Abstract: Improvements in tool holders in the field of machine tools; quick change connectors for replacing one tool holder with another in chucks mounted on rotary machine tool drive shafts and spindles; tool changing attachment permitting rapidly changing or switching one drilling, cutting or milling tool holder or collet for another involving removably engaging connectors employing mere operator hand force in effecting engaging or disengaging of the collets or tool holding adapters with the driving chuck; employing a coil compression spring as a locking element in tool holding apparatus between a chuck and a tool carrying collet, the spring captured in compressive engagement between bearing surfaces carried by the chuck and the collet, one such bearing surface provided in a nut threading on the chuck operable to hold and force the spring against a bearing surface on the collet.