MICRO-FINISHING OF EDM MACHINED DIES BY ABRASIVE JET MACHINING

Abstract

Electro-discharge machining (EDM) is increasingly being accepted as a more effective method for the precision machining of tools and dies. Although, in recent years, much progress has been made in improving the efficiency and in the understanding of the process, there remains a number of application problems, particularly in relation to the surface integrity after machining. The present study examines the surface characteristics of the AISI 01, A2, D2 and D6 tool steels (most commonly used in Singapore), mild steel and AISI 304ss. Micro-structure examination was carried out using nital and picral solutions and optical and scanning electron microscope to reveal the nature and causes of the heat affected zone. Detailed analysis was carried out on the extent and origin of the cracks. The propensity of the four tool steels to cracking was compared. The thickness of the white layer (taken to represent the functionally significant damaged zone) and the surface roughness Ra were quantitatively correlated to the nominal input energy. An empirical relationship was computed between the thickness of the white layer and the surface roughness. Various ways of estimating the depth of damaged ( or white) layer produced by EDM were evaluated. A performance chart interconnecting input parameters such as pulse current and duration, on the objective functions like metal removal rates, roughness and white layer thickness for sample tool steel AISI D6 was developed. Fatigue tests were carried out on notched electro-discharge machined (EDMed) specimens to find the effect of machining conditions on fatigue life. Abrasive Jet Machining (AJM) was evaluated as a post operation. The recast and heat affected layers, which are often detrimental to the performance of an EDMed component occur on all materials under all machining conditions. The recast layer is composed of two distinctive layers: a topmost layer, referred to as a white layer, solidifying inwards from the specimen surface and an intermediate layer solidifying outwards from the base of the molten metal. The depth of the surface cracks correlates well with the thickness of the white layer while the density of surface cracks correlates better with the thickness of the overall recast layer. The depth of the damaged layer correlates well with the energy irrespective of tool steel material. Though the thickness of white layer increases with the roughness, a wide scatter is observed. The fatigue life of finish EDMed components is better than rough machined ones. It is possible to remove the top hard white layer by AJM. But means have to be found for better control of the process, to make it viable as a post operation for EDM.