Drill Selection: Point Geometry, Coatings and Materials
Short answer
- Match the drill point angle to the workpiece: 118° for mild steel, carbon steel and aluminum alloys, a 135° split point for stainless steel, hardened steel and titanium, and 90° to 100° for soft plastics.
- Helix angle controls chip evacuation, with 30° to 40° suiting aluminum and 15° to 20° suiting stainless steel and cast iron.
- Tool material and coating then extend tool life, with HSS for most steels, cobalt-alloyed HSS for stainless steel, solid carbide for hardened steels, and TiAlN holding hardness above 600 °C for high-speed drilling.
When a maintenance technician or reliability engineer selects a twist drill, the first question is: does the point geometry match the workpiece material and the intended hole depth? The wrong point angle or helix can cause walking, excessive wear, or chip blockage, turning a routine hole into a failure point.
Which point angle suits each material?
The point angle determines how the cutting edges engage the workpiece. For most general‑purpose steels, a 118° point provides a balanced blend of cutting force and edge strength. When drilling harder alloys—stainless steel, hardened tool steel, or titanium—raise the angle to 135° and use a split‑point (or self‑centering) design. The split point concentrates the cutting forces on a smaller tip radius, reducing the tendency of the drill to walk across the surface and improving start‑up accuracy.
For soft, non‑metallic substrates such as acrylic, polycarbonate, or low‑density plastics, a shallower angle (often 90° to 100°) reduces the thrust force and prevents the material from deforming or cracking. The flatter tip also spreads the load, which is helpful when the drill is used without a pilot hole.
In practice, match the point angle to the material’s hardness and ductility:
- 118° solid point – mild steel, carbon steel, aluminum alloys.
- 135° split point – stainless steel, hardened steel, titanium, high‑strength alloys.
- 90°‑100° flat point – PVC, acrylic, polycarbonate, other soft plastics.
When a split point is specified, the two tapered lips are offset slightly. This geometry creates a tiny central groove that guides the drill into the material, virtually eliminating walking on smooth surfaces.
How does helix angle affect chip evacuation?
The helix angle governs chip formation and evacuation. A high helix (30°‑40°) produces long, thin chips that are easy to clear from the flutes, making it the preferred choice for aluminum and other soft metals where chip packing can cause heat buildup. Conversely, a lower helix (15°‑20°) yields thicker, more robust chips that are better suited to tough, abrasive materials like stainless steel or cast iron, where the chip must carry away more material without breaking.
When drilling deep holes (depth > 3 × diameter), the helix angle alone may not prevent chip re‑cutting. In those cases, a combination of a moderate helix (20°‑25°) and a peck‑drilling cycle is recommended. The peck cycle withdraws the drill periodically, allowing chips to clear and reducing heat concentration.
Which tool material and coating fit the job?
Tool material selection is a trade‑off between hardness, toughness, and heat resistance. Three families dominate industrial drilling:
- High‑speed steel (HSS) – offers good toughness and can be resharpened. Suitable for most steels, aluminum, and plastics at moderate speeds.
- Cobalt‑alloyed HSS (often 5% or 8% cobalt) – retains hardness at higher temperatures, making it the go‑to for stainless steel, high‑temperature alloys, and applications where heat buildup is unavoidable.
- Solid carbide – the hardest option, delivering superior rigidity and wear resistance. Best for high‑speed machining of hardened steels, aerospace alloys, and composite materials. Carbide’s brittleness demands a rigid setup; any flex in the spindle or workholding can cause premature breakage.
Coatings extend tool life by reducing friction and providing a thermal barrier. The most common PVD coatings are:
- Titanium Nitride (TiN) – gold‑colored, raises surface hardness, and is effective up to ~400 °C. Good for general‑purpose drilling of steels and aluminum.
- Titanium Carbonitride (TiCN) – slightly higher hardness and better wear resistance than TiN, suitable for abrasive alloys and higher cutting speeds.
- Titanium Aluminum Nitride (TiAlN) – dark gray, retains hardness above 600 °C, ideal for high‑speed drilling of stainless steel, titanium, and nickel‑based alloys where heat is a primary failure mode.
When selecting a coating, consider the material’s thermal conductivity and the intended cutting speed. For example, drilling stainless steel at 30 m/min with a TiAlN‑coated carbide drill will stay below the coating’s oxidation limit, whereas a TiN coating would degrade more quickly.
What speed, feed and pecking suit deep holes?
Cutting speed (surface feet per minute, SFM, or meters per minute) and feed rate (inches per revolution, IPR, or mm/rev) are functions of material, drill diameter, and tool material. A useful rule of thumb for HSS drills in steel is 30 SFM for diameters up to 0.25 in (6 mm) and 20 SFM for larger sizes. Carbide drills can run 1.5‑2× faster because of their higher hot‑hardness.
Feed should be set to produce a chip thickness of roughly 0.001‑0.002 in (0.025‑0.05 mm) per tooth. For a 0.5‑in (12.7 mm) HSS drill in mild steel, a feed of 0.004‑0.006 in/rev (0.1‑0.15 mm/rev) is typical. Increasing feed reduces dwell time and heat, but excessive feed raises thrust and can cause breakage, especially with long, slender drills.
Deep holes require special attention:
- Pepcking – drill in 1‑2 × diameter increments, retract, clear chips, then resume. This limits axial load and prevents chip re‑cutting.
- Pilot drilling – start with a smaller diameter (e.g., 30% of final size) to establish alignment, then step up with larger drills. The pilot reduces the effective chip load on the larger drill and improves hole straightness.
- Coolant delivery – use a high‑pressure mist or through‑tool coolant when drilling deep holes in heat‑sensitive alloys. The coolant not only removes heat but also lubricates the flutes, aiding chip evacuation.
When using coolant, select a fluid compatible with the workpiece and coating. For TiAlN‑coated carbide drilling stainless steel, a water‑soluble synthetic cutting fluid with anti‑corrosion additives is common. See our cutting fluids guide for detailed recommendations.
How do workholding and storage support drilling?
Even the best‑chosen drill will underperform if the workpiece or tool is not securely held. Rigid clamping minimizes vibration, which is especially critical for carbide drills that are less forgiving of lateral deflection. Review the options in our clamping and workholding section to select a vise, fixture, or magnetic base that matches the part geometry.
Organizing drills by size, geometry, and coating reduces change‑over time and prevents accidental use of the wrong tool. Our tool sets and storage solutions include modular rack systems and magnetic trays that keep each drill identifiable and protected from damage.
Finally, ensure you have the right drill, tap, and cutting‑tool inventory on hand. The comprehensive catalog of drills, taps and cutting tools provides the reference dimensions needed to match point angle, helix, material, and coating to the job at hand.
By aligning point geometry, helix design, material, coating, and machining parameters with the specific workpiece and setup, you can minimise walk‑off, extend tool life, and achieve consistent hole quality across a wide range of applications.
If you need assistance locating the appropriate drill or matching a coating to a specific material, feel free to reach out for a sourcing or cross‑reference discussion.
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Request a QuoteFrequently asked questions
What drill point angle should I use for stainless steel?
Use a 135° split point for stainless steel, hardened steel, titanium and other high-strength alloys. The split point concentrates cutting forces on a smaller tip radius, which reduces the tendency of the drill to walk across the surface and improves start-up accuracy. A 118° solid point is the general-purpose choice for mild steel, carbon steel and aluminum alloys.
What helix angle is best suited to drilling aluminum?
A high helix of 30° to 40° is preferred for aluminum and other soft metals. It produces long, thin chips that clear easily from the flutes, which prevents chip packing and heat buildup. A lower helix of 15° to 20° suits tough, abrasive materials such as stainless steel or cast iron, where thicker chips carry away more material.
How do you drill deep holes without chip re-cutting?
For holes deeper than 3 × diameter, combine a moderate helix of 20° to 25° with a peck-drilling cycle. Drill in 1 to 2 × diameter increments, retract, clear chips, then resume, which limits axial load and prevents re-cutting. Pilot drilling at about 30% of final size and high-pressure or through-tool coolant also help.
What coating suits high-speed drilling of stainless steel?
TiAlN, a dark gray coating that retains hardness above 600 °C, suits high-speed drilling of stainless steel, titanium and nickel-based alloys where heat is the primary failure mode. TiN is effective up to about 400 °C and suits general-purpose drilling of steels and aluminum. Stainless steel at 30 m/min with TiAlN-coated carbide stays below the oxidation limit.