Experimental Investigation Into Sequential Micro Machining (SMM) for Micro Hole Drilling on SS–304

Introduction

The sequential machining is defined as the strategy where two or more machining processes are executed in sequence on the same or different machine tools. The sequential implementation of dissimilar machining processes minimizes the shortcomings, which would have occurred due to their individual application. The SMM system allows micro-feature cutting and finishing tasks to be completed in a single setting without removing or displacing the tool electrode and workpiece from their original working positions. The system setup acts as a multifunctional machine tool and eliminates the possibility of an error occurring due to repositioning or resetting of the workpiece. The system setup also assures manufacturing of high-quality complex parts with increased precision. Thus, SMM proves to be an efficient and economical manufacturing technique, as it eliminates the requirement of individual machining system setups needed for post-processing of the part.

By considering the importance of micro-holes in critical components, the present work applies micro-ECDM and micro-ECM processes for micro-hole drilling in a sequential manner. The machine components functioning in stringent operating conditions have to face severe thermal fatigue stresses. Induction of such stress results in untimely failure of a component part and a system as a whole. Structuring micro-holes in such components relieve thermal stresses by providing sufficient cooling. The micro-hole drilling also protects components from thermal fatigue failure, which in turn improves functionality and reliability of the system significantly. Bhattacharyya et al. (2004) enumerated that the micro-holes were used extensively in highly sophisticated areas such as cooling holes in blades of power plant turbine, compressor blades used in aircraft application. The strategy of applying sequential machining proves vital for micro part manufacturing therefore; there is a need to explore more and more sequential machining combinations.

The literature survey indicates that very little research work has been carried out in this area. Notwithstanding, some researchers have extended their constructive work in the direction of sequential machining. A tabletop prototype machine tool was developed for conducting µECM and µEDM processes in sequence. Skoczypiec et al. (2014) applied electro-discharge sinking followed by electro-chemical sinking for the generation of star-shaped cavities. Similarly, micro-groove fabrication was also undertaken by applying µECM andµ EDM processes sequentially. Li et al. (2006) used sequential combination of laser beam machining and µEDM for drilling micro-holes in fuel injection nozzle of a Diesel engine. The micro-hole of an average diameter of 80.8µm was drilled by using Nd: YAG laser. The laser drilling generates recast layer in the area of 15µm surrounding the hole surface. To remove this recast layer, µEDM was conducted which removes irregularities from the hole surface. The sequential application of laser and µEDM reduces total drilling time by 70% in comparison to conventional EDM.