Carbon fiber–reinforced plastic (CFRP) composites are prone to delamination, tearing, and burrs in conventional drilling. It has been proven that longitudinal–torsional coupled ultrasonic-assisted drilling (LTC-UAD) can significantly improve the hole quality of CFRP composites, where the amplitude ratio is a key parameter which directly affects the contact state between the cutting edge and the CFRP composite. This paper investigates the effect of amplitude ratios on the hole quality in LTC-UAD of CFRP composites by kinematic characteristic analysis and drilling experiments. The increased amplitude ratio increases the effective rake angle of the main cutting edge, sharping the cutting edge and reducing the thrust force by 29.6%. The increased amplitude ratio also causes the reduction of up to 20.1% and 49.22% in burr factor and hole wall surface roughness, respectively, but the reduction in the delamination factor is not significant. The variable direction shear effect contributes to the shearing fracture of the fibers, thereby reducing the burrs and delamination. The variable stress state makes the fibers more susceptible to shear fracture, but the scraping and polishing effect of the minor cutting edge helps to remove excess fiber fragments and coat the hole wall evenly with the resin to fill the cavities, which in turn reduces the hole wall surface roughness. Additionally, the secondary cutting action of minor cutting edge on the hole wall is enhanced by the increased amplitude ratio, which results in a larger diameter deviation but within the range of 0.012 mm.
