How to Choose a Drill Bit: The Complete Guide for Machinists

Selecting the right drill bit is one of the most overlooked decisions in the workshop, yet it has a profound impact on productivity, hole quality, tool life, and overall machining costs. Many machinists and workshop owners default to grabbing whatever drill is closest to the required size, assuming all drills perform equally. This approach leads to broken tools, poor surface finishes, wasted time, and unnecessary expense.

The truth is that drill bit selection is a technical decision that requires understanding several key factors: the material being drilled, the drill material itself, the point angle, hole depth requirements, accuracy tolerances, coatings, and the machine being used. Each of these variables influences how the drill performs, how long it lasts, and whether your finished holes meet specification.

This guide walks you through a systematic approach to drill selection, giving you the knowledge to choose the right tool for every job. Whether you're running a CNC machining centre, operating a manual drill press, or working in a fabrication workshop, these principles will help you make faster, smarter decisions that improve your results and reduce your costs.

What is a Drill Bit?

A drill bit is a cutting tool designed to create cylindrical holes in solid material. It works by rotating at high speed whilst being fed into the workpiece, with the cutting edges (called flutes) removing material in the form of chips that are evacuated from the hole.

Understanding basic drill anatomy helps you make better selection decisions. Here are the key components:

• Point: The cutting tip of the drill, typically ground to a specific angle (90°, 118°, 135°, or 140°). The point angle affects how the drill enters the material and how efficiently it cuts.
• Flutes: The spiral grooves running along the body of the drill. Flutes remove chips and allow coolant to reach the cutting edges. Most general-purpose drills have two flutes.
• Margin: The outer edge of the flute that maintains the hole diameter and provides guidance as the drill cuts deeper.
• Cutting Edges: The sharp edges where the flutes meet the point. These are where the actual cutting happens.
• Shank: The non-cutting end of the drill that fits into the machine's chuck or spindle.

Step 1: Identify the Material You're Drilling

The material being drilled is the first and most critical factor in drill selection. Different materials have different hardness levels, chip characteristics, and thermal properties, all of which affect how a drill performs.

Here's how common workshop materials influence your choice:

Aluminium: Soft and easy to machine, but prone to built-up edge (material sticking to the drill). Requires sharp tools and good chip evacuation. Drills can be run at high speeds.

Mild Steel: The most common material in workshops. Machines well with standard drills. Moderate speeds and feeds work effectively. Good chip formation with proper coolant.

Stainless Steel: Work-hardens easily and generates significant heat. Requires slower speeds, careful feed rates, and excellent coolant delivery. Standard HSS drills struggle; cobalt or carbide is preferred.

Cast Iron: Brittle and produces sharp, discontinuous chips. Requires careful handling to avoid chatter and breakage. Lower speeds than mild steel. Can be drilled with HSS but carbide offers better results.

Titanium: Extremely difficult to machine. Generates intense heat and work-hardens rapidly. Requires specialised drills, slow speeds, and aggressive coolant. Carbide or cobalt drills are essential.

Hardened Materials: Materials above 45 HRC hardness require carbide drills and specialised techniques. Standard HSS drills will not cut effectively.

Plastics: Prone to melting and cracking. Require sharp tools, high speeds, and careful feed rates to avoid heat buildup and delamination.

Step 2: Choose the Correct Drill Material

Drill bits are manufactured from different materials, each with distinct performance characteristics. Your choice here directly affects tool life, speed capability, and cost.

High-Speed Steel (HSS): The traditional choice for general-purpose drilling. HSS drills are affordable, tough, and forgiving. They can handle interrupted cuts and variable feeds without breaking. However, they lose hardness at high temperatures, limiting speed capability. Ideal for manual machines, job shops, and low-volume work.

Cobalt HSS: An upgraded version of standard HSS with cobalt added to the steel. Cobalt drills maintain hardness at higher temperatures, allowing faster speeds and longer tool life than standard HSS. Better suited to stainless steel and cast iron. More expensive than HSS but cost-effective for production work.

Solid Carbide: Extremely hard and heat-resistant, allowing very high speeds and excellent tool life. Carbide drills are ideal for CNC machines and high-volume production. However, they are brittle and can break if feeds or speeds are incorrect, or if the machine has runout or chatter. Significantly more expensive than HSS.

Through-Coolant Carbide: Carbide drills with internal coolant passages. Coolant is delivered directly to the cutting edges, dramatically improving performance in difficult materials like stainless steel and titanium. Essential for deep-hole drilling and high-speed production. Premium cost but exceptional results.

Step 3: Select the Right Point Angle

The point angle is the angle formed at the tip of the drill. Different angles suit different materials and applications.

90° Point: A sharp, acute angle used primarily for drilling hard, brittle materials like cast iron and composites. The sharp point concentrates cutting force, reducing the risk of chatter. Not suitable for soft materials like aluminium, where it can cause built-up edge.

118° Point: The most common general-purpose angle. Suitable for mild steel, aluminium, and most workshop materials. A good compromise between cutting efficiency and durability. Standard on most off-the-shelf drills.

135° Point: A wider angle that reduces cutting force and heat generation. Better for stainless steel and titanium, where heat is a concern. Also performs well in aluminium, reducing built-up edge. Increasingly popular for production work.

140° Point: The widest angle, designed for maximum heat dissipation and reduced cutting force. Ideal for difficult materials and high-speed CNC work. Requires rigid machines and precise feeds and speeds.

Step 4: Consider Hole Depth

How deep the hole needs to be affects drill selection and technique.

Standard Drilling: Holes up to 3–5 times the drill diameter. Standard drills with two flutes work well. Chip evacuation is straightforward.

Deep-Hole Drilling: Holes deeper than 5 times the drill diameter. Chip evacuation becomes difficult, and heat buildup increases. Specialised deep-hole drills with improved flute geometry, or through-coolant drills, are recommended. Peck drilling (withdrawing the drill periodically to clear chips) may be necessary.

Through-Coolant Applications: When drilling very deep holes or difficult materials, through-coolant drills deliver coolant directly to the cutting edges, dramatically improving chip evacuation and heat control. Essential for production work in stainless steel and titanium.

Peck Drilling: A technique where the drill is withdrawn periodically to clear chips. Necessary for deep holes in tough materials. Reduces tool life compared to continuous drilling but prevents chip packing and breakage.

Step 5: Consider Hole Accuracy Requirements

Not all holes need to be precision drilled. Understanding your accuracy requirements helps you choose the right approach and avoid over-specifying tools.

General-Purpose Drilling: Holes for fasteners, clearance holes, or non-critical applications. Standard drills are adequate. Tolerances are typically ±0.1–0.2 mm.

Precision Drilling: Holes requiring tighter tolerances (±0.05 mm or better). Requires sharp drills, rigid machines, and careful feeds and speeds. Carbide drills often preferred for consistency.

Reaming Operations: For the tightest tolerances, drill slightly undersize, then finish with a reamer. The reamer produces the final size and surface finish. This two-step approach is more reliable than trying to drill to final size.

CNC Production Work: Automated machines allow consistent, repeatable results. Carbide drills with optimised feeds and speeds deliver excellent accuracy and tool life. Through-coolant drills are often justified by the volume and consistency achieved.

Step 6: Select the Right Coating

Coatings are thin layers applied to the drill surface to improve performance. They reduce friction, increase heat resistance, and extend tool life.

Uncoated: No coating. Suitable for general-purpose work in mild steel and aluminium. Lower cost. Adequate for manual machines and low-volume work.

TiN (Titanium Nitride): A gold-coloured coating that improves heat resistance and reduces friction. Extends tool life by 2–3 times compared to uncoated drills. Good for production work in mild steel and aluminium. Moderate cost increase.

TiCN (Titanium Carbonitride): A more advanced coating offering better wear resistance than TiN. Suitable for stainless steel and cast iron. Extends tool life further. Higher cost than TiN.

TiAlN (Titanium Aluminium Nitride): Excellent heat resistance, allowing higher speeds. Ideal for difficult materials and high-speed CNC work. Premium cost but justified by tool life and productivity gains.

AlTiN (Aluminium Titanium Nitride): The most advanced coating for extreme conditions. Maintains hardness at very high temperatures. Used for titanium, hardened steel, and production work at maximum speeds. Highest cost but exceptional performance.

Step 7: Match the Drill to the Machine

The machine you're using influences drill selection. Different machines have different capabilities, rigidity, and speed ranges.

Hand Drills: Low speed, low rigidity. Require tough, forgiving tools. Standard HSS drills are appropriate. Avoid carbide, which can break easily. Keep speeds low and feeds gentle.

Drill Presses: Moderate speed and rigidity. HSS drills work well. Cobalt drills are suitable for production work. Carbide drills can be used if the machine is rigid and runout is minimal.

Lathes: Often used for drilling holes in rotating parts. Require careful setup and rigid toolholding. HSS or cobalt drills are typical. Speeds must be carefully controlled based on the workpiece diameter.

CNC Machining Centres: High speed, high rigidity, excellent repeatability. Carbide drills are the standard choice. Through-coolant drills justify their cost through superior performance and tool life. Optimised feeds and speeds are essential.

Quick Drill Selection Reference Guide

Use this table as a quick reference when selecting drills for common materials and applications:

Common Drill Selection Mistakes

Learning from common errors helps you avoid costly mistakes:

Using HSS for High-Volume Stainless Steel Work: Standard HSS drills break quickly in stainless steel. Cobalt or carbide drills with proper coatings are essential. The initial tool cost is recovered through reduced breakage and downtime.

Incorrect Point Angle Selection: Using a 118° drill on stainless steel or titanium generates excessive heat and causes rapid wear. A 135° or 140° point angle is far more effective. Match the point angle to the material.

Ignoring Coolant Requirements: Many difficult materials require specific coolant types and delivery methods. Dry drilling stainless steel or titanium leads to rapid tool failure. Invest in proper coolant systems, especially for production work.

Choosing Drills Based Only on Price: The cheapest drill is rarely the best value. A slightly more expensive cobalt or carbide drill often delivers better tool life, fewer breakages, and faster cycle times, resulting in lower overall cost per hole.

Incorrect Speeds and Feeds: Running drills too fast causes heat buildup and rapid wear. Running them too slow wastes time and can cause chatter. Learn the recommended speeds for your material and machine, and adjust based on results.

Frequently Asked Questions

What drill bit is best for stainless steel?

Cobalt or solid carbide drills with a 135° point angle are ideal. A TiCN or TiAlN coating improves performance further. Through-coolant drills are best for production work. Run at slower speeds than you would for mild steel, and use aggressive coolant delivery.

Should I use carbide or HSS?

HSS is more forgiving and suitable for manual machines, job shops, and low-volume work. Carbide is ideal for CNC machines and high-volume production, where its superior speed capability and tool life justify the higher cost. For medium-volume work, cobalt HSS is often the best compromise.

What drill coating is best?

For general-purpose work, TiN coating offers good value. For stainless steel and cast iron, TiCN is better. For high-speed CNC work and difficult materials, TiAlN or AlTiN coatings provide the best performance. Match the coating to your material and production volume.

When do I need through-coolant drills?

Through-coolant drills are most beneficial for deep-hole drilling, difficult materials like stainless steel and titanium, and high-volume CNC production. The improved chip evacuation and heat control justify the premium cost in these applications.

What point angle should I choose?

Use 118° for general-purpose work in mild steel and aluminium. Use 135° for stainless steel, titanium, and cast iron. Use 90° for very hard, brittle materials. Use 140° for maximum heat dissipation in high-speed CNC work.

How do I know if my drill is dull?

A dull drill produces a poor surface finish, requires excessive feed pressure, generates excessive heat, and may produce a squealing sound. Replace it immediately. A dull drill is more likely to break and wastes time and material.

Can I use the same drill for multiple materials?

A general-purpose HSS drill with a 118° point works reasonably well in mild steel, aluminium, and cast iron. However, for best results, use material-specific drills. Stainless steel and titanium require specialised drills and techniques.

What's the difference between a jobber drill and a short-flute drill?

Jobber drills have standard flute length and are suitable for general-purpose work. Short-flute drills have shorter flutes and are more rigid, making them better for deep holes and difficult materials. Long-flute drills are used for through-hole drilling where chip evacuation is critical.

How do I prevent drill breakage?

Use rigid toolholding, maintain machine rigidity, avoid interrupted cuts, use appropriate speeds and feeds, and ensure the workpiece is securely clamped. For carbide drills, ensure the machine has minimal runout. For difficult materials, use peck drilling to clear chips.

Should I use a centre punch before drilling?

Yes, especially on flat surfaces. A centre punch creates a small indentation that guides the drill and prevents it from wandering. This is particularly important when drilling by hand or on manual machines.

Conclusion

Choosing the right drill bit is a systematic process that considers the material being drilled, the drill material and coating, the point angle, hole depth, accuracy requirements, and the machine being used. By working through these factors methodically, you'll select tools that deliver better results, longer tool life, and lower overall costs.

Start by identifying your material and production volume. Then select an appropriate drill material—HSS for general work, cobalt for production, carbide for CNC. Match the point angle to your material, choose a coating that suits your application, and ensure your speeds and feeds are correct for your machine.

Avoid the common mistakes of choosing drills based solely on price, using the wrong tool for difficult materials, or ignoring coolant requirements. Invest in quality tools and proper technique, and your workshop will run faster, more reliably, and more profitably.

The next time you reach for a drill, pause and ask yourself: What material am I drilling? How many holes? How deep? What accuracy do I need? What machine am I using? Answer these questions, and you'll choose the right tool every time.