EE was roof bolting in the #4 entry on the MMU. As the EE was drilling the hole, the coupling that connects the drill steel together broke causing the 2 sect.of steel to come apart. The upper piece of drill steel fell out of the hole which bounced off the drill canopy striking the EE in the lower left ribs, caused a small burn on upper side of the left bicep,abrasion on left ribs.