How to get the most out of the drilling system

By Alf Stenqvist, Production Line Manager Tophammer Equipment

A rock drilling system is complex and many parameters decide its performance. Taking into account the rock characteristics, these parameters include: Bit diameter, design and button type, type of flushing media used, drill rod type and geometry, piston mass and geometry, the frequency and velocity of rock drill impact piston‚ rotation speed and feed force. To get the most out of a drilling system, all these factors must work in harmony. The system should also match the overall excavation method, which means that fragmentation for loading and crushing is another consideration.

The bit that counts

A combination of drill bit, drill rods, shank adapter, rock drill and drill rig determine the performance of the rock drilling system - and this is an area in which Atlas Copco has considerable knowledge. Here I want to focus on the drill string, i.e. the drill bit, rods and shank adapter.

Three parameters should be considered when choosing a drill bit. They are:

Penetration rate
Hole straightness
Service life

In 95% of all rock drilling, a button bit is selected to drill the hole to a given diameter. The bit's ability to penetrate the rock efficiently depends on the contact surface of the buttons, their shape and number, the bits' flushing characteristics and the brittleness, or drillability, of the rock.

Ballistic buttons (left) and spherical buttons: The ballistic buttons are generally considered the best choice for most rock drilling applications.

Tests have shown that the relatively smaller contact surface and the larger protrusion of the ballistic button help to break the rock more efficiently giving higher penetration rate than the spherical button. This efficiency also gives the best hole straightness and we recommend ballistic button bits for most applications. Additional reasons are:

The larger protrusion makes it easier to clean the hole, leaving the bit to deal with "fresh" rock, thereby avoiding secondary crushing.

Atlas Copco's patented grinding system, using a profiled diamond grinding wheel, maintains the performance of the ballistic button by restoring its original profile, which is essential for optimum penetration rate and service life.

How the buttons bite: With the same impact energy the ballistic button will penetrate deeper into the rock, since it has a smaller contact area (footprint) than the spherical button.

We should remember, however, that spherical buttons are still preferred in certain types of hard and abrasive rock formations, where spherical buttons can have a superior service life.

A perfect partner

The design of the drill bit, especially the number, placement and shape of the buttons, makes it the perfect partner for the rock drill and its percussive energy to press the buttons into the rock. If the power of the rock drill is insufficient, or the buttons are worn too flat, the penetration rate decreases and hole deviation increases. In addition, the risk of damaging the buttons increases with the size of the wear flat.

Using the Diarot rock drilling simulation system Atlas Copco is able to simulate the performance of different drill bit designs, using data from a worksite or from its extensive database. After the button bit, we should examine the drill rods and shank adapter. The task is to transfer the rock drill piston's energy through the drillstring and drill bit into the rock. The piston impacts on the drillstring 50-60 times a second and, each time, the buttons penetrate 0.5-1.5 mm into the rock.

If the design and setting of the drilling equipment is matched to the rock, energy transmission will be nearly optimal and this, in turn, will positively affect the service life of the drillstring. When the piston in the rock drill strikes the shank adapter the kinetic energy of the piston is converted into a compressive stress wave, which travels through the drillstring and into the rock and breaks it. In a well-tuned rock drilling system, most of the energy in the stress wave is being utilized to break the rock. In a mismatched drilling system, some of the energy in the stress wave will be reflected and returned back up the drillstring as a tensile stress wave. Tensile stress waves are much more detrimental to the drillstring in comparison to compressive stress waves.

The drillstring's joints must be tight enough for energy transmission to be fully effective. Loose joints result in energy loss and drillstring failure which can be simply detected by measuring the temperature on the coupling.

Extension rod joints must be kept tight during drilling. Loose joints cause overheating and energy loss, drastically reducing efficiency.

Rigid rods pay off

There are different types of rods in the Atlas Copco range and we recommend the Speedrod, with integrated couplings for extension drilling. A drillstring with Speedrod gives faster penetration than one with extension rods and couplings, as less energy is lost at the joints. Furthermore, the Speedrod's threads are easy to keep tight during drilling, giving very efficient energy transmission.

Drill rods must be selected to suit the entire system. The largest possible rod diameter for the hole dimension is recommended. A larger-diameter rod has a longer service life and transmits more energy. Furthermore, large rods are also more rigid and, in the same way as TAC guide tubes, will give straighter holes and better blasting results.

Finally, to make the whole system work as efficiently as possible, the drill rods must also suit the rock drill, its piston and shank adapter.

For more information please contact alf.stenqvist@atlascopco.com