The illustration below shows how a zero-point shifts in traditional systems. To compensate for common bore-hole gauge differences that occur in the field (caused by temperature, for instance) a counter weight is required between the basic unit and the stage. Three different types of bolts are usually utilised, all arranged in a way where the zero bolt sets the position, the floating bolt may only be moved in one direction, and the free bolt is movable in both directions.

Now if you imagine that the illustration below shows the rotation axis (of a horizontal center, for instance), the problem quickly becomes obvious: The center of the stage and/or of the workpiece shifts from the rotation axis, slowly leading to a multiple errors in various angular positions.