Roof bolters were going to bolt #4-5 x-cut. 2 rows outby the cut, they stopped to spot 2 or 3 bolts. After installing the first bolt near the rib, the left side operator swung head to spot another bolt. After drilling the hole, he was removing steels when a piece of rock hit the canopy, broke, & part of it came under canopy cutting his left arm with sharp edge.