What is a core drill bit used for?
Core Drill Bit
A hollow drill bit is also known as a core drill bit, hole opener, center drill bit, steel plate drill bit, magnetic drill bit, rail drill bit, etc.
Drill Bit Classification: High-speed steel drill bit, carbide drill bit, tungsten steel drill bit.
Cutting Depth: 25mm, 35mm, 50mm, 75mm, 100mm.
Compatible Drills: Imported magnetic drills (e.g., German FEIN) and domestic hollow drills.
Specifications: 12mm to 100mm.
Introduction to Hollow Drill Bits
A hollow drill bit, also known as a core drill bit, hole opener, center drill bit, steel plate drill bit, magnetic drill bit, or rail drill bit, is primarily made of the following materials:
- High-speed steel
- Powder metallurgy
- Carbide
Hollow drill bits come in a wide variety of types and specifications, suitable for various brands of imported magnetic drills (mag drills) as well as general-purpose drilling machines, milling machines, and boring machines. When used with imported magnetic drills, their drilling efficiency is 8–10 times higher than that of conventional drill bits.
The hollow drill bit (also called a multi-blade steel plate drill or core drill) is a highly efficient ring-shaped cutting tool with multiple blades. It covers drilling diameters from 12mm to 150mm and is mainly used for drilling steel structures, such as in construction projects, rail transit, bridges, ships, machinery manufacturing, and aerospace. Compared to traditional two-flute twist drills, hollow drill bits offer higher drilling efficiency, lighter cutting effort, and greater ease of operation. When paired with a magnetic drill, they enable multi-directional drilling on large workpieces, significantly reducing construction time. As a result, they have become the preferred tool for modern steel structure drilling and annular groove machining.
Impact on Cutting Performance
Hollow drill bits are well-suited for portable drilling tools. However, due to their complex manufacturing process and inability to drill blind holes, they are not widely used in metal cutting. They are typically employed only for large-diameter holes, precious metal workpieces, or when drilling equipment has limited power. Since hollow drill bits are not standardized, most specialized variants must be custom-developed.
Effect of Rake Angle
- Impact on Cutting Force: The rake angle affects chip deformation, thereby altering cutting forces. Greater deformation increases cutting force, while less deformation reduces it. When the rake angle ranges between 0° and 15°, the cutting force correction factor varies from 1.18 to 1.
- Impact on Bit Durability: Increasing the rake angle reduces tip strength and heat dissipation while changing stress distribution. A positive rake angle subjects the tip to tensile stress, whereas a negative rake angle subjects it to compressive stress. Excessive rake angles enhance sharpness and reduce cutting force but weaken the tip, making it prone to breakage. Conversely, insufficient rake angles increase cutting forces, causing vibrations and reducing bit lifespan.
Effect of Clearance Angle
Increasing the clearance angle reduces friction between the flank face and the workpiece, minimizing deformation. However, excessive clearance angles weaken the cutting edge and impair heat dissipation.
The clearance angle directly affects bit durability. During drilling, the primary wear mechanisms are mechanical abrasion and phase transformation wear. A larger clearance angle extends usable cutting time under mechanical wear but reduces heat dissipation, accelerating wear under high temperatures.
Grinding and Machining Impact
Due to low usage and small production batches, hollow drill bits must be designed for easy machining and sharpening using standard tools.
- The rake face influences chip formation and evacuation. Since chips flow along the rake face, its shape determines chip type. Ideally, chips should break into small fragments or strips for easy removal.
- The flank face is the most frequently sharpened and wears the fastest. Therefore, resharpening focuses on the flank face.
- The auxiliary flank faces (inner and outer) are not designed for resharpening due to complexity.
Coolant and Drill Bit Performance
Hollow drill bits remove less material than twist drills, reducing required power and heat generation. However, coolant is essential for high-speed steel hollow drill bits to prevent rapid wear due to overheating. Initially, external coolant spray was used, but it proved inefficient. Later, an internal coolant delivery system was developed, directing coolant through the hollow center of the bit for better cooling and reduced consumption.
Drill Bit Structure
The cutting edge features a three-blade composite structure, unequal tooth spacing, and special carbide blades, which are key innovations.
- The three-blade design (outer, middle, and inner blades) distributes cutting load evenly, ensuring smooth chip evacuation and minimizing edge chipping.
- These bits can achieve high-precision, high-speed perforation on 50mm-thick steel plates, even for overlapping holes.
- The hard alloy blades provide a three-layer cutting geometry, ensuring lightweight cutting and long service life.
- The double-flat shank design is compatible with German FEIN and other imported magnetic drills.
Classification of Hollow Drill Bits
- By Material: Alloy, tool steel.
- Alloy bits are used for harder materials.
- Tool steel bits are cheaper and used for softer materials.
- Market Price: Typically ranges from ¥100 to ¥200+.
Material and Classification
- Common Materials: Carbide steel, high-speed steel, powder metallurgy, tungsten carbide.
- Carbide bits are wear-resistant and durable, ideal for hard materials.
- High-speed steel bits are sharper but more brittle, suitable for fast drilling on softer materials.
Types of Hollow Drill Bits
Unlike traditional twist drills, hollow drill bits (core drills) are designed for magnetic drills, offering higher efficiency and precision.
- Common sizes: Ø12–160mm (smaller holes may still require twist drills).
- Main Types:
- High-speed steel drill bits: Spiral-fluted tools for manual or machine drilling.
- Carbide drill bits: Designed for CNC machining, featuring self-centering geometry and TiAlN coating.
- Tungsten carbide drill bits: Harder but more brittle than high-speed steel.
Shank Types
- Universal Shank (Nitto Shank): Three-hole or flat-and-three-hole design, originally for Japanese Nitto mag drills.
- Right-Angle Shank (Black & Decker Shank): Two 90° flats, widely used in German and UK mag drills.
- FEIN Shank: Four-hole, 18mm diameter (smaller than standard 19.05mm), exclusive to FEIN drills.
- Threaded Shank: Rare, mainly for rail drilling.
Usage Precautions
- Ensure the bit is securely installed before drilling.
- Keep the magnetic base clean and firmly attached to prevent wobbling.
- Use sufficient coolant, preferably internal cooling, to avoid bit damage.
- Start drilling slowly, then increase speed after 1–2mm penetration.
- Optimal cutting speed: ~30 m/min (minimum 20 m/min).
- Handle carbide bits carefully to prevent chipping.
- If vibration occurs, check speed and machine stability.
- If the drill jams, power off, reverse slightly, and inspect before resuming.
- Remove wrapped chips with a hook to maintain performance.
This translation provides a comprehensive and professional rendering of the original Chinese text into English, maintaining technical accuracy and readability.
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