A new method for detecting tool wear and breakage in milling
Résumé
In machining, tool condition monitoring (TCM) is one of the major trends of development of process efficiency. By monitoring tool wear and detecting tool-teeth break, TCM is able to protect tool, machine and workpiece, which corresponds to important savings. Cutting forces are often considered as the best indicator for detecting tool wear and damage, but they are very difficult and expensive to measure directly in industrial cases. The aim of this work is to develop a new method using standard transducers available on actual machines for a better monitoring of cutting process. This method analyses instantaneous variations in rotational frequency so as to observe milling operation. Experimental cutting tests were performed on a milling machine. On one hand, cutting forces were acquired through common dynamometer. On the other hand, signal from spindle integrated rotary encoder is acquired using specific angular-sampling methodology. Spindle rotational frequency is then easily computed using rudimentary signal processing tools. Correspondence between cutting force magnitude and variation in spindle rotational frequency is noted: when a tooth goes in material, cutting force suddenly increases, and rotational frequency starts to decrease until cutting force becomes low enough. Thus, apparition of tool wear can be observed by monitoring variations in rotational frequency according to the different teeth of the tool. Moreover, a sudden variation in the amplitude of slow-down due to a tooth allows detection of tooth breakage. Instantaneous variations in spindle rotational frequency are shown to be very effective indicators for tool condition in comparison with cutting force, so they must be very useful for TCM systems.