ROCK TOOLS
Metres per bit
Paul Moore gets the lowdown on bit wear plus reports on optimisation of drilling through digital tools and other developments in rock drill consumables as well as a new drilling service agreement
A Boart Longyear percussive bit and rod. The company in this article offers a number of proven tips to help miners achieve more metres per bit
drilling through hard materials such as quartzite, or drilling with excessive rotation, the wear tends to be greater on the bit circumference. Hogan states:“ Thus, when the buttons are sharpened, the diameter across the gauge is less than or equal to the diameter of the bit body. In this situation, the bit tends to bind( gauge-out) in the hole. Drillers should consider bit replacement or restoring the gauge button-to-body gap to the original new condition.”
When drilling in non-abrasive materials, carbide wear is minimised and drilling intervals are possibly longer.“ This allows for continued chip removal around the bit and wears away the bit body quicker than the carbides, also referred to as body wash. Similar wear occurs in fractured and loose formations where constant hole cleaning and re-drilling of the materials is required for stabilising the hole and to keep the hole open during retraction. To prevent button loss or shearing under these conditions, the protrusion should be reduced to the original height by scheduling grinding intervals.”
Drilling contractors and mining companies rely on their tooling to get the highest productivity and the lowest cost per metre for every project. Boart Longyear Global Product Manager, Jeff Hogan told IM:“ When purchasing top hammer button bits, for example, choosing a high-quality product designed for the specific ground conditions is only the first step to achieving more metres per bit. Ensuring bits are properly used and maintained is critical to realising a high return on investment.”
Why button bit wear is important
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. Hogan comments:“ As flats develop on the buttons – tungsten carbide inserts – energy utilisation is no longer optimised, 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 a third of the button diameter will maintain penetration / productivity, provide maximum bit utilisation and help extend the life of all drill string components.”
If the button flats are allowed to develop further to where they are considered overdrilled, productivity and the rate of penetration deteriorates. Hogan comments:“ 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.”
Wear by rock type
Button bit wear patterns vary depending on the type of ground and the drilling parameters. If
Over drilling effects
Boart Longyear told IM that the detrimental effects of over drilling button bits may not be immediately apparent. However, running dull bits not only slows down drilling rates but escalates costs by reducing the life of the drill string components, the rock drill and the rig components.“ Premature button bit insert failures are substantially reduced when over drilling is eliminated and correct sharpening procedures are performed.”
In terms of recommendations on getting the most of out of top hammer bits, the attached table summarises Boart Longyear’ s advice to miners. From a complete selection of rods, bits, and shanks to couplings, adapters, and other accessories, the company provides top hammer tooling solutions for any percussive drill and blast application for both surface and underground drilling. Boart Longyear’ s experience with drilling equipment comes from more than 127 years of proven performance and service in a variety of applications. The company recently
( left): Harder ground conditions and drilling with excessive rotation creates wear that tends to be greater on the bit circumference;( right): Body wash often occurs when drilling in nonabrasive materials
42 International Mining | NOVEMBER 2017