644 



Subsurface Geologic Methods 



rotated on the formation. This relationship is beneficial in helping the 

 drilling fluid clean the bit. 



The geometry of the cone-shaped cutters and the position of the 

 bearing pins supporting the cutters are altered to provide more or less 

 twisting-tearing-scraping action of the teeth as they penetrate the forma- 

 tion. When extremely hard and abrasive formations such as tightly 

 cemented sandstone, sandy dolomite, quartzite, chat, chert, and similar 

 rock types, are being drilled, rock bits perform most efficiently with 

 cutters that have an approximately true rolling motion which provides 

 a chipping-crushing action of the teeth on the formation. A minimum of 



Figure 340. Hughes 9%-inch type OSC-3 tricone bit. 



abrading or scraping action of the cutter teeth must be embodied in the 

 hard-formation-rock bit design; otherwise, accelerated wear of the teeth 

 will result in a short bit life. 



The rock bit designed to drill efficiently soft, unconsolidated, low- 

 strength formations retains the desirable chipping-crushing action and also 

 embodies the twisting-tearing-scraping action on bottom, which is essen- 

 tial for fast rates of penetration. 



The different types of tricone bits, developed for drilling the different 

 formations, embody chipping-crushing action. However, the twisting-tear- 

 ing-scraping action varies in the different types, which, along with general 

 operating practice, are discussed in the following pages. 



