Fragments generation characteristics and design of a kind of efficient rock breaking bit during borehole drilling with small diameter at a downward angle of coal mine roadway

Borehole drilled at a downward angle is an important part for roadway bolting support.How well the bolt is anchored depends, in part, on borehole quality.A major factor that can reduce borehole quality is the difficulty of discharging rock fragments from a small diameter borehole drilled at a downward angle.Therefore, to reduce the fragment size to improve the discharging efficiency, theoretical analysis, numerical simulation, laboratory experiment, and filed test was adopted to explore the correlations between the fragment size and blade spacing.A kind of efficient rock breaking bit of the small diameter borehole drilled at a downward angle was developed.The sizes of bit channels were optimized, and the performance of the bit were also analyzed and verified.The results indicate that the central rock pillar will appear in the bottom of the borehole during the drilling process of the normal PDC two-wing drill bits.The sizes of the central rock pillar increase with blade spacing, which is the main source of the fragments with large size and leads to the increase of the mean size of the fragments with the particle sizes greater than 2.5 mm.The fragment size can be reduced by changing the blade structure to eliminate the central rock pillar.The structure of the normal bit could cause local gathering and energy loss of the fragments.Benefiting from the structure advantages the designed bit can reduce the energy loss of mixed flow to make the fragments get higher velocity and is more conducive to the smooth discharging compared with the normal bit.The main and auxiliary blades of the designed bit will make the rock at the bottom of the borehole form a platform.They can ensure that the rock at the bottom of the hole failed due to cutting to and eliminate the central rock pillar.So that the mean fragment size is significantly lower than other types of bits, and it also has higher rock breaking efficiency.The field test results demonstrate that the designed bit can effectively reduce the fragment size, and has higher drilling speed and fragment discharging efficiency.The research results of this paper have a certain guiding role and reference value for the theory and equipment research of the rapid and high-quality formation of a small diameter borehole drilled at a downward angle.