1. Field of the Invention
The present invention relates generally to earth boring rotary rock bits and, more particularly, to rock bits having one piece bit bodies.
2. Description of the Prior Art
A rotary rock bit usually is manufactured from a plurality of subassemblies. Each subassembly comprises mainly a leg having a journal pin formed on the lower extremity thereof. A cone type cutter is rotatively supported on the journal pin and is usually retained thereon by a plurality of ball bearings. Fluid nozzles and self-contained lubrication systems are also contained within each leg. The leg subassemblies are then welded together to form a bit body.
A serious shortcoming with rock bits made in this manner is that a true geometry bit is practically impossible to manufacture. Because of the tolerance buildup in each of the subassemblies and because of the final welding process of the three legs, unwanted bit skew is incorporated into the final product. Moreover, the cone heights of the assembled products are not always the same. In addition, the journal offset and angle are not accurately held using the conventional assembly techniques.
Another problem with manufacturing rock bits from leg subassemblies is that many additional steps are required in the process. First of all, the leg seams must be machined with a 120.degree. milling machine, and dowel holes must be drilled. Moreover, the journal pin bearing surfaces must be machined and having the journal pins integral with the legs makes the machining operation lengthy and cumbersome. In addition, the bearing surfaces of each journal pin must necessarily be carburized while the rest of the leg surface be painted over to prevent the remaining surface from being curburized. However, carbon leakage does occur on this surface and this leakage is deleterious to the subsequent operations of the legs.
Because of the problems associated with the above-mentioned leg subassemblies, one piece bit bodies have been proposed. Also, separate journal pins have been proposed in order to carburize and machine the pins prior to assembling them to the legs.
U.S. Pat. No. 4,043,411 shows one method of having a separate journal pin secured to a leg. However, the shortcoming found in the method of manufacture of the rock bit legs of the referenced patent is that the welding is accomplished on the interior side of the leg, i.e., the cutter receiving surface side of the leg. Such a method of manufacture severely restricts the design capabilities of the journal pin. Moreover, with such a restriction, the cone cannot be preassembled with the journal pin prior to welding the pin to the leg and may require secondary operations to clean welded areas prior to final assembly.
Another U.S. Pat. No. 4,098,150 also discloses separate journal pins. However, the structure disclosed therein does not lend itself to a one piece bit body construction.
U.S. Pat. No. 3,850,256 discloses a modified one bit body construction; however, very few of the problems discussed above are solved with the structure disclosed therein.