You're staring at a piece of stainless steel. It’s thick, cold, and honestly, it looks like it’s going to win. You've already burned through two standard HSS bits, and all you have to show for it is a shallow, shiny dimple and a smell of scorched metal that lingers in the garage. This is usually the moment most people realize they should have just bought an 8mm cobalt drill bit from the jump.
It’s not just about the size. 8mm is that "Goldilocks" diameter—big enough for a heavy-duty M8 bolt, small enough to be the perfect pilot for something larger. But the material? That’s where the physics gets interesting.
The 5% secret: What's actually inside an 8mm cobalt drill bit?
Most people think "cobalt bit" means the whole thing is made of cobalt. It isn't. If it were, the bit would be so brittle it would shatter the second it hit the work surface. What you're actually looking at is an alloy. Specifically, M35 or M42 high-speed steel.
The M35 grade typically contains about 5% cobalt. The M42 bumps that up to 8%. That tiny percentage changes everything about how the tool handles heat. High-speed steel (HSS) is great, but it has a "red hardness" limit. Once the friction of drilling into something like Grade 316 stainless steel pushes the temperature past a certain point, standard HSS softens. It loses its temper. The edge rounds off, and suddenly you aren't drilling; you're just rubbing two pieces of hot metal together.
Cobalt raises that thermal ceiling. It allows the 8mm cobalt drill bit to keep its razor-sharp cutting edge even when it’s glowing dull red. It's the difference between cutting through butter and trying to push a spoon through a brick.
Why 8mm is the "workhorse" size
Go into any serious fabrication shop. You’ll see 8mm bits everywhere. Why? Because the M8 bolt is the backbone of mechanical assembly. Whether you are working on a truck suspension, building a custom gate, or mounting heavy machinery, 8mm is the standard.
But there’s a nuance here. An 8mm hole in stainless steel is a lot of surface area. As the bit spins, the outer edge of the flute is traveling much faster than the center. This creates a massive amount of heat. If you use a cheap bit at this diameter, the heat buildup is exponential. That's why the cobalt alloy is non-negotiable at this specific size. You need that heat resistance to survive the 30-45 seconds of sustained cutting required to pop through a 6mm plate.
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Don't make the "High Speed" mistake
Here is the thing that kills me: people see "High Speed Steel" and think they need to run their drill at max RPM. Stop doing that. If you take your brand new 8mm cobalt drill bit and crank your drill press to 3,000 RPM, you will ruin the bit in four seconds. Cobalt is tough, but it isn't magic. Drilling hard metals is about chip load and surface feet per minute (SFM). For an 8mm bit in stainless steel, you should probably be hovering around 400 to 600 RPM.
You want to see long, curly "pigs' tails" of metal coming out of the hole. If you’re seeing tiny dust-like chips or smoke, you’re spinning too fast or not pushing hard enough. Cobalt bits require significant "feed pressure." You have to commit. You have to lean into it. If you let the bit dwell and rub without cutting, the metal underneath actually "work hardens." It becomes even tougher than it was before you started.
Real-world performance: M35 vs. M42
I’ve spent years around CNC machines and manual lathes. There’s a constant debate about whether the extra cost of M42 (8% cobalt) is worth it over M35 (5%).
Honestly? For most of us, M35 is the sweet spot.
M42 is incredibly hard, which is great for exotic alloys like Inconel or Titanium. But that hardness comes at a price: brittleness. If you’re using a handheld drill and your hand wobbles slightly, an M42 bit is more likely to snap. The M35 8mm cobalt drill bit has just enough flex to forgive a bit of human error while still eating through hardened steel like it’s nothing.
The geometry of the tip
Look at the tip of a high-quality 8mm bit. You’ll usually see a 135-degree split point. Standard bits often have a 118-degree point.
The 135-degree angle is flatter. This serves two purposes. First, it puts more of the cutting edge in contact with the material immediately. Second, the "split point" prevents the bit from "walking" across the metal when you start the hole. It bites instantly. If you're working on a finished surface, that's the difference between a perfect hole and a giant, ugly scratch across your workpiece.
Maintenance: You can actually sharpen these
People treat drill bits like disposable razors. It’s a waste. Because the cobalt is integrated throughout the entire alloy—not just a coating like Titanium Nitride (TiN)—you can sharpen an 8mm cobalt drill bit dozens of times.
When a Titanium-coated bit gets dull, you grind off the coating, and it's basically a junk HSS bit. When you grind a cobalt bit, you’re just exposing fresh cobalt steel. Use a bench grinder or a dedicated sharpener like a Drill Doctor, keep the angles consistent, and that one bit will last you a decade.
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Spotting the fakes
The market is flooded with "cobalt-colored" bits. They look the part—they have that dull, gold/bronze hue—but they’re just surface-treated HSS.
How do you tell? Look at the shank. Real cobalt bits are usually stamped "HSS-Co" or "M35/M42." Also, check the price. If you’re buying a set of ten 8mm bits for five dollars, you aren't buying cobalt. You're buying disappointment. A single, high-quality 8mm cobalt bit from a brand like Bosch, Cle-Line, or Guhring will usually cost more than a whole "budget" set. It’s worth every penny.
Actionable steps for your next project
If you're about to tackle a project involving hard metal, here is your playbook for success:
- Center Punch First: Use a spring-loaded center punch or a hammer and punch to create a divot. This gives the 135-degree split point a place to sit.
- Lubrication is Life: Never drill dry. Even a drop of 3-in-1 oil helps, but a dedicated cutting fluid like Tap Magic or Oatey Dark Threading Oil is better. It carries heat away and prevents the chips from welding to the flutes.
- The "Slow and Heavy" Rule: Set your drill to its lowest gear. Apply steady, downward pressure. If the bit isn't producing chips, stop.
- Clear the Flutes: If you’re drilling a deep hole (more than 15mm), pull the bit out every few seconds to clear the metal shavings. This prevents the bit from binding and snapping.
- Cool Down: If the metal is getting too hot to touch, stop. Let it air cool. Don't quench a hot cobalt bit in cold water; the thermal shock can cause micro-cracks in the alloy.
Basically, treat your 8mm cobalt drill bit like a precision instrument rather than a blunt object. It's an investment in your sanity. When you feel that bit finally "break through" the underside of a thick steel plate with a clean, burr-free exit, you'll get it.
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Check your drill speed settings before you start your next hole. If you're using a hand drill, try to use a handle attachment for better leverage and stability. Most people underestimate the torque required for an 8mm cut, and a sudden "catch" can twist your wrist if you aren't ready. Stay safe, wear eye protection—those cobalt-steel chips are hot and sharp—and let the tool do the work.