An apparatus and method are provided for controlling a machine tool to perform predetermined or programmed operations on a work piece wherein the tool may be subject to variations in its dimensions due to wear or may be variably located with respect to the tool holder depending on how it is mounted. The apparatus provided herein is operable to sense and measure or indicate the location of the end of the tool or cutting edge with respect to the work and, in certain intances, the diameter of the tool to determine the degree it is worn. In a particular form of the invention, a single probe is automatically controlled to indicate the location of the tool end and to measure or indicate its diameter. The same measuring device may also be utilized to detect a broken tool and prevent operation of the machine until the tool is replaced. In a further form, the detection means may be operable to initiate an action which results in the replacement of a broken or substantially worn tool by directly replacing same before the next operation is effected or by controlling a turret to indicate a second tool with respect to the work while the worn or broken tool is retracted away from the work.