Abstract: A vertical milling or boring machine has a tool head supporting column which rides on base mounted rails. A ball, screw extends along the base under the column. Each column has a motor driving a nut carried by the ball screw. The nut is caged by the column so that rotation of the nut moves the column along the rails. To prevent ball screw droop, a slidable block is provided on either side of the column which operate as follows. The column pulls one of the these supports along until it latches to a post proximate the middle of the ball screw whereupon this latch overcomes the connection between the column and the slidable support leaving the slidable support latched to the post. When coming back the other way, the column pulls the other slidable support. The column pushes the first support off the post and latches to the first support when the column encounters the first support and leaves the other slidable support latched to the post once the column passes the post.