Raising the bar

The rock drill bit can be considered the ‘business end’ of a sophisticated engineering system that essentially converts hydraulic energy into impact waves to break rock. Sandvik Mining set out to develop a unique new drill bit to solve persistent tool life problems experienced by a customer in northern Sweden. After three years of intensive development, principal engineer Andreas Rindeskär is visibly proud of the ‘top center’ design.

“We had to come up with a completely new design to be able to create the features that we wanted from the bit,” Rindeskär says.

Product manager Robert Grandin was one of the key drivers of the R&D project.

“It started as a solution for an extreme customer problem, and now it’s a standard solution that can be used everywhere,” Grandin says. “It proved so successful we decided to apply this highly specialized technology for additional diameters and applications.”

“The new bit design essentially delivers more drill metres per shift compared with a standard bit, thanks to less frequent bit changes and faster penetration rates,” Grandin says. “Drill operators say they really like this product because they do not want to get out of the cabin to change the bit more often than they absolutely have to.”

A crucial design issue was lack of space on the bit head.

“Given the designs we had had for the past 20 or so years, there wasn’t much space available to do anything,” Rindeskär says. “We had to come up with a completely new design idea to be able to create the features that we wanted.”

The top priority for Rindeskär and his designer colleague John Hammargren was to increase the service life. As the main criterion for discarding a drill bit is excessive wear on the diameter, the simplest way to lengthen service life is to add more gauge buttons. 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 a few millimetres above the gauge buttons located on the periphery of the bit. The front buttons are furthermore set at a slight angle relative to the symmetric axis of the bit. The raised front creates a slightly recessed hole bottom pattern that alters the rockbreaking action for improved performance.

The penetration rate remains unaffected by the raised front design, or even increases due to decreased recrushing and easier passage of cuttings. Improved understanding of the tool

steel permitted higher stress levels in the bit body. The design life of the bit is around 500 metres in abrasive conditions, which means that the steel fatigue properties have been sacrificed for more wear resistance, to some extent.

Best of both worlds

A key enabler for pushing the stress limits is today’s highly accurate and reliable production process. Continuous improvement of the production technology now allows extremely close

manufacturing tolerances. The top center also features a new cemented carbide grade, the GC80.

“The problem with carbide is that it’s either wear resistant or tough,” says Grandin. “When we developed the GC80, we wanted to combine the best of those two worlds.”