Examination of the formation column shows that rock types fall into four 

 general classifications given below. 



General Classification Bit Group No. (See fig. 29-1) 



1. Soft formations (1) 



2. Medium-soft formations (2) & (3) 



3. Medium-hard formations (4), (5), (6), (7) 



4. Hard formations (8), (9), (10) 



Soft-Formation Bits (Group 1) 



Bits in this classification are designed for drilling soft formations having 

 low compressive strength and high drillability. Normally these bits are run 

 with relatively light weights ranging from 1000 to 3000 pounds per inch of bit 

 diameter. The rotary speeds range from 90 to 200 revolutions per minute. 



The bearing capacity of any rock bit is a critical feature, and every effort 

 is made to design the bearing to outlast the cutting structure; however, the 

 bearings in this bit group do not require as great a capacity as for the other 

 groups. This smaller bearing enables the designer to provide greater tooth 

 depth. The teeth on these bits are widely spaced and relatively thin so as to 

 permit deep penetration into the formation. The geometry is such that this bit 

 subjects the formation to a maximum of gouging-scraping action with a mini- 

 mum of chipping-crushing action. 



Medium-Soft-Formation Bits (Groups 2 and 3) 



The bit types shown in groups 2 and 3 are designed for those formations 

 that are more compact than those just considered, but that still fall in the 

 category of relatively soft formations. These formations may be interspersed 

 with thin layers of medium-hard formations. The weights normally applied to 

 these bits range from 2000 to 4000 pounds per inch of bit diameter with rotary 

 speeds ranging from 80 to 125 revolutions per minute. 



It will be noted from the design feature chart (fig. 29-1) that the bits 

 in group 2 have a greater bearing capacity than those in group 1. Group 2 bits 

 also utilize relatively slim teeth, but with slightly closer spacing and shallower 

 depth. Additional gage hardfacing is also provided. In group 3, the tooth 

 angles have been increased while the tooth spacing and depth have been further 

 reduced. A larger amount of gage hardfacing has been provided than in group 

 2 bits. Groups 2 and 3 are also designed to provide a maximum gouging-scraping 

 action and a minimum of chipping-crushing action. 



Medium-Hard-Formation Bits (Groups 4, 5, 6, and 7) 



The bit types shown in groups 4, 5, 6, and 7 are designed for both abrasive 

 and non-abrasive medium-hard formations. 



638 



