Precautions for Using Rock Drilling Tools
Drill steels used in drilling-and-blasting operations are constrained by hole diameter, so their cross-sectional area is relatively small. These slender components must withstand severe wear, corrosive media, and high-frequency cyclic loads—tension, compression, bending, and torsion—transmitted by high-impact rock drills. Depending on loading conditions and rock properties, service life may range from only a few dozen hours to just over one hundred hours. Rock drilling tools are among the most heavily stressed consumable engineering tools in industry: short-lived, technically demanding, and indispensable in large quantities.
Every tool user wants maximum return at minimum cost, which means reducing tool consumption. For that reason, tool manufacturers and users should work closely together to improve service life through joint technical optimization.
1. From the User’s Perspective: What to Look for When Selecting Rock Tools
(1) High product quality
High-quality shank adapters are expected to last longer, reducing downtime for disassembly and replacement. Drill rods and bits are equally critical: if a rod breaks, the rod and bit may both be lost, and the hole may be scrapped.
On modern hydraulic drill rigs, normal drilling is highly automated and physically less demanding, but replacing shank adapters, fishing broken rods, or regrinding bits wastes labor hours and increases labor intensity and operating cost. Therefore, users care not only about price, but even more about product quality.
(2) Best possible penetration rate
In jumbo drilling operations, apart from labor and tool purchase cost, many other time-based costs are effectively fixed. Since tooling cost is a large share of drilling cost, faster penetration directly lowers total construction cost. This is why users place high priority on drilling speed.
(3) Minimum blast-hole deviation
In medium- and deep-hole blasting, hole deviation reduces explosive loading and hole spacing efficiency, which lowers production output. Users therefore demand high hole straightness, typically within very tight tolerances.
Main causes of deviation include:
collaring error at hole start,
hole-marking/alignment error,
straightness error generated during drilling by tool behavior.
With full-hydraulic or computer-controlled rigs, the first two errors can be greatly reduced, making tool-structure-induced deviation the main remaining factor. Deviation usually increases with depth and can eventually produce scrapped holes. A highly effective way to reduce or eliminate deviation is to use guided rock drilling tools.
2. Analysis of Rock Tool Wear and Failure Causes
(1) Misalignment in the drill string
If the shank adapter, coupling sleeve, and drill rod are not concentric, bending deformation occurs, generating stress and poor fit at connections, which leads to loosening.
(2) Feed pressure
Too low: penetration efficiency drops; the assembly loosens; energy transfer losses increase; repeated micro-separation/impact at contact faces creates high stress. Typical signs: overheating and clicking/chattering at joints. Overheating accelerates thread wear and may cause erosive pits.
Too high: bit rotation speed decreases, risk of jamming rises, and rod bending stress increases.
(3) Impact pressure
Improper impact-pressure setting directly affects rotation behavior, penetration efficiency, and tool life.
(4) Rotation speed
Rotation speed must match bit diameter and drill impact frequency. Larger bits require lower RPM. Excessive RPM damages peripheral cutting structure.
(5) Rotation pressure/torque setting
Proper rotation pressure is essential. It helps prevent sticking and is necessary for stable rotation. Incremental rotation pressure is also key to maintaining string tightness. Insufficient tightness often causes hot joints, thread spalling, premature thread wear, and even fracture.
(6) Improper operating practice
Mixing heavily used tool components with new ones can shorten overall service life. Other damaging practices include poor alignment when making up rods, dirt/sand in threads, and thread makeup without lubricant.
“Blank firing” (hammering without effective bit-rock contact) is especially destructive and must be avoided.
3. Relationship Between Drilling Parameters and Drilling Performance
Impact pressure
Higher impact pressure increases piston velocity and impact energy. When the bit is in good contact with intact hard rock, shock-wave energy is utilized most effectively. If bit-rock contact is poor, energy is reflected back into the drill string as tensile waves instead of entering the rock.
Maximum impact energy per blow can only be fully utilized in sufficiently hard rock. In softer rock, impact pressure/energy should be reduced to limit harmful reflection.
For any given impact setting, higher pressure means higher stress in the drill steel cross-section. To maximize service life of rods and shank adapters, operating pressure must always match the tool string capacity.
Feed pressure
Feed force keeps the bit in firm contact with rock while allowing rotation. Feed force must be properly matched to impact pressure.
Correct feed force gives the most economical drilling.
Too low feed force lowers penetration rate and loosens threaded joints. Overheated couplings and rattling noise indicate feed setting is too low. If drilling continues with loose joints, energy loss and temperature rise increase, leading to thread burn-off and fracture.
Too high feed force reduces RPM and penetration, increases jamming risk, and can increase hole deviation because of string bending tendency.
Rotation speed
Rotation indexes the bit to a new position for the next impact. With button bits, peripheral indexing between blows is typically around 10 mm. Therefore RPM must be adjusted according to impact frequency and bit diameter. Larger diameters require lower RPM. Excessive RPM causes accelerated wear of gauge buttons/edge structure.
4. Practical Operating Precautions
Before adding rods, set appropriate impact pressure, feed pressure, rotation speed, and damping/buffer pressure according to actual geology. Ensure parameter matching with rock hardness and formation type.
During drilling, keep the tool assembly concentric to avoid bending deformation of shank adapter, rod, and bit.
Prevent blank firing. If it occurs, stop drilling immediately to avoid severe tool damage.
After each rod is withdrawn, apply cooling lubricant to the rod thread end and quick-coupling interface.
After blast-hole drilling, thread joints are hot and may contain rock debris and mine water. Cooling lubricant provides lubrication, temperature reduction, corrosion protection, and improved thread wear resistance.During rod removal, avoid repeated rod vibration/shaking, which is one of the most damaging actions for drill rods.
If a threaded joint is difficult to loosen, inject lubricant into the thread area first, then proceed with breakout or controlled vibration. This minimizes direct thread damage.
During rod vibration, the rock drill may transmit impact pressure near 200 bar. Sustained high-frequency impacts cause stress concentration; repeated vibration plus heat can create adhesive wear/cold-weld burn spots and erosive pits on thread ends, and in severe cases local melting pits. These defects become fatigue origins and can cause premature rod failure.
