Workpiece damage generated during conventional drilling (CD) of carbon fibre reinforced plastics (CFRP) (such as delamination, matrix cracking, fibre pull out etc.) results in reduced fatigue strength, poor assembly tolerance and compromised structural integrity of the component. Avoidance of such damage during drilling of CFRP is a challenge for the aircraft and aerospace industries. At present, it requires a lot of resources and huge cost in order to generate damage free holes in CFRP in the industries. Therefore, the industries (such as BAE Systems) are looking for alternative hole producing processes for damage free drilling. It has been reported that thrust force during drilling should be reduced in order to reduce exit delamination. Ultrasonic assisted drilling (UAD) has been found to reduce the cutting and thrust forces during drilling of metals when compared to CD. Although limited attempts employing UAD indicate a reduction in thrust and cutting forces and damage when machining CFRP, this process has not been examined in detail with respect to optimising machining parameters in relation to machining theory. In addition, there has been limited research regarding the effect of the UAD process on overall workpiece damage as a result of drilling of CFRP. The focus of this research was the identification of the mechanism responsible for thrust force reduction in UAD in comparison to CD during drilling of CFRP which would help in selecting the machining parameters resulting in minimum workpiece damage. Consequently, the cutting speeds resulting in reduced forces and damage were investigated. As a fundamental concept in machining theory, higher rake angles result in lower cutting forces and improved surface roughness. Calculations of effective normal rake angle at the cutting edge of a twist drill in UAD revealed a maximum of 62° and 49° effective rake angle at 10 and 100 m/min (40160 Hz, 7.3 μm peak-to-peak amplitude) respectively for a new tool. Employing knowledge of the effective rake angle, experiments were performed at specific cutting speeds in order to examine the effects of UAD on forces and damage during drilling of CFRP and compared to CD. Further work employed a pilot hole to remove the contribution of chisel edges so that influence of effective rake angles at the cutting edges could be examined. Through-hole drilling tests, comparing UAD and CD, employing a constant feed rate of 0.05 mm/rev and two cutting speeds (10 and 100 m/min) were carried out. At 10 m/min, there was a reduction in thrust force and torque of 55 % and 45 %, respectively when utilising UAD with a new tool. 40 % reduction in thrust force and 46 % in torque with 52 μm of tool wear corresponded to 36 % reduction in entrance delamination and 22 % reduction in exit delamination at 10 m/min. At 100 m/min, 20 % reduction in thrust force and 30 % in torque was obtained respectively; however, this did not yield a significant reduction in entrance or exit delamination. Analysis of internal damage did, however, reveal a 55 % reduction in internal damage (i.e. fibre pull-out and fibre disorientation) at 100 m/min. Thus, the key contribution of this research is that low cutting speed is required in UAD in order to achieve the greatest reduction in machining forces (and hence, delamination) at entrance and exit of a hole in comparison to CD. The reason for this was discovered to be higher effective rake angle at low cutting speed in UAD causing the reduction in thrust force and torque. Furthermore, reduction of internal damage in a hole required higher cutting speed in UAD. The results imply that the cutting speed should be varied during drilling a hole in CFRP. In UAD, the cutting speed should be lower at entrance and exit of a hole and higher for drilling the intermediate part. Whereas in CD, the cutting speed should be higher at entrance and exit and lower during drilling of intermediate part keeping the feed rate constant in order to achieve the minimum workpiece damage during drilling of CFRP. The proposed variation of cutting speed during drilling of a hole is possible in the machine used in the present research.