Gravity percussion drill with upper end cocking spring and method of assembly

A multiple-blow gravity drill structure comprising:

• an elongated casing with a longitudinal enclosed bore, the casing having means at its upper end for attachement to a lifting and dropping cable tool apparatus so that the casing will hang in a generally vertical position, the casing also having means at its lower end for detachably holding a drill bit, and the casing having an upwardly facing lower casing anvil in its lower end that projects upwardly into the lower end of the bore;
• a hammer disposed within the bore for reciprocal movement within the casing, the hammer having an elongated cylindrical body with a downwardly facing lower hammer anvil opposed by the upwardly facing lower casing anvil, the hammer also having a neck of relatively shorter length and reduced cross section compared to the body, the neck extending upwardly from the body, the neck having a flange around its upper end and the neck having means at its upper end for being pressed down by a portion of the casing that cocks the hammer;
• spring opposition means disposed in the bore around the neck of the hammer to allow the hammer to slide therethrough, the spring opposition means being restricted against movement in the direction in which the force of gravity pulls the hammer;
• and
• a cocking spring compressed between the flange on the hammer and the spring opposition means when the hammer is cocked against the casing, the cocking spring having a rate of compression holding the lower hammer anvil off the lower casing anvil when the hammer is cocked, while allowing the spring to yield sufficiently when the drill structure is dropped to a point of impact so that the lower hammer anvil will provide a follow-up blow against the lower casing anvil shortly after the drill bit on the lower end of the casing has struck an initial blow.

A method of assembling a multiple-blow gravity drill stem, the method comprising:

• forming a hammer assembly by
• positioning a spring restricting member on a reduced upper portion of an elongated hammer having anvils at its upper and lower ends, the spring restricting member being slidable along the reduced portion of the hammer until meeting a portion of the hammer of full cross section,
• positioning a cocking spring on the reduced upper portion of the elongated hammer above the spring retaining member, and
• mounting a spring retaining member on the reduced upper portion of the elongated hammer above the cocking spring;
• inserting the hammer assembly into a lower portion of a casing for the drill stem in which a ledge is formed by a reduction in the cross section of a casing bore to restrict the depth of insertion of the restricting member, the hammer being of smaller cross section than the bore so as to slide past the ledge;
• pressing a casing cap onto the upper hammer anvil to cock the spring while moving the lower end of the cap downward to meet the upper end of the lower portion of the casing;
• and
• connecting the casing cap to the lower portion of the casing to close the hammer assembly within the bore, to hold the cocking spring in a cocked position in which the lower anvil of the hammer is spaced apart by a gap from an anvil at the lower end of the bore.