A composite rotary cutting tool and adaptor includes a straight cutting tool, such as a drill bit or the like, having a cylindrical shank adhesively bonded within a close fitting axial bore in an adaptor or gripping member which has an outer circumferential gripping surface larger than the cutting tool shank. The cutting tool shank has at least one helical channel or groove formed therein so as to maintain a substantial portion of the original cylindrical surface area. The adaptor is hardened in a manner to prevent scale formation in the bore, and an anaerobic adhesive is applied within the bore and along the shank and into the helical groove. The shank is inserted into the adaptor bore to effect full wetting between the interfacing surfaces. The assembly is then fixtured in a vertical orientation and heat cured so as to obtain a composite cutting tool and adaptor having high torque and low concentricity run-out and angularity error.