Patent Application
20070034415
One type of earth boring bit has cones that rotate as the bit revolves around a bit axis. Each cone has a number of rows of cutting elements. In one type, the cutting elements comprise tungsten carbide inserts pressed within holes formed in the exterior surface of the cones. Each insert has a cylindrical barrel that fits in an interference fit in one of the holes. A cutting end or tip is integrally formed with the barrel and protrudes from the hole. The cutting tip may be of a variety of configurations, such as hemispherical, ovoid, or chisel-shaped.
Typically, in the prior art, the holes are drilled in annular lands formed on the exterior of the cone. The land is normally cylindrical or conical. The depth of the hole sidewall normally equals the length of the barrel. The land is thus flush with the junction between the barrel and the cutting tip. The entire cutting tip protrudes from the land.
In some bit cones, part of the holes may be drilled into individual spaced apart circular counterbores, rather than an annular land. In those cases, the sidewall of the hole has a depth, measured from the base of the counterbore, that equals the length of the barrel. The junction of the barrel with the cutting end is flush with the base of the counterbore.
A number of years ago, reduced projection inserts were used in the nose and inner rows of some bits manufactured by the assignee of this invention, particularly for hard formations. In those bits, the junction between the barrel and the cutting end of the reduced projection inserts was recessed in the hole. The depth of the hole sidewall was approximately 0.060 inch greater than the length of the barrel. As a result, only a portion of the cutting end protruded past the land. The recession was done to reduce the risk of coring due to lost inserts in the nose area. To the inventors' knowledge, inserts with recessed cutting tips were not located in the outer or heel row or the adjacent row in those bits. The design of recessing the cutting tips of the inserts in the nose and inner rows of certain bits was discontinued several years ago by the assignee of this application.
The outer row inserts encounter the most severe conditions during drilling. These inserts are more subject to breakage due to high bending forces imposed on the inserts. Inserts are made less resistant to breakage by increasing the toughness, but this decreases the hardness. A decreased hardness generally results in the insert wearing faster in hard formations. Also, outer row inserts are made more resistant to breakage by having a shorter cutting tip, which reduces the moment arm on the insert. However, in the prior art, it has been generally considered that reduced insert projection reduces the rate of penetration. Also, the loss of inserts, even with a shorter cutting tip, still occurs.
In this invention, the cutting ends of outer row inserts are recessed in holes deeper than the barrel lengths. Each hole has a cylindrical sidewall with a substantially constant diameter. The sidewall has a depth that is greater than a length of the barrel of the insert.
During the press-fit procedure, an entry portion of the hole is distorted. The distorted entry zone is slightly enlarged relative to the remainder of the hole. As a result, the distorted entry zone provides less gripping force than the remainder of the hole. In this invention, at least a majority of the distorted entry zone is above the barrel and encircles part of the cutting tip. As a result, the barrel experiences the full designed gripping force throughout its length. Inspections have determined that the entry zone is typically about 0.080 inch in depth. Preferably, the minimum difference between the barrel length and the sidewall depth is 0.080 inch.
The holes for the outer row may be drilled in an annular land that has a tapered outer edge. If so, at the outer edge, the sidewall depth is less than at the inner edge, 180 degrees thereform. The sidewall depth of the hole at the outer edge should be greater than the length of the barrel and is preferably greater than the depth of the distorted entry zone, so that the barrel is not in contact with any part of the distorted entry zone. In one embodiment, the sidewall depth at the minimum depth point at the outer tapered edge of the land is 0.080 inch greater than a length of the barrel. Preferably, inserts are mounted in holes for the adjacent row in the same manner. Inserts for some of the inner rows may also be mounted in the same manner.
Referring to
Each cone 17 has a generally conical exterior with a gage surface 19 on its outer end and a nose area 23 on its inner end. A plurality of inserts 20, shown schematically in
Referring to
Insert 20 is mounted within a hole 39 that is drilled into the body of cone 17 through land 37. In the preferred embodiment, hole 39 is normal or perpendicular to inner portion 37b of land 37, although it could incline, if desired. Hole 39 has a bottom 39a that is normally conical as a result of the point of the drilling tool used to form hole 39, or it may be flat. Hole 39 has a cylindrical sidewall 39b with a depth measured from its intersection with bottom 39a to land 37. In the example of
Prior to installing insert 20, the diameter of hole sidewall 39b is initially the same measured from its bottom edge to land 37, within conventional manufacturing tolerances. This initial diameter is slightly smaller than the outer diameter of barrel 31 to create an interference fit. During the press fit procedure, insert 20 is pushed into hole 39 with a high force. Inspection has shown that an entry portion 40 of sidewall 39b becomes slightly permanently distorted. Entry portion 40 flares out and becomes slightly enlarged in diameter relative to the remaining portion. The amount of outward deflection is slight, thus the diameter of hole 39 still is considered constant from the bottom edge of sidewall 39b to land 37. Nevertheless, it has been found that entry portion 40 fails to grip insert 20 as adequately as the remaining portion of sidewall 39b. The portions of sidewall 39b below entry portion 40 are only elastically deformed, thus provide a desired confining stress to barrel 31. In the prior art, the permanent deformation of entry portion 40 may still be in contact with barrel 31, but does not provide adequate confining stress because it is permanently deformed.
The depth of entry portion 40 along sidewall 39b can be predicted by finite element analysis or physically observed by laboratory instruments by removing previously installed inserts 20 before the bit is run. Inspections have shown that distorted entry portion 40 normally has an average depth of about 0.080 inch, although variations do occur, of course. The depth of entry portion 40 does not appear to be dependent on the diameter or depth of sidewall 39b.
In this invention, hole 39 is drilled deeper than in the prior art to preferably assure that substantially the entire entry portion 40 is entirely above barrel 31. Barrel 31 is thus preferably entirely retained within a portion of hole 39 that has only undergone elastic deformation. The minimum sidewall 39b depth Dmin is made greater than the length L of barrel 31, preferably by at least the average depth of distorted portion 40, which has been found to be about 0.80 inch. The depth Dmax at the innermost point is even greater than barrel length L. A portion of hole sidewall 39b thus completely encircles cutting end 33.
Cutting end 33 protrudes past land 37 at all points, even where sidewall 39b has its maximum depth Dmax. The amount of protrusion of cutting end 33 depends on a number of factors, such as the drilling rotation speed and the type of earth formation being drilled. Generally, in very hard formations, the amount of penetration of the bit into the borehole bottom is only about 0.050 inch or less per revolution. For the heel row 21, the penetration is even less because three redundant rows cover that portion of the borehole bottom. If bit 11 is to be used in such a formation, the difference between sidewall maximum depth Dmax and barrel lengh L should be limited to result in cutting end 33 protruding past land 37 at least 0.050 inch at any point.
Referring to
Referring to
The invention has significant advantages. The deeper hole provides increased retention by positioning substantially all of the deformed entry portion around the cutting end, rather than around any portion of the barrel. The deformed entry portion provides additional support to the cutting end by serving as a buffer to reduce the load and impact forces on the insert. The reduced projection of the cutting end does not diminish the rate of penetration yet reduces breakage. The reduced projection allows a harder or more wear resistant grade of carbide to be used for the inserts due to better confining stress around the entire barrel section of the insert.
While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, it may be useful in some applications to have conventional depth holes for the outer row and reduced projection inserts in the adjacent row.
