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1、Chapter 7 EDM machine tools,7.1 Sinker EDM machine tools 7.2 Wire EDM machine tools,7.1 Sinker EDM machine,7.1.1 Work principle of EDM The basic principle of EDM is electrical erosion, which is known as removing metal from workpiece surface by electrical spark thus obtaining certain shape, dimension
2、s, surface qualities of a workpiece.,Tool electrodes,Sinker EDM processing,Power (2) generates electrical pulses on workpiece and tooling electrode (4); Automatic feeding mechanism (3) keep a certain clearance between tool and workpiece,Stages of EDM process,Fig 7-4 electrical erosion surface,During
3、 each pulse peak, electrical arc will be produced where the insulation is the weakest or the clearance is minimum to vaporize materials both on workpiece and tool.,Machining speed,Each spark produces a temperature between 8,000 and 12,000C; Machining speeds of EDM depends on spark energy. Material r
4、emoval rates range from 1 to several thousand mm3 per minute. Energetic sparks result in fast machining but rough surface finish.,7.1.2 Features of EDM 1. Advantages,(1) Tool material hardness is unimportant since ultra-high temperature of electrical arc is able to melt or vaporize any material. The
5、refore, EDM is suitable for machining any conductive material regardless of hardness, brittleness, toughness （韧度）and melting point.,Limits of hardness in conventional machining,No hardness limit in EDM,(2) No cutting force during machining since no mechanical contact between electrode and workpiece,
6、 therefore, it is suitable for machining those workpiece that has special structure, e.g. thin-wall, narrow slot, non-round holes etc. (3) Rough and finish machining can be continually performed by adjusting pulse width.,Alloy mould of car headlight,Easy automation of control and machining due to di
7、rect machining by electric energy. Reduced risk of hazardous dust particles from material since particles are flushed away to the filter.,Machining under fluid,Development of a product by EDM,Task: Making plastic spoons on a mass production,2) The electrode is then mounted on an EDM machine, togethe
8、r with the workpiece in steel that is going to become the plastic injection mold.,1) The design has to be worked out, and the shape of the spoon reproduced on a copper or graphite electrode.,5) Finally the workpiece with the impression is mounted on a plastic injection machine. After everything is s
9、et up, production can start.,3) The electrode will penetrate into the workpiece by electrical discharge erosion.,4) Creating a negative impression of the spoon.,2. Shortages of EDM,Electrode must be made first. It is undesired since electrode making is expensive and time consuming. Machining area ar
10、e unfavorably hardened. Though machining area is small, hardened layer due to fast cooling after heating of ultra high temperature may crack. Discharge clearance produces error. The clearance must exist for electrical arc generation, however, it affects machining accuracy. Machining accuracy depends
11、 on electrode erosion. Eroded electrode needs repair or replacement. Though EDM has many disadvantages, it is more suitable for metal cutting compared to conventional machine tools,7.1.3 Application of EDM in mold manufacturing Holes and cavities manufacturing Orifice manufacturing (d0.1mm) (3) Surf
12、ace hardening (4) Pattern carving,7.1.4 Basic construction of EDM machine An EDM machine consists of four systems:,Fig 76 Basic construction of EDM machine,Machine tool reality Pulse generator (power unit) Automatic adjusting system Circulation and filtration system for work fluid.,1. Machine tool r
13、eality Machine tool reality consists of bed, column, worktable and spindle head. It is a framework to ensure relevant position between electrode and workpiece.,Fig 7-7 machine tool reality and mechanical transmission system of an EDM machine,(1) Column and bed As the basic components, they have the
14、requirements: Enough Rigidity; Perpendicularity between bed, worktable and column;,(2) Worktable It holds work fluid flume, and clamps workpiece; Worktable is driven by servo motor through ball screw-nut transmission.,(3) Spindle head As the actuator of the automatic adjusting system that controls t
15、he clearance between electrode and workpiece, Spindle head is a key component of an EDM machine. Requirements of spindle head: Simple structure Short transmission chain Enough acceleration for feeding and withdrawing Good linearity of motions,2. Pulse generator (power unit) Pulse generator converts
16、industrial AC to unilateral pulse current to supply energy for EDM. Pulse properties determine efficiency, stability, accuracy and roughness of EDM. Basic requirements of a pulse generator are: (1) Enough discharge energy to ensure metal melting or vaporization; (2) Wide range of pulse parameters (e.g. pulse width, pulse interval, peak current) for rough and finish machining; (3) Durable electrode. Relevant wearing should less than 0.5%; (4) Rectangular wave pulse is adopted. Steep edge
