Optimization of Machining Parameters for Multi-performance Characteristics in Milling of Composite Solid Propellants Using RSM

Abstract

The solid rocket motors (SRMs) produced with case-bonded composite solid propellant (CSP) grains are formulated with metallic aluminium (Al) powder as the fuel, ammonium perchlorate (NH4ClO4) as the oxidizer and hydroxyl-terminated polybutadiene (HTPB) as a polymer binder. These CSPs are sensitive to fire hazard by mechanical stimuli such as friction, heat, impact load and static charge which are inevitably present in conventional machining operations. In order to machine this ‘hazardous to machine’ CSP material safely, using custom build cutting tool called ‘turbine cutter’ with minimum cutting power and for maximum material removal rate (MRR), experimental studies are carried out. The main objective of this study is to identify the optimum process input parameters for low cutting power (CP) and high MRR and further to enhance the safety in machining of ‘hazard to machine’ materials. To achieve this objective, the effect of machining parameters on CP and MRR was investigated. Full factorial experiments were carried out on live propellant grain using the turbine cutter on CNC vertical turn mill (VTM). In order to investigate the influence of process parameters in machining CSP material, two-factor interaction (2FI) models are developed, and subsequently, ANOVA is performed to evaluate the significant process parameters. Response surface methodology (RSM) is used to develop the mathematical models and also for multi-response optimization, using commercial software, Design-Expert. The optimum values of machining parameters attained with a desirability value of 0.88 are as follows: cutting velocity (CV) is 125 rpm (196 m/min), table feed rate (TFR) is 24 deg/min (0.418 m/min), and depth of cut is 4 mm for minimum CP and maximum MRR, and their values are 15.75 × 10−2 kW and 1092.11 × 10−6 m3/min, respectively.