A STUDY ON THE REDUCTION OF CHIP EVACUATION TORQUE IN ULTRASONIC ASSISTED DRILLING OF SMALL AND DEEP HOLES

This paper presents an experimental study to examine the effectiveness of ultrasonic assisted drilling (UAD). The cutting torque has been typically considered as independent from the hole depth. It has been found that the cutting torque in drilling can be reduced by 25% by applying ultrasonic assistance. As deeper depths, a new component named as chip evacuation torque is generated, making the total torque exponentially increase. The substantial increase of the total torque can lead to a risk of drilling breakage. A critical depth is defined as the depth where the total torque is still lower than the allowance torque, which was determined from the broken condition of the drill bits. Using response surface technique in design of experiments, this study found that UAD can increase the critical depth in small and deep drilling by 1.4 times compared to conventional drilling, with a safety factor of 5. This helpfulness of UAD can be further developed to combine with other common techniques such as peck drilling or grooving tool for deep and small drilling.