Three Cone Rock Bit Design and Metallurgical Considerations
Authors: Laird Jack B. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the Improved Oil Recovery Symposium of the Society of Petroleum Engineers of AIME, to be held in Tulsa, Okla., March 22–24, 1976. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made.
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Abstract
In rock bit design, the main limitation is one of space, since the tool has to go into a cylinder of the desired size. The designer has to rely on past history and experience. Little formal use is made of the fundamentals of machine design.
A bearing pin angle is selected based on past experience and a general knowledge of the type of formation which is to be drilled. This angle determines the outside contour of all three cones and influences the sizes and dimensions of all cutter and bearing elements.
Offset is introduced by offsetting the bearing pin in the direction of rotation. This is generally built into soft or medium formation bits. Offset causes a gouging and scraping action to be imparted to the cutter teeth, increasing the penetration rate.
Tungsten carbide insert bits are made by employing many of the same techniques which are used to make steel tooth bits. The main difference is in the cutting elements. These consist of tungsten carbide inserts of various shapes pressed into holes which have been machined into the surface of the cones.
Bearing arrangements are usually subject to severe space limitations. A series of compromises are made in order to obtain the best overall design. Many types are equipped with lubrication systems and seals which exclude the abrasive laden drilling fluid. Journal or friction type bearings may also be utilized. These afford a more uniform loading of the bearing pins. Special alloy materials are used in the bearings to help absorb radial and thrust loads.
Metallurgical requirements dictate the use of alloy steels in the manufacture of cutters, heads and bearings. All parts must be carefully processed. Ball and roller bearings must be made to resist breakage. Cutters and head bearings must be made to have a hard, abrasive resistant case with a strong, tough core material to resist shock loading.
Abrasive resistant hardfacing materials are employed on the gage surfaces of the cutters to maintain full gage hole. Hardfacing is also used on many designs on the inner rows of teeth to retard abrasive wear and to maintain good penetration rates.

Introduction
The design criteria of the three cone type bits have been under development since the introduction of the first "unit" bit design in 1933.