Member had installed 1st row of bolts in #4 entry. He then installed a bolt in the next row and swung the drill head out to spot a bolt, when coal/rock rib measuring 14' L, 20"" T and H was 68"" struck members left leg, pinning his leg against the drill head near a panel switch. Left leg above knee received laceration & abrasion wound. Surgery repaired muscles and tendons.