Rotary drilling core barrel knowledge explanation

The rotary drilling rig's auger drill bit has a simple structure, which only consists of a cylinder and teeth. It is a type of drill bit with relatively low manufacturing costs among the rotary drilling rig's drill bits. Generally, the teeth used in the auger are cutting teeth, and when crushing rocks with extreme hardness, cone teeth or other ultra-high hardness alloy teeth can be installed on the auger. Although the structure of the auger is simple, its role in crushing rocks is unparalleled to other drill bits. The principle of the auger drilling and crushing rocks is ring grinding and crushing, which is to cut out a ring-shaped free surface around the pile hole by welding the teeth on the lower ring-shaped section of the bottom during drilling into the rocks. Because the contact area between the drill bit and the bedrock is small (only a narrow ring-shaped area), the force and work continuously exerted by a single tooth on the rocks are relatively large, resulting in high efficiency in crushing rocks. The auger is currently the most commonly used drill bit for crushing hard rock strata and gravel strata in the rotary drilling rig.

The simulated drilling rig has a high safety factor of 3.13 under the operating conditions of applying maximum pressure and torque during rotary drilling. The maximum displacement is only 0.207mm, indicating that the overall stiffness and strength of the drill bit are relatively large, meeting the structural design requirements.

Layout of Teeth on the Auger Drill Bit of the Rotary Drilling Rig

The principle of arranging the teeth on the auger bit is similar to other rock-embedded drill bits, which is arranged in the most favorable way for crushing rocks. Generally, the principle of arranging teeth in three groups, namely internal, middle, and external groups, is followed. The teeth on the auger bit are divided into internal cutting teeth, ring cutting teeth (middle part), and external cutting teeth according to the size of the tooth tip's rotation diameter from inside to outside, and arranged in an alternating manner. There is also a certain height difference among them, with the ring cutting teeth protruding the longest, followed by the external cutting teeth, and the shortest being the internal cutting teeth.

This arrangement is designed to create conditions for the leapfrog-like crushing of rocks. The principle is as follows: first, the ring cutting teeth cut into the rocks to create a wedge-shaped free surface, and then the external and internal cutting teeth follow up. The free surface cracks cut by the three groups of teeth interconnect (the rotation spacing and height difference between the internal, middle, and external tooth tips are designed based on the laws of crack interconnection in different rocks), creating a deep groove-shaped free surface around the bottom of the hole, completely separating the rock core in the center from the surrounding rock mass.

Positioning and Guiding of the Auger Drill Bit of the Rotary Drilling Rig

The cylinder of the auger bit is entirely straight, and guide strips are arranged on the outer side of the cylinder wall, which are opposite to the rotation direction and inclined upward. The auger bit is generally used for drilling into hard bedrock or gravel strata, which are generally坚硬 and have slow footage. The straight cylinder-shaped bit has excellent guiding and positioning functions. The main role of the guide strips around the cylinder wall is to guide the rock debris at the bottom of the bit upward, avoiding the accumulation of debris in the grooves that hinder the cutting of fresh rocks by the teeth. Another role of the guide strips is to protect the cylinder wall and enhance wear resistance.

Design of Other Important Parameters for the Auger Drill Bit of the Rotary Drilling Rig

The main parameter of the auger bit is its diameter, which is the diameter of the pile hole that can be drilled. The second parameter is the height of the cylinder, which is usually larger than the diameter of the auger bit. This structural relationship between size and height is necessary to meet the need for twisting and breaking the rock core. After the rock core is ring-cut by the auger bit, as the cutting depth continues to increase, the rock core will be twisted and broken along a 45-degree angle with the axial direction. Therefore, the height of the cylinder is required to be greater than the diameter of the rock core, so that the rock core can be twisted and broken and then brought out of the pile hole.

Setting and Calculation of the Angle of the Teeth on the Auger Drill Bit of the Rotary Drilling Rig

During the process of drilling into the stratum by the rotary drilling rig, the teeth are the key parts that directly contact the stratum. The angle and spacing of the teeth directly determine the working efficiency of the rotary drilling rig. Therefore, during the design of the drill bit, research should be conducted on the mechanical model of the teeth cutting soil and rocks to find ideal angles and spacing distances for the teeth