Reamer – a drilling tool to impact wellbore walls. Consists of steel body with mounted (inserted) rock cutting elements. To prevent wear-out of rock cutting elements (diamond crowns, bits) the well should be gauged by diameter. Diamond and roller reamers of diameter exceeding the main rock cutting elements by several millimeters are used to prevent wear-out, to stabilize the drill stem operation, to decrease vibration and wellbore deviation.