EDM MACHINES – ELECTRICAL DISCHARGE MACHINING
An electrical discharge (EDM) machine is used for profiling (contouring) components. The principle behind EDM involves machining material by generating a series of sparks.
History
The method of processing metals using electrical sparks (electrical erosion) was introduced in 1943 by the Soviet scientists Natalya and Boris Lazarenko. The technology quickly found industrial applications, and Switzerland began producing and exporting these machines in 1952. The introduction of computers and numerical controls in the 1980s provided a substantial and enduring advancement to this technology.
Machine Description
The EDM machine consists of a large tank where all the machining operations take place. This tank has a front door that allows for loading the material and adjusting the machine. At the bottom of the tank is a table to which the material trays (raw material plates) are fixed. The table is motorised and can move longitudinally and laterally (X and Y axes). A pre-drilled hole penetrates the raw material outside the contour of the component to be cut. The tray is positioned and securely fixed to the machine’s table. Depending on the material type, the thickness of the trays, and the complexity of the component’s profile, multiple trays can be stacked on the machine’s table, allowing the cutting of as many components as there are trays stacked. Vertically positioned above the tank, the machine has two large spools of wire, similar to a tape recorder. One spool holds the new wire, while the other collects the used wire. The wire is made of brass or coated copper, with a diameter ranging from 0.02mm to 0.3mm, depending on the machine’s capacity and the component to be cut. A system of rollers guides the wire and maintains the ideal tension. Upon exiting the machine, the wire is vertically guided by the operator through the pre-drilled hole in the tray. It is then guided back up outside the tank to reach the collection spool. Once the component’s contour is input into the machine’s computer system, the operator can close the machine’s front door. Before the machine begins machining the component, the first step is to fill the tank with a dielectric solution (water or mineral oil). A dielectric has the property of weakly conducting electricity but allows electrostatic forces to operate freely.
Operating Principle
The tray is immersed and threaded with the wire, which acts as the system’s electrode. During machining, the wire continuously unwinds from one spool to the other. The machine sends short electrical current pulses to the wire. By perfectly controlling the current intensity, a spark is generated with each pulse, creating an arc between the wire and the workpiece (the inner wall of the hole drilled in the tray). The temperature caused by the succession of sparks can reach 8000 to 12000°C. With each pulse, the material locally transforms into plasma, which is expelled by the dielectric, leaving behind a crater. The succession of craters creates the component’s cut. Throughout the process, the machine’s table and tray simultaneously move along the Y and Z axes to accurately and precisely follow the component’s contour (the wire’s position remains fixed). Some machines allow the table to tilt, enabling the cutting of inclined planes.
Advantages and Disadvantages
Wire EDM cutting offers several advantages over traditional machining technologies. The very small diameter of the wire is up to ten times smaller than the diameter of the smallest milling cutters. This allows the wire to cut much more complex contours with greater precision. Additionally, there is no contact, and therefore no mechanical stress, between the tool (the wire) and the component. This makes it possible, for example, to manufacture long, thin components like springs without the risk of breaking the component, as could happen with a milling machine or machining centre. The number of components produced in the same cycle can be multiplied by stacking trays. However, this number is limited, and the machine’s feed rate is not very fast (0.2-10mm/min). As a result, when technically possible, stamping is preferred over EDM for mass-producing components. Lastly, only electrically conductive materials are logically eligible for this technology.
Examples of Components Produced by EDM
As long as the material is conductive, the list of applications is virtually endless. From the middle part of a watchcase to the smallest and thinnest springs in a movement, the advantages of the technology often justify the choice of EDM. EDM is also widely used to produce dies and punches for stamping. This technology has significantly reduced the manufacturing costs of stamping tools, which were previously entirely machined from solid. Finally, it is particularly appreciated for the production of small, thin components with complex profiles, as well as for small series and prototyping because it does not require the manufacture of specific tooling, as is the case with milling or stamping operations.