Solving a problem a customer experienced when drilling in abrasive rock conditions has led to a “groundbreaking new design for drill bits”, according to Sandvik. The new top centre drill bits “which incorporate largest upgrade to face drilling bits in decades, are now available as standard products in three sizes.” Primary applications include face drilling and bolting in both mining and tunnelling environments, where long bit life is essential in cutting costs and improving productivity. “The top priority when developing the new top centre drill bit was to increase service life. Since the main reason for discarding a drill bit is excessive wear on the diameter, the simplest way to achieve longer service life is to add more gauge buttons. However, this can prove problematic because of the minimal space available. Furthermore, an increase in the number or size of the carbide buttons generally decreases the penetration rate: the same impact force yields a lower net force per button.”
The new design solves these problems with a so-called raised front, elevating two or three front buttons – depending on diameter size – a few millimetres above the gauge buttons located on the periphery of the bit. Furthermore, the front buttons are set at a slight angle relative to the symmetric axis of the bit. The raised front creates a somewhat recessed hole bottom pattern that alters the rock-breaking action to achieve improved performance.
In addition to the new design, the top centre bit also features a new, innovative cemented carbide grade, the GC80. “The problem with the carbides that exist on the market today is that they are either wear-resistant or tough,” says Robert Grandin, Product Manager Top Hammer Tools at Sandvik Mining. “When developing the GC80, we wanted to combine the best of those two worlds in order to get as much as possible out of the top centre design.”
The key to this according to Sandvik is a completely new production method, which makes it possible to produce a button that improves wear resistance on the outside, and yet combines toughness with a softer centre, pushing the service life and long grinding intervals even further.
Tool life improvements brought about by the Sandvik top centre design also yield significant health and safety benefits, since operators spend less time near an unreinforced face. “The new bit design essentially delivers more drill metres per shift compared with a standard bit, thanks to less frequent bit changes,” Grandin says. The bits offer up to 80% longer grinding intervals and up to 60% longer bit life, meaning higher productivity and a safer work environment with fewer bit changes. More than 1,000 hours of testing in widely varying conditions and sites in countries including China, Turkey, Bulgaria, Georgia, Zimbabwe, Australia, Mexico and Sweden “have shown operators logging more drilled metres and more holes between each regrind. Less regrinding means less wear on the drill diameter. An additional benefit is more accurate collaring, which directly impacts the accuracy of drilled holes and consequently blasting results. Top centre drill bits are available in bit sizes 43, 45 and 48 mm with 2-3 raised end buttons and 7-8 gauge buttons in grade GC80 and connections R32, Sandvik Alpha 330 and R35.