<< Figure 7: Turbine wheel. >>
<< Figure 8: Typical PEM machining. >>
The turbine wheel of figure 7 is PEMed from a solid disk. There are 30 airfoils in the outer ring and 14 in the inner ring of the 34 mm diameter wheel. Six turbine wheels are machined in one fixture in an 18-minute cycle. The electrode is made from a 0.75 mm thick stainless steel disk with the airfoil shapes wire EDM cut with the required machining gap allowance. The disk is mounted in a holder that allows electrolyte to flow through each airfoil shape as it plunges into the workpiece. A typical machining operation is shown in figure 8 where electrolyte is seen flowing radially outward from the tool as it oscillates at 50 Hz. In similar fashion, the Nitinol part of figure 9 is machined to lengths up to 32 mm with a tolerance of 7 microns. 60 of these can be machined in one operation.
The diesel injector component of figure 10 has an internal oval- shaped connecting passage machined using the oval electrode on the end of an arm that extends into the component main bore. After positioning the electrode, electrolyte is flushed into the
<< Figure 10: Diesel injection component with electrode. >>
<< Figure 9: Nitinol medical implant component. >>
main bore to allow electrolyte flow between the electrode and workpiece. A radius is created at the oval hole entrance and exit by the machining current. Fixturing for this part consists of a nest for 48 parts that are machined simultaneously using 24 double- ended electrodes. Machining time for the 48 parts is 11 minutes or 14 seconds machining time per part.
A variety of automotive components have features produced by the PEM process. Components similar to the one shown in figure 11 require several different feature shapes and good surface finish to be machined into a hardened blank.
The PEM process has limited ability to produce very sharp edges. However, there are exceptions based on material, part geometry and electrode design. Typically, edge radii are limited to approximately 0.05 mm. Some applications such as surgical components and rotary shaver heads are PEMed to sharp cutting heads.
44 | commercial micro manufacturing international Vol 6 No.6
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