A cutting face of a drill bit is defined as a plane, truncated cone, or other surface positioned immediately behind the leading cutting edge of the drill bit. The cutting face structurally directs the cuttings or tailings away from the leading cutting edge of the drill bit toward a slurry point.
Two types of excavation cutting faces are classified by the angle of attack the cutting face makes with the direction of travel of the cutting head as it excavates through the formation. These two cutting face types are high angle and low angle. A high angle cutting face pushes the newly excavated cuttings ahead of the cutter, while a low angle cutting face directs the excavated material inwardly toward the return flow of drilling fluid.
Examples of high angle cutting faces can be found on currently manufactured Polycrystalline Diamond Compact (PDC) bits. Rotation of a PDC drill bit generates a spiral as the circular motion of the bit combines with the penetration of the bit along the borehole. Material which is excavated by each pass of the cutting edge accumulates ahead of the high angle cutting face where the material is compressed and pushed aside as additional material is excavated ahead of the accumulated material. Although drilling fluids injected behind the cutting face slurry the excavated material for transport away from the excavation area, a portion of the excavated material tends to become compacted in front of the high angle bit, thereby decreasing efficiency, and hence the penetration rate, of the cutter.
An example of a low angle cutting face is shown in FIG. 16 of U.S. Pat. No. 5,622,231, issued to the inventor of the present application. Further examples of low angle cutting faces are shown in FIGS. 1-3 of the present application, as described in greater detail below. As seen from these illustrations, low angle cutting faces typically terminate at an angle point where the inclined surface of the cutting face stops. In this manner, low angle cutting faces direct newly excavated material inward, toward a drilling fluid return flow, rather than pushing the excavated material ahead of the cutting face as with the above-described high angle cutting faces. An example of a drilling fluid return flow used with a low angle cutting face is shown in U.S. Pat. No. 5,622,231, which describes the use of orifices positioned behind the cutting face to direct the drilling fluid to the forward end of the borehole to slurry the soil loosened by the low angle cutting face for excavation. Typical drilling fluids may be a liquid, gas, foam, or a mixture having solids suspended in the mixture.
Low angle cutting faces are typically used in loose or unconsolidated soils, whereas high angle (rotary) drill bits are typically used in more firm or rocky soils. However, low angle cutting faces are relatively inefficient and often become clogged due to the tendency of soil to become compacted ahead of the angle point along the inclined surface of the cutting face.
Additionally, low angle cutting heads are used almost exclusively in place of high angle (rotary) cutting heads for delicate drilling operations (such as drilling a borehole immediately beneath an Aboveground Storage Tank (AST) for purposes of determining whether the AST is leaking). Low angle cutting heads are preferred for these delicate operations because they do not typically induce undesirable vibrations or impacts to the adjacent floor of the AST. However, prior low angle cutting heads (such as that shown in U.S. Pat. No. 5,622,231) direct drilling fluid ahead of the cutting face to help slurry the excavated material, and this drilling fluid tends to migrate into the surrounding formation where it can weaken the formation and present a potential hazard to the AST.
Thus, there is a need for more efficient low angle cutting heads which are less susceptible to becoming clogged due to soil compaction along the cutting face. Additionally, there is a need for improved low angle cutting heads which may be used safely for delicate drilling operations such as drilling boreholes beneath ASTs.
It is with regard to this background information that the improvements available from the present invention have evolved.