The point angle of a drill is a critical geometric parameter that directly dictates cutting performance, chip formation, tool life, and hole quality in metal machining. Selecting the optimal angle is contingent on workpiece material properties (hardness, ductility, toughness), hole requirements (precision, depth, surface finish), and machining conditions. This article dissects the three most common drill point angles—118°, 135°, and 90°—and their targeted applications in metalworking.
118° Point Angle: The Universal Workhorse
The 118° point angle is the industry standard for general-purpose drilling, characterized by a balanced combination of cutting efficiency, chip evacuation, and versatility. Its symmetric design features two primary cutting edges with a moderate included angle, making it adaptable to a wide range of ferrous and non-ferrous metals.
Key Applications:
• Carbon Steel & Low-Alloy Steel: Ideal for machining mild steel (A36, 1018), medium-carbon steel (4140), and low-alloy steel. The 118° angle generates manageable cutting forces and produces continuous, curly chips that evacuate smoothly, reducing clogging and tool wear.
• Cast Iron (Gray & Ductile): Performs well in gray cast iron (GG25, ASTM A48) and ductile iron (GJS400) due to its ability to penetrate without excessive pressure, minimizing workpiece fracture or edge chipping.
• Aluminum & Non-Ferrous Alloys: Suitable for non-heat-treated aluminum (6061, 7075) and copper alloys (C11000) when paired with proper lubrication. The angle prevents excessive material deformation and ensures clean hole walls.
Limitations: Less effective in hardened steel (HRC > 35) or high-toughness materials, as the moderate angle may cause edge wear, chipping, or poor hole straightness.
135° Point Angle: The Hardened & High-Performance Specialist
The 135° point angle, often referred to as a "split point" or "self-centering" angle, is engineered for high-strength materials and precision machining. Its steeper angle concentrates cutting forces closer to the center, enhancing rigidity and reducing deflection, while the split-point design improves centering accuracy and eliminates the need for pre-drilling.
Key Applications:
•Hardened Steel & Tool Steel: Excels in machining hardened steel (HRC 35–60), tool steel (D2, H13), and stainless steel (304, 316). The steep angle reduces cutting edge loading, preventing chipping and extending tool life when using carbide or coated HSS drills.
• Deep-Hole Drilling: The self-centering capability minimizes hole runout in deep-hole applications (depth-to-diameter ratio > 5:1), critical for aerospace and automotive components where precision is paramount.
•Automated Machining: Widely used in CNC machining centers and automated lines due to its stability and reduced setup time (no pre-centering). The efficient chip evacuation also suits high-speed machining (HSM) of tough materials.
Limitations: Less efficient in soft, ductile materials (e.g., pure aluminum, copper) as the steep angle may cause chip adhesion, poor surface finish, or increased cutting torque.
90° Point Angle: The Spotting & Countersinking Expert
The 90° point angle, also known as a "countersink angle," features a shallow, wide cutting profile designed for specific hole preparation and finishing tasks. Unlike 118° and 135° angles, its primary function is not deep drilling but creating tapered surfaces or precise starting points.
Key Applications:
• Spotting Holes: Used to create a shallow, conical starting hole before drilling with a larger or longer drill. The 90° angle ensures the subsequent drill aligns perfectly, eliminating runout and improving hole straightness—essential for machining hard or uneven workpieces.
• Countersinking & Chamfering: Ideal for countersinking screws (flat-head, oval-head) to ensure flush seating. It also creates 90° chamfers at hole edges to remove burrs, prevent material stress concentration, and facilitate component assembly.
• Soft Materials & Thin Sheets: Suitable for soft metals (pure aluminum, brass) and thin-walled components, where the wide angle distributes cutting forces evenly, avoiding material deformation or tear-out.
Limitations: Not intended for deep drilling. The wide angle generates higher cutting forces and poor chip evacuation in deep holes, leading to excessive tool wear and reduced hole quality.
Summary: Selecting the Right Point Angle
Choosing the correct drill point angle hinges on aligning geometric advantages with machining requirements:
• 118°: Go-to for general-purpose machining of mild steel, cast iron, and non-ferrous alloys—versatility for low-to-medium hardness materials.
• 135°: Preferred for hardened steel, stainless steel, deep holes, and precision CNC applications—stability and self-centering for tough materials.
• 90°: Specialized for spotting, countersinking, and chamfering—critical for hole alignment and finishing tasks.
Complementary factors such as drill material (HSS, carbide), coating (TiN, TiAlN), and cutting parameters (speed, feed) further optimize performance, but the point angle remains the foundational design choice for successful metal drilling.
