- Working principle
Comparing to closed-loop wire cut EDM, normal wire cut EDM could be called open-loop wire cut EDM, which doesn’t have error detection ability and compensation of error. So performance of accuracy controls and restraining disturbance is not good, furthermore, it is sensitive to systemic parameters change. While, for closed-loop wire cut EDM, no matter what reasons (internal or external change), once error happens, servo motors system can compensate error to increase accuracy and stability. i.e. there is feedback and compensation of closed-loop wire cut EDM machine, oppositely, there is no such kind ability of normal wire cut EDM.
Regularly, closed-loop wire cut EDM uses servo motors, fast feed speed; and normal wire cut EDM uses stepper motors, low feed speed.
There is not detection device of normal wire cut EDM, and structure is pretty simple;
There is detection device of closed-loop wire cut EDM.
- Stability of performance
Closed-loop wire cut EDM has good stability of accuracy and surface finish;
Normal wire cut EDM is not good enough in stability of accuracy and surface finish.
- Accuracy keeping
Closed-loop wire cut EDM can keep long term accuracy;
The accuracy of normal wire cut EDM may lose after long term use.
For better communication of wire cut EDM machining technology, there is a whole set terminology in the market.
- 1. Wire cut EDM machining
It refers to technology that utilizes spark discharging to erode material to do shape machining upon workipeice.
- Discharging gap or discharging clearance
It refers to the difference between electrode and workpiece while discharging happens. To wire cut EDM machining, discharging clearance is usually around 0.01mm
- Electrical erosions
It refers to stuff that release while electrical discharging. Electrical erosions mainly include metal chip and gas from working solution.
- CNC high speed feed wire cut EDM machining(WEDM-HS)
It refers to high speed feed reciprocal wire cut EDM machining, with electrode wire feed speed 7-12/s
- CNC low speed feed wire cut EDM machining(WEDM-LS)
It refers to low speed single direction feed wire cut EDM machining, with electrode wire feed speed 10-15m/min.
CNC high speed feed wire cut EDM is consisted by main machine and CNC controller. Figure 1-1 is the photo of Suzhou Kingred’s KD series CNC high speed feed wire cut EDM machine. Hereby, we take Kingred’s wire cut EDM as a sample to introduce structure of CNC high speed feed wire cut EDM machine
Figure 1-1; ①-main machine; ②- CNC controller
- CNC controller
CNC controller is composed of CNC system and impulse power system.
① CNC system includes control system and auto programming system. Kingred’s wire cut EDM Controller is equipped with brand PC, which has wire cut EDM machining control and program software, it can design machining track, to realize automatic control of whole machining process. , see figure 1-10,
② Impulse power system, the impulse power source of CNC high speed feed wire cut EDM machine is also called high frequency power, it is the key part of machine, impulse power system convert AC electrical current to high frequency impulse power, to provide a series of rectangular pulse with different frequency width. Impulse power system is very important to wire cut EDM machine, it is the main reason to affect machining performance and quality of machine. It is relative to machining speed and surface finish directly. Impulse power source of CNC high speed feed wire cut EDM machine should be: (1) peak current should be proper; (2) width of high frequency is adjustable; (3) frequency of impulse should be as high as possible; (4) parameters of impulse is adjustable in a large range. See Figure 1-11, the controller of Kingred’s CNC wire cut EDM machine.
CNC high speed feed wire cut EDM is consisted by main machine and CNC controller. Figure 1-1 is the photo of Suzhou Kingred’s KD series CNC high speed feed wire cut EDM machine. Hereby, we take Kingred’s wire cut EDM as a sample to introduce structure of CNC high speed feed wire cut EDM machine.
- Main machine
CNC high speed feed wire cut EDM machine is composed of machine bed, column, working table, wire running system, taper unit, working solution recycle system, Z axis, water-proof cover. As show in figure 1-2
① Machine bed and column
Machine bed and column are two basic structure of CNC high speed wire cut EDM machine, see figure 1-3, it adopt large “T” shape machine bed, “C” structure column, HT250 casting with resin sand casting technology, compact structure, strong rigidity of whole machine. Column is fixed on machine bed as constructional element, machine bed hold whole machine as a base. Machine bed and column had been age treated to remove inner stress, to reduce deformation. Machine bed is strong enough take whole machine, good ability of anti-vibration, less thermal deformation. Manufacturing and assembly of machine bed and column must meet strict requirement of geometric and mechanical accuracy.
② Working table
Working table supports and holds workpiece. Its movement is controlled by two stepper motors(servo motors of closed-loop control wire cut EDM). The while wire cut EDM machining is finished by relative movement between working table and electrode wire. The working table of high speed feed wire cut EDM machine includes X and Y axis slides, adopts precious linear guideway and precious ball screw as moving components. XY cross structure had been used for decades, the mechanical rigidity and controllability of Cross structure had been proved and widely accepted, its design and manufacturing technology is pretty mature today, widely used for many machine tools. See figure 1-4 of cross structure XY axes slides.
③ Wire running system
Wire running system controls electrode wire feed at a certain speed and tension, electrode wire feed reciprocally, and coil electrode wire on the wire drum without overlap, figure 1-5 is diagram of wire running system of high speed feed wire cut EDM machine, this system mainly includes guide, wire drum, power contact, tension control units.
1- Main guide; 2-electrode wire; 3- auxiliary guide; 4-linear guide; 5-guides of tightening; 6-slide block; 7- slide of linear guideway; 8-fixed pulley; 9-wire drum; 10-rope; 11-weight drop; 12-electical block(power contact)
While CNC high speed feed wire cut EDM machine is working, guides need to bear fast feeding speed of electrode wire, normally use rolling bearing as guide. There is strict accuracy requirement of “V” groove of guide, the radius of arc at the bottom of groove must be smaller than radius of electrode wire to avoid transverse movement of electrode wire. Basically, reduce the weight of guide at the condition of that guide has met required strength. Beside, the working surface should have enough hardness and low surface roughness.
Wire drum is one of key units to ensure electrode feeding quickly and where to store electrode wire, see figure 1-6. Wire drum does axial moving while turning at high speed, this way, it can coil electrode wire on the wire drum evenly and tidily without overlap, wire drum is able to turn clockwise and counterclockwise to realize wire feed reciprocally. In order to ensure wire drum turn smoothly, inverter was brought into wire cut EDM machine to control the speed of wire drum turning.
For CNC high speed feed wire cut EDM machine, electrode wire obtain cathode electricity from power feed contact(electrical block), the contact resistance of power feed contact must be very small, otherwise, power feed contact touch the fast feeding for long time, so it must be wear resistant, normally, hard alloy, with good wear resistance and conductivity, is chosen as material of power feed contact.
During process of wire cut EDM machining, electrode is getting loose under impact of direction change, discharging, this situation will affect machining accuracy and surface finish. If there is no tension control unit, it is necessary to tighten electrode manually, when cut large thickness workpeice, cutting quality will getting worse. At present, some high rank CNC high speed feed wire cut EDM machine has weight drop tension control system, it can keep constant tension, save a lot work of operator and increase cutting quality obviously, see figure 1-7 and figure 1-8 wire tension control system of Kingred’s CNC wire cut EDM machine.
④ Taper unit
Taper cutting is realized through taper unit(see figure 1-9).the move axes of taper unit are U and V axes. Usual taper cutting principle is central axis of down guide doesn’t move, fixed, up guide is driven by stepper motor U and V axis and move in four directions. Working table and taper unit are moving simultaneously to realize some angle of electrode.
⑤ Working solution recycle system
Who it works? Pump sucks working solution out of tank, then working solution goes through filter to get rid of unclean stuff; and filtered working solution goes into up pipe and then up and down nozzle separately, then go back to tank through filter net.
It is necessary to change working solution and filter if they affect cutting quality.
According to different wire feed speed, there are two different type wire EDM machine in the market, CNC low speed feed wire cut EDM machine and CNC high speed feed wire cut EDM machine, this classification method is widely accepted in the market.
CNC high speed feed wire cut EDM machine, see figure 1-1, its electrode wire feeds reciprocally at fast speed, regularly 4-11m/s, and electrode is used reciprocally. But fast feed speed of electrode wire will cause vibration and interval stop of direction change, machine quality goes down. High feed speed wire cut EDM machine is original from China, and widely used in domestic market. Nowadays, many China made high speed feed wire cut EDM machine has ability of multi-time cutting, which is unique for low speed feed wire cut EDM machine before, with multi-time cutting ability, surface finish had been improved apparently, up to around 1μm. This kind new machine is called medium speed wire cut EDM machine.
CNC low speed feed wire cut EDM machine, see figure 1-2, its electrode wire feeds slowly and continuously to one direction, regularly 0.2m/s, electrode won’t be used again after electrical discharging, work smoothly, evenly, less vibration, cutting quality is pretty good, such king wire cut EDMs are mainly produced in Japan, Switzerland, Japan and etc. CNC low speed feed wire cut EDMs are mainly used for machining products with very strict requirement of accuracy and surface finish.
Wire cut EDM machining is mainly used to process various punch tie, plastic mold, Powder metallurgy mold and etc, which have 2D and 3D faces combined, or components. It can also cut various sample plate, magnetic steel, Silicon Steel Sheet, semi-conductive material or precious metal. Furthermore, it is able to do tiny machining, abnormal shape groove or machining of standard defect of sample parts, widely used in electrics, precious machine tools, light industry, army industry and so on.
- Mold machining
Wire cut EDM is widely used to machine various molds, such as punch die, squeezing die, powder metallurgy mold, bend mold, plastic mold(see figure 1-1). Among these different kind molds, cutting punch die take a great share, to precious punch die machining, wire cut EDM machining is a indispensable technology. By adjusting different compensation value while programming, wire cut EDM can cut terrace die, punch plate, stripper plate and etc, it is easy to meet the requirement of mold fitting clearance and machining accuracy.
Advanced punch dies have characteristics of complicated structure, big producing difficult, high accuracy, long life, high efficiency and low consumption ratio of material. These new advanced technology of mold also bring continuous development of wire cut EDM technology,
To mechanical parts machining, wire cut EDM is suitable to machine parts with large varieties and small quantity, special material which is not easy to processed by conventional machines, special gear, forming cutting tools, various shape holes, please check figure 1-2.
When try to make new product, use wire cut EDM machine to cut parts out of workpiece, there is no need to make molds, producing time is shortened obviously, at the same time, cost is reduced. Besides, as it is easy to adjust electrical parameters of wire cut EDM, so it is possible to overlap thin plate and cut together to increase machining efficiency.
And wire cut EDM machine can also machine parts with tiny gap, narrow slit and complex shape.
Advantage of wire cut EDM machining:
- Wire cut EDM machine is able to machine material, with high hardness, high strength, high fragility and high tenacity, which is not easy to be or can’t be machined by conventional method, even some semi-conductive materials are also can be processed by wire cut EDM machine.
- Thanks to tiny size of electrode wire, wire cut EDM can machine small abnormal shape holes, small gap or job with complicated shape.
- There is very little heat impact on the surface of workpiece, suitable to process some heat sensitive material; at the same time, as impulse energy concentrate on very small area, machining accuracy is higher.
- During machining, electrode wire doesn’t touch workpiece, there is no physical cutting force, it is good to process workpiece with low rigidity.
- As the cutting path is very small, there is very less removal ratio of material, so use ratio of material is pretty high.
- Comparing to EDM spark erosion machine, electrode wire replaced formed electrode, saving a lot of cost of designing and making electrode and prepared period is shortened obviously.
- Using water-based working solution, clean and safe, especially environment-friendly.
- Utilize electricity to do machining, electrical parameters are controllable, it is convenient to realize whole process automatic.
Disadvantage of wire cut EDM machining:
As the electrode needs to go through workpiece, so it can’t machine blame hole kind or step plane kind workpiece, besides, machining efficiency of wire cut EDM machine is compare to other conventional metal processing equipments.
Main difference between wire cut EDM and EDM sinker machine:
Wire cut EDM uses electrode wire to do axial feeding;
EDM sinker machine uses machined electrode with same shape as of cavity need to make.
CNC wire cut EDM machine puts impulse voltage between electrode wire and workpiece through impulse source, controlled by servo system, to get a certain gap, and realize impulse discharging in the working liquid between electrode wire and workpiece. Numerous tiny holes appears due to erosion of impulse discharging, and therefore get the needed shape of workpiece( as show in figure 1-1).
Electrode wire is connecting to cathode of impulse power source, and workpiece is connecting to anode of impulse power source. When workpiece is approaching electrode wire in the insulating liquid and gap between them getting small to a certain value, insulating liquid was broken through; very shortly, discharging channel forms, and electrical discharging happens. And release huge high temperature instantaneously, up to more than 10000 degree centigrade, the eroded workpiece is cooling down swiftly in working liquid and flushed away. As show in figure 1-2
Three basic conditions that wire cut EDM work correctly:
- The gap between electrode wire and workpiece should be certainly maintained in a required range. Within this range, not only impulse power can break through insulating liquid to create spark discharging, but also the eroded workpiece can be flushed away after discharging process. If gap is too big, insulating liquid can’t be break through,, and there will be no spark discharging; if gap is too small, short circuit is easy to happen, no spark discharging neither.
- The procedure should happen in the liquid with insulate capacity, for example saponification and deionized water, the liquid could act as medium of discharging channel and provide cooling and flushing.
- Electrical discharging should be short time impulse discharging,. As with short discharging time, the released heat won’t affect inside material of workpiece, and limits energy to a tiny field and keep characteristics of cool machining of wire cut EDM machine.