HSS-R, HSS-G, HSS-E and HSS-Co: What the Codes on a Drill Bit Mean

Two boxes on a shelf. One says HSS-G, one says HSS-TiN, and the TiN one is gold and costs more. Most buyers take the gold one. In stainless it will often be the one that dies first — because HSS-G describes how the bit was *made*, and TiN describes a coating three microns thick sprayed on whatever was underneath. The codes answer different questions, and reading them properly is worth more than any single spec on the box.

The letter after HSS: how the bit was made

The line that matters is between rolled and ground. A rolled bit has a cutting edge formed by deformation; a ground bit has one cut into hardened steel. Under a loupe the difference is obvious — the rolled edge is rounded, the ground edge is crisp. In practice a rolled bit needs more thrust to start cutting, which means more heat, which is exactly what kills HSS. On mild steel a competent operator gets away with HSS-R. On stainless it work-hardens the surface before it cuts, and then nothing will drill it.

The steel grades: M2, M35 and M42

"Red hardness" is the whole argument. Every HSS bit is hard enough to cut steel at room temperature; the question is what happens at the cutting edge when the temperature climbs. M2 starts to lose hardness around 550 °C and, once softened, never comes back — the bit is finished even after it cools. Cobalt raises that threshold, which is why M35 survives stainless: drilling stainless generates heat faster than any other common material because the chips do not carry it away.

M42 is harder and correspondingly more brittle. In a hand drill it chips; in a rigid machine with proper feed it outlasts M35 comfortably. That is why M42 shows up mostly as bi-metal hole saw teeth and machine drills rather than as a general-purpose bit.

From our range

Industrial M35 Cobalt HSS Twist Drill Bit for Stainless Steel (Inox) – OEM

M35 cobalt, 5% Co through the body — the grade for stainless, not a coating over M2.

HSS-G vs HSS-Co: the comparison people search for

They are not competing options — they answer different questions. HSS-G tells you the bit was ground, which is about accuracy and edge quality. HSS-Co tells you the steel contains cobalt, which is about heat. A good cobalt bit is also ground; a cheap one may not be, and a rolled cobalt bit is a genuinely poor product wearing a premium name.

• Drilling mild steel, aluminium, brass: HSS-G. Cobalt adds cost and brittleness you will not use.
• Drilling stainless, cast iron, hardened plate: HSS-Co (M35). Not optional — plain HSS work-hardens the surface and stops.
• Hand drill, thin material, occasional use: HSS-G. Cobalt chips when the bit is levered sideways, and hand drills lever.
• Deep holes, production runs, anything with heat: HSS-Co, and use cutting fluid.

When a customer describes their job in one sentence, the material is usually the only word you need. "Stainless" means cobalt. Everything else means ground HSS and a sharper price.

HSS-TiN vs HSS-Co: what the gold layer really does

Titanium nitride is a 2–4 micron hard ceramic coating. It genuinely reduces friction and lets chips slide out cleanly, which lowers heat and can extend life by 2–3× on abrasive but non-heat-critical work. What it cannot do is change the steel underneath.

• The coating covers the flanks. The cutting edge itself wears through the coating within the first few holes — after that you are drilling on the base steel with a coated body.
• On a sharpened bit, the coating is gone from the edge entirely. TiN bits do not survive regrinding as coated bits.
• A TiN bit over an M2 body is still an M2 bit against heat. Against stainless it will fail at the same temperature a plain M2 does.
• Where TiN pays for itself: abrasive materials, high-volume production in mild steel, and any job where chips packing in the flutes is the limiting factor.

A coating does not upgrade the steel. It buys time for the steel you already have.

TiAlN (violet-black) handles more heat than TiN and suits dry, high-speed machining; black oxide is not a hard coating at all — it is a surface treatment that resists rust and holds a little cutting fluid, worth having on a bit that lives in a humid coastal store and worth nothing to the cutting edge.

From our range

Industrial Titanium Coated HSS Twist Drill Bits (DIN338, Metric Sizes) – OEM

TiN-coated DIN 338 in the sizes that see abrasive, high-volume work — where the coating actually earns its cost.

Cobalt vs carbide: where the line falls

Carbide is harder than any HSS and holds an edge at temperatures that would ruin M42. It is also brittle enough that a moment of sideways deflection snaps it. The decision is not about material hardness — it is about rigidity:

The practical rule: carbide belongs in a machine spindle, cobalt belongs in a workshop. A CNC or a solid pillar drill with proper feed will make carbide pay for itself several times over. A cordless drill on a ladder will break a carbide bit before the hole is through. The fuller job-by-job comparison is in HSS, cobalt or carbide.

Reading a box on a counter

What a buyer should look for, in the order it matters: DIN 338 (the dimensional standard — length, flute, shank), then the grade (M2 / M35 / M42), then ground or rolled, then the coating, then the point angle. A box that lists a coating and a colour but not a grade is telling you which fact it would rather you did not check.

For distributors this is the sales script that works, because it is checkable: a customer who has bought gold bits that failed in stainless will buy M35 from whoever explains why. We supply M2 ground, M35 cobalt and TiN-coated lines from the same factories with the grade marked on the shank, in bulk or under your own brand — tell us the mix and we quote in 24 hours.