Abstract: A tool post has a central bore formed with a normal screw thread. A hub is turnably received in the bore and has a thread cooperating with the thread of the post; it also has one or more steeply pitched helical tracks. One or more undercut grooves are formed in the post, each having a portion open to the bore. Longitudinally diverging wedges are slidably received in these grooves, each having a follower portion projecting into a respective helical track, so that turning of the hub causes the wedges to be shifted axially of the tool post. A sleeve extends from one end through the tool post bore. A slide block has a threaded stud which extends from the other end into the bore of the post and with a tapped blind bore of the sleeve.