Self-excited drill bit sub

A drill bit sub attachable between a running string and a drill bit for use in drilling a well, the sub comprising cooperatively threaded upper and lower tubular members defining an oscillation chamber therebetween,

• the upper tubular member comprising an upper section having an axial bore with a first diameter, the axial bore being bounded at a lower end by a substantially flat, downwardly facing, first annular surface, a middle section having a second coaxial bore defined by a sidewall with an upper end terminating at the first annular surface, the second coaxial bore having a second diameter from about 3 to about 5 times greater than the first diameter, an axial length ranging from about 1.6 to about 5.6 times the first diameter, and a lower, downwardly facing, threaded section,
• the lower tubular member comprising an upwardly extending portion cooperatively threaded for engagement with the downwardly facing, threaded section of the upper tubular member, the upwardly extending portion having a third coaxial bore with a diameter greater than the first diameter of the upper tubular member, the upwardly extending portion terminating in an upwardly facing, substantially flat, second annular surface having a width of at least about ⅛inch, the second annular surface being surrounded by a downwardly and outwardly extending truncated conical surface, the conical surface having a base surrounded by a third, substantially flat annular surface having an outer perimeter coextensive with the sidewall of the cylindrical bore of the middle section of the upper tubular member,
• the first, second and third annular surfaces, sidewall and truncated conical surface cooperating to define an oscillation chamber within the drill bit sub whenever the upper and lower tubular members are threadedly engaged,
• the lower tubular member further comprising a downwardly opening, coaxially aligned, threaded section attachable to a drill bit.