Machining processes

Topics: Machining, Electrical discharge machining, Ultrasonic machining Pages: 6 (1767 words) Published: January 1, 2014
Machining processes
Electrochemical machining (ECM) is a method of removing metal by an electrochemical process. It is normally used for mass production and is used for working extremely hard materials or materials that are difficult to machine using conventional methods.[1] Its use is limited to electrically conductive materials. ECM can cut small or odd-shaped angles, intricate contours or cavities in hardened exotic metals, such as titanium aluminised, Inconel, Waspalloy, and high nickel, cobalt, and rhenium alloys. Both external and internal geometries can be machined.ECM is often characterized as "reverse electroplating," in that it removes material instead of adding it. In the ECM process, a cathode (tool) is advanced into an anode (work piece). The pressurized electrolyte is injected at a set temperature to the area being cut. The feed rate is the same as the rate of "liquefaction" of the material. The gap between the tool and the work piece varies within 80-800 micrometers (.003 in. and .030 in.) As electrons cross the gap, material from the work piece is dissolved, as the tool forms the desired shape in the work piece. The electrolytic fluid carries away the metal hydroxide formed in the process.

Advantages and Disadvantages
Because the tool does not contact the work piece, its advantage over conventional machining is that there is no need to use expensive alloys to make the tool tougher than the work piece. There is less tool wear in ECM, and less heat and stress are produced in processing that could damage the part. Fewer passes are typically needed, and the tool can be repeatedly used. Disadvantages are the high tooling costs of ECM, and that up to 40,000 amps of current must be applied to the work piece. The saline (or Acidic) electrolyte also poses the risk of corrosion to tool, work piece and equipment.

Electric discharge machining (EDM), sometimes colloquially also referred to as spark machining, spark eroding, burning, die sinking or wire erosion, is a manufacturing process whereby a desired shape is obtained using electrical discharges (sparks). Material is removed from the work piece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool-electrode, or simply the ‘tool’ or ‘electrode’, while the other is called the work piece-electrode, or ‘work piece’. When the distance between the two electrodes is reduced, the intensity of the electric field in the volume between the electrodes becomes greater than the strength of the dielectric (at least in some point(s)), which breaks, allowing current to flow between the two electrodes. This phenomenon is the same as the breakdown of a capacitor (condenser) (see also breakdown voltage). As a result, material is removed from both the electrodes. Once the current flow stops (or it is stopped – depending on the type of generator), new liquid dielectric is usually conveyed into the inter-electrode volume enabling the solid particles (debris) to be carried away and the insulating proprieties of the dielectric to be restored. Adding new liquid dielectric in the inter-electrode volume is commonly referred to as flushing. Also, after a current flow, a difference of potential between the two electrodes is restored to what it was before the breakdown, so that a new liquid dielectric breakdown can occur.

Advantages and disadvantages
Some of the advantages of EDM include machining of:
Complex shapes that would otherwise be difficult to produce with conventional cutting tools Extremely hard material to very close tolerances
Very small work pieces where conventional cutting tools may damage the part from excess cutting tool pressure. There is no direct contact between tool and work piece. Therefore delicate sections and weak materials can be machined without any distortion. A good surface finish can be obtained.

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