Circular indexing plate for a machine tool

In a machine tool having a circular indexing plate adapted to support a load, said plate being formed with an upper and lower face, means mounting said plate for rotation about a vertical axis, a plurality of jacks disposed along the periphery of said plate and bearing against portions thereof to support said plate, each of said jacks including a body portion and a movable portion, said movable portion being adjustable vertically, at least between at low end of stroke and a high end of stroke, the stroke of each so disposed jack movable portion in the direction of said plate being capable of exceeding the horizontal position of said plate without load each of said jacks including control means for determining the vertical position of each said movable portion including at least two sets of contacts mounted on each of said jacks, one of said sets being arranged to provide control contact at the level position of said plate without load and the other set being arranged to provide control contact at the high end-of-stroke of said jack movable portion and means mounted on each said jacks responsive to the movement of the movable portion with respect to said body to actuate one of said contact sets.