A clamp assembly for selectively clamping a movable cross-head assembly to each column of a press having a plurality of columns. The clamp assembly includes an inner clamp sleeve positioned concentrically about each column. The inner clamp sleeve is slidably longitudinal with respect to the column and has an inner generally cylindrical surface for engaging the column. An outer clamp sleeve is secured to the cross-head assembly concentrically about the inner clamp sleeve. The inner and outer clamp sleeves are threadeably secured together to limit longitudinal movement of one clamp sleeve with respect to the other. Although threaded together, there is sufficient clearance between the opposing threads of the inner and outer clamp sleeves to define a helical chamber therebetween. A hydraulic fluid system is provided for selectively introducing pressurized hydraulic fluid into the helical chamber. The pressure of the hydraulic fluid causes the inner clamp sleeve to shrink so that its inner surface is urged substantially uniformly against the column to limit longitudinal movement of the cross-head assembly with respect to the column.