Working Characteristics of DTH Drill Bits

Table of Contents

The rapid growth of the global economy has led to the increasingly widespread extraction and use of mineral resources. Humans have had to expand the collection area from shallow mines to deep, complex formations. Compared with rock drilling tools for shallow mines, DTH hammers can guarantee the construction cycle while conducting high-quality hole-breaking operations and can adapt to the construction of alpine and drought and water shortage regions, which are widely used in hole-breaking operations such as mining blast hole construction, hydrological water well drilling, geological exploration, trenchless pipeline laying, construction foundation, geotechnical engineering, and oil drilling, and are highly popular in the drilling industry.

As a direct tool for DTH hammer rock breaking, the DTH drill bit has a vital role in the process of DTH hammer impact rotary rock breaking. Through the research on the working characteristics of the DTH drill bit, we can improve its performance and service life while promoting the development of the DTH hammer and improving economic efficiency.

The structure of DTH drill bit

The structure of DTH drill bit

The working characteristics of the DTH drill bit are inseparable from every part of the DTH drill bit. The DTH drill bit consists of carbides, exhaust holes, head, splines, and impact end faces.

Carbides

Cemented carbides mainly bear the high-frequency stress wave from the hammer, and directly transmit the high-frequency stress wave to the rock surface to participate in the cutting work. The stress of carbide is extremely complicated during work, especially the gauge button, under the action of high-frequency impact force, generates bending moments and torques, which can easily cause a broken button. Therefore, the carbide grade and button shape as well as the distribution of carbides should be considered comprehensively when designing the drill bit, and the corresponding carbide should be selected for different rock hardness, different geological structures, and the actual working conditions of the drill bit.

Exhaust holes

The wear of DTH drill bits and carbides during drilling is related to the material properties of the skirt body and carbides and the size and flow velocity of cuttings at the bottom of the hole. The wear rate of carbides directly determines the service life of the DTH drill bit. Additionally, cuttings movement at the hole’s bottom primarily relies on high-pressure gas inside the vent hole to blow them to the ground. Consequently, designing an exhaust hole should consider factors such as DTH drill bit strength, venting resistance, and rock chip-blowing performance.

Head

The head is a crucial component of the DTH drill bit and can be designed in different forms to meet specific requirements. 

The crown shape of DTH drill bits includes two airfoil, three airfoil, and four airfoil. The selection of the crown shape of the DTH drill bit is closely related to the field working conditions, the depth of the single hole, and the diameter of the hole.

Spline

The DTH drill bit both impacts and rotates to break rock. Additionally, the drill pipe provides a torsional moment through the spline set at the end of the drill bit. However, these splines are subject to back-torque from the rock at the bottom of the hole as well as the vibration and friction brought by high-frequency impact during the process of impact rotation of the drill bit to break the rock, which is prone to plastic deformation after long time use and eventually leads to spline failure. Spline failure will lead to the inability of the drill bit to rotate and the impactor to operate normally.

Impact end face

The arrangement of the carbides of the DTH drill bit mainly depends on the shape of the end face, which directly affects the energy transfer efficiency and strength of the DTH drill bit. The end face shape of the DTH bit contains four types: flat face, convex face, tower type face, and concave face.

Working characteristics of DTH drill bits

DTH drill bit

The DTH drill bit breaks rock through impact and rotation, and the forces acting on the DTH drill bit include impact force, drilling pressure, and torque. 

Impact force is the main force of the DTH drill bit to break the rock. It hits the DTH drill bit through the piston, and the energy is transferred to the rock at the bottom of the well in the form of a shock wave, and the rock at the bottom of the well is broken by high-frequency reciprocating impact.

Drilling pressure is provided by the drill pipe, which mainly suppresses the rebound of the DTH drill bit and ensures the stability of the drilling. Drill pipe is connected to the DTH hammer through the outer casing,  and its rotation drives that of the DTH hammer, and the torque is transmitted through the spline of the DTH drill bit to perform rotary cutting on the rock at the bottom of the hole. DTH drill bit completes rock-breaking drilling under the combined action of these three loads.

The end of the DTH drill bit is connected to the DTH hammer, and the piston hits the impact end face of the DTH drill bit. Under the action of the impact load and impact speed with shortest action time, the rock surface is partially broken to form a one-time crushing pit. At the same time, as the DTH drill bit rotates, it provides a second impact that further breaks down the rock. This continuous impact and rotation create cuttings that remove material during hole formation until a circular blast hole is completed. Thus, the rock-breaking and hole-forming process of the DTH drill bit constitutes a compound movement of impact crushing, shearing, and scraping.

In short, the DTH drill bit is a crucial tool that requires careful consideration of various parameters to achieve optimal results. The design of the drill bit should minimize the friction between the tool and the rock to improve efficiency. During manufacturing, it is essential to precisely match the components to ensure the stability of the entire tool system.

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