration and electrostatic filtration. Fixture accessories include special clamps for electrodes, oil cups, trajectory processing devices (translational heads), electrode rotating heads, electrode dividing heads, etc.
Wire-cut electric discharge machining
Wire-cut electric discharge machining is a branch of electric discharge machining. It is a process method that directly uses electrical energy and thermal energy for processing. It uses a The moving wire (electrode wire) is used as a tool electrode to cut the workpiece, so it is called wire cutting. In wire-cutting processing, the relative movement of the workpiece and the electrode wire is realized by digital control, so it is also called CNC wire-cut EDM, or wire-cutting processing for short.
(1) According to wire cutting speed: it can be divided into slow wire cutting machine tools and high speed wire cutting machine tools.
(2) According to processing characteristics: it can be divided into large, medium, small and ordinary straight wall cutting type and taper cutting type wire cutting machine tools.
(3) According to the form of pulse power supply: it can be divided into RC power supply, transistor power supply, grouped pulse power supply and adaptive control power supply for wire cutting machine tools.
Commonly used terms for electric discharge machining
(1) Tool electrode
The tool used for electric discharge machining is one of the electrodes during electric spark discharge, so it is called It is a tool electrode, sometimes referred to as an electrode. Since the material of the electrode is often copper, it is also called copper.
(2) Discharge gap
The discharge gap is the distance between the tool electrode and the workpiece during discharge. Its size is generally between 0.01 and 0.5 mm. The gap is larger during rough machining. , smaller during finishing.
(3) Pulse width ti (μs)
Pulse width is referred to as pulse width (also commonly expressed by symbols such as ON and TON), which is added to both ends of the discharge gap on the electrode and workpiece The duration of the voltage pulse. In order to prevent arc burn, EDM can only use intermittent pulse voltage waves. Generally speaking, a larger pulse width can be used during rough machining, and only a smaller pulse width can be used during finishing.
(4) Pulse interval to (μs))
Pulse interval is referred to as interpulse or interval (also commonly expressed as OFF and TOFF), which is the interval between two voltage pulses . If the interval time is too short, the discharge gap will not have time to eliminate ionization and restore insulation, which will easily cause arc discharge and burn the electrode and workpiece; if the pulse interval is too long, the processing productivity will be reduced. When the processing area and processing depth are large, the pulse space should also be slightly larger.
(5) Discharge time (current pulse width) te (μs)
The discharge time is the time for the discharge current to flow in the discharge gap after the working fluid medium breaks down, that is, the current pulse width , which is slightly smaller than the voltage pulse width, and the difference between the two is one breakdown delay td. ti and te have a great influence on the productivity, surface roughness and electrode loss of EDM, but what actually matters is the current pulse width te.
(6) Breakdown delay td (μs)
After the pulse voltage is applied from both ends of the gap, it usually takes a short period of time td before the working fluid medium can be Breakdown discharge, this short period of time td is called breakdown delay (see Figure 3-2). The breakdown delay td is related to the size of the average discharge gap. When the tool underfeeds, the average discharge gap becomes larger, and the average breakdown delay td becomes larger. On the contrary, when the tool overfeeds, the discharge gap becomes smaller, and td becomes smaller. Small.
(7) Pulse period tP (μs)
The time between the start of one voltage pulse and the start of the next voltage pulse is called the pulse period. Obviously tP=ti to
(8) Pulse frequency fP (Hz)
Pulse frequency refers to the number of pulses emitted by the power supply per unit time. Obviously, it is the reciprocal of the pulse period tP, that is,
(9) Effective pulse frequency fe(HZ)
The effective pulse frequency is the effective discharge occurring on the discharge gap per unit time. times, also known as working pulse frequency.
(10) Pulse utilization factor λ
Pulse utilization factor λ is the ratio of effective pulse frequency fe to pulse frequency fp, also known as frequency ratio, that is, effective spark pulse per unit time The ratio of the number to the total number of pulses in the unit time.
(11) Pulse width coefficient τ
The pulse width coefficient is the ratio of pulse width ti to pulse period tp.
(12) Duty cycle ψ
The duty cycle is the ratio of pulse width ti to pulse interval to, ψ=ti/to. The duty cycle is generally larger during rough machining and should be smaller during finishing machining. Otherwise, the discharge gap will not have time to eliminate ionization and restore insulation, which may easily cause arc discharge.
(13) Open circuit voltage or peak voltage (V)
The open circuit voltage is the highest voltage between the electrodes within td before gap opening and gap breakdown (see Figure 3-2) . Generally, the peak voltage of transistor square wave pulse power supply is 60~80 V, and the high voltage peak voltage of high and low voltage composite pulse power supply is 175~300 V. When the peak voltage is high, the discharge gap is large and the productivity is high, but the forming replication accuracy is poor.
(14) Spark sustaining voltage
The spark sustaining voltage is the sustaining voltage during spark discharge on the discharge gap after each spark breakdown. It is generally around 25 V, but it It is actually a high-frequency oscillating voltage (see Figure 3-2).
(15) Processing voltage or average gap voltage U(V)
Processing voltage or average gap voltage refers to the average voltage across the discharge gap indicated on the voltmeter during processing. It It is the average of multiple open circuit voltages, spark discharge sustaining voltages, short circuit and pulse interval voltages.
(16) Machining current I(A)
The machining current is the average current flowing through the discharge gap indicated on the ammeter during machining. It is small when finishing, large when roughing, small when the gap is open, and large when the gap is reasonable or short.
(17) Short-circuit current Is(A)
Short-circuit current is the average current indicated on the ammeter when the discharge gap is short-circuited. It is 20 to 40 times larger than the average current during normal processing.
(18) Peak current (A)
The peak current is the maximum value (instantaneous) of the pulse current during gap spark discharge, and is commonly expressed as Ip in Japan, the United Kingdom, and the United States. Although the peak current is not easy to measure, it is an important parameter that affects processing speed, surface quality, etc. When designing and manufacturing a pulse power supply, the peak current of each power amplifier tube is calculated in advance. Selecting the peak current actually means selecting several power tubes for processing.
(19) Short-circuit peak current (A)
The short-circuit peak current is the maximum value of the pulse current when the gap is short-circuited. It is 20 to 40 times larger than the peak current and is the same as the short-circuit current Is The difference is a multiple of the pulse width coefficient.
With the development of digital control technology, EDM machine tools have been numerically controlled and controlled by micro-electronic computers. The machine tool functions are more complete and the degree of automation is greatly improved, realizing automatic positioning of electrodes and workpieces, automatic conversion of processing conditions, automatic exchange of electrodes, automatic feeding of the worktable, multi-directional servo control of the translation head, etc. The adoption of low-loss power supply, micro-finishing power supply, adaptive control technology and complete fixture system has significantly improved the processing speed, processing accuracy and processing stability, and expanded the scope of application. EDM machine tools are not only developing in the direction of small, precise and specialized, but also in the direction of super large ones that can process automobile bodies and large stamping dies.
Electrical discharge machining technology
(1) Electrical discharge machining machine tool processing technology single electrode method
Using a single electrode to process workpieces, generally used for simple shapes and high precision Undemanding workpiece. Single-electrode machining can also use a horizontal head to achieve rough, medium and fine machining of the workpiece.
(2) Multi-electrode method of EDM machine tool processing
The same workpiece uses multiple electrodes, which are generally divided into coarse, medium and fine processing in sequence. Precision processing.
(3) Electrode decomposition method for EDM machine tool processing
According to the geometry of the workpiece, the electrode is decomposed into several parts, and the main cavity electrode is used to process the main part of the cavity. Then use the auxiliary cavity electrode to process sharp corners, narrow slit cavities and other parts.
(4) EDM machine tool processing standards
Rough machining generally uses larger current and larger on time.
For medium processing, medium current and medium on time are generally used.
Finishing generally uses smaller current, high frequency and smaller on time. Electrical discharge machining is a method that directly uses electrical energy to process parts. EDM equipment should be composed of the following parts: pulse power supply, automatic gap regulator, machine tool body, working fluid and its circulation filtration system. The automatic gap adjuster automatically adjusts the distance between the electrodes to adapt the feed speed of the tool electrode to the corrosion speed. Spark discharge must be carried out in an insulating liquid medium. Foreign laser processing equipment and processes have developed rapidly. We now have 100kW high-power CO?2 lasers and kW-level high-beam-quality Nd:YAG solid-state lasers. Some of them can be equipped with optical fibers for multi-station and long-distance work. Laser processing equipment has high power and high degree of automation. It has generally adopted CNC control, multi-coordinate linkage, and is equipped with auxiliary systems such as laser power monitoring, automatic focusing, and industrial TV display.
The minimum aperture diameter of laser hole making has reached 0.002mm, and automatic six-coordinate laser hole making special equipment has been successfully used to process aircraft engine turbine blades and combustion chamber air film holes, achieving no recast layer and no microscopic holes. Crack effect. Laser cutting is suitable for parts made of heat-resistant alloys, titanium alloys, and composite materials. At present, the cutting speed of thin materials can reach 15m/min, the slit is narrow, generally between 0.1 and 1mm, the heat-affected zone is only 10 to 20 times the slit width, and the maximum cutting thickness can reach 45mm. It has been widely used in three-dimensional aircraft skins. , frames, ship hull frames, helicopter rotors, engine combustion chambers, etc.
Laser welding of thin plates is quite common and is mostly used in the automotive, aerospace and instrument industries. Laser micro-welding technology has become an important means of micro-connection of micro-packaging nodes in avionics equipment and high-precision mechanical equipment.
Laser surface strengthening, surface remelting, alloying, and amorphization processing technologies are increasingly used. The application of laser micromachining in electronics, biology, and medical engineering has become an irreplaceable special processing technology. Laser rapid prototyping technology has developed from the research and development stage to the practical application stage, and has shown broad application prospects.
Domestic research on the application of laser processing began in the early 1970s, but the development rate was slow. Although it has certain applications in laser hole making, laser heat treatment, welding, etc., the quality is unstable. At present, a solid-state laser processing system with optical fiber transmission has been developed, and the simultaneous welding of fiber-coupled three beams and the laser welding of quartz watch cores have been realized. We have completed the development of a laser sintering rapid prototyping principle prototype, and used epoxy polyester and resin sand to sinter powder materials to quickly form typical parts, such as impellers and gears.
In the next few years, laser processing technology should be combined with the pre-research results achieved and based on demand, focusing on laser processing of defect-free air film holes and real-time inspection and control technology, high-strength aluminum (including aluminum lithium, aluminum magnesium ) Research on laser welding technology of alloys, laser powder sintering rapid prototyping technology of metal parts, laser precision processing and laser shock strengthening of important components. Achieve defect-free processing of air film holes in high-temperature turbine engines, which can extend the service life of blades to more than 2,000 hours; use welding instead of CNC machining of aircraft secondary load-bearing components, and use welding instead of riveting for ribbed wall panels; achieve surface strengthening of important parts , improve safety, reliability, etc., so that advanced laser manufacturing technology can play a greater role in the military industry. Electron beam processing technology is becoming increasingly mature internationally and has a wide range of applications. The 40kV ~ 300kV electron guns produced abroad (mainly 60kV and 150kV) have generally adopted CNC control, multi-coordinate linkage, and a high degree of automation. Electron beam welding has been successfully used in special materials, dissimilar materials, complex curves in space, variable cross-section welding, etc. We are currently studying automatic weld tracking, wire filler welding, non-vacuum welding, etc. The maximum welding penetration can reach 300mm, and the weld depth-to-width ratio is 20:1. Electron beam welding has been used for the combined welding of large structures of main load-bearing components such as launch vehicles and space shuttles, as well as important structural parts such as aircraft beams, frames, landing gear components, engine integral rotors, casings, power shafts, and nuclear power plant pressures Manufacture of containers. For example: the F-22 fighter jet adopts advanced electron beam welding, which reduces the weight of the aircraft and improves the performance of the entire aircraft; a large number of load-bearing components in the "Su-27" and other series of aircraft, such as landing gear, load-bearing bulkheads, etc. , all adopt high-voltage electron beam welding technology.
Electron beam welding has been adopted for various types of domestic aircraft and engines, as well as various types of missile casings, fuel tanks, tail nozzles and other structural parts. Therefore, the application of electron beam welding technology is becoming more and more widespread, and the demand for electron beam welding equipment is also increasing.
Foreign electron beam welding machines, represented by Germany, the United States, France, Ukraine, etc., have reached engineering production. It is characterized by the use of variable frequency power supply, which greatly improves the size, noise, and high-voltage performance of the equipment. In terms of the control system, it uses advanced computer technology and adopts advanced CNC and PLC technology to make the control of the equipment more reliable. , the operation is easier and more intuitive.
Foreign vacuum electron beam physical vapor deposition technology has been used in high-temperature anti-corrosion and heat-insulating ceramic coatings on aeroengine turbine blades, improving the thermal shock resistance and lifespan of the coating. Electron beam etching and electron beam irradiation curing resin matrix composite material technologies are in the research stage.
In the future, electron beam processing technology should actively expand its professional fields, closely follow the development of international advanced technologies, and focus on research on key technologies of electron beam physical vapor deposition, electron beam welding of main load-bearing structural parts, and research on electron beam welding of main load-bearing structural parts. Research on electron beam irradiation curing technology, research on key technologies of electron beam welding machines, etc. The surface functional coating has high hardness, wear resistance and corrosion resistance, which can significantly increase the life of parts and has a wide range of uses in industry. Most of the United States and European countries currently use microwave ECR plasma sources to prepare various functional coatings.
Plasma thermal spray technology has entered engineering applications and has been widely used in wear-resistant coatings, sealing coatings, thermal barrier coatings and high-temperature protective layers for key parts of products in aviation, aerospace, shipbuilding and other fields.
Plasma welding has been successfully used in the welding of 18mm aluminum alloy storage tanks. Plasma welding equipped with robots and seam tracking systems has also been put into practical use for welding complex seams in space. Micro-beam plasma welding is widely used in the welding of precision parts. Plasma spraying in my country has been used in the development of weapons and equipment, mainly for wear-resistant coatings, sealing coatings, thermal barrier coatings and high-temperature protective coatings.
Vacuum plasma spraying technology and all-round ion implantation technology have begun to be studied, but there is still a big gap with foreign countries. Although plasma welding is used in production, the welding quality is unstable. In the future, ion beam and plasma processing technology should be combined with the achieved results and focus on new technology research on thermal barrier coatings and ion implantation surface modification according to needs. At the same time, on the basis of the preliminary results achieved, further plasma Welding technology research. The basic principle of ultrasonic processing: Add abrasive suspension between the workpiece and the tool, and the ultrasonic oscillation wave is generated by the ultrasonic generator, which is converted into ultrasonic mechanical vibration by the transducer, so that the abrasive particles in the suspension continuously hit the processing surface, and the hard and The brittle ones are partially damaged by the processed materials and hit down. The machining process is strengthened by the instantaneous positive and negative alternating positive pressure shock waves and negative pressure cavitation on the workpiece surface. Therefore, ultrasonic processing is essentially the comprehensive result of the mechanical impact of abrasives, ultrasonic impact and cavitation.
Rotary ultrasonic processing has been developed on the basis of traditional ultrasonic processing, that is, the tool is constantly vibrating and rotating at a certain speed, which will force the abrasive particles in the tool to continuously impact and scratch the surface of the workpiece , crush the workpiece material into very small particles and remove them to improve processing efficiency.
Ultrasonic processing has high precision, fast speed, wide range of processing materials, and can process complex cavities and profiles. During processing, the contact between the tool and the workpiece is light, the cutting force is small, and no burns, deformation, or damage will occur. Residual stress and other defects, and the ultrasonic processing machine tool has a simple structure and is easy to maintain. Traditional machining is done by manual operation of ordinary machine tools. During processing, the machine tool is moved by hand to cut the metal, and the accuracy of the product is measured by eyes using calipers and other tools. Modern industry has long used computer digitally controlled machine tools for operations. CNC machine tools can automatically and directly process any products and parts according to programs programmed by technicians in advance. This is what we call "CNC machining".
CNC machining is widely used in all fields of mechanical processing. It is also the development trend of mold processing and an important and necessary technical means. "CNC" is the abbreviation of Computerized Numerical Control in English. CNC technology, referred to as CNC (Numerical Control). It is a method that uses digital information to control machine tool movement and processing processes. Machine tools that use CNC technology to implement processing control, or machine tools equipped with a CNC system, are called CNC (NC) machine tools.
The CNC system includes: CNC device, programmable controller, spindle driver and feeding device. CNC machine tools are highly integrated products of machinery, electricity, liquid, gas and light. To control the machine tool, it is necessary to use geometric information to describe the relative motion between the tool and the workpiece, and use process information to describe some process parameters that the machine tool must have for processing. For example: feed speed, spindle speed, spindle forward and reverse rotation, tool change, coolant switch, etc. This information forms a processing file (i.e., a normal CNC processing program) in a certain format and is stored on an information carrier (such as a disk, punched paper tape, magnetic tape, etc.), and is then read in by the CNC system on the machine tool (or directly through the CNC system). Keyboard input, or input through communication), by decoding it, the machine tool moves and processes parts. Modern CNC machine tools are typical products of mechatronics and are the technical basis for a new generation of production technology, computer integrated manufacturing systems, etc. .
The development trend of modern CNC machine tools is high speed, high precision, high reliability, multi-function, composite, intelligent and open structure. The main development trend is to develop intelligent full-function general-purpose CNC devices with open structures in both software and hardware. CNC technology is the foundation of mechanical processing automation and the core technology of CNC machine tools. Its level is related to the country's strategic status and the level of reflecting the country's comprehensive strength. It has evolved with the development of information technology, microelectronics technology, automation technology and detection technology. develop.
A CNC machining center is a CNC machine tool with a tool magazine that can automatically change tools and can perform a variety of processing operations on workpieces within a certain range. The characteristics of parts processed on a machining center are: after the processed parts are clamped once, the CNC system can control the machine tool to automatically select and replace tools according to different processes; it can automatically change the machine tool spindle speed, feed amount, and the motion trajectory of the tool relative to the workpiece. And other auxiliary functions, it can continuously and automatically perform multi-process processing such as drilling, countersinking, reaming, boring, tapping, and milling on each processing surface of the workpiece. Because the machining center can centrally and automatically complete a variety of processes, it avoids human operating errors, reduces workpiece clamping, measurement and machine tool adjustment time, as well as workpiece turnover, transportation and storage time, and greatly improves processing efficiency and processing accuracy. , so it has good economic benefits. Machining centers can be divided into vertical machining centers and horizontal machining centers according to the position of the spindle in space