The changes of cutting speed, surface roughness and surface topography were researched. Adopting cutting speed as characterization of cooling liquid failure is more intuitive and accurate. The relation among the variation of the conductivity, ablation product concentration, pH value, viscosity and cutting speed was compared and the reason of the change of conductivity was analyzed from the aspects of ablation products, concentration and electrochemistry angle. It is proposed iron ions generated by electrolysis play a leading role in increasing the conductivity of the cooling fluid. Conductivity has great relevance to cutting speed and it can be considered as the index of cooling liquid failure measurement. ��