PSI - Issue 14

Rohit K. Mahalle et al. / Procedia Structural Integrity 14 (2019) 945–951 Rohit Kumar Mahalle / Structural Integrity Procedia 00 (2018) 000–000

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1. INTRODUCTION Solid Composite Propellant are widely used in propulsion application of rockets & missiles for various purpose in defence and space programs. Thrust characteristics of solid rocket motor are largely determined by initial internal configuration i.e. initial ignition surface of propellant grain to meet specific requirement [1]. Grain configuration, which is exposed to initial ignition, is directly exposed surface area. This surface area is either pre-casted as per the requirement or produced by machining operation. Machining of propellant grain is a critical operation. Also propellant being a visco-elastic, behaves very differently than the material on which machining is carried out conventionally ex. Steel, copper etc. Conventional machining indulges in heat generation and associated with transfer of energy to material to be machined & to the tool by which the machining is to be carried out. This is due to friction between tool and work piece, shearing of work piece resulting in chip formation and rubbing of chip over machined surface, if any. HTPB (hydroxyl terminated poly-butadiene) based propellant used in solid rocket motor is very sensitive to these factors and machining conditions. If energy is transferred to initiate the ignition during machining, mass fire and explosion hazard may take place. This will result in damage to man, machine and material. Also due to hygroscopic nature of propellant, poor mechanical and ballistic properties in presence of moisture above a permissible level, coolant of any type cannot be used. Hence vacuum suction is used for machining. For machining of propellant hollow contouring cutter is used with HSS (High speed steel) M2 grade conical inserts. This cutter is developed specially for propellant machining operation. Speed, feed & depth of cut are main parameters for any material machining. Trials were conducted keeping feed and speed of cutter same and varying the depth of cuts in axial and radial direction, as per the machining requirement. Objective of these trials were to analyze the effect of depth of cut on propellant machined grain and to arrive at a mathematical expression in order to predict the propellant machined depth in both radial and axial directions during Research over machining of propellant is done by various researchers worldwide. Although, many research have been conducted earlier, but none did give a way to carry out propellant machining operation effectively. The three machining parameters speed, feed and depth of cut is studied by Albert J. Shih, Mark A. Lewis and John S. Streokowski “ End milling of elastomers – fixture design and tool effectiveness for material removal” [2]. In this, researchers have made an attempt to carry a research on elastomer, but the major drawback is that the material is not as hazardous and sensitive as HTPB- based solid propellant. Kishore Kumar Katikani in his research work “HSS tool wear mechanism in machining of HTPB based composite propellant grain” [3] has made an attempt to investigate over the mechanism of tool wear during propellant machining operation. In this the machining parameters were kept constant and over a period of time the tool wear mechanism was analyzed. This gave an idea to further proceed in the related field in order to analyze the machining parameters effect during machining of propellant grain and & to arrive at a stage, so as to predict the behavior of propellant during machining operation. 3. EXPERIMENTAL METHOD Experimental set up for conducting study comprises of following:  Special Purpose CNC Turn Mill center (make : MTAR, model: Special purpose CNC vertical Boring and facing machine)  Hollow contouring cutter (Patent titled “A milling cutting tool” bearing application no. 3023/DEL/2013 dt. 10 Oct’13) [4]. Customized four conical HSS inserts are attached to the cutter which performs cutting action on propellant grain Due to explosive nature of material and coming under hazard class 1.3, remote machining operation is carried out. Cutter is attached to a hollow arbor which in turn is connected to a CDCS (Chip and Dust Collection System, propellant machining operation. 2. RELATED RESEARCH