Optimizing the EDM hole-drilling strain gage method for the measurement of residual stress

When using the electrical discharge machining (EDM) hole-drilling strain gage method to measure the residual stress within a component, the metallurgical transformation layer formed on the wall of the EDMed hole induces an extra stress, which can lead to significant measurement errors. Accordingly, the objective of the present work was to explore and determine the optimal EDM parameters which reduce the thickness of the metallurgical transformation layer and therefore minimize the magnitude of the hole-drilling induced stress. The experimental results demonstrated that by maintaining the relative stability coefficient of the discharge duty ratio at a value greater than 0.99, the induced stress emerged in EDM hole-drilling measurement can be reduced substantially and becomes insensitive to the parameters of the pulse current and pulse-on duration. Further investigations revealed that when the residual stress is to be measured accurately, using a hollow electrode instead of the usual solid electrode and the following parameters are recommended. The pulse current and pulse-on duration are in the ranges of 4-12 A and 9-23 μs, respectively, and the pulse-off duration needs to be longer than the value required to ensure that the relative stability coefficient of the discharge duty ratio exceeds 0.99.