Abstract: The process enables, while starting from the same basical members, assembled in two different arrangements, to drive an earth-boring tool such as a bit, a milling cutter, a core barrel or the like with a reduced speed or a fast speed, according to the prevailing boring conditions. The downhole motor comprises a shaft and a body constituted by two helicoidal gears arranged within each other, providing an encapsulation preferably of right-handed pitch and having K and K+1 teeth. For providing a reduced speed the body is rotatably mounted and the tool is connected to said body. The shaft is angularly fixed and free to undergo a nutation movement. The resulting speed is the relative speed of rotation between the said body and the said shaft. For providing a fast rotation speed, the body is angularly and radially fixed. The helicoidal shaft meshes with the body and is supported in order freely to rotate around a crank system. The tool is connected to the crank.