Cut-surface quality – the pluses and minuses of EDM Essay

Electrical discharge machining (EDM) can produce cut-surfacequality that rivals or even surpasses surfaces finished by conventionalmethods. In fact, one of the major benefits of the latest generation ofEDM equipment–both wirecut and diesinking–is that they often eliminatethe need for secondary finishing operations. This is especiallyimportant with contours and shapes that are difficult to polish. Although EDM is still relatively slow, particularly when ahigh-quality cut surface is required, the newest equipment is capable ofmuch higher cutting speeds than was possible only a few years ago, withno sacrifice in surface quality. Furthermore, advances such as computercontrol, sophisticated software, and automation allow untended EDMoperations, releasing operators for other duties or automaticallymachining workpieces overnight or during a weekend. Defining quality No machined surface is perfectly smooth. It doesn’t matterhow the surface was machined.

An enlarged workpiece cross-section willshow peaks and valleys. These microscopic undulations determine surfacequality and whether a component will function as intended or even matewith another component. Before talking about EDM cut surfaces though, we must reachagreement on what we mean by “high surface quality.” Sincethere are various ways of specifying quality, and since the values canbe expressed in either English or metric, we must be careful inevaluating and comparing surface-quality measurements. For example,expressing surface roughness as so many microns is meaningless unlessthe measurement method also is stated. One way of expressing surface quality is the root-mean-square (rms)method, which consists of squaring measurements taken over all the peaksand valleys, adding the numbers, and taking the square root of the sum.

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This sounds cumbersome and it is, but the result indicates surfaceroughness and can be compared with similarly derived numbers to contrastthe quality of one surface against another. Although this method iswidely used, it overemphasizes maximum deviations that may only rarelyoccur across the surface. Another method gaining acceptance measures the arithmetic mean ofall peaks and valleys. This method measures all deviations and derivesa mean between the most prominent peaks and deepest valleys. For a givensurface, this measurement will be slightly larger than the rms value. A third view of surface quality is maximum roughness, whichmeasures the distance between the highest peak and the deepest valley.Obviously maximum roughness, R.

sub.max in metric or H.sub.

max inEnglish, will be considerably higher than the corresponding rms orarithmetic mean measurement. The accompanying chart compares therelative value of various finish measurements. Process principles To understand how EDM affects cut-surface quality and integrity, wemust first look at how the process works.

Metal is removed by erosioncaused by a controlled electrical spark. There is no direct contactbetween electrode and workpiece. The rate at which metal is removeddepends on the electrical conductivity of the workpiece. One terminal of the power supply is connected to the workpiece, theother to the electrode. The workpiece and electrode are separated by adielectric fluid–usually deionized water for wire-cutting and a specialhydrocarbon for diesinking–which acts as an electrical insulator untilthe spark occurs. The dielectric also cools the work area after thespark ends, and flushes away metal particles before the next sparkoccurs. DC voltage is applied between the electrode and the workpiece. Atfirst, no current flows because the two pieces are insulated by thedielectric fluid, however, an electrical field does build up across thegap.

As the electrode approaches the part and the gap narrows, a pointis reached where the voltage ionizes the dielectric fluid and a sparkjumps the gap. When the spark first occurs, a large amount of energy is released,vaporizing material from the work surface. As current continuesflowing, intense heat melts additional material. When the voltage dropsto zero, current stops flowing, and the spark is quenched. At thispoint material removal ceases.

As soon as the spark is quenched, the vapor bubble begins tocollapse. The dielectric cools the area, solidifying the material thatwas melted. Some of this material is carried away by movement of thedielectric fluid, leaving a small crater at the point where thedischarge occurred.

Some of the melted material is redeposited into thecrater. This recast layer is an important factor in EDM surfacequality. To some extent, this oversimplifies the sequence of events. Theaccompanying drawings provide more detail. There are two importantpoints concerning EDM that should be emphasized here. First, itisn’t simply the current flow that removes material–it isswitching the current on and off that actually vaporizes and melts themetal. Second, the most efficient part of the sequence is the initialdischarge when material is vaporized.

The longer current flows, themore heat builds up and the more material is melted rather thanvaporized. Also, some of the melted material always will be recast intothe cavity. There is presently no EDM technology to entirely eliminatethe recast layer. Newer power supply designs, however, are effective inminimizing the recast layer as well as any heat-affected materialdirectly below it. Pulses and quality Metal removal, as already stated, depends on starting and stoppingthe spark. The amount of metal removed is a function of the energyreleased during the spark, which, in turn, is determined by frontal-gapvoltage (i.

e., voltage between the workpiece and electrode), by theelectric current that flows, and by the time (pulse width) current flowsbefore the spark ends. Older EDM equipment used a bank of capacitors in the power supplyto store energy for the spark. This design is known as a”capacitant-discharge” power supply. The capacitors gatherand store electrical energy until the equipment senses properfrontal-gap voltage, which creates the spark and releases this energy.After the capacitors “dump” their charge, the spark isextinguished and the capacitors begin a recharge cycle in preparationfor the next spark. The problem is that the spark is initiated solely on the basis offrontal-gap voltage sufficient to ionize the dielectric. For one sparkthe capacitors may have become fully charged and will release a maximumamount of energy thereby removing a significant amount of material.

Foranother spark the frontal-gap voltage may be reached before thecapacitors achieve full charge. This spark will remove less material.Consequently, metal removal across a workpiece surface may be quiteuneven–there will be large craters at some points, small craters atother points. The pulse-type power supply was developed to eliminate thisproblem. To control the spark more accurately, engineers designed apower supply where each spark releases the same amount of energy. Inaddition, each new spark is delayed until all the energy in the previousspark has been used. The energy bursts may now occur at a more randomrate, but each time the spark creates an electrical current, the sameamount of energy is released and the same amount of material is removed.

The pulse-generator power supply provides more precise control ofthe spark and enables more efficient use of each spark. This meansevery eroded cavity will have essentially the same diameter and depth.And, although there always will be some material recast into eachcavity, the depth of this recast layer is greatly reduced.

The latest generation of pulse-generator power supplies provideshighly refined cut-surface quality. The power supply furnishes a largenumber of low current, extremely short pulses. Thus each spark is usednearly 100 percent.

Because of the low current and short sparkduration, material is removed primarily by vaporization. The recastlayer and underlying heat-affected zone is now about one-tenth of thatgenerated by a capacitant-discharge power supply. Surface finish qualityis such that secondary finishing operations are usually unnecessary. Ifthis recast layer must be removed from the workpiece in subsequentsteps, the amount of material will be less than with the irregular cutproduced by a capacitant-discharge power-supply system.

Surface quality is especially important when diesinking EDM is usedto make injection molds, pressure casting dies, and other componentswhose cut surfaces must range from smooth-matte to highly-polished.Modern pulse-control power supplies benefit both diesinking and wirecutEDM. With the pulse-generator used in diesinking EDM equipment, ahighly-polished surface with an arithmetic mean roughness of 7 to 10microinches is possible. Such EDM sink-polishing provides a consistent,refined surface which is particularly desirable on delicate materialsand highly detailed components that could be damaged or destroyed bytraditional polishing methods. Machining sintered materials There is no problem with EDM cutting of conventional, homogeneousmetals such as tool and high-speed steel, or even titanium. Nonferroussintered materials, on the other hand, present an entirely different setof problems. A major consideration with EDM cutting of sinteredmaterials is not so much a concern for surface quality as for surfaceintegrity.

These materials include any of the various forms of carbide (i.e.,carbides of silicon, tungsten, titanium, tantalum, and chromium). Also,cubic boron nitride (CBN), because it has a hardness approaching diamondand is less costly than diamond, is being used in some criticalapplications. And then there is the “miracle material of the’80s,” polycrystalline diamond.

Many people feel that EDM and sintered materials are incompatible;that carbide and the other materials cannot be successfully machined bythis method. Others feel that the materials can be EDMed, but that thenecessary allowances, precautions, and limitations create so muchuncertainty that the chances for success are slim and unpredictable.Unfortunately, there is some truth in both viewpoints, although certain”wizards” seem to know a few secrets that they haven’tshared with the rest of us.

In the case of proprietary materials, such as polycrystallinediamond, material composition and special techniques for successfullyEDMing it have been withheld as trade secrets. Only recently has somemeager information surfaced. The problem with electrical discharge machining any of thesesintered products is the structure and composition of the materialitself. Granules (or crystals) of the primary material are heldtogether by a matrix (or binder). The binder is generally much moreelectrically conductive than the granules. Thus, during EDM cutting,electrical current from the spark flows through the binder and aroundthe granules, eroding the binder and cutting the material. The binder, however, does much more than merely hold the granulesin place. The sintering process captures the granules within the binderunder great tension.

When the EDM spark flows through the binder, itreacts electrochemically to the current, releasing some of thattensional force. Following a high-energy EDM pulse, some of thegranules are totally free and will simply fall away. Others are onlypartly held in place and may flake away under moderate pressure. In studying the effects of EDM cutting on carbide materials,researchers discovered that the shape and duration of the electricalpulse had a marked influence on the surface integrity of EDM-cutcarbides. With older power supplies, varying amounts of energyavailable in the EDM spark had inconsistent effects on the cobalt bindermaterial. The pulse-type spark generators that release an identicalamount of energy in each spark limit the electrochemical destruction ofthe binder, resulting in improved surface integrity. For more information on EDM equipment, circle E19.

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