Extra wide multiple spindle drilling heads may need guides to prevent slight rotations.  A loose quill that rotates only a fraction of a degree can cause a wide head to move significantly at its' ends.  Some drilling machines are "tight" enough to eliminate this possibility. When the application tolerances are tight and there is any possibility that the drilling machine has some slight rotational slack rod guides should be added to the head.  Guides, lined with bronze bushings, can be easily added during the head's manufacture.  These guides, which are usually located on the out-board ends of a head, will eliminate any rotational movement.