The four basic actions of percussion drilling:
• Percussion: Drives the drill bit into the rock.
• Advancement: Maintains close contact between the drill bit and the rock.
• Rotation: Rotates the drill bit to a new position before the next percussion stroke.
• Hole Flushing and Cuttings Removal: Clears rock cuttings from the borehole while cooling the drilling tools.
The impact energy generated by the down-the-hole hammer’s impact piston is transmitted to the rock through the drill bit.
The impact energy transmitted through the drill string by a down-the-hole hammer includes impact force and impact frequency. The impact power depends on the set impact pressure.
One of the fundamental factors in drilling is the drill string’s ability to transmit impact energy. A drill string of a given specification has a defined maximum energy transmission capacity. If the transmitted impact energy exceeds this maximum limit, it will lead to premature failure of the drill string.
The impact energy required for drilling depends on the type of rock being drilled. If the rock to be drilled is relatively soft, the lower limit of the drill’s pressure range should be selected;
conversely, if the rock is relatively hard, the upper limit of the pressure range should be selected.
The principle for selecting a reasonable impact pressure is to balance both penetration rate and the service life of the drill string.
The feed action ensures that the drill bit remains in constant contact with the rock being drilled. As the drill rig’s impact pressure increases, the feed pressure also rises.
The appropriate feed pressure setting depends on the impact pressure, the hardness of the rock, the drilling depth, and the type of drill string.
When drilling in fractured rock formations, lower impact and feed pressures should be selected.
The optimal feed pressure can be determined by observing visual indicators and listening for sounds.
The feed process is smooth and stable (no bouncing or shaking of the feed beam bracket) – The drill pipe rotates smoothly (relatively stable rotational speed) – The rotary head feeds smoothly, with no up-and-down bouncing – The wire rope shows no noticeable bouncing – The penetration rate of the drill string is relatively stable
The impact sound is solid and muffled (no abnormal clicking sounds or vibration noises occur)
During drilling operations, it is important to note that simply increasing the feed force will not improve penetration rate. Blindly increasing feed pressure will ultimately cause the hole to deviate, and the resulting bending of the drill pipe will in turn increase rotational resistance. Conversely, if the thrust is too low, the drill bit will not make full contact with the rock. This prevents the impact energy from being fully transferred to the rock, and some of the energy (drilling energy) will rebound, causing damage to the drill bit, drill pipe, and thrust beam bracket.
In the rock drilling process of percussive drilling, the function of rotation is to alter the contact position between the drill bit’s cutting edges and the rock with each impact. The ideal rotation speed depends on the drill bit’s geometry, type, and diameter, as well as the rock’s drillability (hardness, abrasion resistance, etc.). Additionally, the rotation speed of the drill pipe is related to the drilling parameters. Too low a rotation speed results in energy loss, as insufficient rotation causes the drill bit to repeatedly crush the rock, thereby reducing penetration rate. This causes the drill bit and drill rod to grind against each other rather than breaking up the rock. Too high a rotation speed leads to rapid wear of the drill bit, as the rock is abraded by rotation rather than broken by impact. Additionally, excessively high rotation speeds can cause the threaded connections of the drilling tools to become too tight, making it difficult to disassemble the drill rod.
The purpose of flushing the hole with compressed air is to expel the broken rock cuttings from the bottom of the hole. Compressed air is delivered through the air passages in the drill string and then expelled through the air ports in the drill bit. The compressed air mixed with rock cuttings is blown out through the annular gap between the drill string and the borehole wall.
If cuttings are not flushed out effectively, it will result in reduced penetration rate (increased cuttings), shorten the service life of the drill string, accelerate bit wear, and increase the likelihood of a stuck pipe. After the drill bit reaches the bottom of the hole, it should be rapidly raised and then rapidly advanced to complete a flushing cycle.
Note: After each drill bit completes its run, the hole must be flushed at least once.
If any abnormalities occur during drilling, stop drilling immediately to troubleshoot the issue or have maintenance personnel investigate. Monitor drilling performance, paying particular attention to the following points:
1 Rotary Head:
1.1 Check for abnormal vibration in the impact hose;
1.2 Check other hoses for abnormal vibration;
1.3 Monitor the lubricant level;
1.4 Check for abnormal leaks from the rotary head;
1.5 Hydraulic system
1.6 Monitor all pressure gauges during drilling.
1.7 Ensure that the readings on these pressure gauges do not exhibit large fluctuations or sudden abnormal pressures.
1.8 Check the temperature of the hydraulic oil; it should not exceed 60°C.
Note: During drilling, constantly monitor for signs of encountering groundwater; if detected, immediately
shut off the dust collector.
1.9 During drilling, constantly monitor whether the flushing and cuttings removal are functioning normally.
1.10 If necessary, move the rotary head up and down to flush the borehole. During drilling, monitor the compressed air pressure. If the pressure rises, it is likely due to the air holes on the drill bit becoming blocked (rock cuttings in the borehole are not being expelled in a timely manner). If the volume of rock cuttings expelled decreases or ceases entirely during drilling, proceed with caution and verify that the flushing is functioning properly. In this case, it is best to stop the percussion and raise the rotary head to clear the borehole.
