To carry out the deciding of the tool exchange timing in various tools and processing conditions accurately by using a fuzzy inference, and furthermore to renew a membership function adapting to the deciding standard of the tool exchange timing of a user and the like.
In a tool exchange timing deciding method in which the cutting condition is made as the input variable, and the tool exchange timing is made as the output variable so as to set a fuzzy inference rule, and the tool exchange timing is decided by a fuzzy inference, the fuzzy inference rule and a label names are made common in all the using tools, while the membership functions of the fuzzy inference are set individually to each using tool, the data of the cutting condition such as the main shaft rotation number, the feed speed, the main shaft drive torque, and the work material, in the cutting processes are collected and memorized by a cutting condition data collecting and memorizing means 41, a fuzzy collection is extracted from the memory data of a specific cutting process number, and the membership function of the fuzzy inference is produced and renewed automatically by a membership function automatic producing and renewing means 43.