Abstract: A collar for axially clamping tools, spacers and the like on a shaft having a smooth portion and a threaded end portion includes an internally threaded attachment ring to fit on the threaded end of the shaft, an adjustment ring adjacent the members to be clamped and tightening screws threaded through the attachment ring and abutting the adjustment ring to axially urge the latter against the clamped members. The attachment and adjustment rings are interconnected by annular flexible connecting portions and an intermediate ring through which the screws freely pass. A second embodiment includes a second intermediate ring and additional connecting portion. A third embodiment includes an outer ring and clamping screws to urge the outer and attachment rings together to improve the grip of the collar on the threaded end of the shaft.