The drill guide can be secured to a workpiece and has one or more drill guide bushes (14). A screw spindle (1) has one end provided with right-handed (2) and the other end with left-handed thread (3) of equal pitch, each end engaging a corresponding thread (6,8) in a clamp jaw (7,9). An un-threaded cylindrical portion between the ends engages a bore (10) in an element (11) containing the bushes (14). This element is axially located on the spindle, e.g. by circlips (12,13). The jaws, or the jaws and the element, may be provided with aligned bores (15-17) so that they are slidable on a (releasable) rod (18) parallel to the spindle.
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