Problems and improvement measures of rock drilling tools

Date: Oct. 24, 2024

Rock drilling and blasting technology and the use of rock drilling equipment are the main means of mining production. With the continuous development of rock drilling equipment, it has also promoted the development of rock drilling tools. The existing enterprises in China that produce drilling tools have formed a certain production scale, and their product varieties basically meet the needs of mining production.

Drill tools include drill bits, drill rods, connecting rod sleeves, and integral drill bits. They are a slender rod system that can withstand high-frequency impacts of tension, compression, bending, torsion, and piston under severe rock wear and mine water corrosion conditions. A single impact can reach 25-350J, and they often fail in the form of stress corrosion fatigue fracture. Their service life is generally only a few minutes to a few hours. Therefore, rock drilling tools have the most demanding stress conditions among all mechanical tools. The shortest service life and essential consumable tool for basic industries. In addition, some manufacturers have outdated production equipment and manufacturing processes, use inferior steel, lack modern design methods and means, resulting in unstable product quality, poor reliability, and low service life. Therefore, improving the manufacturing quality and service life of drill tools has great practical significance. Analysis of Failure Modes and Causes of Drill Tools

1.1 Drill bit
The main forms of damage to drill bits include detachment (teeth), fragmentation (teeth), bulging pants, cracking pants, waist breakage, inverted cone, and broken edges and corners, among other abnormal forms of damage. The sheet-like series (straight, three blade, X-shaped) drill bits are limited in diameter (generally less than or equal to 89mm) and frequently ground due to their radial distribution. During the rock drilling process, the central needle blade at the bottom of the hole repeatedly breaks the rock, and the two end drill blades bear the entire circumference of the rock breaking work. Moreover, the high linear velocity causes rapid damage to the front and radial surfaces of the blade tip, forming an inverted cone. The tapered hole is too shallow, and there is no straight wall gap area at the bottom of the hole, which deteriorates the stress on the drill tip and the pants wall, causing the pants to swell. The relative wing thickness (total circumference of blade/drill bit diameter) is very small: the fixed plate is not firm, the borehole is not round, the geometric stability is poor, it is not durable, and radial wear is fast. If the steel hardness of the pants body is too high, it will cause cracks in the pants. The ball tooth drill bit emerged with the hydraulic chiseling equipment. The breakage and detachment of edge teeth, cylindrical deformation, or fatigue fracture of the ball teeth and drill bit body together, especially for extremely tough mineral rocks (uniaxial compressive strength δ ≥ 300MPa), a sharp decrease in drilling speed and early edge tooth breakage are unavoidable. Due to the inclination angle between the edge teeth and the drill bit axis, the edge teeth are subjected to eccentric force, and the stress distribution inside the teeth is uneven, which deteriorates the stress condition of the edge teeth and causes edge tooth breakage; Due to the fixed teeth of the press fit, the clamping force of the ball tooth drill bit is related to the interference fit. A small interference fit reduces the clamping force, causing the edge teeth to fall off. A large interference fit causes the tooth hole to expand and crack, reducing the clamping force and causing the alloy teeth to fall off. In addition, when the edge teeth break rocks, the high circumferential speed can also cause the edge teeth to break. Poor fit between the drill bit and the drill bit pants, with a small fitting length and point or line contact between the drill bit and the pants, resulting in maximum stress at the boundary between the inner surface of the drill bit pants and the end of the drill bit. The stress on the pants deteriorates, leading to stress concentration and early breakage. In addition, improper heat treatment can cause the hardness of the drill bit pants to be too high, and micro brittle fatigue cracks are prone to occur on the inner surface of the pants at the stress concentration point, which can penetrate and propagate outward to crack.

1.2 Drill rod
The early failures of drill rods include: breakage at the collar, middle of the rod body, and tip; Tail explosion and top stacking; Brittle fracture; In the form of thread damage, the drill rod is subjected to comprehensive alternating stresses mainly caused by impact, bending, and corrosion during operation. If the drill rod does not have high fatigue strength, impact toughness, corrosion resistance, low notch sensitivity, and crack propagation rate, then under the repeated action of these stresses, the drill rod will accumulate damage and produce cracks, which will then propagate to fatigue fracture and mostly occur at the root of the thread. The brittle fracture of the drill rod shows a bright crystal surface state without fatigue cracks at the fracture surface. The main reason is the presence of local defects in the manufacturing process, such as impurities, unreasonable cross-sectional shape, horn mouth during forging, improper heat treatment, and other factors that result in insufficient strength, poor plasticity, and stress concentration of the drill rod. The cone connecting drill rod has long adopted outdated forging technology, with drill tips that are thicker and shorter, resulting in poor geometric dimension accuracy and cone surface roughness of the drill tips. This artificially increases the diameter of the drill bit cone hole and pants by 2mm, allowing for a radial loss of 2mm in the cone head diameter, reducing the theoretical life of the drill bit by one-third. In addition, forging the drill tip requires heat treatment, which some manufacturers do not do, resulting in “metallurgical gaps” in the forged rod body of the drill tip, deterioration of the metallographic structure of the drill tip, poor surface quality, blockage of water holes at the tip, and difficulty in ensuring the quality of the finished product. Fracture of the drill rod is the most common form of damage; Secondly, due to improper material and heat treatment, such as excessive hardness of the core of the carburized drill rod, poor quenching, etc., it is easy to cause the tail of the drill rod to explode, and insufficient hardness of the small drill rod can lead to the top of the pile. In addition, poor manufacturing quality can lead to poor fitting between the sleeve and the threaded part, surface corrosion, poor lubrication, and improper operation, all of which can cause early wear of the threads. In short, the low service life of rock drilling tools is due to the unreasonable design of tool structural parameters, and most enterprises have not fully processed according to national standards. Due to factors such as excessive geometric dimensions and positional tolerances, low fitting accuracy, outdated manufacturing processes, improper selection of steel and heat treatment methods, outdated manufacturing processes and equipment, and low usage technology.

2.1 Reasonable selection and development of high-quality materials are the material basis for improving the service life of drilling tools
According to the force analysis of the brazing tool during operation, the material of the brazing tool must have good stiffness, high fatigue strength, low fatigue notch sensitivity, high ability to clamp alloy sheets, good corrosion resistance, toughness and wear resistance, as well as good manufacturing processability, strong hardenability, hardenability and weldability. In addition, the hardness of the drill body directly affects the service life of the drill rod, so the quality of the hollow steel must be strictly controlled when selecting materials, with small dimensional tolerances, compliance with national requirements, good appearance, and hardness ranging from HRG28 to 43 and evenly distributed. The steel 55SiMnMo, 35SiMnMoV, and 40SiMnCrNiMo used for small drill rods in China have high fatigue strength and have been transformed from austenite to bainitic steel with good toughness after annealing, normalizing, and quenching. 40MnMoV alloy steel is used for small drill bits with induction brazed solid plates and fixed teeth, and 45NiCrMoV alloy steel is used for ball tooth drill bits with hot embedded fixed teeth. The selection of hard alloy pieces (teeth) on the rock drill bit and drill rod must be based on the mechanical properties of the rock and the type of rock drill. 2.2 Improving Design and Structure

(1) It is necessary to ensure that the geometric dimensions and positional tolerances of the rock drilling rod, especially the perpendicularity of the end face and the straightness of the rod body, meet national standards. Improve the cross-sectional shape, use composite arc transition in the transition area between the drill rod collar and the rod body, and design a transition arc retreat groove at the root of the threaded connection drill rod. Reduce stress concentration and improve the ability to resist fatigue damage. The taper connecting drill rod should ensure the length of the drill tip, cone angle, and surface roughness of the cone surface. The introduction of advanced foreign technologies, such as the rolling forming method for fully threaded drill rods in the UK, and surface hardening treatment have improved their toughness and wear resistance, thereby increasing their service life. Sandvik’s Speedrod drill rod from Sweden uses threaded connections, eliminates the need for connecting rod sleeves, eliminates the clearance between joint surfaces, improves the alignment and stiffness of the connection, and ensures the straightness of the blast hole.
(2) (2) Sandvik rock drilling tool department has designed a CASE shock wave computer simulation program, which clearly shows that the correct selection of drill bits is very important for ensuring high productivity and extending the life of the entire tool system.

(3) The improvement of the single tooth drill bit is difficult and should be eliminated as soon as possible. A newly developed composite disc tooth drill bit should be adopted, which has the following advantages:

(1) It retains and expands the advantages of good overall solidity of the sheet-like structure, high efficiency in breaking rocks with sharp blade edges, strong resistance to frontal impact and radial damage, stable geometric shape, unrestricted minimum diameter, drillability of extremely tough mineral rocks, long passivation cycle and self sharpening properties of the drill blade, convenient regrinding and recyclability. (2) Overcoming the shortcomings of poor overall robustness, edge tooth breakage, fast diameter reduction, low alloy utilization, difficult manufacturing, and poor quality stability of interference fit fixed tooth ball drill bits.

(3) The ball tooth drill bit has free tooth arrangement, the middle teeth only bear compressive stress, strong impact resistance, slow blunting of the tooth crown, and does not require grinding or requires frequent grinding! The advantages of less wear, longer lifespan, unrestricted maximum diameter, and the ability to accelerate project progress and improve labor conditions for workers are more prominent. It is also possible to use the popular deep cone hole new type of connecting drill bit from international companies such as Sweden’s Secoroc and Japan’s Mitsubishi and Toyo. The impact force of the rock drill piston is evenly distributed on the contact surface of the larger tip hole. The slender drill tip improves the coaxiality between the tip and the hole, making the connection more reliable and easy to disassemble. It can also increase the thickness of the waist wall of the drill bit while further reducing the diameter of the drill bit body, improving the powder discharge effect and increasing radial wear, thereby extending the service life of the drill bit. The improvement in the design of ball tooth drill bits is to strengthen the edge teeth, correctly select the geometric parameters of the teeth, and appropriately reduce the inclination angle to improve the stress condition and enhance the impact resistance of the edge teeth; Reasonably determine the gap between welds and the interference fit of fixed teeth to increase the fastening force of fixed teeth; Try to increase the number of edge teeth on the step teeth, improve the powder discharge system, reduce the repeated crushing of rock powder, lower energy consumption, and thus extend the service life of the drill bit.

2.3 Improve manufacturing processes and enhance manufacturing quality. Forged drill tips have the following drawbacks: low geometric dimension accuracy and surface roughness, shallow insertion depth of drill tips, inability to meet the matching needs of modern cone connections, and inability to achieve correct connection between drill tips and cone holes. The correct connection between the drill tip and the taper hole is related to extending the service life of the drill tip and drill bit, and improving the energy transfer efficiency of impact rock drilling. Moreover, forging the drill tips damages the overall uniformity of the metallographic structure of the drill rod, which is detrimental to its lifespan. Therefore, it is necessary to completely eliminate forging drill tips. Alloy drill steel that has undergone controlled rolling and controlled cooling should be used, and the surface of the rod can be controlled between HRG3040. The drill tips can be machined by precision turning or precision grinding and can be directly used. Mechanical cutting of drill bits improves the surface quality of products, enhances the accuracy of taper holes and threads, and ensures the mechanical properties of drill bits and steel bodies. For small and medium-sized hard alloy drill tools, efforts should be made to improve the quality and variety of steel and welding materials used in rock drilling hard alloy drill tools. It is recommended to use high-power low-frequency (8-10KHz) low-temperature sinking induction brazing technology to effectively fix teeth. The surface of the drill rod is subjected to heat treatment, such as shot blasting, rolling, anti-corrosion, etc., to improve the strength of the surface layer and reduce the sensitivity of fatigue notches. Shot blasting treatment is also required for the surface of the drill bit, which can effectively reduce surface defects such as oxidation and decarburization, and improve the fatigue resistance of the drill bit. Processing the cone of the drill bit, spraying zinc, and spraying aluminum are beneficial for reliable connection, ensuring even contact between the drill bit and the wall of the hole, eliminating stress concentration, effectively preventing drill bit swelling, fracking, and drill rod breakage, and isolating the corrosion of the steel body of the hole wall by flushing water.
2.4 Grinding: Grinding or surface polishing of hard alloy pieces (teeth) on rock drilling bits or drill rods can eliminate micro cracks on their surfaces, significantly extend the life of the drill bit by 100% to 500%, and also extend the service life of the drill rod extension sleeve and drill rod tail. This reduces the failure of the drill rod caused by excessive internal stress, lowers the internal stress in other components of the rock drilling equipment, improves drilling speed, and makes drilling more straight. In general, when the drilling speed decreases, the diameter of the flattened part of the ball teeth on the drill bit approaches or reaches one-third of the diameter of the ball teeth, inverted cones and “snake skin” appear, or the alloy protrudes too much due to the wear of the drill bit, it is necessary to grind it. For broken or damaged teeth caused by other reasons, they must be ground off to prevent debris from falling and damaging other teeth.
2.5 Reasonable operating techniques can effectively extend the service life of rock drilling tools. The service life of rock drilling tools depends on their quality and usage techniques. Rocks form a series of discontinuous surfaces such as different directions, bedding planes, joints, and faults in the complex geological development process. These differences in physical mechanics require that the selection of drilling tools must achieve a reasonable match between the rock characteristics of the rock drill and the drilling tools. In addition, with reasonable operating techniques, such as the need for a suitable axial thrust during rock drilling, the rock drill must ensure stable contact between the drill bit and the rock; Do not manually hammer the drill bit during disassembly. Use a specialized tool to remove the drill bit, which can artificially reduce the early damage of the drill bit and effectively extend its service life.

According to the 10 principles of “effective rock breaking, timely powder discharge, stable geometric shape, sturdy and wear-resistant, economical material utilization, flexible rotation and advance and retreat, easy disassembly and reliable connection, straight drill holes, easy manufacturing, and beautiful appearance” in drill tool design, we can only make a qualitative leap in the quality and variety of domestic drill tools and move towards the world’s advanced level by continuously improving and innovating in the geometric structure parameters, materials, manufacturing processes, and usage technologies of rock drilling tools.