What is the angle of twist drill bit grinding?

Core Grinding Angles of Twist Drill Bits

1. Point Angle (Tip Angle)

• Standard Values: The most common point angle in general-purpose drill bits is 118°, favored for its versatility across wood, plastic, and medium-carbon steels. For harder materials like stainless steel, cast iron, and high-alloy metals, a 135° point angle is prevalent. This wider angle distributes cutting forces more evenly, minimizes edge chipping, and reduces friction-induced heat buildup. Specialized applications, such as drilling thin sheet metal, may use angles as narrow as 90° to prevent material deformation, while masonry drills often adopt angles between 130° and 140° to handle the abrasive nature of concrete and stone.
• Functional Impact: A 118° angle creates a sharper tip, allowing for quicker penetration in softer materials but generating higher localized heat in hard metals. In contrast, the 135° angle’s flatter tip spreads the cutting load across a larger area, reducing stress on the cutting edges and extending tool life in demanding applications. Some premium drill bits, designed for multi-material use, feature a split point—a modified 135° angle with a smaller inner angle (typically 90°) at the tip—to enhance centering accuracy and reduce “walking” (drifting off the intended starting point) on smooth surfaces.

2. Rake Angle (Helix Angle)

• Variations by Material and Diameter: Rake angles are not fixed but vary with the drill’s diameter and intended material. For small-diameter drills (≤3mm), a more positive rake angle (often +10° to +15°) is used to compensate for lower rigidity, ensuring smooth cutting in delicate materials like wood or plastic. Larger drills (≥10mm) typically adopt a less positive or neutral rake angle (+5° to 0°) to strengthen the cutting edges, critical for handling the higher forces encountered in metal drilling. For abrasive materials like cast iron, a slightly negative rake angle (-3° to -5°) prevents premature edge wear by increasing the thickness of the cutting lip.
• Helix Angle Correlation: The rake angle is closely linked to the drill’s helix angle—the angle of the flute spiral around the drill body. A steeper helix angle (30°–45°) creates a more positive rake, ideal for soft materials where rapid chip evacuation is key (e.g., wood, aluminum). A shallower helix angle (15°–25°) results in a less positive rake, better for hard materials like steel, as it reduces the risk of edge deflection and improves chip control in high-heat conditions.

3. Clearance Angle (Relief Angle)

• Optimal Ranges: Clearance angles typically range from 5° to 15°, with values adjusted based on material hardness. For soft materials like wood or plastic, a larger clearance angle (10°–15°) is used to minimize friction and ensure smooth cutting. For harder materials like steel and stainless steel, a smaller clearance angle (5°–8°) is preferred to maintain the structural integrity of the cutting lip, as a larger angle would weaken the edge, leading to chipping under high stress. In precision applications, such as aerospace machining, clearance angles are often fine-tuned to ±1° to balance heat management and edge strength.
• Secondary Clearance: Many high-performance drill bits feature a secondary clearance angle—a smaller angle ground behind the primary clearance angle. This additional angle further reduces friction by creating a gap between the drill’s flank and the hole wall, particularly important in deep-hole drilling where heat buildup is more pronounced. Secondary clearance angles typically measure 2°–5° and are common in professional-grade bits from brands like Bosch and DeWalt.

4. Chisel Edge Angle and Relief

• Chisel Edge Relief: To further optimize performance, many modern drill bits include chisel edge relief—a small grind that reduces the contact area of the chisel edge with the workpiece. This modification lowers friction, reduces heat, and prevents “digging” in soft materials, where a large chisel edge can cause uneven hole initiation. Premium drill bits, such as those from Irwin or Makita, often feature precision-ground chisel edges with tailored relief angles to enhance performance across diverse materials.

Material-Specific Angle Optimization

Wood and Soft Plastics

• Point Angle: 118° is standard, though some wood-specific bits use angles as low as 90° for faster penetration.
• Rake Angle: Highly positive (+15° to +20°) to create a sharp cutting edge that slices through fibers cleanly.
• Clearance Angle: 10° to 15° to minimize friction, as these materials generate little heat but can cause binding if the flank rubs against the hole wall.
• Helix Angle: Steep (35°–45°) to quickly evacuate wood chips or plastic swarf, preventing clogging in deep holes.

Mild Steel and Low-Alloy Metals

• Point Angle: 118° is optimal, providing sufficient sharpness for penetration while distributing forces to avoid edge wear.
• Rake Angle: Moderately positive (+5° to +10°) to reduce cutting resistance without sacrificing edge strength.
• Clearance Angle: 7° to 10° to manage heat buildup, as steel conducts heat more slowly than wood, increasing the risk of tool overheating.
• Helix Angle: 25°–35° to balance chip evacuation and structural rigidity, preventing deflection in longer drills.

Stainless Steel and High-Alloy Metals

• Point Angle: 135° is standard, as its wider profile reduces localized stress and distributes heat across a larger area.
• Rake Angle: Neutral to slightly negative (0° to -5°) to strengthen the cutting edge against the high forces of tough metals, preventing micro-chipping.
• Clearance Angle: 5° to 8° to limit flank contact, reducing friction and heat generation in materials that retain heat.
• Helix Angle: 20°–30° to slow chip evacuation, allowing chips to carry away more heat before exiting, and to enhance rigidity in high-stress drilling.

Cast Iron and Abrasive Materials

• Point Angle: 130°–140° for cast iron, providing a robust tip to withstand abrasive wear; masonry bits often use 135°–150° angles to 破碎 hard particles.
• Rake Angle: Negative (-3° to -8°) to create a thick, wear-resistant cutting edge that resists abrasion.
• Clearance Angle: 5° to 7° for cast iron, minimizing flank wear; masonry bits may have larger clearance angles (8°–12°) to facilitate chip removal in porous materials.
• Helix Angle: Shallow (15°–25°) for cast iron to reduce vibration, and variable or stepped helixes in masonry bits to enhance debris evacuation.

Leading Brands and Their Angle-Optimized Models

Bosch: Precision Engineering for Versatility

• Bosch HSS-Co Series (High-Speed Steel with Cobalt)
  Designed for high-performance metal drilling, the HSS-Co series incorporates 5%–8% cobalt into the steel alloy, enhancing heat resistance and wear resistance. Key models include:
  • Bosch 2608585225: A 3mm diameter drill bit with a 135° point angle, ideal for stainless steel and high-alloy metals. Its helix angle of 30° balances chip evacuation and rigidity, while a slightly negative rake angle (-2°) strengthens the cutting edges. The clearance angle is set to 6° to minimize friction in heat-retentive materials.
  • Bosch 2608585230: A 5mm diameter HSS-Co8% bit (8% cobalt content) engineered for aerospace-grade alloys. It features a 135° split point angle—with a 90° inner tip for centering accuracy—and a secondary clearance angle of 3° to reduce flank wear. The rake angle is neutral (0°) to handle the high stress of drilling hardened metals.
• Bosch Professional Multi-Material Bits (HSS-G)
  For general-purpose use across wood, plastic, and mild steel, Bosch’s HSS-G (high-speed steel, ground) bits prioritize versatility:
  • Bosch 2607010577: A 6mm diameter bit with a 118° point angle, optimized for balanced performance. Its rake angle is +8° for sharp cutting in wood and plastic, while the clearance angle is 9° to manage heat in steel. The helix angle of 35° ensures efficient chip evacuation in both soft and medium-hard materials.

DeWalt: Durability for Heavy-Duty Applications

• DeWalt DW1263 Series (HSS with TiN Coating)
  The DW1263 series targets metalworking professionals, featuring titanium nitride (TiN) coating for reduced friction and increased hardness.
  • DeWalt DW1263-8: An 8mm diameter bit with a 118° point angle, designed for mild steel and cast iron. Its rake angle of +6° balances cutting efficiency and edge strength, while the clearance angle of 8° minimizes heat buildup. The 30° helix angle ensures consistent chip flow, preventing clogging in deep holes.
  • DeWalt DW1362: A 10mm diameter bit with a 135° point angle, tailored for stainless steel. Its TiN coating reduces friction by up to 30% compared to uncoated bits, while a negative rake angle (-3°) and 5° clearance angle enhance durability in tough materials. The helix angle is 25° to slow chip evacuation, aiding heat dissipation.
• DeWalt Max Impact Bits
  For use with impact drivers, these bits feature reinforced angles to withstand vibration:
  • DeWalt DWA1240: A 4mm diameter impact-rated bit with a 135° point angle and reinforced cutting lips. The rake angle is neutral (0°) to resist chipping under impact, while the clearance angle of 7° ensures smooth operation in metal and wood.

Makita: High-Cobalt Solutions for Hard Materials

• Makita D-46473 Series (HSS-Co5%)
  This series contains 5% cobalt, offering superior heat resistance for continuous heavy-duty use:
  • Makita D-46473-6: A 6mm diameter bit with a 135° point angle, optimized for high-alloy steels and stainless steel. Its rake angle is -4° to strengthen the cutting edges, while the clearance angle of 6° reduces friction. The helix angle of 22° slows chip flow, allowing heat to dissipate through the chips rather than the bit.
  • Makita D-46475: A 10mm diameter HSS-Co5% bit with a 135° split point, featuring a 90° inner tip for precise centering. The secondary clearance angle of 2° minimizes flank wear, and the oxidization coating (black finish) provides additional heat resistance, making it suitable for drilling in high-temperature environments.

Irwin: Innovation in Cutting Geometry

• Irwin 3018002 Series (High-Speed Steel with Laser Edge)
  These bits feature a “Laser Edge” design, a precision-ground cutting lip geometry that improves sharpness and centering:
  • Irwin 3018002-3: A 3mm diameter bit with a 118° point angle, ideal for wood, plastic, and mild steel. Its rake angle of +10° ensures sharp cutting in soft materials, while the clearance angle of 10° reduces friction. The 34° helix angle facilitates rapid chip evacuation, preventing clogging in wood.
  • Irwin 3018005: A 5mm diameter bit with a 135° point angle and Laser Edge sharpening, designed for multi-material use. The rake angle transitions from +5° at the tip to 0° at the outer edge, balancing sharpness and strength, while the clearance angle of 8° works with the TiN coating to reduce heat.

Hitachi: Affordable Reliability for General Use

• Hitachi 721038 Series (HSS General Purpose)
  These uncoated high-speed steel bits are versatile enough for wood, plastic, and mild steel:
  • Hitachi 721038-6: A 6mm diameter bit with a 118° point angle, featuring a rake angle of +7° for sharpness in soft materials and a clearance angle of 9° for heat management in steel. The 32° helix angle ensures efficient chip removal, making it a popular choice for home projects.
  • Hitachi 721042: An 8mm diameter bit with a 118° point angle, designed for general metal drilling. Its simplified geometry—with a slightly rounded cutting edge—reduces the risk of chipping during casual use, while the 28° helix angle balances chip flow and rigidity.

Grinding Techniques and Maintenance

Key Grinding Considerations

• Symmetry: Both cutting lips must have identical angles to ensure balanced cutting forces. Asymmetry causes uneven wear, vibration, and off-center holes. Professional grinders use jigs or laser alignment tools to ensure symmetry within ±0.5°.
• Consistency: The point angle, rake angle, and clearance angle must be maintained across the entire length of the cutting lip. Inconsistent grinding leads to localized stress points, accelerating wear.
• Coating Preservation: Coated bits (e.g., TiN, AlCrN) require specialized grinding wheels to avoid stripping the coating, which protects against heat and wear. Diamond or cubic boron nitride (CBN) wheels are recommended for coated bits, as they grind cleanly without excessive heat.

Manual vs. Machine Grinding

• Manual Grinding: Done with a bench grinder, this requires skill to maintain angles. Users must steady the bit against the grinder’s wheel, rotating it to shape the lips. Guides or angle templates help amateurs achieve approximate angles, but precision is challenging.
• Semi-Automatic Grinders: Tools like the Tormek T-8 or Drill Doctor 7500 automate angle control, allowing users to set desired point angles (118° or 135°) and rake angles. These machines ensure symmetry and consistency, making them popular in small workshops.
• CNC Grinding: Used in manufacturing, CNC grinders program angles with micron-level precision, incorporating complex geometries like split points and secondary clearances. Brands like Bosch and DeWalt rely on CNC grinding for their premium models to ensure uniform performance across batches.

Impact of Angle Degradation and Wear

• Increased Cutting Force: Dull bits require more pressure to drill, straining tools and increasing the risk of user fatigue or injury.
• Heat Buildup: Blunt edges generate excess heat, discoloring the bit (blueing) and weakening its structure. In extreme cases, heat can melt coatings or anneal the steel, permanently reducing hardness.
• Poor Hole Quality: Ragged edges, oversized holes, or elliptical shapes indicate uneven wear on the cutting lips, often due to asymmetric angle degradation.
• Chip Evacuation Issues: Worn rake or helix angles disrupt chip flow, causing clogging, which further increases heat and friction.

Conclusion