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Top1. Introduction
Heated treaded steels are preferred due to their higher wear resistance and high strength. They are machined to the final shape and size in the hardened state. These hardened steels have been traditionally machined to finish geometries by abrasive processes such as grinding and polishing and in few cases by electrical discharge machining. The current trend is to finish the hard part directly in CNC machines for industrial requirement. The present research work involves an experimental investigation on machining of hardened AISI M2 high speed steel. M2 steel is a molybdenum based high speed steel, having a density of 8.16 g/cm3 and a melting point of 4680°C. The researchers have worked upon many aspects of hard turning and came up with their own recommendations about the process. The process is essentially a high speed, low feed rate and low depth of cut finishing process. The cutting speeds used for machining of hardened AISI 4340 steel were from 100 m/min to 150 m/min, feeds in the range of 0.1 mm/rev to 0.2 mm/rev, whereas depth of cut was maintained as 0.25 mm (Abhijeet et al., 2006). The suitable machining parameters for minimum surface roughness while machining of using Nimonic alloy was found to a cutting speed of 54 m/min with a feed of 0.05 mm/rev. Machining parameters have significant role on surface quality (Ezilarasan et al., 2011).
Several researchers have reported that the cutting forces are influenced by a number of factors such as cutting conditions, cutting time, work piece hardness and tool geometry (Thakur et al., (2014), Ezilarasan et al., (2013)). As the machining involves a workpiece material of above 45 HRC, the forces generated are expected to be high, so as per convention a harder tool material with low wearing capabilities is required. Mostly the researchers have used CBN, PCBN and coated CBN tool inserts for the purpose. Few researchers have even used ceramics as well as WC coated with TiN or CBN–TiN cutting tools (Abhijeet et al., (2006), Grezesik et al., (2006), Dessoly et al., (2004)). Lima et al., (2005) reported that the three machining force components, namely, cutting force, feed force and thrust force decrease with increase in cutting speed. The highest tool life was obtained at the lowest feed & speed combination. At low cutting speeds, high cutting forces are encountered due to low cutting temperature and built up edge formation. High speeds result in high cutting temperature thus reduced forces due to thermal softening of the workpiece material (Ebrahimi et al., 2009; Lin et al., 2008). Many researchers reported that the force conditions in hard turning were different from those in conventional turning; i.e. the radial force was dominant rather than the tangential force for most of the tool geometries except variable edge hone radius tool.