Micro-hole drilling by electrical discharge machining

Rajadasa Ramachandra Hebbar, Purdue University

Abstract

The drilling of small holes, typically less than 300 $\mu$m in diameter, is most commonly accomplished by using micro-hole electric discharge machining (MH-EDM). An important application of this technology is in the drilling of fuel injector nozzle holes for diesel engines. Past research in die-sinking and wire-cut EDM has established that the single most important factor influencing the EDM process is the spark-gap condition. While adaptive control systems have been developed and utilized successfully in die-sinking and wire-cut EDM applications, the state of art of EDM for drilling micro-holes is still in a primitive stage in terms of spark-gap control. This is because of the lack of knowledge pertaining to the spark-gap of the MH-EDM process. The adaptive control systems developed for conventional applications can not be directly used with MH-EDM because the discharges occur at very high frequencies in MH-EDM. In the present work, an attempt has been made to understand the spark-gap conditions in MH-EDM. A high speed measurement system capable of sampling up to 250 MSamples/sec is used to measure the gap-voltage, gap-current and radio frequency emissions from the gap and the data is stored for off-line analysis. A classifier has been developed using digital signal processing techniques to classify the discharges into good and bad machining discharges. Parametric studies have been conducted to understand the effect of the controllable EDM parameters on the spark-gap conditions and on the resulting quality of the micro-hole. Replica and sectioning techniques have been used to evaluate the quality of these micro-holes with respect to diameter, taper and surface finish. The suitability of the replica technique for measuring the quality of micro-holes has also been assessed. The results of the study show that, by optimizing conditions in the spark-gap, the MH-EDM process can be substantially improved. Schemes for the on-line monitoring of the gap conditions are discussed along with a brief description of the schemes currently being considered for implementation in MH-EDM.

Degree

Ph.D.

Advisors

Chandrasekar, Purdue University.

Subject Area

Industrial engineering

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