Cobalt vs M35 vs M42 Drill Bits: What’s the Difference for Drilling Steel?

Cobalt drill bits are high-speed steel alloyed with cobalt — and the grade you pick, M35 at 5% or M42 at 8%, decides how hot the bit can cut and how likely it is to snap.

A hardware store bin labeled “cobalt” hides a family of grades, not one material. M35 and M42 both add cobalt to high-speed steel for heat resistance, but they trade toughness for hardness at different points. Pick wrong for your setup and you get either a bit that dulls on stainless or one that shatters mid-hole.

Cobalt vs M35 vs M42: What Actually Changes?

M35 carries about 5% cobalt and M42 about 8%, and that gap drives everything else. More cobalt raises red hardness — the bit keeps cutting when friction pushes the edge toward glowing. It also makes the steel more brittle, so the extra durability on hard metal comes at the cost of fracture tolerance.

Both are built to the same geometry standard, DIN338, so the difference you feel at the chuck is the alloy, not the shape. Coinchtools’ grade comparison and the cobalt drill bit guide from Aims Industrial both frame it the same way: cobalt content is a dial you turn toward heat resistance or toward forgiveness, never both at once.

Which Grade Cuts Which Metal?

M35 handles common stainless grades like 304; M42 is built for 316 stainless, 17-4PH, titanium, and HRC 35–45 alloy steel. The dividing line is heat at the cutting edge. Tougher alloys push temperatures higher, and the 8% cobalt bit holds its edge longer before it starts rubbing instead of slicing.

Here’s the catch people miss: M42 earns its keep on a rigid machine, not in your hand. Its lower fracture tolerance means a drill press or CNC setup — where the bit stays perfectly perpendicular — gets the full benefit. Handheld drilling flexes the bit slightly, and M42 is the grade more likely to chip under that side load. For a hand drill, M35 is usually the safer all-round pick.

How Do You Drill Stainless Without Burning the Bit?

Low RPM, firm feed, and cutting oil — those three rules matter more than which grade you bought. Stainless work-hardens the moment a dull edge rubs instead of cuts, and heat is what kills cobalt bits early. Pro Tool Reviews’ metal-drilling coverage lands on the same advice: slow the spindle, lean into the cut, and keep the chips flowing.

Running dry or fast is the most common way to wreck a good bit. If you want a shortlist of bits that hold up on steel, this roundup of tested bits for drilling steel skips the guesswork. Either grade rewards the same technique — steady pressure, coolant, and no impatience.

Does More Cobalt Always Mean a Better Bit?

No — the better grade depends on setup rigidity, material hardness, and how much breakage risk you can absorb. M42 is the wrong answer on a wobbling hand drill even though it’s the harder steel. M35 is the wrong answer in a CNC running 17-4PH all day, where the extra red hardness pays for itself.

Match the grade to the job, not the spec sheet. Rigid machine plus hard alloy points to M42; hand drill plus common stainless points to M35. Buy one of each if you drill both ways, and stop treating “cobalt” as a single material.