A drill rig consisting of a rail-track carriage on which is mounted a counterbalanced boom 10 ft (3 m) in length. For two-drill mounting, a crossbar is attached to the boom, and drill cradles can be fixed over or under the bar by swinging it through 180 degrees . For a four-drill mounting, two additional vertical columns are used; these are jacked to the floor and the drill carriage clamped to the track. The crossbars can be clamped to any required position on the vertical columns.