When a construction project involves drilling into steel beams, stainless steel handrails, or heavy-gauge metal panels, standard high-speed steel (HSS) drill bits dull within a few holes. The heat generated by friction softens the cutting edge, causing the bit to stop cutting and simply rub against the metal. Cobalt drill bits solve this problem by using a steel alloy that contains 5 to 8 percent cobalt, giving the bit the ability to stay sharp under high temperatures where standard HSS would fail. While diamond-tipped bits excel on tile and stone, cobalt bits are the right choice for drilling ferrous metals, stainless steel, and cast iron on the construction jobsite. Understanding the grades, geometry, and drilling techniques for cobalt bits makes the difference between clean holes and broken, wasted tooling.
What Makes Cobalt Steel Different from Standard HSS
Cobalt drill bits are not coated. The cobalt is mixed into the steel during the melting process, producing an alloy that stays hard even when friction raises the temperature of the cutting edge well past the point where standard HSS would soften. This property is called red hardness – the ability of a cutting tool to retain its cutting hardness at elevated temperatures where most steels would anneal and soften.
Standard HSS (M2 tool steel) begins to lose cutting hardness around 250 degrees Celsius. Cobalt alloy steel maintains its cutting structure up to approximately 500 degrees Celsius. On a jobsite where multiple holes must be drilled in structural steel or stainless pipe, this extra heat tolerance means fewer bit changes and faster completion of the drilling task. For concrete and masonry, carbide-tipped bits handle the abrasive mineral aggregate. For metal, cobalt provides the heat management necessary for production work.
Cobalt Grades – M35 versus M42
The two common cobalt alloy grades used in drill bits are M35 (5 percent cobalt) and M42 (8 percent cobalt). The number refers to the molybdenum content in the steel; the percentage of cobalt is the key variable that affects performance and cost.
M35 bits are the standard choice for most construction metal drilling. They handle stainless steel, cast iron, titanium alloys, and high-strength steel up to Rockwell C35 hardness. M35 provides a balance between hardness and toughness – the bit is hard enough to cut efficiently but not so brittle that it snaps under side load or chatter during hand drilling. Most cobalt drill bit sets sold for general construction and fabrication use M35 steel.
M42 bits contain 8 percent cobalt, making them harder and more heat resistant. They can drill through hardened steels up to Rockwell C65. The tradeoff is reduced toughness – M42 bits are more brittle and require steadier drilling with less side pressure. They work best in drill presses where the workpiece is clamped securely.
| Grade | Cobalt Content | Typical Hardness (HRC) | Best For | Relative Toughness |
|---|---|---|---|---|
| HSS M2 | 0% | 60-62 | Mild steel, aluminum, wood | High |
| HSS M35 | 5% | 64-66 | Stainless steel, cast iron, titanium | Moderate |
| HSS M42 | 8% | 66-68 | Hardened steel, tool steel, Inconel | Low |
Red Hardness in Practice
The practical benefit of red hardness shows up when drilling at speeds above 2,000 RPM in hard metals. Drilling a 3/16-inch hole in 304 stainless steel with a standard HSS bit at 2,500 RPM softens the edge within 5 to 10 holes. A cobalt M35 bit under the same conditions drills 40 to 50 holes before degrading. This means fewer tool changes on production jobs.
Bit Geometry and Cutting Tip Design
The shape of the cutting tip determines how the bit engages with the metal surface. Two geometry factors matter most: point angle and whether the tip uses a split-point design or a conventional chisel point. These factors affect hole accuracy, required feed pressure, and the risk of bit breakage.
Point Angle – 118 Degrees versus 135 Degrees
The point angle of a drill bit is the angle formed between the two cutting edges at the tip. A shallower angle (118 degrees) produces an aggressive cut with less downward pressure required, while a steeper angle (135 degrees) distributes cutting forces more evenly across the edge.
118 degrees is the standard for soft materials – wood, plastic, and mild steel. This angle allows quick penetration with minimal pressure.
135 degrees is the standard for harder materials – stainless steel, hardened steel, and titanium. The flatter angle spreads the cutting load, reducing the tendency for the bit to grab or chatter on entry. Most cobalt drill bits use a 135-degree point angle as the default, and this is the correct choice for metal fabrication work.
Split Point versus Conventional Point
A conventional point (also called a chisel point) has a flat web at the tip that must be forced through the material. This creates walking – the bit skids across the surface before engaging – which mars the work surface and produces off-center holes. Operators often resort to center-punching every hole to compensate.
A split point (also called a self-centering point) has the tip ground with an additional relief cut that eliminates the flat chisel edge. The bit starts cutting immediately with minimal pressure. Every cobalt drill bit used in professional metal fabrication benefits from a split point design, as it reduces operator fatigue and prevents the bit breakage that happens when a wandering bit catches the edge of the workpiece. Split-point bits also eliminate the need for center punching in most applications.
Speed, Feed Rate, and Lubrication Requirements
Drilling metal with cobalt bits follows different rules than drilling wood or concrete. The correct spindle speed and feed pressure determine whether the bit cuts cleanly or burns, smokes, and breaks. Using the wrong speed is the most common cause of premature bit failure among operators new to cobalt tooling.
Recommended RPM by Material and Bit Diameter
The rule for metal drilling is straightforward: the harder the material and the larger the bit diameter, the slower the spindle speed. A 1/16-inch bit in aluminum can spin at 5,000 RPM without issue, but the same bit in stainless steel at 5,000 RPM would overheat and fail within seconds.
| Material | 1/16 inch | 1/8 inch | 1/4 inch | 3/8 inch | 1/2 inch |
|---|---|---|---|---|---|
| Mild Steel | 3,000 | 2,500 | 1,800 | 1,200 | 900 |
| Stainless Steel | 2,000 | 1,500 | 1,000 | 750 | 500 |
| Cast Iron | 2,500 | 2,000 | 1,400 | 950 | 700 |
| Aluminum | 5,000 | 4,000 | 3,000 | 2,000 | 1,500 |
Running a bit too fast generates heat that can crack the cutting edge or work-harden the metal. Running too slow increases torque and risks snapping the bit.
Feed Pressure and Chip Formation
Cobalt bits require steady, moderate pressure – not heavy force. Apply enough pressure to maintain continuous chip formation. Thin, feathery chips indicate proper feed. Blue or burned chips mean the speed is too high or the lubrication is insufficient. Ceramic and stone drilling techniques rely on different tooling and abrasive action, but the principle of matching feed rate to material hardness applies across all hard-surface drilling applications. Too little pressure causes the bit to rub rather than cut, generating heat without penetration.
Lubrication
Cutting fluid is necessary when drilling metal with cobalt bits, particularly in stainless steel and other gummy materials. A few drops of cutting oil at the entry point reduces friction, carries away heat, and flushes chips out of the flute. Drilling dry in stainless steel is the fastest way to ruin a cobalt bit.
For aluminum, use kerosene or aluminum-cutting fluid. For stainless steel, use sulfur-based cutting oil. For cast iron, minimal lubrication is needed since graphite provides self-lubrication. Never drill dry in any ferrous metal. Even a drop of oil every few seconds doubles the hole count before sharpening.
Preventing Bit Breakage and Overheating
Cobalt bits are harder than standard HSS bits, but hardness comes with brittleness. Techniques that produce clean holes in hard materials help extend bit life and prevent the type of damage that comes from improper technique.
Peck Drilling
Peck drilling means withdrawing the bit periodically during the cut to clear chips and allow the bit to cool. For holes deeper than three times the bit diameter, peck drilling is essential. Without pecking, metal chips pack into the flute and increase torque, which can snap the bit or weld the chips to the cutting edge. A typical peck cycle for a 1/4-inch bit drilling 1-inch deep into stainless steel is: advance 1/8 inch, withdraw fully, clear chips, apply lubricant, advance another 1/8 inch.
For through-holes, reduce feed pressure as the bit exits the far side of the material. The cutting edge can catch on the breakout burr and snap the bit if the operator maintains full pressure through the exit.
Signs of Overheating
Three visual indicators tell you the bit is overheating:
- Discoloration: a blue or black tint on the cutting edge indicates the bit has exceeded 300 degrees Celsius. The temper of the steel has been affected and the edge will not hold its sharpness as long.
- Smoking: visible smoke from the drilling point means the bit is burning the metal surface rather than cutting it. This is often accompanied by a burnt-oil smell and indicates immediate action is needed.
- Glowing: any red glow at the cutting edge means the bit has reached temperatures above 500 degrees Celsius, which will soften even cobalt steel if sustained. Stop immediately and allow the bit to air cool.
When any of these signs appear, stop drilling immediately, allow the bit to air cool (do not quench in water – rapid thermal shock can crack the steel), apply fresh lubricant, and resume at a lower speed with lighter feed pressure.
Extending Bit Life Through Storage and Maintenance
A quality cobalt drill bit set costs more than standard HSS bits, but proper storage and maintenance allow them to last through hundreds of holes before needing replacement or sharpening. Drill guides and jigs help maintain perpendicular alignment, reducing side loads that cause chipping and edge damage.
Storage and Cleaning
Cobalt bits should be stored in a case or roll that keeps each bit separated. Bits rubbing against each other during storage dull the cutting edges and can chip the fragile split-point tips. A silica gel pack in the storage case prevents rust in humid conditions. Keeping bits organized also saves time on the jobsite – reaching for a 1/4-inch bit and grabbing the wrong size is a common frustration that proper indexed storage eliminates.
After each use, clean the flutes with a wire brush to remove metal chips. Residual chips left in the flute can corrode over time and make the bit harder to sharpen later. For stainless steel drilling, the chips are especially tenacious – a quick wipe with a rag and cutting oil after cleaning prevents any corrosion from starting.
Sharpening Cobalt Bits
Cobalt bits can be resharpened using a bench grinder with a cool-cutting wheel or a specialized drill sharpener. The key is to maintain the original point angle (typically 135 degrees for cobalt bits) and not overheat the cutting edge during grinding. Overheating during sharpening damages the temper of the steel and reduces cutting performance – a freshly ground bit that was overheated during sharpening will cut worse than a dull bit that still has its original heat treatment intact.
A drill sharpening tool designed for the construction jobsite allows workers to restore cutting edges in minutes rather than discarding worn bits. A properly sharpened cobalt bit performs essentially like new, making sharpening one of the most cost-effective practices for any shop or crew that regularly drills metal. Given the cost premium of M35 and M42 bits over standard HSS, learning to sharpen them properly pays for itself within the first few sharpening cycles.