Automating the diesinking process.
* High wear necessitated frequent electrode changes.
* Less than reliable technology made predicting results almost impossible.
* Unsophisticated generator controls resulted in DC arcing, which frequently caused fire or damaged workpieces.
Happily, recent advancements in die-sinking technology have overcome these problems and untended operation is becoming quite common. In some cases, continuous machining for up to 300 hours without operator assistance is both possible and practical with virtually no risk to the machine or the workpiece.
Many of the difficulties associated with untended operation were first overcome in 1977 with the introduction of EDM units with orbiting quills and integrated adaptive control. The finished cavity in traditional diesinking is slightly tapered with the deeper portions having a rougher surface finish than the part nearer the surface. This is caused by residual particles that continue the machining process as they are flushed along the sides of the electrode (Figure 1).
This phenomenon is accentuated during finishing operations because of high-wear characteristics in low-amperage finishing modes. Since the frontal gap (sparking distance) is smaller than the lateral gap, most machining is done across the leading electrode surface. The subsequent wear creates a distortion in cavity shape (Figure 2).
Advantages provided by an orbiting system include improved flushing, reduced electrode wear, and shorter finishing times. It also minimizes the effect of residual particles reducing the tapering action. Another advantage is that the sides and bottom of the cavity will have identical 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 ideal machining conditions seldom exist, conditions that result in short-circuits, abnormal discharges, and other operating difficulties can develop quickly. These and other dangers associated with DC arcing ruled out untended machining. The solution seemed to lie in optimizing effective machining current. This quest led to developing fully automatic adaptive controls, which today are capable of performing the following functions:
* The control breaks down the succession of machining pulses into short trains of pulses with an interval of "off" time. This is done while maintaining the total pulse interval time. Through control of these pulses effective machining current is optimized.
* The electrode is given a high-speed automatic pulsation movement until any machining abnormalities are eliminated.
* Machining is stopped before the workpiece is damaged if the above operations are unsuccessful.
Adaptive controls also adjust secondary parameters resulting in less electrode wear and faster machining rates. Setting errors are reduced and repeatability is enhanced. Abnormal discharges, which can degenerate into destructive arcs, are totally eliminated.
Figure 4 shows a cavity being machined for a universal joint forging 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 the EDM process. These holes left small posts in the finished cavities that had to be removed in a secondary operation.
The ability to orbit the electrode under fully automatic adaptive control eliminated the need for secondary operations. One electrode was used 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 adaptive control came the integration of CNC for table movement, orbit, and generator 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 setup time and increases the efficiency of both the operator and the machine.
An excellent example of the capabilities of CNC applied to EDM is the production of a keyboard mold, Figure 5. The mold consists of 96 cavities and required 300 hours to machine. Estimates are that production costs were cut in half by using CNC to automate the EDM operation. Apart from initial programming, every stage of production was automatic with no need for supervision.
A wirecut EDM unit produced the electrolytic copper electrodes for the diesinking operation. The 96 cavities on the cavity half of the mold 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 contained in a pocket 0.520" x 0.520" x 0.480" deep with a 2-degree taper on all sides. Accuracy was specified within 0.0004", both in size and distance between cavities. Surface finish was 1.12 micron RA.
This precision mold was finished two months ahead of the expected date. The total machining time of 300 hours--a record in its own right--is even more impressive because there was no operator intervention. Electrodes were changed automatically.
A third example
Another application for CNC diesinking EDM is multiple-workpiece production. Figure 6 shows how knob mold workpieces are fixtured in a holder block that was designed for multiple-cavity work. The electrodes are mounted on shanks. Flushing is through the electrode. A graphite electrode is used to rough cut the cavities. The graphite roughing electrode is exchanged for a copper finishing electrode automatically. A 0.40 micron RA surface finish is attained.
All generator settings, table movements, tool changes, and translations are preprogrammed. The total machining time is 1.5 hours per cavity. This includes EDM polishing of the cavities that eliminates the need for hand polishing. Another benefit of polishing with EDM is that it preserves the dimensional integrity of the machined cavity.
Recent developments in controls and software have shattered many of the old concepts of diesinking EDM. At the forefront is contour EDMing, which permits simultaneous machining of the X, Y, and Z axes in either a positive or negative direction. This is a significant contrast to the traditional "sinking" application.
Programmed cycles such as vectoral, conical, orbital, directional, and others have simplified EDM diesinking. Further, advances in tooling, fixturing programming, and integration of machining strategies will continue to expand use of CNC diesinking EDM.
In the machine-tool industry, there has often been an inherent fear of anything that breaks away from tradition. This is especially true for EDM. Today, however, the impact of CNC on EDM is reshaping the framework of practical applications.
Everyday we see new applications, and new approaches to old applications, that eliminate many former restraints. It's a time of change and overwhelming growth in an industry whose potential is just being realized.
For more information on EDM equipment, circle E67.
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|Publication:||Tooling & Production|
|Date:||Jan 1, 1985|
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