As a commonly used cutting tool, the geometry of a drill bit directly affects machining accuracy, chip evacuation efficiency, and tool life. This article systematically reviews the main body and rear structure of drill bits, covering key parameters such as helix angle, point angle, clearance angle, flute design, and shank types, helping you make more informed decisions in design and selection.
Image source: Parts of drill
Drill Body
Drill Body refers to the main part of the drill bit, which includes all the geometric shapes and structural features responsible for bearing cutting forces and performing the actual cutting operation. The design of the drill body directly affects the quality of the drilled hole, cutting efficiency, and tool life.
The geometry and size design of the drill body directly impact the performance of the drill bit during the machining process, such as cutting speed, hole accuracy, and chip removal efficiency.
Drill Diameter
- Function: Theoretical diameter formed by the outer cutting edges, determines hole size. In actual machining, consider elastic deformation (steel rebound: 0.005–0.02 mm, aluminum alloy: 0.02–0.05 mm).
- Standard: ISO tolerance h8–h11; precision grade requires ±0.005 mm control.
Flute
- Chip Removal Mechanism: Spiral angle and cutting speed generate centrifugal force to eject chips along the groove.
- Flute Types:
- Straight flute: For hard and brittle materials (cast iron/ceramics).
- Parabolic flute: For deep-hole drilling (depth-to-diameter ratio >10).
- Variable lead flute: Suppresses harmonic vibration (e.g., titanium alloys).
Flute Width
- Design Principle: Flute width = 0.25–0.35 × drill diameter.
- Extreme Conditions: For magnesium alloy, increase flute width by 20% to prevent chip clogging and combustion.
Lead (Pitch)
- Geometry: P = πD / tanβ (D: diameter, β: helix angle).
- Optimization Example: For Φ10 mm drill with 30° helix angle, theoretical lead ≈ 54.4 mm.
Helix Angle
- Material Matching Table:
| Material Type | Recommended Angle | Function |
| Low carbon steel | 30° | Balances chip evacuation and rigidity |
| Stainless steel | 25° | Reduces work hardening |
| Aluminum alloy | 40° | Rapid evacuation of ribbon-like chips |
| Composites | 15° | Reduces delamination |
Point Angle
Mechanical Characteristics:
- 118°: Axial force to radial force ratio = 1:0.35
- 135°: Axial force increases by 15%, but heat dissipation area increases by 30%
- Special Designs:
- Double point angle (e.g., 118° + 140°): Increases tool life by 50%
- R-type chisel edge: R0.5 arc replaces straight edge, reduces feed force by 30%
Flank
- Geometry: Primary relief angle (α) and secondary relief angle (α₁) provide dual support.
- Grinding Note: Carbide drills should retain 0.05–0.1 mm land width to enhance strength.
Clearance Angle
- Dynamic Compensation: Theoretical value: 8–12°; actual grinding increases by 2–3° to compensate for machine rigidity.
- Material Matching:
- Soft materials: 12–15° (e.g., aluminum)
- Hard materials: 6–8° (e.g., hardened steel)
Outer Corner
- Chamfering: R0.1–R0.3 mm radius transition to avoid stress concentration.
- Reinforcement Process: PVD coating (e.g., TiAlN) improves high-temperature resistance.
Height of Point
- Regrinding Standard: h = 0.015D – 0.03D (D = diameter).
- Inspection Method: Use projector; height difference between main cutting edges ≤0.02 mm.
Shank Area
Shank Area refers to the rear part of the drill bit, which is typically connected to the machine tool’s clamping system. Its primary function is to connect the drill bit to the machine spindle and transmit torque and cutting forces. The design of the shank area plays a crucial role in the stability of the drill bit, clamping force, and the efficiency of torque transmission during machining.
The dimensions and design of the shank area are crucial for the performance, application range, and machining precision and efficiency of the drill bit.
Body
- Heat Treatment: HSS drills require triple tempering (560°C × 1 hr) to reach 63–65 HRC.
Neck
- Stress Optimization: Transition radius ≥0.3D; deep-hole drills require special reinforcement.
Taper Shank
- Morse Taper Standards:
| Taper No. | Large End Diameter | Applicable Drill Diameter |
|---|---|---|
| MT2 | 17.78 mm | Φ6–15 mm |
| MT3 | 23.825 mm | Φ15–25 mm |
Straight Shank with Tang
- Anti-Rotation Design: Flat width = 0.1D; depth = 0.05D.
Tang
- Torque Transmission: Tang cross-sectional area must satisfy
τ = 16T / (πd³) ≤ allowable shear stress (typically ≥300 MPa).
Central Axis
- Concentricity Requirement: Precision grade ≤0.01 mm/100 mm; check using V-block + dial indicator.
Front-End View Structure
Front-End in drill bit design refers to the front part of the drill bit, typically the section that makes contact with the workpiece and performs the cutting. It includes elements such as the cutting edges, point angle, and chisel edge, which directly affect the cutting performance, chip removal efficiency, and hole quality.
The front-end part of the drill bit plays a crucial role in the axial force, cutting force, and chip removal during the cutting process. A well-designed front-end structure can significantly improve hole precision, reduce vibration, decrease cutting force, and extend tool life.
In short, the Front-End is a critical part of the drill bit, directly determining its cutting performance and hole quality.
Chisel Edge Angle
- Centering Improvement: S-type chisel edge reduces axial force by 40%.
Cutting Edge
- Edge Treatment:
- Edge rounding radius: Carbide R0.02 mm; HSS R0.05 mm
- Chamfer width: 0.05–0.1 mm (enhances edge chipping resistance)
Land Width
- Stability Mechanism: Width = 0.1D–0.15D; too narrow causes vibration, too wide increases friction.
Margin
- Guiding Precision: Gap between margin and hole wall = 0.005–0.015 mm (equivalent to H7/g6 fit)
Margin Width
- Optimized Balance: Standard 0.2–0.3 mm; increase to 0.5 mm for deep-hole drills.
Body Clearance
- Gradient Design:
- Cutting area: 0.1–0.3 mm
- Transition area: 0.05–0.1 mm
Depth of Body Clearance
- Chip Control: For deep-hole machining, depth should be ≥3× chip thickness.
Dimensional Definitions
Front-End in drill bit design refers to the front part of the drill bit, typically the section that makes contact with the workpiece and performs the cutting. It includes elements such as the cutting edges, point angle, and chisel edge, which directly affect the cutting performance, chip removal efficiency, and hole quality.
The front-end part of the drill bit plays a crucial role in the axial force, cutting force, and chip removal during the cutting process. A well-designed front-end structure can significantly improve hole precision, reduce vibration, decrease cutting force, and extend tool life.
In short, the Front-End is a critical part of the drill bit, directly determining its cutting performance and hole quality.
Overall Length
- Safety Margin: Should be 10–15 mm longer than the machine’s maximum stroke to prevent collision.
Flute Length
- Empirical Formula: Lf = 1.2 × hole depth + (3–5) mm.
Functional Length
- Rigidity Guideline:
- Steel machining: L/D ≤ 8
- Aluminum machining: L/D ≤ 12
Shank Length
- Clamping Standard:
- Spring collet: ≥3 × diameter
- Hydraulic holder: ≥2 × diameter
Neck Length
- Vibration Reduction Design: Long-neck type (L = 2D) for tight spaces; feed rate reduced by 30%.
By understanding the design essentials of each drill bit component and their corresponding material compatibility, you can significantly improve drilling stability and quality. Choosing the right drill bit structure not only extends tool life but also enhances machining efficiency, making it a vital factor in achieving high-precision and efficient manufacturing.
