Figure 29-10 shows a bit with off-center wear and has rotated about some 

 center other than its own. Little is known of what actually causes a bit to run 

 off-center, but such is introduced with drilling certain shales where crooked-hole 

 problems necessitate the use of light weights on the bit. Apparently under these 

 conditions, the wall of the hole is not strong enough to keep the bit from 

 wandering. When this happens, the cutting structure no longer covers the 

 entire bottom of the hole, and ridges of uncut bottom build up and rub against 

 the cone shells. Once this condition starts, it progressively becomes worse 

 because the ridges of formation rubbing on the cone shells further decrease 

 the penetration rate and cause the bit to drill an even larger hole. This condition 

 can best be prevented by increasing the weight on the bit. 



Figure 29-11 illustrates an extremely dull bit that has been used long past 

 its economical life. In addition, the severe gage wear shows that the considerable 



Figure 29-11. A bit showing extreme wear. 



651 



