Patent Publication Number: US-6655480-B1

Title: Cutting insert for percussion drill bit

Description:
TECHNICAL FIELD 
     The present invention relates a cutting insert for a percussion drill bit. 
     BACKGROUND ART 
     In general, there are four different face designs used in the hammer bit industry. They are: drop center, concave, flat face and convex (sometimes called double bevel or double gauge). Each has its own advantages and disadvantages for different applications. The placement, diameter, shape and number of tungsten carbide inserts used on these bits vary by face design and manufacturer. In general, the smaller the carbide diameter, the faster the penetration rate. The trade-off with the smaller carbides is that they wear out faster and require more frequent re-sharpening. Larger carbides, while giving up penetration rate, will last longer and are less prone to shear failure. 
     Most face designs incorporate large diameter carbides on the gauge row for greater wear resistance and smaller diameter carbides on the inner rows where wear resistance is minimized as a result of reduced speed of the inserts closer to the longitudinal axis of the bit. The most popular carbide designs are dome-shaped and conical-shaped. Dome-shaped carbides are standard on most bits and conical carbides can be ordered as optional equipment. The advantages of conical carbides are higher penetration rate, bigger chip size and more efficient rock breakage. The primary disadvantage of conical carbides is that they are more fragile than dome-shaped carbides and are therefore used primarily in soft consolidated formations only. 
     DTH (down the hole) carbides are subject to various types of wear during drilling. The majority of the wear is caused by abrasion as the bit rotates against the bottom of the hole and against the hole wall, causing “wear flats.” If wear becomes excessive, bit life and hammer performance can deteriorate. When a carbide is excessively worn, stress is higher on the carbide and can lead to premature failure. 
     The two types of wear addressed by the present invention are referred to as “frontal wear” and “gauge wear.” Frontal wear occurs when drilling in hard rock, such as granite. In this case, the gauge row carbides will wear faster than front carbides because of the greater distance covered around the outside of the bit during rotation. Gauge wear occurs when drilling in abrasive rock with a high quartz content. An “anti-taper” develops which diminishes the clearance of the bit body caused by unusually high wear to the gauge row. The typical solutions to these problems are: changing the grade of carbide used in the gauge row; increasing the diameter used in the gauge row; re-sharpening the inserts; or decreasing the penetration rate. 
     Accordingly, it is desirable to provide an improved gauge row cutting insert with extended service life without loss of penetration rate. 
     DISCLOSURE OF INVENTION 
     The present invention overcomes the above-referenced shortcomings of prior art cutting inserts by providing a cutting insert having the service life expectancy of a dome-shaped insert and the rate of penetration (ROP) of a cone-shaped insert. The insert has a cutting head which is a non-symmetrical dome shape, wherein carbide is reduced in the trailing portion and increased in the working portion of the cutting head. 
     More specifically, the present invention provides a cutting insert for a gauge row (or other location) of a percussion drill bit, including a generally cylindrical mounting portion extending along a central axis and a cutting head extending from the mounting portion. The cutting head has an exposed surface forming a non-symmetrical dome shape such that a working portion of the cutting head is enlarged with respect to a standard dome shape and a trailing portion of the cutting head is reduced with respect to a standard dome shape. The cutting head has an inner edge and an outer edge. The exposed surface is non-symmetrical along a central plane which bisects the working portion and trailing portion. The exposed surface is symmetrical in a second plane perpendicular to the central plane such that the exposed surface forms an arc of constant radius from the inner edge to the outer edge. 
     Preferably, the exposed surface is symmetrical in any intersecting plane which is perpendicular to the central plane such that the exposed surface forms an arc of constant radius in any such plane from the inner edge to the outer edge. 
     The invention also provides a percussion drill bit including a shaft with a boring head positioned at a distal end of the shaft and having a longitudinal axis. The boring head has an upwardly facing upper end with a plurality of holes formed therein. The holes include a gauge row of holes. A cutting insert, as described above, is positioned in each hole of the gauge row of holes. 
     Accordingly, an object of the invention is to provide an improved cutting insert for a percussion drill bit wherein the cutting insert has an exposed surface which is non-symmetrical in a plane bisecting the working portion and trailing portion, and symmetrical in a second plane perpendicular to the central plane such that the exposed surface forms an arc of constant radius from the inner edge to the outer edge in the second plane. 
     Another object of the invention is to provide a cutting insert for a gauge row of a percussion drill bit having a cutting head with an exposed surface which is non-symmetrical along a central plane bisecting the working portion and trailing portion, and symmetrical in any intersecting plane which is perpendicular to the central plane such that the exposed surface forms an arc of constant radius in any such plane from the inner edge to the outer edge. 
    
    
     The above objects and other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a perspective view of a percussion drill bit in accordance with the present invention; 
     FIG. 2 shows a side view of a cutting insert for use with the drill bit of FIG. 1; 
     FIG. 3 shows a front view of the cutting insert of FIG. 2; 
     FIG. 4 shows a rear view of the cutting insert of FIG. 2; 
     FIG. 5 shows an overhead plan view of the cutting insert of FIG. 2; 
     FIG. 6 shows an overhead plan view of the cutting insert of FIG. 2; 
     FIG. 7 shows a cross-sectional view taken at line  7 — 7  of FIG. 6; 
     FIG. 8 shows a cross-sectional view taken at line  8 — 8  of FIG. 6; 
     FIG. 9 shows a sectional view taken at line  9 — 9  of FIG. 6; 
     FIG. 10 shows a sectional view taken at line  10 — 10  of FIG. 6; 
     FIG. 11 shows a sectional view taken at line  11 — 11  of FIG. 6; 
     FIG. 12 shows a sectional view taken at line  12 — 12  of FIG. 6; and 
     FIG. 13 shows a side view of the cutting insert of FIG. 2, with a standard dome-shaped configuration shown in phantom. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     FIG. 1 shows a perspective view of a percussion drill bit  10  in accordance with the present invention. The percussion drill bit  10  includes a shaft  12  with a boring head  14  positioned at a distal end of the shaft  12  and having a longitudinal axis  16 . The boring head  14  has an upwardly facing upper end  18  with a plurality of holes formed therein. These holes are configured to receive tungsten carbide inserts  20 . A gauge row  22  of inserts is provided around the peripheral edges of the upper end  18  of the boring head  14 . 
     A gauge row insert  22  in accordance with the present invention is shown in more detail in FIGS. 2-13. Each gauge row insert  22  includes a generally cylindrical mounting portion  24  extending along a central axis  26 , and a cutting head  28  extending from the mounting portion  24 . The cutting head  28  has an exposed surface  30  forming a non-symmetrical dome shape such that a working portion  32  of the cutting head  28  is enlarged with respect to a standard dome shape D, as shown most clearly in FIG. 13, and a trailing portion  34  of the cutting head  28  is reduced with respect to a standard dome shape D, as also most clearly shown in FIG.  13 . 
     As shown in FIG. 5, the cutting head  28  has an inner edge  36 , an outer edge  38 , a forward edge  40 , and a rearward edge  42 . 
     The exposed surface  30  is non-symmetrical along a central plane  44  which bisects the working portion  32  and the trailing portion  34  (i.e., the central plane splits the working portion  32  and trailing portion  34  each in half, and is oriented tangential to a radius extending from the longitudinal axis  16  of the drill bit  10 .). The central plane  44  is co-extensive with the cross-section  10 — 10 , shown in FIG.  10 . As shown in FIG. 10, the exposed surface  30  of the cutting head  28  does not have a constant radius, and is therefore non-symmetrical in this plane  44 . 
     The exposed surface  30  is symmetrical in any intersecting plane which is perpendicular to the central plane  44  such that the exposed surface  30  forms an arc of constant radius in any such plane from the inner edge  36  to the outer edge  38 . Examples of such intersecting planes through which the exposed surface  30  is symmetrical are shown in FIGS. 7,  8  and  9 . FIG. 9 illustrates such a perpendicular intersecting plane that intersects the central axis  44  of the cutting insert. FIG. 7 illustrates another intersecting plane that is perpendicular to the central plane and is more near the central axis than it is near the forward edge  40  or rearward edge  42 . The intersecting plane illustrated in FIG. 8 is also more near the central axis but on the opposite side of the central axis. As shown in FIGS. 7,  8  and  9 , the exposed surface  30  has an arc of constant radius from the inner edge  36  to the outer edge  38 . 
     Preferably, each insert  22  comprises a single tungsten carbide component. 
     As shown in FIG. 2, the inner and outer edges  36 , 38  are non-linear in side view orthogonal to the central axis  26 . The inner and outer edges  36 , 38  curve from the forward edge  40  upwardly along the working portion  32  and taper downwardly along the trailing portion  34  in side view. Accordingly, the inner and outer edges  36 , 38  have a curved hump  45  and a flat portion  46  which tapers downwardly. In this configuration, after the tool has been worn, it may be machined down to a standard dome shape for further use. 
     In essence, the present invention moves material from the trailing portion to the working portion of the cutting head to provide substantially the same benefits of a dome-shaped and cone-shaped configuration by providing improved bit rate of penetration and improved part life. 
     Wherein at least a substantial majority of planes perpendicular to the central plane are symmetrical in shape. As illustrated in FIGS. 7-9, any plane perpendicular to the central plane  44  is symmetrical in shape. As illustrated by FIG. 10, the exposed surface  30  is non-symmetrical in the central plane  44 . Also, the exposed surface  30  is non-symmetrical in any plane coextensive with the central axis and non-perpendicular to the central plane  44 , such as the planes illustrated by the sectional views of FIGS. 11 and 12. This configuration helps relieve stress on the trailing portion and increase the wear life of the working portion of the insert. 
     While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will appreciate alternative designs and embodiments for practicing the invention within the scope of the appended claims.