Anti-tracking Earth Boring Drill

Abstract

In an earth boring drill, a cutter is disclosed with cutting teeth arranged to engage a selected annular area of the bore hole bottom in a non-tracking and cutter shell erosion preventing manner during bit rotation. The spacing of the teeth in different circumferential rows of the cutter is changed to maintain an optimum distance between the teeth. Further the teeth are arranged in groups of interrupted spacing, and interruption teeth are used selectively to arrange the pattern of teeth to prevent tracking and cutter shell erosion.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates in general to earth boring drills, and in particular to rotatable cutters that are adapted to effectively traverse an annular area on the bore hole bottom in a manner that prevents tracking of the teeth.

Definition of Terms

To help clarify some of the terms that may be susceptible to varying interpretation, the following definitions are provided:

A. "Tracking" is a condition which results when a cutter tooth engages a previously made depression in a bore hole bottom. As a result, a crest of rock may be generated on the bore hole bottom, which may lead to disadvantages such as erosion of the cutter shell or premature tooth disintegration.

B. "Circumferential row" refers to a row of teeth on a cutter shell that generally lie on a circle defined by a plane perpendicular to the axis of a conical cutter and extending through the cutter shell. Such rows may be spiraled, or otherwise deviate from a circular pattern, and if so, fall within the term "substantially circumferential."

C. "Spacing" of a cutter teeth relates to the angular measure between teeth. For example, a 36 tooth spacing means that 10.degree. separate the center lines of adjacent teeth in a circumferential row.

D. "Pitch" is the distance between the centerlines of adjacent teeth of a circumferential row, measured generally between the intersections of the centerlines with the surface of the cutter shell that supports the teeth.

E. "Integer" is a whole (not fractional or mixed) number.

F. an "independent cutter" is one having teeth arranged to remove an entire annular area of earth from a bore hole bottom without assistance from another cutter, even though two or more independent cutters may cover the same bottom hole area in the interest of faster earth removal.

DESCRIPTION OF THE PRIOR ART

It has long been known that consideration must be given to the sizing and geometrical form and placement of a rotatable cutter in an earth boring drill for the purpose of avoiding tracking. Tracking may cause the above mentioned premature wear of the cutter shell or its teeth, and thus may significantly increase drilling costs. Tracking is a greater problem in some types of drill bits then in others. If the cutters of a drill bit are designed for contact that approaches true rolling upon a bore hole bottom, tracking is more likely to occur. Yet true rolling contact is often advantageous to cutter life in rock drilling, especially in bits that utilize for example hard metal inserts or compacts of tungsten carbide for teeth which disintegrate rock due to a crushing action produced by relatively blunt protrusions of the compacts. Non-true rolling, or sliding contact with a bore hole bottom in such bits results in increased abrasive wear of the compacts without adding significantly to rock disintegration.

If true rolling contact with the bore hole bottom is utilized in drilling rock, it is known that proper sizing of the cutters helps avoid tracking. Specifically, regardless of tooth spacing, it is known that the ratio of the circumference described on the bore hole bottom by a row of cutter teeth to the circumference of that row on the cutter should not be an integer.

In addition, if the teeth are evenly spaced, it is known that the ratio of the circumference described on the bore hole bottom by a row of cutter teeth to the arc length on the cutter shell surface between the teeth should not be an integer.

Also, it is undesirable to have a tooth in a row of equally spaced teeth aligned with a tooth in an adjacent row such that they tend to act as one large tooth. This is especially disadvantageous if such alignment also leaves a large exposed surface area on the cutter shell which is susceptible to erosion through contact with the bore hole bottom.

Prior art drill bits have utilized a large variety of cutter teeth arrangements for the purpose of preventing tracking. Yet the problem still exists, especially in shaft cutters in the mining industry, where cutters are often designed to be independent, i.e., capable of independently removing the rock from an annular area of the bore hole bottom. Such cutters must have teeth distributions especially adapted to prevent tracking, since there may be no dissimilar cutters that tend to destroy any rock ridges that result from tracking.

SUMMARY OF THE INVENTION

It is the general object of the invention to provide in a drill bit one or more cutters having an improved cutter tooth arrangement that engages the bore hole bottom in a nontracking manner.

Another object of the invention is to provide in a drill bit one or more cutters with improved tooth distribution to produce uniform loading of the teeth as the cutter is rotated on bottom.

Another object is to provide in a drill bit one or more cutters with improved tooth distribution to reduce cutter shell erosion from the bore hole bottom.

The invention may be summarized as a drill bit having one or more cutters with rows of teeth of varying spacing. The teeth in selected rows are arranged in at least two groups of substantially equally spaced teeth, such groups being interrupted by a space that may contain a tooth. None of the grouped teeth of adjacent rows are allowed to be aligned on the same cone element, except for an interruption tooth which breaks the pattern of the groups of teeth to prevent tracking. This arrangement is especially advantageous when the cutter teeth are formed of wear resistant inserts or compacts in enabling tooth spacing that minimizes cone shell erosion or fracture.

Additional objects, features, and advantages of the invention will become more fully apparent with reference to the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective end view of a large diameter drill bit having cutters constructed in accordance with the principles of the invention.

FIG. 2 is a schematic layout showing a preferred cutter teeth spacing arrangement.

FIG. 2-A is a fragmentary cross sectional view containing the axes of two adjacent cutter teeth in a row.

FIG. 3 is a schematic side elevation view that shows one half of a cutter shell having a preferred geometry, with one tooth of each row rotated into a common plane.

FIG. 3-A is a schematic side elevation view of a conically shaped, true rolling cutter.

FIG. 4 is a perspective view of a cutter constructed in accordance with the principles of the invention, after having served its useful life.

FIG. 5 is a perspective view of a typical prior art cutter, after having served its useful life.

DESCRIPTION OF A PREFERRED EMBODIMENT

With reference initially to FIG. 1, the numeral 11 designates the body of a drill bit, which in this instance comprises a face plate 12 supporting a plurality of bearing and mounting means 13 that rotatably support cutter shells 15 from which protrude a plurality of cutter teeth 17.

The bearing and mounting means each include legs 18 that support load pins 19 which extend through the cutters and support bearing elements (not shown). The load pins are retained by suitable fasteners 20 and the mounting means are secured to the plate 11 by suitable means such as weld 21.

The bit illustrated in FIG. 1 is of the reaming type having a sleeve 23 surrounding a drill stem 25 that is threaded at 27 to receive a driving means (not shown). The innermost cutters 29 of the drill bit assembly have their load pins supported by the sleeve 23, while the intermediate cutters 31 and outer cutters 33 have their load pins supported by inner legs 18 and an outer leg (not shown).

As previously stated, it is sometimes advantageous to have true rolling contact between each cutter and the bore hole bottom. As seen in FIG. 3-A, the apex 34 of cone 29 is as close as practicable to the center line or axis of rotation 37 of the bit body.

For the purpose of preventing tracking in a true rolling cutter, the radius R from the outer edge 35 of the cutter to the center line 37 of the bit body, when divided by the radius r from the outer edge 35 of the cutter to its axis of rotation 39, must produce a non-integer. If an integer or a quotient approaching an integer is produced, the teeth will track. The cutter 29 illustrated schematically in FIG. 3 has a forty degree conical angle .alpha., or an angle .alpha./2 of 20.degree., but this angle may vary depending upon cutter location on the bit body and the configuration of the bore hole bottom.

The intermediate cutters 31 in FIG. 1 also preferably have an imaginary apex that falls on the bit axis or centerline 37 of the bit, as do the outer cutter 33. However, it has been found that exactly true rolling contact is not always feasible from the manufacturing point of view, and hence, some variation occurs from the aforementioned conditions requiring for true rolling. For example, satisfactory results have been obtained with the particular bit illustrated where apexes of the cutters fall between the bit axis 37 and the outside diameter of the drill stem 25.

The cutter shell 15 is substantially a cone, with truncated nose and heel regions 40, 41 as shown in FIG. 3. For the purpose of eliminating sharp corners, however, and facilitating better compact retention, a bevel 43 may be included on the inner end region, and a double angled bevel 45 on the outer end region. Irrespective of cutter shell geometry, true rolling contact may be maintained (if the above described limitations are not) so long as the outer edges of cutter teeth 17 fall on an imaginary conical surface that includes the cone element 47, as shown in FIG. 3. Manufacturing limitations may prevent the placement of all teeth in this manner, but this is the preferable arrangement.

After cone geometry and placement on the bit body are established, tooth placement must be considered in accordance with the principles of the invention to further decrease the possibility of tooth tracking.

A feature of the invention is to provide a group of substantially evenly spaced teeth relative to an adjacent row of equally but differently spaced teeth such that all teeth of the group are no closer than a minimum.

The bit illustrated has tungsten carbide inserts or compacts with cylindrical bodies retained by interference fit in mating apertures formed in the cutter shells. The minimum distance between compacts (X in FIG. 2-A) cannot be too close or else the cutter shell metal may fracture prematurely during drilling. Also, the distance between compacts, as measured on the cutter surface (Y in FIG. 2-A) must be small enough to prevent premature cutter failure.

With reference to FIG. 2, the innermost or nose row R 1 of cutter teeth is illustrated as having a starting tooth 51 (illustrated in heavier line) which is one of a group of equally spaced teeth forming a circumferential row. The adjacent row R2 of FIG. 2 comprises a partial row of teeth, specifically nine teeth in this instance, that are on the same spacing as the teeth of row R1 being preferably centered midway between the teeth of row R1. Thus, row R2 is a partial one on a selected cutter shell region 52 on approximately one side of a plane 53 containing the axis 39 of the cutter. Similarly, the cutting teeth of row R3 define a partial row on another region 54 of the cutter, there being eight equally spaced teeth in this instance on the same spacing of rows R1 and R2 located preferably midway between the teeth of adjacent row R1. As shown, the partial rows R2 and R3 are staggered relative to one another to achieve better tooth distribution on the bore hole bottom.

On rows progressing outwardly toward the outer end of the cutter, it ultimately becomes necessary to decrease the spacing, since as shown in FIG. 2-A, the distance Y on the cutter surface between compacts in a row must not become too great. This change of spacing first occurs in row R4, in which the starting tooth or compact 55 is aligned as close as practicable to a cone element 57 that passes through the center of compact 59 of row R2. For the bit illustrated compacts 55 and 59 should be spaced no closer than about one compact diameter.

The compacts of row R4 may be on a nineteen tooth spacing. Thus compact 61 is further from compact 63 of row R2 than compact 55 is from 59; compact 65 of row R4 is further from compact 67 of row R2; compact 69 of row R4 is about midway between compacts 67 and 71 of row R2; but compact 73 of row R4 is relatively close to the opposite side of compact 71 of row R2. To prevent alignment of compact 75 of row R2 and any compact of an equally spaced group in row R4, the nineteen tooth spacing of row R4 is interrupted for a distance to begin another group of compacts starting with compact 79, followed by additional equally spaced compacts 81, 83, and 85, (identified as Group B) which are thus spaced relative to the compacts of row R2 in a manner similar to the spacing of the group of compacts 55, 61, 69 and 73 (identified as group A) relative to their adjacent compacts in row R2.

Compact 79 of Group B in row R4 is misaligned with the closest compact 80 of the adjacent row R2, just as starting compact 55 of row R4 is misaligned relative to the closest compact 59 of row R2. Hence, the relationships pattern between groups A and B of row R4 and the compacts of row R2 is produced.

It has been found advantageous to use an interruption compact 77 in the space separating groups A and B, even though such interruption compact is aligned with a compact of an adjacent row. For example, compact 77 of row R4 falls on cone element 89 passing through the center of compact 75 of row R2. Since compact 77 of row R4 is confronted with a different pattern of rock crests on the bore hole bottom, the compacts 75 and 77 cannot act as one, but function efficiently as separate teeth on different rock configurations.

In selection of the rows R1 - R16, the spacing is changed from the previous row and the compacts divided into two interrupted groups such that no compact of a group is aligned with a cone element passing the center of a compact in an adjacent row, except for the interruption compact, which is permissible for the reason explained above. This spacing pattern is followed on intermediate rows, but is altered in adjacent rows R14 and R16, which have some of the compacts of adjacent rows aligned. The compacts of these rows are not grouped, but Row R16 has a selected number of compacts on one cone section (for example, thirteen compacts on a 31 tooth spacing on cone section 52) and another selected number of teeth on another cone section (for example, fourteen compacts on a 25 tooth spacing on cone section 54). Row R14 has teeth on a selected spacing such as, for example, thirteen teeth on a 25 tooth spacing on cone section 52. The alignment of some compacts of rows R14 and R16 is advantageous since the heel row has a large number of compacts in one complete row formed of partial rows of teeth of widely different spacing. Hence, any tracking pattern in the heel row R16 and with row R14 will be broken. It is, however, within the broad scope of the invention to arrange the compacts of rows R14 and R16 in groups of interrupted teeth as previously described in use on the intermediate rows.

FIG. 5 illustrates a typical prior art cutter shell 91 having a plurality of compacts 93 protruding therefrom. The spacing of such a prior art bit results in the grouping of compacts such that they tend to form pairs as indicated by the numeral 95 with resulting void spaces that lead to cone erosion 97.

FIG. 4 illustrates a cutter assembled in accordance with the principles of the invention, after completing its useful life, and shows that the compacts 99 are in a worn condition without erosion of the metal 101 of the shell. The compacts exhibit the desirable evenly worn condition.

While the invention has been shown and described in only one of its embodiments, it should be apparent to those of skill in the art that it is not thus limited but it is susceptible to various changes and modifications without departing from the spirit thereof. It is possible within the broad scope of the invention that the spacing within groups of compacts may not be exactly equal, but may be varied to still fall within the term "substantially equally spaced."

Claims

I claim:

1. For an earth boring drill, an improved cutter comprising:

a cutter shell;

a first row of substantially circumferential and substantially equally spaced teeth secured to said cutter shell;

a second and adjacent substantially circumferential row of teeth secured to the cutter shell in two groups of teeth of spacing differing from that of the first row, the groups being separated by an interruption space;

none of the teeth of said groups lying on a cutter shell cone element passing through the center of any tooth of the first row.

2. The cutter defined by claim 1 in which an interruption tooth is secured to said cutter shell in said interruption space.

3. For an earth boring drill, an improved cutter comprising:

a substantially conically shaped cutter shell, truncated to form a nose region and a heel region;

a nose row of teeth on said nose region;

intermediate rows of teeth on said cutter shell, at least one of which has spacing differing from that of the nose and heel rows;

among rows adjacent row of teeth of differing spacing, a first row of substantially circumferential teeth of selected spacing;

a second and adjacent row of substantially equally spaced teeth of different spacing from the first row, comprising two groups being separated by an interruption space containing an interruption tooth;

none of the teeth of said groups lying on a cutter shell cone element passing through the center of any tooth of the first row;

a heel row of teeth on said heel region.

4. The cutter defined by claim 3 in which selected interruption teeth are aligned with teeth in groups of adjacent rows.

5. The cutter defined by claim 4 in which said teeth are wear resistant compacts secured in mating apertures in the cutter shell.

6. For an earth boring drill, an improved cutter comprising:

a cutter shell;

a first row of substantially circumferential and substantially equally spaced teeth;

a second and adjacent substantially circumferential row of at least two groups of substantially equally spaced teeth separated by an interruption space and having a spacing different from that of said first row;

With the first and last teeth of each group being no closer than a selected minimum distance from any tooth of the first row.

7. The cutter defined by claim 6 which further comprises an interruption tooth separating said groups.

8. The cutter defined by claim 7 in which the teeth are compacts of cylindrical body form, and the selected minimum distance is about one compact diameter.

9. An earth boring bit comprising:

a bit body;

a plurality of cutter shells sized and mounted to said bit body for substantially true rolling contact;

a fist row of substantiAlly circumferential and substantially equally spaced teeth on said cutter shells;

a second and adjacent substantially circumferential row of two groups of teeth secured to the cutter shell in a tooth spacing differing from that of the first, the groups being separated by an interruption space;

none of the teeth of said groups lying on a cutter shell cone element passing through the center of any tooth of the first row.

10. The earth boring bit defined by claim 9 in which said cutter shell includes an interruption tooth in said interruption space.

11. The earth boring bit defined by claim 10 in which said teeth are wear resistant compacts secured in mating apertures in the cutter shell.