652 Subsurface Geologic Methods 



Hard and Hard, Abrasive Formation Bits 

 The W7 and W7R Tricones 



For hard sandy or chert-bearing limestone and dolomite, quartzite, 

 novaculite, chat, chert, granite, and other hard, abrasive rocks the W7 

 (fig. 347) and W7R (figs. 343 and 348) tricone bits are favored. The W7R 

 differs from the W7 in its gage design. The W7 has the conventional tri- 

 angular-gage surface design, and adjacent heel teeth are spaced apart at 

 gage. The W7R tricone bit has a gage design fortified by a circumferential 

 cutting edge at gage, formed by a web, which joins adjacent heel teeth 

 in pairs. 



The web-type gage surface developed for the W7R has several dis- 

 tinctive features. A gage surface is formed on which a much larger volume 

 of tungsten carbide is applied to resist the severe abrasion encountered in 

 drilling hard and abrasive formations. The conventional triangular-gage 

 teeth form a rock tooth on the formation, which is supported by its 

 cementation with the wall of the hole and the bottom of the hole. This 

 support has an angular connection of approximately 270° and offers a 

 great resistance to the removal of the rock teeth by the heel teeth on the 

 cutters. The W7R bit, with its webbed-gage surface design, cuts a smooth 

 wall, eliminates the junction of the rock teeth with the wall, and readily 

 prevents the buildup of strong rock teeth on bottom. 



In drilling hard, abrasive formations that have a high compressive 

 strength, a minimum of twisting-tearing-scraping action is a prerequisite 

 for the design of a well-engineered rock bit. Any twisting-tearing action 

 results in a fast rate of wear on the cutting teeth with a corresponding 

 reduction in footage. The cones on W7R and W7 tricones are designed 

 to minimize the twisting-tearing-scraping action and provide the essential 

 chipping-crushing action. The cone-bearing pins are located on center 

 for approximately true-rolling action to prolong the gage life and cutting 

 structure. The included angle of teeth is greater to provide the strength 

 required in preventing broken or battered teeth. The teeth are more closely 

 spaced and are arranged to prevent tracking and insure a long-cutting life. 



The rows of teeth interfitting into grooves in mating cones are re- 

 tained, as in other Hughes rock bits, to obtain mechanical cleaning of 

 the cutting structure, a suitable depth of teeth, an essential thickness of 

 cone shell, and a strong bearing. 



The selection of the W7 or W7R is contingent upon the particular 

 characteristics of the formation being drilled. When gage wear is not 

 serious on the conventional triangular-gage surface and only moderate 

 weight on the rock bit is required to establish an effective chipping- 

 crushing action by the cone teeth contacting the formation, the W7 may be 

 the preferred bit. When gage wear is a factor or when heavy weight 

 must be applied on the bit to establish the desired chipping-crushing action 

 on bottom, the W7R is the most suitable design of rock bit. 



