A cutter robot used to operate in a horizontal pipe having a horizontally extending main body, a stand gear to support the main body aloft, and a head bearing a rotary grinder assembly, and this head consists of: (i) a motor having an upward output shaft to bear the grinder assembly locked about its end, (ii) two holders, and (iii) two vertically held piston cylinders, which are arranged such that the first piston cylinder connected to the main body has an upward piston rod, and the first holder means supported at the end of the upward piston rod of the first piston cylinder holds the second piston cylinder, and the second holder means supported at the end of the downward piston rod of the second piston cylinder holds the motor.