The applicability of various indirect monitoring methods to tool condition monitoring in drilling

Condition monitoring of cutting tools is important for a number of reasons. The unmanned use of flexible manufacturing systems is not possible without a reliable system for tool condition monitoring. Tool wear affects the surface quality of processed workpieces. Tools cannot be optimally used based on tool change policy which relies on time and which easily leads to too frequent change of tools from which it follows that valuable production time is lost and the tool cost becomes high. There is great variation in how well different monitoring methods work in tool condition monitoring. It is well known and accepted that cutting forces increase as a function of tool wear and consequently thrust force and torque are often monitored in drilling. Feed drive and spindle current actually also measure the same thing as feed force and torque transducers although through a longer measuring chain. Tool wear also changes the dynamics of cutting processes and consequently drift forces, vibration and sound have been used for tool wear monitoring. Cutting dynamics change also at higher frequencies i.e. ultra sonic vibrations and acoustic emission are also used for tool wear and failure monitoring. In the paper some physical reasons behind the use of various indirect monitoring methods of tool condition in drilling are presented and the benefits and drawbacks of each method are discussed.