A DTH hammer casing bit is suitable for loose formations and sand layers, effectively solving borehole stability issues. By synchronizing the advancement of a protective casing with the drill bit, this technology has become indispensable in fields like geological hazard prevention and tunnel engineering.Casing drilling tools are primarily categorized into ​​retractable-diameter bits​​ and ​​fixed-diameter bits​​. Retractable-diameter bits account for approximately 95% of market use.

​​01 How Casing Bits Work and Their Core Function​​

The working principle of a casing bit is based on ​​simultaneous casing advancement technology​​. While the bit cuts through the rock, an outer casing advances synchronously with the bit, providing immediate support to the borehole wall.Upon completion of drilling, the casing remains in place to stabilize the hole, facilitating subsequent operations such as charging (in drilling and blasting) or other procedures. The casing is later extracted using a pipe puller.In geotechnical drilling, geological conditions are often complex and unstable, containing mixtures of gravel, clay, and rock. Conventional drilling techniques struggle with wall collapse, whereas casing tools ​​effectively prevent borehole collapse​​ through the protective casing, ensuring borehole integrity.

​​02 Main Classifications and Technical Features​​

Casing drilling tools can be classified into two main types based on bit structure:

• ​​Retractable-Diameter Bits​​: These include designs like single-eccentric, double/triple-eccentric, and radially retractable bits. Their key feature is the ability to change diameter, allowing for reaming and subsequent retrieval through the casing.
• ​​Fixed-Diameter Bits​​: These employ inner and outer bit structures, typically requiring the casing to rotate; they are concentric casing tools.

Eccentric bits, as the mainstream type, have distinct technical characteristics. For instance, a single-eccentric bit uses a kidney-shaped long hole and a pin to transmit torque. The bit body only needs to rotate a small angle (about 8°) to expand or retract its diameter.Compared to fixed-diameter tools, retractable bits avoid the problem of the annulus between the inner and outer pipes getting clogged with rock cuttings, thereby reducing the probability of downhole incidents.

​​03 Significant Advantages and Application Value​​

Casing bits demonstrate multiple advantages in practical engineering applications:

1. ​​Increased Drilling Efficiency​​: Proven reliable in various formations and conditions, they effectively prevent frequent hole collapse, shorten construction cycles, and improve overall drilling efficiency.
2. ​​Simplified Drill String Design​​: Utilizing a floating structure where the outer casing does not rotate during drilling (only moves longitudinally) reduces resistance and increases the penetration rate.
3. ​​Adaptability to Complex Formations​​: Particularly effective in unstable ground like gravel and boulder fields, they handle geological conditions that are challenging for conventional methods.
4. ​​Reduced Incident Rate​​: Optimized designs minimize downhole accidents such as stuck bits, broken casing shoes, or broken casings, enhancing operational safety.

​​04 Key Construction Points and Safety Considerations​​

Strict adherence to operational procedures is essential for safe and effective casing drilling:

• ​​Pre-Drill Inspection​​: Before lowering into the hole, carefully inspect the DTH hammer, casing tools, casing, and casing shoe connections for tightness. Ensure the eccentric bit moves freely and the air passage is clear.
• ​​Parameter Control​​: In complex formations, apply the principle of "low rotation speed, low feed pressure, high air return velocity." Control the drilling rate based on rock hardness, looseness, and water content.
• ​​Cuttings Management​​: Closely monitor the casing's advancement and the cuttings removal status. Perform strong blow-downs to clean the hole every 0.3-0.5 meters of drilling to maintain a clean borehole.
• ​​Bit Retrieval​​: When drilling is complete or the center tool needs replacement, first clean the hole by blowing out residual cuttings. Then, stop rotation, slowly raise the center tool, reverse rotate at low speed to retract the eccentric bit, and then retrieve it conventionally.
• ​​Accident Prevention​​: When encountering extremely loose or fractured strata, reduce feed speed and pressure, and clean the hole repeatedly. If the tool gets stuck, avoid forceful handling; analyze the cause and take targeted measures.

As an effective solution for challenging formations, casing bit technology has broad development prospects. With ongoing national economic development, these tools will continue to evolve to meet more engineering demands.Future casing bits will develop towards ​​greater intelligence, higher efficiency, and broader adaptability​​, providing even more reliable technical support for geotechnical engineering and geological exploration.

For more knowledge and experience about drilling tools, please refer to the relevant topics.

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