Until recently, it was considered impractical, if not downrightfoolhardy, to operate an EDM unit untended around the clock. Threefactors prompted this feeling: * High wear necessitated frequent electrode changes. * Less than reliable technology made predicting results almostimpossible.
* Unsophisticated generator controls resulted in DC arcing, whichfrequently caused fire or damaged workpieces. Happily, recent advancements in die-sinking technology haveovercome these problems and untended operation is becoming quite common.In some cases, continuous machining for up to 300 hours without operatorassistance is both possible and practical with virtually no risk to themachine or the workpiece. Many of the difficulties associated with untended operation werefirst overcome in 1977 with the introduction of EDM units with orbitingquills and integrated adaptive control. The finished cavity intraditional diesinking is slightly tapered with the deeper portionshaving a rougher surface finish than the part nearer the surface. Thisis caused by residual particles that continue the machining process asthey are flushed along the sides of the electrode (Figure 1).
This phenomenon is accentuated during finishing operations becauseof high-wear characteristics in low-amperage finishing modes. Since thefrontal gap (sparking distance) is smaller than the lateral gap, mostmachining is done across the leading electrode surface. The subsequentwear creates a distortion in cavity shape (Figure 2). Advantages provided by an orbiting system include improvedflushing, reduced electrode wear, and shorter finishing times.
It alsominimizes the effect of residual particles reducing the tapering action.Another advantage is that the sides and bottom of the cavity will haveidentical surface finishes and electrode wear will be evenly distributed(Figure 3). Prior to the introduction of a fully automatic adaptive control,EDM required continuous monitoring and fine tuning.
Because idealmachining conditions seldom exist, conditions that result inshort-circuits, abnormal discharges, and other operating difficultiescan develop quickly. These and other dangers associated with DC arcingruled out untended machining. The solution seemed to lie in optimizingeffective machining current.
This quest led to developing fullyautomatic adaptive controls, which today are capable of performing thefollowing functions: * The control breaks down the succession of machining pulses intoshort trains of pulses with an interval of “off” time. Thisis done while maintaining the total pulse interval time. Throughcontrol of these pulses effective machining current is optimized. * The electrode is given a high-speed automatic pulsation movementuntil any machining abnormalities are eliminated.
* Machining is stopped before the workpiece is damaged if the aboveoperations are unsuccessful. Adaptive controls also adjust secondary parameters resulting inless electrode wear and faster machining rates. Setting errors arereduced and repeatability is enhanced. Abnormal discharges, which candegenerate into destructive arcs, are totally eliminated.
An example Figure 4 shows a cavity being machined for a universal jointforging die using both orbiting and fully automatic adaptive control.In the past, these dies were produced using traditional techniques.Holes in the electrode were necessary to facilitate flushing during theEDM process. These holes left small posts in the finished cavities thathad to be removed in a secondary operation. The ability to orbit the electrode under fully automatic adaptivecontrol eliminated the need for secondary operations. One electrode wasused for both roughing and finishing each cavity to a 4.5 micron RA(arithmetic mean) finish. Machining was performed completely untended.
Shortly after the introduction of orbiting and automatic adaptivecontrol came the integration of CNC for table movement, orbit, andgenerator settings. The EDM time for single-cavity molds can be greatly reduced by CNC.All electrodes necessary to rough and finish a cavity can be shank mounted for use with automatic electrode changers. This reduces setuptime and increases the efficiency of both the operator and the machine. Another example An excellent example of the capabilities of CNC applied to EDM isthe production of a keyboard mold, Figure 5. The mold consists of 96cavities and required 300 hours to machine. Estimates are thatproduction costs were cut in half by using CNC to automate the EDMoperation.
Apart from initial programming, every stage of productionwas automatic with no need for supervision. A wirecut EDM unit produced the electrolytic copper electrodes forthe diesinking operation. The 96 cavities on the cavity half of themold are square and measure 0.160″ x 0.160″ x 0.240″deep.
On the core half, they are U, L, or square shaped and containedin a pocket 0.520″ x 0.520″ x 0.480″ deep with a 2-degreetaper on all sides. Accuracy was specified within 0.
0004″, both insize and distance between cavities. Surface finish was 1.12 micron RA. This precision mold was finished two months ahead of the expecteddate. The total machining time of 300 hours–a record in its ownright–is even more impressive because there was no operatorintervention. Electrodes were changed automatically.
A third example Another application for CNC diesinking EDM is multiple-workpieceproduction. Figure 6 shows how knob mold workpieces are fixtured in aholder block that was designed for multiple-cavity work. The electrodesare mounted on shanks. Flushing is through the electrode. A graphiteelectrode is used to rough cut the cavities. The graphite roughingelectrode is exchanged for a copper finishing electrode automatically.A 0.
40 micron RA surface finish is attained. All generator settings, table movements, tool changes, andtranslations are preprogrammed. The total machining time is 1.
5 hoursper cavity. This includes EDM polishing of the cavities that eliminatesthe need for hand polishing. Another benefit of polishing with EDM isthat it preserves the dimensional integrity of the machined cavity. Recent developments in controls and software have shattered many ofthe old concepts of diesinking EDM. At the forefront is contour EDMing,which permits simultaneous machining of the X, Y, and Z axes in either apositive or negative direction. This is a significant contrast to thetraditional “sinking” application. Programmed cycles such as vectoral, conical, orbital, directional,and others have simplified EDM diesinking. Further, advances intooling, fixturing programming, and integration of machining strategieswill continue to expand use of CNC diesinking EDM.
In the machine-tool industry, there has often been an inherent fearof anything that breaks away from tradition. This is especially truefor EDM. Today, however, the impact of CNC on EDM is reshaping theframework of practical applications. Everyday we see new applications, and new approaches to oldapplications, that eliminate many former restraints. It’s a timeof change and overwhelming growth in an industry whose potential is justbeing realized.
For more information on EDM equipment, circle E67.