To adjust the relative positions of two elements mounted for guided sliding relative motion in a first direction and in a contact plane thereof, a cylindrical adjusting tool is fitted into a reference bore in one element and rotationally and non-slippably engaging an edge of the other element. A manually operable button on the tool may be depressed to remove a cam permitting adjustment rotation of the tool, or released to secure the tool in the bore while clamping means are operated integral with the two elements, and the tool can then be removed. Substantially one-handed adjustment is thus possible leaving an operator's other hand free to operate other apparatus.
