The milling bit rotates during the bit drilling cement plug and the milling head cuts the edge of the plug to form cement column.

• 新型套铣钻头在钻塞时钻头旋转,使套铣头旋入灰塞边缘后形成灰柱,单向铣牙做圆周运动切削灰柱,实现钻塞。