The electrochemical discharge machining (ECDM) process has a potential in the machining of silicon nitride ceramics. This paper describes the development of a second order, non-linear mathematical model for establishing the relationship among machining parameters, such as applied voltage, electrolyte concentration and inter-electrode gap, with the dominant machining process criteria, namely material removal rate (MRR), radial overcut (ROC) and thickness of heat affected zone (HAZ), during an ECDM operation on silicon nitride. The model is developed based on response surface methodology (RSM) using the relevant experimental data, which are obtained during an ECDM micro-drilling operation on silicon nitride ceramics. We also offer an analysis of variance (ANOVA) and a confirmation test to verify the fit and adequacy of the developed mathematical models. From the parametric analyses based on mathematical modelling, it can be recommended that applied voltage has more significant effects on MRR, ROC and HAZ thickness during ECDM micro-drilling operation as compared to other machining parameters such as electrolyte concentration and inter-electrode gap.Keywords ANOVA 9 Micro-electrochemical discharge machining (ECDM) 9 Response surface methodology (RSM) 9 Silicon nitride

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Anterior sagittal anorectoplasty (ASARP) was used for the definitive correction in 107 cases of anovestibular fistula (AVF) between 1996 and 2005. These cases were subjected to three different types of treatment regimes during the same period. Majority of the cases (78) were operated in one stage where postoperatively an early oral feed was started (A). Cases were discharged in 2-4 days. In the second group (B), there were ten cases who were also operated in one stage but with prolonged fasting of 9-10 days postoperatively. Nineteen cases (C) were operated under cover of colostomy during the same period. In the immediate postoperative period, among the group A, one case had a major wound disruption requiring a colostomy and a redo surgery. Three cases had subcutaneous leak. In seven cases there was premature dehiscence of mucocutaneous or skin sutures. In groups B and C, there were no significant complications in the immediate postoperative period. In the follow-up period, out of 107 cases, 63 (58.8%) had constipation at the end of 3 months. However, at the end of one year, only 24.3% (26 cases) cases had constipation. Regarding fecal continence, 86 cases (90.5%) were totally continent. Seven had history of occasional soiling and in two cases, soiling was more frequent. As far as repair or correction of AVF or vestibular anus is concerned, we feel that anterior sagittal approach is more suitable as it requires less pelvic dissection. Separation of posterior vaginal wall from rectum, which is considered, is the most important step of the operation, takes place under direct vision. We also feel that AVF can be repaired in one stage with an early postoperative oral feed, provided we are meticulous in pre and postoperative bowel management. It reduces hospital stay and the cost of treatment. This provides a good option to cases who are not able to afford prolonged hospitalization (fasting) or are not willing for a colostomy.