The classification of jet bits is much the same as that for conventional bits. 

 The three-cone, two-cone, cross-section, and drag types are most popular. The 

 three-cone bit, the most widely used, is illustrated in Figure 30-1. It has three 

 conical-toothed cutters, which rotate on bearings. This bit normally has three 

 nozzles located around the outside of the bit and between the cones. The nozzles 

 are placed within a few inches of the cutting plane of the bit so that a minimum 

 of energy is dissipated before the mud stream impinges on the bottom of the 

 hole. The two-cone bit, which is shown in Figure 30-2, is similar in most re- 



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Figure 30-1. Three cone jet rock bit 

 (courtesy Hughes Tool Company). 



Figure 30-2. Two cone jet rock bit 

 (courtesy Hughes Tool Company). 



spects to the three-cone, except that it has only two cones and two nozzles. The 

 cross-section-type jet bit is shown in Figure 30-3. It consists of two elongated 

 roller-shaped cutters, and two narrow cutters at the circumference, all of which 

 turn on bearings and carry cutting teeth. Normally, two nozzles are placed in 

 this bit so that the mud stream is directed to the bottom of the hole. 



The jet drag bit (fig. 30-4) is sometimes used for drilling in extremely 

 soft formations. It is becoming less popular and will not be considered in the 

 following discussion. It may have two, three, or four blades and a comparable 

 number of nozzles. The cutting action of this bit is one of scraping and peeling 

 the formation. 



As early as 1921, attempts were made to utilize the jet rock-bit principle. 

 Comparatively little success was attained until about 1947, when it became 



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