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The ECM technology
Manufacturing the complex geometry of femoral implants in difficult to machine materials such as Titanium and cobalt chrome can be complicated and labour intensive. Hand finishing or polishing can alter the finished shape and thus produce small, but important differences between each implant. Machining techniques include electrodischarge machining (EDM) and electrochemical machining (ECM).
ECM uses the principles of electroplating in reverse in that the work piece dissolves into the sodium nitrite electrolyte. The advantages of ECM over EDM are that it is faster (reportedly 100 times faster), eliminates the need for multiple roughing and finishing electrodes, and can machine any metal, including tungsten carbide and Titanium, irrespective of its hardness.
Closing the gap
Until now, the limitation of ECM has been the large gap that is required between the electrode and the work piece. This gap is needed for electrolyte flushing and makes it virtually impossible to produce fine detail with the process. To combat this problem, an electrochemical machine has been developed that uses an impulse technique that simultaneously oscillates the electrode and pulses the current. This allows the machine to operate with electrode gaps as small as 10 microns. The small gap enables extremely fine detail to be transferred from the electrode to the part. Furthermore, the machine retains the major advantages of ECM, which are rapid metal removal rates, zero electrode wear and high quality surface finish. Steve Duffield, Managing Director of Electrosion, the developer of the machine, comments, “Our new machine can achieve metal removal rates of 1000 mm3/min, routinely produce surface finishes of 0.1 Ra and, with care, 0.03 Ra and, because there is no sparking or contact between the electrode and the part, zero electrode wear, which makes it possible to manufacture many parts or cavities with just one electrode.”
The electrode
The electrodes for the machine can be made from any conductive material, which allows their rapid manufacture using conventional machining techniques. In addition, their long life and the need for only one to complete both roughing and finishing will produce significant cost savings. The low temperature of the electrolyte, which runs at 20 °C, and the absence of contact between the electrode and the work piece produces further benefits because it eliminates the localised stresses that can be introduced into the surface of the part through milling or grinding.
Applications
Duffield envisages a wide range of medical applications for the new technology. “Because of the polished finish that we can achieve straight off the machine, it is possible to produce mould cavities for critical products such as contact lenses, whose moulds require precision and surface quality. Thin and fragile parts in hard materials will also lend themselves to manufacture on the machine, because the low operating temperatures and zero load on the part will eliminate the possibility of distortion or the introduction of stress. We have successfully produced surgical scalpels, including the final cutting edge, in one operation on this machine.”
The controls employ fuzzy logic principles, which enable the machine to adapt itself to the requirements of each material and shape being manufactured. By simplifying the operation of the machine, the new control eliminates the need for skilled operators and ensures consistent results in a production environment.
Efficient cutting
The combination of speed, accuracy and surface quality offers some significant advantages, including lower tooling costs, and the ability to economically and rapidly machine exotic materials and easily produce fragile components. At the same time, being able to achieve nano levels of precision with submicron finishes facilitates the manufacture of many components and moulds.
For more information contact Steve Duffield, Managing Director, Electrosion Ltd, Unit 17 Mapplewell Industrial Estate, Mapplewell, Barnsley S75 6BP, UK, tel. +44 1226 380 336; e-mail: steve@electrosion.co.uk; www.electrosion.co.uk
