FIELD: oil and gas industry.;SUBSTANCE: blade underreamer includes a body of cylindrical form and rock destruction tool made as blade symmetrical in all directions, wider in the central part and narrowing gradually to ends. The blade end parts are separated from the main middle part by springing inserts, for example, by plates perpendicular to the blade rotation axis. At that the blade end parts are made as a half of ellipsoid of rotation with embedment of cutter elements, in particular, single-point diamonds along its whole surface. The blade is made so that it can be rotated in vertical plane at horizontal pin penetrating the body and blade in their middle. The body has two diametrical openings shaped to lengthwise cavities in width equal to the biggest thickness of the blade in retractable part. One opening is higher than the second one; it allows burial of one half of the blade inside the body through the lower opening and in the other part of the blade through the upper opening in transport position. When the blade is retracted fully into operating position through openings it is placed perpendicular to longitudinal axis of the body. At that blade the bottom part of the blade medium top part is placed on upper opening lower surface while blade top part upper surface thrusts against bottom opening upper surface thus locking the blade in operating position. The top part of the body has a groove with thread made at its conical lateral side for the purpose of its connection to the pipe string. Below this thread, through groove bottom inside the body there is a cylindrical stepped channel placed in parallel to longitudinal axis of the body and offset in regard to this axis, in wide top part of it there is a piston (12) closed from the top by cover with two openings: small feedthrough opening and large opening connected to return valve at the cover top. The piston is made as integrated unit with pusher passing through narrow lower part of the channel and having a groove contacting the blade top part at free end, bottom surface of the groove is made as an arc. Cylindrical bore shifted relative to body lengthwise axis is arranged nearby said stepped channel, in parallel to it, to feed flushing fluid to processing zone.;EFFECT: improved productivity of well reaming, improved reliability of the equipment and its maintainability, increased reaming size of the well.;4 dwg
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