content
When a top hammer button bit is sharp, the percussive energy transferred into the rock is at its optimum, resulting in effective rock fracture and a maximum penetration rate for the given drilling parameters.
As flats develop on the buttons (tungsten carbide inserts), energy utilization is no longer optimized, leading to a lower penetration rate and reduced productivity. As buttons wear, the bit is less effective at fracturing the rock and the energy is dispersed over a larger surface area. Instead, rock is ground, rather than cut, producing smaller cuttings and energy (no longer used for breaking virgin rock) is reflected back up the drill string. This reflective energy contributes to increased loads on the tooling, drifter, and rig. As a general guideline, refurbishing the buttons before the flats reach 1/3 of the button diameter will maintain penetration/productivity, provide maximum bit utilization and help extend the life of all drill string components.
If the button flats are allowed to develop further to where they are considered over-drilled, productivity and the rate of penetration deteriorates. In this situation, much of the rock in contact with the buttons is re-drilled and the steel bit face may make contact with the bottom of the hole. Fewer and smaller rock chips are produced and much of the energy is reflected back up the drill string. This sacrifices the life of all the drill string components is a major contributor to hole deviation, increases wear and tear on drill rig components and results in higher operational costs overall. Top hammer button bits that are over-drilled exhibit broken buttons and impact drilling productivity.
Jeff Hogan of Boart longyear
