bit types shown in group 4 are for the softer end of the range of medium-hard 

 formations. The teeth on group 4 bits are slightly slimmer and more widely 

 spaced than those on bit types in groups 5, 6, and 7. The amount of gage 

 hardfacing is comparable to those bits in group 3; however, the cutting 

 action is essentially the same in all four groups. 



Weights used on group 4 bits normally vary from 2000 to 4000 pounds 

 per inch of bit diameter. Rotary speeds vary from 75 to 125 revolutions per 

 minute and are generally decreased as the weight is increased. 



The bits shown in groups 5, 6, and 7 are designed to permit the use of 

 the heavier weights needed to penetrate harder formations effectively. The 

 rotary speeds are in the range of 40 to 60 revolutions per minute when heavier 

 weights are used. 



Hard-Formation Bits (Groups 8, 9, and 10) 



The bit types shown in groups 8, 9 and 10 are designed to drill hard for- 

 mations that often have very abrasive properties. 



The bearing size in these bits has been increased to provide maximum 

 capacity. The tooth angles have been increased and teeth are more closely 

 spaced. A sufficient increase in gage hardfacing has been provided to strengthen 

 this part of the bit against abrasive wear. In addition, the geometry of these 

 bits provides a maximum chipping-crushing action with a minimum gouging- 

 scraping action. 



The bits in groups 8 and 9 are designed for those formations generally re- 

 quiring heavy weights, such as 4000 to 5000 pounds per inch of bit diameter 

 and rotary speeds in the range of 40 to 60 revolutions per minute. 



Bit types in group 10 are relatively new to the drilling industry. They 

 have tungsten carbide compacts for the cutting elements instead of the more 

 conventional chisel-shaped teeth. A three-cutter design of this type of bit is 

 illustrated in Figure 29-2 and was designed primarily for the extremely hard 

 and abrasive formations such as the chert sections in west Texas and New 

 Mexico, and the hard quartzitic formations in Oklahoma and the Rocky Moun- 

 tains. In these formations it is not unusual for one of these bits to drill 4 to 10 

 times the footage normally obtained with conventional hard-formation rock 

 bits, and in some instances, 15 to 20 times the footage. The drilling rate of this 

 bit will normally equal that of conventional hard-formation bits and has, in 

 some instances, exceeded the penetration rate of regular bits by 50 to 100 

 percent or more. This type of bit usually drills at a constant rate of penetration 

 throughout its life if the formation is uniform. 



This bit may be considered a spot-bit inasmuch as it was designed primarily 

 to drill those formations that not only are very abrasive but also have a high 

 compressive strength. Although the cost of this bit is considerably more than 



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