A RSS method for estimating hole dimension error in a batch micro-EDM process

In an attempt to estimate the spread of errors in an EDM hole making process, a new Root-Sum-Square (RSS) method is proposed to combine the dimensional spread of a batch of electrodes with the over-cut variation in the micro-EDM process. Two sources of errors are commonly associated with an EDM process and contribute to the dimensional accuracy of the EDMed hole: the dimensional variation of the electrodes and the process over-cut error and its variation. Especially in a micro-EDM process, it is often difficult and time-consuming to measure the geometric dimension and tolerance of either a batch of electrodes or holes of small dimensions. By quantitatively establishing the relationship among the spreads in geometric errors of the electrodes and holes and the process capability, this new method provides an analytical tool in predicting hole error and allows allocating the tolerance budget when selecting the appropriate electrode making process, the EDM machine and process parameters. A series of experiments are carried out to establish and verify the RSS method. Given a set of EDM parameters and a batch of electrodes, the process error in the average over-cut and its spread is first obtained by the RSS method. The process error is then verified by separate experiments with electrodes of fixed dimension under the same EDM conditions. The validity of RSS method is further confirmed by experiments under different electrode dimensions. The RSS method is shown to well represent the contribution of both electrode and process errors to the Statistical characteristics of the hole dimension. The establishment of this statistical error model should facilitate the design and control of hole quality by balancing the requirements for the dimensional accuracy of the electrodes and the process accuracy in a batch production environment.