Expert Guide: 3 Factors for Choosing the Right Drill Bit for Hard Metal in 2025

Abstract

The act of drilling into hardened metals presents a significant engineering challenge, where conventional tools often fail due to extreme thermal and mechanical stresses. This article provides a systematic examination of the principles governing the selection of an appropriate drill bit for hard metal applications. It posits that an optimal choice is contingent upon a triad of interconnected factors: the intrinsic material composition of the bit, the application of advanced surface coatings, and the specifics of its geometric design. The analysis moves from a foundational discussion of high-speed steel to a more detailed exploration of cobalt alloys and solid tungsten carbide, evaluating their respective capacities for heat resistance and hardness. It further investigates how coatings like Titanium Aluminum Nitride (TiAlN) enhance performance by creating thermal barriers and reducing friction. The geometric aspects, including point angle and split-point design, are assessed for their role in ensuring precision and mitigating mechanical failure. The objective is to equip professionals and enthusiasts with a coherent framework for making informed decisions, thereby improving efficiency, ensuring safety, and prolonging tool life when working with materials such as stainless steel and other hardened alloys.

Key Takeaways

• Select cobalt (M35, M42) or carbide bits for their superior heat and wear resistance.
• Choose a 135-degree split-point tip to prevent bit "walking" and reduce required pressure.
• Use a TiAlN coating for a protective thermal barrier during high-heat applications.
• Operate at slow speeds with firm, consistent pressure to avoid work hardening the metal.
• Always use a high-quality cutting fluid to cool the workpiece and the drill bit for hard metal.
• Employ a pecking motion to clear chips and allow lubricant to reach the cutting edge.
• Firmly clamp your workpiece to eliminate vibration, a primary cause of bit fracture.

Table of Contents

• The Nature of the Challenge: Why Hard Metals Resist Conventional Drilling
• Factor 1: The Core Identity – An Examination of Drill Bit Materials
• Factor 2: The Protective Shield – Understanding Advanced Surface Coatings
• Factor 3: The Form of the Cut – Drill Bit Geometry and Its Implications
• From Theory to Practice: A Methodical Approach to Drilling Hard Metals
• Frequently Asked Questions (FAQ)
• A Final Reflection on Material and Method
• References

The Nature of the Challenge: Why Hard Metals Resist Conventional Drilling

Before we can properly evaluate the tools for a task, we must first develop a deep respect for the task itself. Working with hard metals is not merely a more difficult version of drilling into wood or soft steel; it is a fundamentally different encounter between tool and material. Metals like hardened steel, stainless steel series 304 or 316, titanium alloys, or Inconel possess a material integrity that actively fights against being cut. Their high tensile strength and hardness, often measured on the Rockwell C scale (HRC), mean they resist deformation with immense force.

When a standard drill bit, typically made from High-Speed Steel (HSS), attempts to penetrate these materials, a cascade of failures begins. The first and most immediate enemy is heat. The friction generated at the tip of the bit is immense, and hard metals are often poor conductors of heat, meaning the thermal energy concentrates right at the cutting edge. A standard HSS bit will see its temper drawn out, softening the steel until its cutting edges dull and round over. You might see the tip turn a straw yellow, then brown, then a tell-tale blue, signaling that it has been permanently damaged and has lost its hardness.

A second phenomenon, work hardening, adds another layer of difficulty. Many hard metals, particularly stainless steels, have the peculiar property of becoming even harder in the localized area being worked. If your drill bit spins in place without cutting, or if the feed pressure is too light, the friction and pressure effectively "cold-work" the surface of the metal, making it significantly harder than it was initially. You are, in effect, creating a super-hardened layer that your bit must then try to penetrate, a battle it will almost certainly lose. This leads to a vicious cycle of more friction, more heat, and rapid tool failure. The consequences extend beyond a ruined drill bit; they include inaccurate holes, potential damage to the workpiece, and a considerable waste of time and resources. Understanding this complex interplay of heat, pressure, and material response is the first step toward selecting a tool that is not just a participant in this struggle, but its master. A proper drill bit for hard metal is not an expense but an investment in efficiency and precision.

Factor 1: The Core Identity – An Examination of Drill Bit Materials

The capability of any cutting tool begins with its fundamental composition. The material from which a drill bit is forged dictates its baseline hardness, its toughness (resistance to chipping or breaking), and, most profoundly, its ability to maintain that hardness at elevated temperatures. This property, known as "hot hardness" or "red hardness," is perhaps the single most important attribute for a drill bit intended for hard metals. Let us explore the hierarchy of materials available.

High-Speed Steel (HSS): The Foundational Material

High-Speed Steel is the bedrock of modern machining tools. It represented a monumental leap forward from simple carbon steels because of its ability to maintain a sharp cutting edge at speeds and temperatures that would instantly ruin older tools. HSS is an alloy of steel with elements like tungsten, molybdenum, and chromium. These elements form hard carbides within the steel matrix, providing good wear resistance.

However, when faced with truly hard metals, the limitations of standard HSS become apparent. Its hot hardness is insufficient to withstand the extreme temperatures generated when drilling stainless steel or hardened tool steel. The heat causes the steel's tempered structure to break down, leading to a rapid loss of edge sharpness. While HSS is an excellent, cost-effective choice for mild steel, aluminum, and wood, it is simply outmatched in the arena of high-hardness alloys.

Cobalt Alloys (M35 and M42): The Resilient Workhorse

The introduction of cobalt into the HSS alloy matrix represents the next significant step in performance. Cobalt does not form carbides itself; instead, it improves the high-temperature performance and mechanical properties of the entire steel matrix (García-León et al., 2019). It allows the bit to retain its hardness at much higher temperatures, directly combating the primary failure mode seen in standard HSS. Two grades are prevalent.

M35 grade contains approximately 5% cobalt. This addition provides a noticeable improvement in hot hardness and is an excellent choice for drilling into tough materials like stainless steel. It offers a good balance of performance and cost, representing a significant upgrade over standard HSS for demanding applications.

M42 grade contains a higher percentage of cobalt, typically around 8%. This makes M42 bits even more resistant to heat and capable of cutting into harder materials, including tool steels and other difficult-to-machine alloys. The trade-off for this increased performance is a slight reduction in toughness, meaning M42 bits are somewhat more brittle than M35. They demand a more rigid setup and consistent feed pressure to avoid chipping. For professionals who regularly work with the toughest metals, M42 cobalt is often the go-to choice.

Solid Carbide: The Apex of Hardness

At the pinnacle of drill bit materials sits solid tungsten carbide. Carbide bits are not steel alloys at all. They are created through a process called sintering, where microscopic particles of tungsten carbide (one of the hardest known materials) are fused together within a metallic binder, usually cobalt. The result is a tool with extraordinary hardness and wear resistance, capable of operating at speeds and temperatures far beyond the limits of any HSS or cobalt bit.

This extreme hardness allows carbide bits to cleanly shear through the hardest and most abrasive materials with exceptional tool life, making them ideal for high-volume production environments, especially in CNC machines. However, this hardness comes with a significant compromise: brittleness. Unlike steel, which has some flex, carbide is very rigid and will fracture rather than bend if subjected to shock, vibration, or bending forces. Using a solid carbide drill bit requires a very rigid setup—a drill press or milling machine is almost always necessary. Any "chatter" or vibration from a less stable hand drill can easily shatter the bit. Therefore, while a carbide drill bit for hard metal offers the ultimate in performance, it must be used with a deep understanding of its mechanical limitations.

Factor 2: The Protective Shield – Understanding Advanced Surface Coatings

If the material of the bit is its heart, the coating is its shield. A coating is an exceptionally thin (a few microns) but very hard layer of ceramic material applied to the surface of the bit. Its purpose is multifaceted: it increases surface hardness, reduces friction between the tool and the workpiece, and acts as a thermal barrier, insulating the core material of the bit from the intense heat of the cut. The right coating can dramatically extend the life of a bit and allow it to perform at higher speeds than its uncoated counterpart.

The Logic of a Sacrificial Layer

Think of the coating as an ablative shield on a spacecraft re-entering the atmosphere. It is designed to bear the brunt of the extreme environment, protecting the critical structure underneath. In drilling, the coating provides a harder, more lubricious surface than the substrate material. This "lubricity" means the chips formed during drilling are less likely to stick to the bit (a condition called "built-up edge"), allowing them to be evacuated from the hole more efficiently. This reduces friction, which in turn reduces heat generation. The coating's role as a thermal barrier is also vital; it slows the transfer of heat from the cutting zone into the cobalt or HSS substrate, helping the bit maintain its hot hardness for longer.

A Spectrum of Modern Coatings

Not all coatings are created equal. They are engineered for specific applications and temperature ranges.

• Titanium Nitride (TiN): Recognizable by its bright gold color, TiN was one of the first widely adopted PVD (Physical Vapor Deposition) coatings. It is significantly harder than uncoated HSS and provides good lubricity. While it represents a major improvement for general-purpose drilling in softer steels, its effectiveness diminishes at the very high temperatures generated by drilling hard metals.

• Titanium Carbonitride (TiCN): This coating has a characteristic violet-gray or copper color. By incorporating carbon into the titanium nitride matrix, TiCN achieves greater hardness and wear resistance than TiN. It is particularly effective in abrasive materials but, like TiN, its performance can degrade at the highest temperatures.

• Titanium Aluminum Nitride (TiAlN) and Aluminum Titanium Nitride (AlTiN): These are the premier coatings for high-heat applications and are the best choice for a drill bit for hard metal. They have a violet-gray to black appearance. Their remarkable quality comes from the inclusion of aluminum. At the high temperatures encountered when drilling hard steel or stainless steel, the aluminum in the coating oxidizes and forms a microscopic layer of aluminum oxide (Al₂O₃) right at the cutting edge. This layer is chemically stable and acts as an exceptional thermal barrier, protecting the bit even as temperatures soar (Aihua & Hongsheng, 2007). AlTiN coatings, which have a higher aluminum-to-titanium ratio, generally offer even better performance at extreme temperatures.

• Black Oxide: This is not a high-performance coating in the same category as the others. It is a surface treatment that provides mild corrosion resistance and helps the bit retain cutting fluid. While better than an uncoated bit, it offers minimal benefit in terms of hardness or heat resistance and is not suitable for demanding hard metal applications.

Factor 3: The Form of the Cut – Drill Bit Geometry and Its Implications

Beyond material and coating, the very shape of the drill bit is a critical piece of engineering. The geometry of the tip and the flutes determines how the bit engages with the material, how forces are distributed, how chips are formed and evacuated, and how accurately the hole is placed. For hard metals, specific geometric features are not just beneficial; they are necessary for success.

The Point Angle: A Question of Engagement

The point angle is the angle formed at the very tip of the drill bit. The two most common angles are 118 degrees and 135 degrees.

• A 118-degree point is sharper and more aggressive. It works well in softer materials like aluminum and mild steel, as it centers easily and requires less force to initiate the cut. However, this sharp point creates high stress on a small area and its long cutting edges can chip when drilling hard, brittle materials.
• A 135-degree point has a flatter profile. This geometry is vastly superior for drilling hard metals for two key reasons. First, the shorter cutting edge is stronger and more resistant to chipping under high pressure. Second, and perhaps more importantly, the flatter angle reduces the tendency of the bit to "walk" or wander across the hard, smooth surface of the metal when starting a hole. It provides a more stable engagement, which is paramount for precision.

The Split Point: The Key to a Clean Start

Imagine looking at the very tip of a standard drill bit. You will see a small, flat area at the center where the two cutting edges meet, known as the "chisel edge." This edge does not cut; it scrapes and pushes material out of the way, requiring significant downward force, or "thrust," to get the hole started.

A split-point geometry corrects this deficiency. It features additional cutting edges ground into the chisel edge, effectively creating a point at the absolute center of the bit. This design allows the bit to begin cutting immediately upon contact with the material, almost like it has its own built-in pilot hole. The benefits are enormous: it dramatically reduces the required thrust, it is self-centering (eliminating the need for a center punch in many cases), and it produces a more accurate and rounder hole. For any handheld drilling operation in hard metal, a 135-degree split point is the unequivocal best choice.

Flute Design and Web Thickness: Strength and Evacuation

The spiral grooves running up the body of the bit are called flutes. They serve two purposes: they form the cutting edges, and they act as channels to evacuate chips from the hole.

• Helix Angle: The angle of these flutes matters. A "fast" spiral (high helix angle) is aggressive and removes chips quickly, which is good for soft materials. For a drill bit for hard metal, a "slower" spiral (lower helix angle) is often preferred. This creates a stronger cutting edge and is better suited for the smaller, more brittle chips produced when drilling hard materials.
• Web Thickness: The web is the central column of metal at the core of the bit. A thicker web provides greater torsional strength and rigidity, making the bit less likely to break under the high torque required for hard metals. The downside is that a thicker web also means a larger chisel edge (unless it has a split point), which increases the required thrust. Manufacturers of high-performance bits carefully balance web thickness and point geometry to optimize strength and cutting efficiency. Our commitment to quality craftsmanship ensures our tools are designed with this balance in mind.

From Theory to Practice: A Methodical Approach to Drilling Hard Metals

Possessing the correct drill bit is only half the battle. The technique used to wield that tool is just as important. Drilling hard metal is a process that rewards patience, preparation, and a feel for the machine, not brute force. The guiding principle is to manage heat and pressure effectively.

Preparation is Paramount

Before the drill even starts spinning, your success is already being determined. First, the workpiece must be absolutely secure. Any movement or vibration will transfer directly to the fragile tip of your cobalt or carbide bit, risking fracture. Use robust clamps, a vise, or whatever means necessary to lock the material in place. Second, create a starting point. While a 135-degree split point is self-centering, a small divot made with a sharp center punch provides an even more secure starting seat for the bit, completely eliminating any chance of wandering.

The Trinity of Success: Speed, Feed, and Coolant

Mastering the relationship between these three variables is the art of machining.

• Speed (RPM): This is the most common mistake. Intuition might suggest that faster is better, but with hard metals, the opposite is true. High rotational speeds generate exponential amounts of friction and heat. You must use slow RPMs. A large diameter bit drilling into hardened steel might require a speed as slow as 200-300 RPM. A smaller bit can go faster, but the rule is always: if you see the bit or the chips turning blue, or if you see smoke instead of vapor from the coolant, your speed is too high. Start slow, and only increase if the cut feels smooth and controlled.

• Feed Rate (Pressure): The pressure you apply must be firm and consistent. Too little pressure is a grave error. It allows the bit to rub instead of cut, which generates friction and leads to the work-hardening phenomenon we discussed earlier. You will dull your bit and make the material impossible to drill. Too much pressure can overwhelm the cutting edges and cause a cobalt bit to chip or a carbide bit to shatter. You are looking for the sweet spot where the bit is producing a continuous chip or small, consistent flakes of metal. Let the tool do the work, but keep it engaged.

• Coolant/Lubricant: This is not optional. A continuous flow of a quality cutting fluid is your primary weapon against heat. It performs three functions: it cools the bit and the workpiece, it lubricates the cutting action to reduce friction, and it helps flush chips out of the hole. For stainless steels and other tough alloys, a sulfurized or chlorinated cutting oil is highly effective. Even for a quick hole, a thick tapping fluid is far superior to a general-purpose oil like WD-40.

The Peck Drilling Technique

When drilling a hole deeper than its diameter, chips can become packed in the flutes, preventing coolant from reaching the tip and causing heat to build rapidly. The solution is "peck drilling." Drill for a short distance—perhaps half the bit's diameter deep—then completely retract the bit from the hole. This action clears the chips from the flutes and allows a fresh supply of coolant to flood the bottom of the hole. Then, plunge back in and drill another short distance. This rhythmic pecking motion is essential for maintaining a cool, clean cut in deep holes and drastically extends the life of your drill bit for hard metal. You can find a variety of these specialized tools by exploring our catalog of specialized attachments.

Frequently Asked Questions (FAQ)

Can I sharpen a cobalt or carbide drill bit?

Sharpening cobalt (M35, M42) bits is possible with the right equipment, typically a bench grinder with a specialized fine-grit wheel dressed to the correct angle. However, recreating the precise 135-degree split-point geometry by hand is extremely difficult. Sharpening solid carbide bits requires a diamond grinding wheel and is generally not feasible outside of a professional tool grinding shop. For most users, once a high-performance bit is dull or chipped, it is best to replace it.

What is the best drill bit for stainless steel specifically?

For stainless steel (like 304 or 316), an M42 (8% cobalt) drill bit with a 135-degree split point is an excellent choice. The high cobalt content resists the heat generated, and the split point prevents the bit from walking on the slick surface. A TiAlN coating is highly recommended as it will form a protective aluminum oxide layer to further combat the high heat and abrasive nature of stainless steel.

Is a more expensive drill bit always better for hard metal?

Generally, in the world of cutting tools, you get what you pay for. A higher price often reflects a superior base material (carbide vs. HSS), a more advanced coating (TiAlN vs. black oxide), and more precise grinding of the geometry. While a hobbyist might get by with a less expensive cobalt bit for a single hole, a professional whose time and accuracy are valuable will find that a premium drill bit for hard metal from a reputable manufacturer is a wise investment that pays for itself in speed, accuracy, and tool life.

Why did my drill bit for hard metal turn blue and stop cutting?

A blue or purple discoloration on the tip of the bit is a clear sign of overheating. The heat has exceeded the material's critical temperature, causing the temper to be drawn out and the steel to soften. It has lost its hardness and will no longer hold a cutting edge. This is almost always caused by running the drill at too high a speed (RPM) or failing to use adequate cutting fluid.

Do I really need cutting fluid or lubricant?

Yes, absolutely. Attempting to drill hard, heat-resistant metals like stainless steel or hardened alloys without a coolant is the fastest way to destroy even the most expensive drill bit. The fluid's primary job is cooling. It carries away the intense heat generated at the cutting point, preventing the bit's tip from softening and failing. Its secondary job, lubrication, reduces friction and helps with chip evacuation.

What is work hardening and how can I avoid it?

Work hardening is a phenomenon where a metal becomes harder and more brittle as it is plastically deformed. With metals like stainless steel, this can happen very quickly. If your drill bit is allowed to spin on the surface without cutting (due to dullness or insufficient feed pressure), the friction and pressure will harden the spot you are trying to drill. To avoid it, use a sharp bit, apply firm and consistent feed pressure so the bit is always cutting, and use a slow RPM to minimize excess friction.

A Final Reflection on Material and Method

The engagement with hard metals is a dialogue between force and finesse. It reveals that sheer power is an inefficient and often destructive approach. True mastery comes from a nuanced understanding of the materials in play—the resilience of the workpiece and the engineered capabilities of the tool. The selection of the proper drill bit is not a trivial preliminary step but a central part of the craft itself. It requires a thoughtful consideration of the bit's core material, from the reliable hot hardness of cobalt to the supreme wear resistance of carbide. It involves an appreciation for the technological marvel of modern coatings, which form protective shields against the onslaught of heat. It demands attention to the subtle but powerful influence of geometry, where the angle of a point can mean the difference between a clean, precise hole and a fractured tool. By integrating this knowledge of material, coating, and form with a methodical practice of slow speeds, firm feeds, and constant cooling, the challenge of drilling hard metals transforms from a frustrating struggle into a predictable and rewarding act of precision engineering.

References

Aihua, L., & Hongsheng, Z. (2007). Oxidation behavior of TiAlN and CrAlN hard coatings. Surface and Coatings Technology, 201(12), 5949-5953.

García-León, R. A., Contente, A., & Paternina-Arboleda, C. D. (2019). Mechanical properties of M2, M35 and M42 high-speed steels. Contemporary Engineering Sciences, 12(1), 1-8. https://doi.org/10.12988/ces.2019.812111

Stephenson, D. A., & Agapiou, J. S. (2018). Metal cutting theory and practice. CRC press. https://doi.org/10.1201/9781315374829

Trent, E. M., & Wright, P. K. (2000). Metal cutting. Butterworth-Heinemann.

Uhlig, H. H., & Revie, R. W. (2008). Corrosion and corrosion control: An introduction to corrosion science and engineering. John Wiley & Sons.