The objective of using heat assisted machining have been to reduce manufacturing cost and improve machinability of hardened materials. Oxyacetylene flame as a heating mechanism is dominantly used in heat assisted machining using single point turning tools. Even so, investigation of the heat source parameters was not reported in the wide published literatures. In this work, mathematical model of the flame spot size was obtained and optimal spot diameter was develop to reconcile the heat sourced from oxyacetylene flame for application with 40mm diameter end mill tool holder fitted with honed edge insert. Heat source parameters namely focus height, oxygen pressure, acetylene pressure and resident time (FOAR) are investigated as the independent variables responsible for varying spot diameter. Analysis of variance (ANOVA) was used to investigate the influence of the flame’s variables on the resulting spot diameter. A response surface methodology was used for the design and finding the optimum flame spot size which will yield minimum heat affected area and superior machinability improvement. The established empirical model suggested that flame spot diameter is influenced by focus height, oxygen gas pressure, acetylene gas pressure and resident time in the same order. Optimal flame spot diameter of 20mm was obtained at focus height 7.75mm, oxygen pressure 25.88psi, acetylene pressure 10.11psi and resident time of 10.37seconds.