Solving a problem a customer experienced when drilling in abrasive rock conditions led to the new design of Sandvik's top-hammer drill bits. According to the manufacturer, they incorporate the largest upgrade to face-drilling bits in decades.

The company also said 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.

Sandvik explained that 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.

In addition, 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. The top centre bit also features a new cemented carbide grade, the GC80.

Robert Grandin, product manager for top-hammer tools at Sandvik Mining, commented: "The problem with the carbides that exist on the market today is that they are either wear-resistant or tough. 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."

According to Sandvik, the key to this is a 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.

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, the company claimed.

Top centre drill bits are available in bit sizes 43mm, 45mm and 48mm, with 2-3 raised end buttons and 7-8 gauge buttons in grade GC80 and connections R32, Sandvik Alpha 330 and R35.