ROCK DRILLING WITH IMPREGNATED DIAMOND MICROBITS - AN EXPERIMENTAL-STUDY

An instrumented laboratory rig was used to drill a single rock type "Bushveld norite" with impregnated diamond microbits. Tests were conducted under both set thrust and set rate of advance conditions to determine the effect of varying the bit pressure, rate of advance and diamond mesh size on the dependent drilling variables. These were power consumption, torque, reactive load, penetration rate, bit wear, diamond wear and rock fracture. Drilling efficiency was monitored by calculating the specific energy of drilling from the power consumption corrected for machine losses, and independently from the generated torque. The diamond wear on used bits was evaluated and classified under the optical microscope. The rock fracture was evaluated by scanning electron microscopy of the drilled surfaces and by particle size distribution analysis of the drilling detritus. A performance model for impregnated diamond bit drilling is presented on the basis of the experimental results. A critical stress threshold per actively drilling diamond must be exceeded for steady drilling to take place. The optimum drilling thrust at which the specific energy is at a minimum and the penetration rate at a maximum, occurs marginally above the bit pressure required for all the exposed diamonds to be in contact with the rock. The transition from suboptimal to steady drilling conditions is described in terms of the characteristic diamond wear, the specific energy, and the coefficient of friction between the bit and the rock. In norite this transition takes place at a calculated diamond pressure of approx. 400 MPa irrespective of diamond size or mode of control. © 1990.