How Coprod realised the
rock driller's dream

By Karl-Axel Stjernström,
Product Manager,
Research and Development,
Atlas Copco

The traditional methods of percussive drilling in benching operations - top hammer and down-the-hole - have proved effective over many years. But they both have limitations. To get the best of both worlds and avoid the drawbacks of each, Atlas Copco created the Coprod drilling system, which has set new productivity standards in benching operations since its introduction in 1993.

In the simplest of terms, percussive drilling systems go back to manually hitting a steel rod with a bit at one end with a heavy hammer and, as recoil makes the rod jump back, rotating it at a small angle between blows to ensure that the hole becomes round.

Drilling for benching operations has mostly been carried out with top hammers, using extension rods connected by threaded coupling sleeves and an exchangeable drill bit at the bottom end.

This equipment works well for smaller hole diameters in solid rock. But it is not so effective in larger hole diameters or deteriorating ground conditions. There are problems in transmitting sufficient energy to the bit, especially in deeper holes, and in obtaining satisfactory flushing.

Some 150 years ago, when the great railroad tunnels were being built under the Alps and the Appalachian mountains of the USA, machines began to deliver the percussive impact and rotation to the drill steel. Productivity took a great leap forward, but hole diameters and depth stayed more or less the same.

The breakthrough towards more efficient blasting operations in benching came with extension rod steel and separate bits, freeing the operators from the limitations of hole depth and diameter. And tungsten carbide inserts in bits in the early 1950s was the next step towards efficient bench drilling operations.

Straighter holes

The most important advantage of the down-the-hole system over top hammer drilling is that it leads to straighter holes. In tophammer drilling, the thrust has to be applied from the top to keep the bit in contact with the bottom of the hole. This causes the relatively slender drill string to bend, steering the bit off its intended course. Rigid guide tubes with a large outer diameter were developed to keep the drill string on a straight course and improve flushing.

The Coprod system: Integrating two types of drillstring for percussive drilling by means of a tophammer. The inner drill rods transmit the power and thrust and the outer tubes transfer the rotation.

With a down-the-hole hammer, a series of tubes offer far greater stiffness and run closer to the hole walls, resulting in considerably less deviation than with a top hammer drill string.

Hydraulically-powered rock drills arrived and became increasingly powerful, sending more percussive energy down the drill string and allowing larger hole diameters in benching - but also requiring more and more thrust.

There was no way to beat down-the-hole drilling for straightness, but the technique fell behind the hydraulic top hammers in drilling capacity. Efforts to combine the advantages of the two techniques were unsuccessful - and it was not until the development of Coprod that the dream was realised.

Coprod is not only a combination of positive features, it also integrates two types of drillstring for percussive drilling by means of a top hammer. Inner drill rods transmit power and thrust to the drill bit and and outer tubes transfer rotation, adding stiffness to the string and improved flushing efficiency.

These assets achieve high top hammer drilling rates and large hole diameters.

Energy loss

Extension rods in the drill string are threaded together and, due to the microscopic movements between mating parts as the percussive shock wave passes the joint, energy is lost and heat develops. The energy loss may be considerable by the time the shock wave reaches the bit and there will be thread wear and reduced life of the drill string components.

But the rods in the Coprod system have no threads and are simply stacked one on top of the other. Laterally, they are centred by the guide bushes in the tubes which surround them and, longitudinally, contact between rod ends is maintained by the thrust from the top. Thanks to the unique double recoil damping system of the rock drills developed for the system, the rod ends remain in permanent contact, energy losses are almost nil and drilling efficiency is maintained from start to finish of the hole.

A comparison of test results achieved with conventional  drillingequipment and with the COPROD system in both  hard and soft  rock.

Rock: Limestone Compressive strength: 600-1100 bar  Location:France

Rock: Basalt Compressive strength: 5500 bar  Location:Australia

Coprod's rock drills also save rod ends from damage that "empty" blows may cause. If the bit enters a cavity and drops down in its splines in the bit chuck, the hammer senses it and percussion is interrupted. But rotation is maintained and percussion restarts automatically when the bit meets resistance again.

Flushing air enters the bit via a centre channel, which connects to the cylindrical surface in the bit rod. A small amount of air, containing a little oil, escapes via the splines in the chuck and the bit to lubricate them. On its way up, the flushing air travels between the smooth outside of the tubes and the hole wall, providing a constant cross section and ideal conditions for flushing the drilling fines.

Coprod offers unique features for drilling holes fast and straight. And the more troublesome the ground becomes, the more the incomparable drilling system comes into its own.

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