The application of acoustic emission for precision drilling process monitoring

Precision drilling is a process where a close tolerance hole can be produced with a special drill bit without subsequent reaming. Producing a hole without reaming results in less overall processing time during hole preparation. Precision drilling is best accomplished by a robot with a computer controlled drilling end effector due to the high degree of process control required. Some aspects of the process, such as spindle speed, feed rate, and peck cycles, can easily be controlled by a computer controlled end effector. Other variables, such as drill bit wear, chipping, and point geometry variation, cannot be controlled with the end effector. These variables affect the diameter of the hole but cannot be detected unless the hole or the drill bit is manually inspected. It is not practical to stop the process and check the diameter after every hole. Therefore, a means to perform real time drilling process monitoring is required to detect if an oversized hole is being drilled. The primary objective of this research was to correlate the diameter of a hole drilled in steel with any acoustic emission (AE) signal measurement parameter. The secondary objective was to correlate drill bit lip height variation, which has a significant influence on the diameter of a hole, with any AE signal measurement parameter. The results of this study showed that acoustic emission could only be correlated to hole diameter variations if those variations were related to the lip height variations. However, AE energy and RMS were correlated to lip height variations under a wide variety of conditions.