WIRECUT EDM - RELIABILITY, COST EFFICIENCY AND VERSATILITY
Wirecut EDM (Electrical Discharge Machining) is a thermal machining process. Sparks discharged from a small-diameter, tensioned wire erode the workpiece without any contact between the tool and the part produced. This process, also called traveling-wire EDM, produces a straight, narrow-kerf cut. The slowly moving wire brings a fresh, constant diameter electrode to the cutting gap, thereby enhancing kerf size control. Usually a programmed or numerically controlled motion guides the cutting, while the width of the kerf is maintained as a constant by the wire size and settings on the spark discharge controls. The dielectric is deionized water under pressure, introduced through concentric nozzles at the top and bottom of the workpiece. In a wirecut EDM system, the wire feeds from a spool, travels over tension rollers, along upper and lower guides, over electrical contacts and into a takeup reel. Note that the EDM wire must be threaded through a starter hole in the workpiece. These holes are normally created for the wire by drilling before parts are hardened.
As the wire discharges sparks for thermal machining, the table on which the workpiece is mounted moves to generate X and Y coordinates. Here, small pulse increments (on the order of .00004") assure accurate cutting. For tapered cuts, some wirecut EDM machines use U and V axes on the upper wire guide. These axes can be used to generate tapers up to 30 degrees per slide. Wirecut EDM can be up to a five-axis process.
Conventional EDM equipment first appeared in the early 1950s, and performed simple machining utilizing the phenomenon of electrical spark but utilizes five heads mounted above discharge. Wirecut EDM machines a single table. The ability to stack came on the market in the early workpieces under each head to depths 1970s. The first five-head wirecut EDM arrived in the United States in December, 1980.
Until then, wirecut EDM had generally been used only for tool and die manufacture, prototype parts or intricate machining jobs that could be accomplished in no other way. Typical tolerances of +/- 0.0002" have made the process highly attractive for precision machining. The reduction in machining steps has also reduced production times and costs for many operations.
The programmer provides dimensional data to a computer via a keyboard. Then the software within the computer will automatically generate either a punched paper tape or cassette, which will drive the wire EDM machine.
A five-head wire EDM system operates on the same single program, but utilizes five heads mounted above a single table. The ability to stack workpieces under each head to depths of seven inches further multiplies the time-efficiency of the process. It increases production by 400 percent, allowing the wirecutting of five workpieces in the time it used to take to produce one piece, and at half the cost.
This innovation often makes wire EDM the most cost-efficient machining method available. Fewer machining steps, no special cutting tools and lower scrap rates combine with the faster production of five-head equipment to make EDM highly attractive, especially for jobs involving precise tolerances, intricate shapes or exotic materials.
Wirecut EDM has advanced into every industry by offering the reliability, cost efficiency and versatility that challenge conventional machining methods, including low-volume stamping. Manufacturing, aerospace, automotive, jewelry, communications and other high-tech industries which have utilized wire EDM have generally found it to be one of the most effective precision-cutting methods for producing intricate tooling, dies, molds and prototype parts.
A simple wirecut EDM operation produced an intricate Ampco 45 bronze mount jaw, which requires tolerances to within +/-.0005". The manufacturer saved an estimated $300 on special cutters, and scrap rates dropped from 22 percent for conventional machining to 3 percent using wirecut EDM. Conventional machining techniques would also have required end mills, ground form cutters, deburring and other secondary operations to produce this workpiece.
The finished part features nine different surfaces. Using conventional machining methods, each surface would normally require a separate setup. Drilling, boring and jig grinding would introduce three additional setups and machining operations. Finally, the teeth of the mount jaw would be cut. This conventional machining process required 4.8 hours per part.
Wirecut EDM made this operation faster and less complicated. First, the workpiece was Blanchard ground. Next, 12 starter holes were drilled, and the workpiece was mounted for wirecut EDM. A parts programmer entered specifications into the computer, which converted those figures into a CNC program for the machine. The unattended machine then produced the mount jaws from five pieces of Ampco bronze blanks.
The same program used to cut the mount jaws also produced a nest type fixture for holding the parts while two starter holes were drilled. These holes were then wirecut to final size in a separate operation. Wirecut EDM also produced the teeth in a final operation. Hole position tolerance was held to .0005" total, the hole size to .0003" total, and tooth position and size to .0005" total.
This procedure would not have been faster than conventional machining without the multiple production capabilities of the five-head equipment. Using five-head wirecut EDM to cut the blanks and fixtures, the process took a total of 2.8 hours per part and produced five parts simultaneously instead of one a 58% reduction in time compared to conventional machining.
A major advance in the manufacture of dies and punches has been the use of wirecut EDM. This die is part of a three-unit pierce die assembly. The complete assembly includes 32 dies, 32 punches and 16 stripper plates. Each unit was produced in a single-step wirecut EDM process, eliminating the problem of transfer-error.
Done conventionally, this project would require approximately $600 in special cutters. The close tolerances required to match dies and punches would create additional expenses in scrap. For instance, each pierce die features five dowel pin holes and five slots, which must line up precisely. However, conventional machining would require jig grinding the holes on a separate machine, making alignment difficult.
The one-step wirecut EDM process aligned holes and slots precisely, resulting in zero scrap. In addition, no special cutters were needed. Wirecut EDM also saved time. Conventional machining would require 544 hours for complete production of the 32 die details. Single-head EDM would take longer, though it would be at a far lower cost than conventional machining. The job was completed using a five-head machine in only 228 hours less than half the time required by conventional machining. By reducing operating costs, wirecut EDM generates added savings for die users.
In cases where single-head wirecut EDM is more efficient than conventional machining, five-head is making wirecut EDM technology even more attractive. Parts such as the hammer blanks would normally be made by stamping, but that was highly impractical in this instance. Composed of annealed T15 high-speed steel, the hammer blanks would generate rapid wear on both punch and die. Required tolerances of +/-.0005" would be impossible to maintain. Stamping would also create burrs and jagged edges that were unacceptable for this precision work.
The manufacturer considered fineblanking, but this did not achieve required tolerances and finish. As a result, these blanks were initially made using single-head wirecut EDM, which was able to perform within tolerances.
The single-head completed stacks of 35 hammer blanks in approximately one hour.
The introduction of five-head wirecut EDM has made this process more efficient, multiplying the quantity simultaneously produced to 175, with little reduction in feed rate. Five-head EDM technology allowed an order for 100,000 units to be filled within 2 1/2 months, at a production rate of approximately 2,000 per day. This order would have tied up a single-head wire EDM machine for a year.
Wirecut EDM has proved superior to conventional machining methods for precision cutting of difficult-to machine materials. Spark erosion works on any electrically conductive material, regardless of hardness. This is especially useful for heat-treated parts, since the workpiece may be prehardened, this eliminates the age old problem of heat distortion. Exotic metals machined by conventional methods tend to experience work hardening, which interferes with cutting and reduces overall accuracy. Since EDM involves no contact between tool and workpiece, there is no work hardening with wirecut EDM.
As with any thermal machining process, however, a recast layer of hardened material is created when sparks, which are pure heat, vaporize a miniscule portion of the workpiece. Typical recast layers for wirecut EDM are approximately .0001" to .0005". A proprietary process developed by our company corrects this recast layer. This proprietary process removes the recast metal, leaving the exposed surface totally undisturbed. It remains just as the metal was produced with the same grain structure and stresses that were inherent in the virgin material. The ability to eliminate the recast layer while controlling tolerances as close as +/-.0001". This has made production of aerospace components possible and cost-effective using wirecut EDM.
Engineering know-how and computer-controlled wirecut EDM technology have provided almost limitless solutions to intricate precision machining problems. The ability to produce work to required levels of accuracy, increased productivity and bottom-line cost efficiency results in a very competitive manufacturing environment for wirecut EDM.
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