Milling is performed by construction equipment called milling machines or cold planers. These machines use a large rotating drum removing and grinding the road surface. The drum consists of scrolls of tool holders. The scrolls are positioned around the drum such that the ground pavement is moved toward the center and can be loaded onto the machines conveyor belt. The tool holders can wear out over time and can be broken while milling if highway structures like manholes are encountered while milling.(source)