produced by this bit would be undergage and would require reaming by the 

 following bit, thus reducing its life in terms of new penetration. 



Figure 29-5 shows a dull bit with excessive tooth breakage, which can be 

 caused by excessive weights and/or rotary speeds that produce high impact 

 loads. Breakage may also occur if the formation is too hard for the bit. A 

 moderate amount of tooth breakage or chippage is not uncommon and is not 

 particularly detrimental. In fact, the complete absence of chipping and breakage 

 sometimes indicates that a softer formation bit should be used, or that the 

 operating conditions are not correct for best bit performance. If tooth breakage 

 appears detrimental to bit performance, the selection of a harder type, or a 

 change in operating conditions (lighter weights, slower rotary speeds, or both) 

 is indicated. In general, the hardfacing that is applied to the teeth to resist 



Figure 29-6. A bit showing severely chipped and upset teeth. 



646 



