On the effect of load on vibration amplitude in ultrasonic-assisted drilling

The ultrasonic-assisted drilling (UAD) process is a technology that applies ultrasonic vibrations in the feed direction of the drill to improve drilling conditions and productivity. In recent years, UAD has been the subject of extensive research due to its advantages over the conventional drilling (CD) process, these being reported as thrust force and torque reduction, burr height reduction, improvements in surface roughness, and hole accuracy. The results of the research also showed that these benefits do not necessarily equally occur for all drilling conditions but can vary with machining and ultrasonic conditions. In particular, it is found that the drill vibration amplitude is a key factor in determining UAD benefits. However, a critical difficulty is that the all-important matter of the drill vibration amplitude at the point of material engagement, the so-called "down-the-hole" amplitude, is not known due to inaccessibility. This difficulty, not faced, for example by the ultrasonic-assisted turning (UAT) process due to its visual access, has prevented measurement of load effects on UAD, as well as verification of analytical models of the process. In this study, a technique to measure the effect of load on drill vibration amplitude during UAD is presented. The technique is based on examining the surface of the drilled hole, where minute, oscillatory traces of the vibrating drill tip are found that correspond to drill tip vibrational amplitude. This data, when coupled with in-process laser vibration measurements at the drill chuck, as well as no load amplitude measurements, permit the effects of load on vibration amplitude to be determined. Results have shown that drilling loads cause measurable reductions in drill vibration amplitude and must be accounted for in UAD procedures.