M42 vs. M35 Cobalt Drill Bits for Stainless Steel Drilling
Introduction: M42 and M35 are both cobalt high-speed steels, but the cobalt level in each grade sets different expectations for heat and wear in stainless steel.
The two grade names look almost the same on a label. Both say cobalt, both are sold as a step up from ordinary high-speed steel, and both get recommended the moment someone mentions 304 stainless. The decision gets harder once price enters the picture, because the gap between a lower-cobalt and a higher-cobalt grade can be wide enough to matter on a bulk order. A shop that buys M42 for every stainless job may be paying for margin it never uses, while a shop that buys M35 for long, hot production runs may find itself resharpening sooner than planned. The pages below explain what cobalt actually does inside high-speed steel, where M42 and M35 genuinely differ, and why neither grade cancels the basic rules of drilling stainless.
What Cobalt Content Means in High-Speed Steel Drill Bits
Cobalt is an alloying element inside the steel, not a surface coating. That distinction matters on the shop floor: a cobalt drill that has been resharpened several times still presents the same cobalt-bearing material at the cutting edge, because the element is distributed through the matrix rather than layered on top. Its main contribution is hot hardness, often called red hardness, which is the ability of the cutting edge to stay hard as temperature rises. In low-carbon steel, a good share of the cutting heat leaves with the chip. Stainless steel behaves differently. It conducts heat poorly, so heat accumulates near the tip, and an edge that softens at that temperature starts rubbing instead of cutting. Cobalt is added specifically to push that softening point higher. Cobalt content is expressed as a range, not a fixed number, and the range is what separates the grades. M42 high-cobalt drill bits are listed at approximately 8% cobalt, while M35 is generally understood as a lower-cobalt grade, commonly cited near 5%. The difference sounds small in percentage terms, but tool steel is judged in narrow windows, and a few points of cobalt shift how much hardness the edge retains when it gets hot. More cobalt also raises the cost of the raw steel and makes the grade harder to process, which is part of why it sits in a higher price band. What the number cannot tell you on its own is how the rest of the alloy is balanced, since molybdenum, tungsten, vanadium, carbon level, and heat treatment all shape how the finished drill behaves.
How M42 and M35 Differ for Stainless Steel Drilling
The comparison is easier to hold onto when it is broken into the four things that actually change a cutting decision. Cobalt content is the starting point, but heat resistance, wear behavior, and the hardness ceiling of the workpiece each carry their own weight, and a machinist who only looks at the cobalt number misses half the picture.
- Cobalt content sets the grade position. M42 carries roughly 8% cobalt, while M35 is a lower-cobalt grade commonly cited around 5%. Higher cobalt generally means more alloy in the matrix, more hot hardness potential, and a higher cost per drill, which shows up quickly when you compare drill price across a full size range.
- Heat resistance expectations follow the cobalt level. Both grades hold hardness better than plain high-speed steel when stainless steel pushes heat into the edge, but M42 keeps more margin in long, continuous cuts where the tip never gets a chance to cool. That margin only pays off when speed and feed are kept inside a sensible window.
- Wear awareness shapes how often the drill comes out of the spindle. Higher-cobalt grades are typically chosen for harder alloys and longer runs, while lower-cobalt grades handle routine stainless work well when the drill is kept sharp and resharpened on schedule. Dull edges wear faster in every grade, and stainless steel punishes them hardest.
- Workpiece hardness limits the choice from the other side. The M42 line-up in question is intended for 304 stainless steel and workpieces up to HRC 45, with straight shanks, a 135° directional point, and diameters from 0.30 mm to 13.30 mm. Hardness is a property of the material being drilled, so no cobalt level moves that ceiling by itself.
Put together, the two grades are not separated by a single headline advantage but by how much heat and wear margin a job really needs. General stainless fabrication, repair work, and intermittent drilling on 304 are usually well served by a lower-cobalt grade run at correct parameters. Long production runs, tougher stainless variants, and work where tool changes interrupt the schedule are where the extra cobalt in M42 starts to justify itself. A lower-cobalt drill run at the right speed will outperform a higher-cobalt drill run at the wrong speed almost every time.
Why Neither Grade Removes All Stainless Steel Drilling Limits
Stainless steel work hardens, and that single behavior does more to determine drill outcome than the grade printed on the shank. When a drill rubs instead of biting, the surface beneath it hardens, and the next revolution is cutting a tougher skin than the one before. Austenitic grades such as 304 combine that tendency with low thermal conductivity, so heat concentrates at the tip instead of escaping into the workpiece or the chip. Cobalt buys an edge more time under those conditions, but it does not change them. That is why the same drill can cut beautifully in one setup and burn in another with only speed and feed changed. The rest of the limits live in the setup rather than in the steel grade. Spindle rigidity, runout, coolant reaching the tip, chip evacuation, hole depth, and whether the drill is guided or wandering all influence how much work the edge must do. Deep holes trap chips, and recutting chips doubles the heat. Hand-fed operations with light pressure are the classic case where stainless steel wins: the drill polishes the surface, the material hardens, and a sharp bit dulls within a few holes. A workpiece above HRC 45 sits outside the intended range of the M42 specification and stays outside it regardless of cobalt content. Choosing a grade is one decision among several, and the other decisions still have to be made well.
Conclusion
M42 and M35 are both cobalt high-speed steels built for stainless steel, separated mainly by cobalt level and the heat and wear margin that comes with it. M42 high-cobalt bits are listed at approximately 8% cobalt and intended for 304 stainless and workpieces up to HRC 45; M35 sits lower in cobalt and covers plenty of everyday stainless work when the feed, speed, and coolant are handled properly. The practical takeaway is to match the grade to the job rather than to a blanket rule, especially when comparing drill price across drill manufacturers and size ranges. Anyone weighing the M42 option can review the full specification before deciding.
FAQ
Q：What is the main difference between M42 and M35 cobalt drill bits?
A：The main difference is cobalt content and the hot hardness that comes with it. M42 is a high-cobalt grade listed at approximately 8% cobalt, while M35 is generally understood as a lower-cobalt grade, commonly cited near 5%. Higher cobalt helps the cutting edge retain hardness when stainless steel forces heat into the tip, which is the reason M42 is often picked for longer runs and harder stainless work, and the reason it costs more.
Q：Are M42 drill bits always better than M35 for stainless steel?
A：No. M42 offers more heat and wear margin, but that margin only matters when heat and wear are the limiting factors. For general fabrication, repair work, and short runs on 304 stainless, M35 performs well when the drill is sharp and the speed and feed are right. A lower-cobalt drill run at correct parameters usually beats a higher-cobalt drill run too fast with light feed pressure.
Q：Why is cobalt content important in drill bits for stainless steel?
A：Stainless steel conducts heat poorly, so cutting heat stays near the drill tip and softens the edge faster than it would in carbon steel. Cobalt raises the temperature at which the steel loses hardness, so the edge keeps cutting instead of rubbing. That is what makes cobalt grades suitable for stainless, and it is also why the workpiece hardness ceiling and the feed plan still decide the final result.
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