Wire Electric Discharge (ED) Machining Tampere University of Technology Tuula Höök Wire electric discharge (ED) machining is based on the same principle as die-sink ED machining. The basic elements in all ED machining methods are dielectric fluid, a workpiece and an electrode. In the die-sink ED machining method the electrode has the same shapes as the wished machining results. In the wire ED machining method the electrode is a moving wire made from some electrically conducting material. The workpiece is cut with the electrode wire. It is possible to produce complicated shapes, but the shapes need to extend through the workpiece material. (See image 1.) Image 1. Core machining with wire ED machine. Wire extends through the workpiece. Core walls are drafted and flat. The wire ED machine does basically two things: It produces frequent electric shocks and guides the electrode wire. The workpiece and the electrode wire are connected to an electric circuit as two opposite poles. If the wire is near enough to the workpiece, the electric shock lets the dielectric fluid to change from an electric insulator to an electric conductor. At that instant a high electric current passes through the dielectric fluid and heats the workpiece surface from a very small area. The corresponding workpiece area melts and what is left, is a small round cavity. The cavity size depends on the electric current and potential. After frequent electric shocks the electrode wire passes through the workpiece and produces the programmed shapes. (See image 2.) Principle of the wire EDM method Image 2. Principle of wire ED machining. The distance between the electrode wire and the workpiece is called a sparking gap. The electrode produces shapes that are a sparking gap dimension larger than the programmed shape through which the electrode wire passes. This is usually taken into account during the wire ED machine programming. The absolute minimum inner corner radius is the wire radius added with the sparking gap width. Wire Electric Discharge Machining - 1
The wire ED machines have 2-5 programmable axes. The machines that are used in mould making applications typically have 5 programmable axes. These axes are: Wire guide, wire tilting in x and y directions and workpiece or wire system movements in x and y directions. (See image 3.) Image 3. Programmable axes in wire ED machine. The electrode wire moves between two coils with a moderate speed. The part of the wire that actually machines the workpiece is constantly changing. There is no time for the wire to heat up. For that reason the problems with electrode wear are not an issue like in the case of die-sink ED machining and it is possible to use wire ED machining also for materials with high melting ranges. The material hardness sets no restrictions. The only restriction is that the material needs to be electrically conductive. Despite of the minimum wear, ED wires are usually disposed after one usage. Sparking and high temperature during the machining reduces the wire tensile strength and the wire could easily break if re-used. ED machining wire is usually brass either zinc-coated or uncoated. Brass wire can be purchased in different hardnesses and different diameters. Soft wires are useful in applications with complex shapes, where the machine changes the wire tilting angle several times. The harder grades are used in automatic re-threading mechanisms and also if the machined shapes contain high flat surfaces. Hard wires resist change in direction and for that reason are likely to produce nice flat surfaces. Zinc coated wire is used in machining high melting point workpiece materials. The zinc coating vaporises in lower temperature than the brass core. Vaporisation reduces the amount of heat that transmits to the brass and the core wears less. The dielectric fluid in die-sink ED machining is usually some petroleum product. In wire ED machining it is most common to use deionised water. The most typical wire ED machining applications in mould making are: Machining ejector holes Shaping and cutting the ejector pin ends to follow mould cavity surface shapes Machining cores and corresponding fastening holes in the mould plates and inserts Wire Electric Discharge Machining - 2
Ejector hole machining There are different options in machining the holes for ejector pins. Most common method is to take the following steps: 1. Drill start holes through the mould plate and/or the mould insert plate 2. Drill loose holes starting from the back side of the insert and/or mould plate. These holes end 20 40 mm before the mould cavity surface. 3. Optional: Take the whole mould plate or just the insert plates to a heat treatment plant to get the steel hardened. 4. Use wire ED machine to produce the tight 20 40 mm ends of the ejector pin holes. Automatic re-threading mechanisms are useful. (See image 4.) Image 4. Different machining phases of ejector holes. It is important to harden the insert or mould plate steel before making the tightly tolerated ends in the ejector pin holes, because the heat treatment tends to change the workpiece shapes at a certain degree. Some manufacturers finish the ejector holes before the hardening operation, but in this case the manufacturer must be sure that the heat treatment is successful and that the shapes change as estimated. Ejector cutting and shaping Ejectors are sold in standard dimensions and shapes. The most common shapes are circular and rectangle. The length of the pins, diameter and other dimensions are standard. There is a need to cut the ejector to a right dimension and if the cavity surface is shaped there is also a need to shape the ejector end. (See images below.) Image 5. Cutting and shaping electrodes with a wire ED machine. Wire Electric Discharge Machining - 3
The wire ED machine is one of the most accurate machines in mould shops and usually there is also free machining time available. Another option is to cut and shape the ejectors in a milling machine, but the most accurate milling machines are used in mould cavity machining operations and normally they are rather busy for long periods of time. Core and core fastening hole machining Basically there are two options for making the fixed cores: Machine the cores directly to the mould insert plate or mould plate Machine the cores to separate pieces of mould steel and attach the piece to the mould assembly Which one of the options is more practical depends on the core dimensions and shapes. High and narrow cores or cores with sharp shapes are easiest to manufacture with separate parts. Special cases are core pins. Core pins are used in making small diameter holes to castings. (See images below.) Image 6. Deep and narrow holes in a casting and core pins for shaping the holes. In the image on the right there is a core pin assembly. The core pin is fastened to the mould insert with a flange. The selection between the two options described below has an influence on the rounding of the mould cavity corners. This selection has an influence on the moulded part. (See images 7 and 8.) Image 7. Milled and wire ED machined cores.pay attention to the rounding between the core and the workpiece. Wire Electric Discharge Machining - 4
Image 8. Wire ED machined core and milled core in the mould cavity: Results in castings. Like every wall in the mould opening direction, the core walls are also drafted. It is rather difficult to make high drafted walls with a milling machine. Especially in the case of injection moulding or high pressure die casting moulds where the accuracy requirements are very high. The wire ED machine produces these shapes accurately with ease. There are different options for fastening the fixed cores to the mould assembly. Core pins are sold as separate mould standard parts. The core pins have straight, undrafted walls. They have a collar with which it is possible to fix the core between the insert plate and the mould plate or between the mould plate and the back plate. The similar technique can also be used in the case of other fixed cores. (See image 9.) Image 9. A high core fitted to a insert plate. If the core shapes are complex, it can be more convenient to attach the core to the mould assembly by using the draft angle as the fixing element. It is necessary to take into account the minimum corner radius. (See image 7.) References E. C. Jameson, Electrical Discharge Machining, Society of Manufacturing Engineers, Michigan, 2001. Wire Electric Discharge Machining - 5