Design of spin-reinforced drill bit teeth with increased vibration resistance

Abstract

Recently several designs have been developed for spin-reinforced teeth for drill bits. However, all these designs differ only in either the geometry of the cutting elements or how they are manufactured. As a result, how the geometry of the reinforced zone affects the lifetime and the efficiency of the teeth has not been investigated. The most common shape of the reinforced zone is that shown in Fig. la. However, this shape is not effective if the teeth must operate under significant vibrating impact loads. This is primarily because surface layers of the abrasive grains are dissolved when they are impregnated into the steel matrix of the spin-reinforced composite material, which embrittles some of the working part of the cutting elements. As a result, the working part of the spin-reinforced tooth partially slips and is blunted during the drilling, which decreases the contact pressure on the rock and lowers the rock-cutting efficiency of the bit. In order to make the spin-reinforced tooth more resistant to vibrating impact loads, we designed a new cutting element in which the geometry of the reinforced zone is fundamentally different. The essence of the new design is that the layer of abrasive material is only placed under the leading edge (Fig. lb). This placement increases the resistance of the working part of the spin-reinforced tooth to bending loads because more of the working part is made out of steel which has not be embrittled by secondary carbides which are formed when the surface layer of the grains melt when the working part of the cutting element is impregnated with the abrasive material. Placing the abrasive material behind the leading (but not the trailing) edge makes it necessary to strengthen this surface of the cutting element, because it gets the largest loads from the rock. At the same time this placement of the abrasive material increases the self-sharpening of the working parts of the cutting elements during drilling, because the wear rate of the unstrengthened trailing edge will be higher than for the leading edge. The reinforced zone must be placed some distance behind the surface of the leading edge for the following reasons~ Crushing the grains of the initial abrasive material (relite or tungsten carbide for example) creates microcracks. As a result, the grains which are a good abrasive material become brittle and cannot withstand impact and bending loads. Thus, if the outer boundary of the abrasive material layer is applied on the surface of the leading edge, the grains will chip rapidly under vibrating impact loads during drilling. Then the lower layer of grains will be exposed and all the abrasive material will be chipped. Also a protective layer of metal prevents the abrasive grains from oxidizing and chipping during the thermochemical processing used to make the spin-reinforced tooth. A manufacturing process had to be designed for the spin-reinforced tooth with the new geometry. To do this we had to establish how the geometry of the reinforced zone is affected by 1) the rotational speed of the centrifugal casting machine, 2) the forces acting on a particle of the reinforcing component, and 3) the location of the surface of the mold which forms the working part of the cutting elements during the spin reinforcement. The spin reinforcement process can be described mathematically by describing how a grain of the reinforcing component moves in liquid steel under centrifugal forces. The forces acting on a grain of abrasive material in liquid steel can be described in a general cylindrical coordinate system, the axis of which is perpendicular to the centrifuge axis of rotation (Fig. 2): F c, the centrifugal inertial force; F h, the hydraulic resistance force; Fg, the force of gravity; Fe, the extrusion force; and F k, the Coriolis force. The forces F c and F h have the greatest effect on the displacement of the grains. By starting from conditions of force equilibrium on the grain, its equation of motion can be written in the following form: