The effect of grain refining practice on the machinability of 4140 steel was investigated using processed bar stock from two industrially produced heats. The first heat employed a vanadium micro-alloy fine grain practice and the second heat employed an aluminum fine grain practice. Progressive flank wear on the machining tool was measured during machining tests to obtain tool life curves for each trial condition. The tool wear was evaluated at three different cutting speeds to produce a Taylor's curve to fully characterize the relative machinability of the two steels. Metallography was performed to document the microstructure and automated SEM/EDX analysis was used to characterize the non-metallic inclusions in the two steels. The microstructure of the machining chips and the surface condition of the tools after machining were also documented. While both steels possessed a bainitic microstructure, the aluminum treated steel exhibited better machinability than the vanadium treated steel.