Relationship between Workpiece Wall Temperature and Discharge Instability in Small-hole EDM

Small-hole electrical discharge machining (EDM) using a pipe electrode is a non-contact machining technique that facilitates the machining of high-aspect-ratio holes. For example, machining a steel workpiece 100 mm in height with an aspect ratio of 100 using a 1.0 mm diameter pipe electrode is relatively straightforward. However, it is well known that as the machining depth increases, the discharge stability may decrease depending on the material and machining conditions. This instability can lead to reduced machining speeds or even process stagnation. In small-hole EDM, the machining progresses as water-based machining fluid is injected through the center of the pipe electrode. However, electrode wear during operation makes it impossible to determine the actual machining depth by simply measuring the main-axis feed. This contrasts with die-sinking EDM, where the electrode wear rate is typically about 1% or less. To address this challenge, we attempted to estimate the actual machining depth during small-hole EDM. Specifically, an analytical model was developed to predict the wall temperature near the workpiece during machining. The wall temperature was then measured during actual machining, and the machining depth was estimated based on the analytical model. This study also investigated the relationship between unstable discharge conditions during deep-hole machining and workpiece wall temperature measurements in small-hole EDM.