Cobalt Drill Bit Guide: M35 vs M42, Stainless & Selection

The single most-asked question in any Australian metal-working workshop: "What drill bit do I use for stainless steel?" The answer, ninety percent of the time, is a cobalt drill bit. Understanding why takes a few minutes — and getting the wrong cobalt drill bit (or the right cobalt drill bit but using it incorrectly) is the difference between drilling 304 stainless cleanly all day and replacing burnt bits every five holes.

Cobalt drill bits are not a coating. They are not "cobalt-coloured" because of a finish. The cobalt is alloyed into the high-speed steel itself — typically 5% (M35 grade) or 8% (M42 grade) — which raises the steel's hot hardness and lets the cutting edge survive the heat that ordinary HSS can't handle. That's the whole engineering story, and it's why cobalt is the standard for stainless, hardened steel, cast iron and high-tensile bolts.

This guide covers what cobalt drill bits actually are, the grade differences (M35, M42, HSS-PM), the materials they're designed for, the technique that makes them last (and the technique that wastes them in five seconds), brand selection in the Australian market, and the cost reality of when cobalt pays back vs staying with premium HSS or upgrading further to solid carbide.

For the broader drill bit selection guide covering all materials and bit types, see our Choosing the Right Drill Bit guide and Types of Drill Bits reference. This article focuses specifically on cobalt as a substrate choice.

What is a cobalt drill bit?

A cobalt drill bit is a high-speed steel (HSS) drill bit with cobalt alloyed into the steel itself. The cobalt is part of the steel — not a surface treatment, not a coating. You cannot scrape the cobalt off; it is the steel.

The two main cobalt grades are designated by their cobalt content:

• M35 grade — 5% cobalt. The standard cobalt drill bit. Significantly better than plain HSS in heat-generating cuts. The default upgrade choice from M2 HSS for stainless steel and most hardened metals.
• M42 grade — 8% cobalt. The premium cobalt grade. Higher hot hardness still, longer life in stainless and very hard steels. About 30–40% more expensive than M35; chosen for production-volume work or particularly demanding materials.

The cobalt addition raises the steel's red hardness — the temperature at which the steel begins to soften and lose its cutting edge. Standard M2 HSS softens around 600°C; M35 cobalt holds its edge to about 650°C; M42 to about 700°C. That additional 50–100°C is the difference between cutting cleanly through stainless steel and burning the cutting edge off in three holes.

Visually, cobalt drill bits typically have a duller, more golden or bronze tint than bright HSS — but colour alone is not a reliable indicator. Cheap drill bits sometimes use surface colouring to imply cobalt content that doesn't exist. The only reliable indicator is the manufacturer's marking and the brand reputation behind it.

M35 vs M42 vs HSS-PM — cobalt grade selection

The practical selection rule:

• Drilling stainless 304 or 316 occasionally in a workshop? Use M35. The cost premium over M2 HSS is small; the performance gain is enormous.
• Production drilling in stainless, repetitive work, or material harder than 304? M42. The extra 30–40% cost is paid back many times over in tool life.
• Hardened steel above 45 HRC, or any application where M42 still struggles? Solid carbide. Cobalt's red hardness ceiling has been reached.
• HSS-PM is specialist territory — interrupted cuts in hardened material, shock-loaded applications, high-precision sharpenability. Most workshops never need it.

Cobalt vs HSS, cobalt vs carbide — where each fits

Three substrates, three different sweet spots. The choice isn't "best material" — it's matching the substrate to the application.

The pattern: as you move from M2 HSS → cobalt → carbide, hardness goes up but toughness goes down. Cobalt sits in a sweet middle position — hard enough for stainless and hardened steel up to about 45 HRC, tough enough to survive hand drilling and interrupted cuts that would shatter solid carbide. For most Australian workshop drilling needs above mild steel, cobalt is the right answer.

For the analogous decision on end mills, see our Carbide vs HSS End Mill deep-dive — the substrate logic is similar but the application differences (rotational drilling vs side-cutting milling) shift the breakpoints.

Stainless steel: the cobalt sweet spot

Austenitic stainless steel — 304 and 316, the most common AU industry grades — has one specific behaviour that defeats ordinary HSS drill bits and makes cobalt the right choice: work-hardening under heat and pressure.

When a drill bit cuts stainless steel, friction generates heat at the cutting edge. The stainless steel surface beneath the cutting edge responds to this heat by becoming harder — a layer typically 0.05–0.2 mm thick that's measurably harder than the parent material. If the drill bit can't cut through this hardened layer, it rubs instead of cuts; rubbing generates more heat; more heat creates a deeper hardened layer; and the bit either burns its cutting edge off (HSS) or simply skates across a now-hardened surface.

Cobalt's higher red hardness lets the cutting edge stay sharp at the temperatures that work-harden stainless. The bit cuts through the work-hardened layer faster than a new layer forms. Combined with correct technique, cobalt drills stainless cleanly.

The same cobalt-HSS logic applies up the scale: for larger-diameter structural stainless holes that exceed twist drill capacity (16-50mm+), the cobalt-grade annular cutter in a magnetic drill uses the same red-hardness advantage with the same technique rules — slow RPM relative to mild steel, continuous coolant, solid steady feed, never pause mid-cut.

The correct stainless steel drilling technique:

1. Slow speed. A 6 mm cobalt bit in 304 stainless wants approximately 200–400 RPM, not 1,000+. A 12 mm bit wants 60–120 RPM. The general rule: about one-third the speed you'd run for mild steel.
2. Firm consistent pressure. Push the bit hard enough that it's continuously cutting (chips coming off, not dust). Light pressure equals rubbing equals work-hardening.
3. Continuous cutting fluid. Even a few drops of thread-cutting oil or dedicated stainless fluid (Trefolex, Tap Magic) makes a substantial difference in bit life and finish quality. Soluble oil also works on benchtop drilling.
4. Don't peck or pause. Once the bit is engaged, keep cutting until the hole is through. Lifting the bit creates a perfect work-hardened ring at the depth you stopped — then the next cut hits hardened material before any fresh cutting can start.
5. Start with a centre punch. Cobalt bits don't like wandering; a centre-punched divot keeps the bit on target from the first revolution.
6. Use a sharp bit, not a tired one. A dull cobalt bit on stainless is just creating heat and hardening the material. Resharpen or replace.

For full speeds and feeds reference across all material/bit combinations, see our Cutting Speeds and Feeds Chart. For cutting fluid selection by application, see our Cutting Fluids Guide.

Hardened steel: cobalt's outer limit

Cobalt drill bits handle hardened steel up to approximately 45 HRC reliably with M42 and 30–35 HRC with M35. Above that, you're approaching the cobalt ceiling and solid carbide is the appropriate next step.

The "stuck bolt" scenario. Snapped grade 8.8 bolts, broken taps, hardened studs — the classic AU workshop "I need to drill out something hard" job. Forum consensus from Practical Machinist and Reddit r/Machinists is consistent: M42 cobalt is the standard first attempt; if M42 won't bite, the bolt is harder than 45 HRC and it's solid carbide territory. For broken tap removal specifically, see our Broken Tap Removal Guide.

Cast iron, high-tensile bolts and abrasive materials

Beyond stainless and hardened steel, cobalt drill bits are the right choice for several other Australian-workshop materials:

Cast iron — abrasive but not particularly hard. Cobalt's wear resistance pays off; the chip is short and crumbly which doesn't load the flutes. Drill cast iron dry — adding cutting fluid creates an abrasive paste that wears the bit faster than dry cutting. M35 is usually sufficient.

High-tensile bolts (grade 8.8, 10.9, 12.9) — bolt grades 8.8 and above are heat-treated and run 30–45 HRC. Cobalt is the appropriate substrate; M42 for the harder grades. Drill slowly with cutting fluid, don't peck.

Spring steel and music wire — heat-treated, 50–55 HRC. M42 cobalt occasionally works at very low speeds; solid carbide is more reliable.

Inconel and high-temperature alloys — work-hardens severely, generates high heat. M42 cobalt is workable for one-off holes; production volume = solid carbide with appropriate coatings.

Titanium and titanium alloys — low thermal conductivity means heat stays in the bit. M42 cobalt is marginal; solid carbide with AlCrN or similar coating is the standard production choice.

Abrasive composites (fibreglass, carbon fibre) — pure abrasive wear regardless of hardness. Solid carbide or PCD-tipped is the long-life choice; cobalt works for small volumes.

Speed, feed, and cutting fluid for cobalt

Cobalt's hot-hardness advantage only delivers if you give the bit the conditions to use it. Wrong speed and feed make a $30 cobalt drill bit perform like a $5 HSS — burning out fast and leaving a rough hole.

The general rule for cobalt vs HSS speeds: cobalt runs at the same speed as HSS in mild steel, about 1.5× HSS speed in stainless (because HSS shouldn't really be used in stainless), and well below HSS speed in hardened material. Slower than you might think — many DIY drilling failures come from running cobalt at the same speed you'd use for mild steel HSS.

Feed rate matters as much as speed. Light feed (low pressure on a hand drill, low feed setting on a drill press) creates rubbing rather than cutting and burns the bit. A cobalt drill bit wants firm, consistent feed pressure that produces continuous chips — a chip should be coming out of the hole every revolution.

Hand drill vs drill press technique

Cobalt drill bits work in both, but the technique differs.

On a drill press: set the speed correctly for the material and bit diameter; clamp the work; apply firm continuous feed; use cutting fluid liberally. Drill press technique is mostly about the setup — once the speed and clamping are right, the cutting itself is straightforward.

In a hand drill (battery or corded): the challenge is the operator. You need to maintain consistent feed pressure, hold the drill straight (no wobbling), and stop cleanly through the back side of the work. Hand-drilling stainless with cobalt is achievable but requires:

• The drill set to low speed — most cordless drills have a Hi/Lo selector. Use Lo. If yours has variable trigger, run it at maybe 30–40% trigger pull.
• A centre punch on the marked spot before starting (cobalt bits hate wandering)
• Both hands on the drill, body weight behind it, feeding firmly
• A squirt of cutting fluid applied before starting and re-applied if you stop
• No stopping mid-hole. If you must stop, lift the bit clear and re-apply fluid before continuing.
• Watch the chip colour. Bright silver chips = correct technique. Blue or brown chips = too much heat — slow down or apply more fluid.

Hand-drilling thicker stainless (above 6 mm) with a small cobalt bit is hard work. For repetitive work in stainless, a drill press or magnetic base drill makes the job dramatically easier.

Cobalt drill bit identification — markings and what to look for

How to tell whether a drill bit is genuinely cobalt — and what grade:

Manufacturer marking. Premium cobalt drill bits are laser-marked or stamped with the grade designation (M35, M42, HSS-Co5, HSS-Co8). Sutton, Bordo, Tivoly, Dormer, Cleveland, Triumph and other premium brands all mark their cobalt clearly. Unmarked or vaguely-marked drill bits should be assumed to be standard HSS regardless of seller claims.

Standards markings.

• Cobalt drill bits to DIN 338 = jobber-length cobalt twist drill
• DIN 1869 or DIN 340 = long series cobalt
• DIN 1897 = stub series cobalt
• The DIN number is followed by the grade: HSS-Co5, HSS-Co8

Colour is unreliable. Genuine cobalt is typically a duller gold/bronze tint than bright HSS, but cheap drill bits can be artificially coloured to look the same. Don't rely on colour alone.

Magnetism. Cobalt drill bits remain magnetic (it's still steel, just alloyed). Solid carbide drill bits are barely magnetic — if a "carbide" drill bit sticks to a magnet strongly, it's likely not solid carbide.

Brand reputation. A Sutton-marked, Tivoly-marked or Dormer-marked cobalt drill bit is genuine cobalt at the stated grade. An unbranded or generic-branded "cobalt" drill bit on eBay or marketplace at one-third the price of premium is a quality risk regardless of marketing claims. The Practical Machinist thread "Where can you get a REAL M42 cobalt drill?" exists because so much "M42" tooling on the cheap end of the market is mis-marked.

Brand selection in the Australian market

Sutton Tools (manufactured in Thomastown, Victoria) — Australia's premium cobalt drill bit manufacturer. Comprehensive M35 and M42 ranges across jobber, stub and long-series formats. The standard recommendation for most Australian workshops. AIMS stocks the Sutton range — see the Sutton Tools collection.

Bordo — Australian-distributed range, strong on M35 cobalt and HSS. Good value for hand-drill use and moderate workshop volumes. AIMS-stocked.

Tivoly (France) — premium European cobalt manufacturer, available in AU through specialist tool distributors. Excellent quality, premium price.

Dormer (UK/Sweden, now Dormer Pramet) — premium European brand with full cobalt range. Available through industrial distributors.

Other premium brands available in AU on order: Cleveland (USA), Triumph (USA), OSG (Japan), Mitsubishi (Japan), YG-1 (South Korea — value premium).

Avoid: unbranded cobalt drill bits on eBay, marketplace listings, or budget retailers at unrealistic prices. Forum-validated reality: cheap "M42 cobalt" sets often turn out to be standard HSS coloured to look like cobalt, or M35 sold as M42, or cobalt content well below the marked grade. A premium HSS bit ($10) typically outperforms a $5 fake-cobalt every time.

Cost reality: when does cobalt pay back vs premium HSS?

The premium for genuine cobalt over premium HSS is real but smaller than people assume:

The payback math: A premium HSS bit drilling stainless will drill maybe 5–10 holes before the cutting edge is gone, then needs resharpening or replacement. A premium M35 cobalt in the same stainless will drill 50–100 holes before resharpening is needed. The cobalt's life is roughly 10× longer in stainless. At 1.5–2× the bit cost, cobalt pays back the upgrade cost on the second hole and everything after that is pure savings.

In materials where HSS works fine (mild steel, aluminium, brass, timber), cobalt is overkill — you're paying 1.5–2× for performance you don't need. Run HSS for the easy materials and keep cobalt for stainless, hardened steel, and the genuinely hard materials.

Build a mixed kit: the right Australian workshop drill bit kit is a mix — premium HSS in common sizes for general work, plus M35 cobalt in 4–10 mm sizes for stainless and hardened material work. Add M42 for production-volume stainless. Solid carbide for jobs where cobalt has reached its limit.

Common mistakes that kill cobalt drill bits early

Cobalt drill bits should last a long time. When they fail prematurely, it's almost always one of these mistakes:

1. Running too fast. The most common error. Cobalt's heat-resistance advantage doesn't help if you generate so much heat that even cobalt's ceiling is exceeded. In stainless, slower than your instinct says.
2. Pecking — lifting and restarting. Each lift creates a work-hardened ring; the next cut hits the hard ring and burns out the cutting edge. Once you start, finish.
3. No cutting fluid. Stainless and hardened steel need fluid. A few drops of thread-cutting oil makes the difference between a bit that lasts and a bit that burns out.
4. Light feed pressure. Rubbing instead of cutting work-hardens the material and overheats the bit. Push firmly enough to be cutting continuously.
5. Using cobalt where HSS would do. Not a failure, just waste — you're consuming expensive bits on jobs that don't need them.
6. Using HSS or "cobalt"-marked-cheap-bit on stainless. Burns out, gets blamed on the work, gets replaced with another cheap bit, repeats. The real fix is genuine cobalt.
7. Drilling cast iron with cutting fluid. Counter-intuitive but true — fluid + iron dust creates abrasive paste. Drill cast iron dry.
8. Not centre-punching. Cobalt bits wander if not started in a punch mark. Wandered bits make oversized, off-centre holes and stress the cutting edge.
9. Letting the bit stop in the hole. Battery dying mid-cut, drill press tripping, hand drill clutch slipping — all create the work-hardening trap.
10. Buying unverified cheap "cobalt" bits. If it's not genuine, the cobalt benefit isn't there regardless of the marking.

Cobalt drill bits at AIMS Industrial

AIMS stocks Sutton M35 and M42 cobalt drill bits (Australian-made), Bordo M35 cobalt, and selected premium imports. Browse the full range in our dedicated Cobalt Drill Bits collection.

For other drill bit types and the broader cutting tool range:

• Jobber Drill Bits collection — HSS jobber-length twist drills for general metal and timber work
• Sutton Tools collection — full Sutton range including HSS, cobalt and solid carbide drill bits, taps, reamers and end mills
• Carbide Drill Bits collection — for the next-step upgrade above cobalt's limit (hardened steel above 45 HRC, titanium, abrasive composites)

For specific cobalt grades, sizes, or premium imports we don't show online, call us on (02) 9773 0122 or use our contact page. We can source most premium cobalt and carbide drill bits available in the Australian market.

For the broader drill bit selection guide covering all materials, see our Choosing the Right Drill Bit Guide and Types of Drill Bits reference.

Frequently Asked Questions

What is the difference between HSS and cobalt drill bits?

Cobalt drill bits are high-speed steel with cobalt alloyed into the steel itself — typically 5% (M35) or 8% (M42). The cobalt isn't a coating; it's part of the steel. The cobalt addition raises the steel's red hardness (the temperature at which the cutting edge begins to soften) from about 600°C for plain M2 HSS to 650–700°C for cobalt grades. This extra heat resistance makes cobalt the right choice for stainless steel, hardened steel, cast iron and high-tensile bolts where ordinary HSS burns the cutting edge off in a few holes.

Are cobalt drill bits worth the extra cost?

For drilling stainless steel, hardened steel, cast iron, or high-tensile bolts — yes, by a wide margin. A cobalt drill bit in stainless typically lasts 5–10× longer than HSS at 1.5–2× the price, paying back the upgrade cost on the second hole. For drilling mild steel, aluminium, brass, or timber where HSS works fine, cobalt is overkill — premium HSS is a smarter spend. Build a mixed kit: HSS for general work, cobalt M35 for stainless and hardened material, M42 for production-volume hard work, solid carbide for above 45 HRC.

What is the difference between M35 and M42 cobalt drill bits?

M35 contains 5% cobalt; M42 contains 8% cobalt. The higher cobalt content gives M42 a slightly higher red hardness (~700°C vs ~650°C for M35), longer tool life in stainless and hardened steel, but at 30–40% higher price. For occasional stainless drilling, M35 is sufficient and cost-effective. For production-volume stainless work or hardened steel above 35 HRC, M42 pays back the cost premium through longer tool life. Above 45 HRC, both are at their limit and solid carbide is the right next step.

What is the best drill bit for stainless steel?

A genuine M35 or M42 cobalt jobber drill bit from a verified premium brand (Sutton, Bordo, Tivoly, Dormer), used with correct technique: slow speed (about one-third of mild steel speed), firm consistent feed pressure, continuous cutting fluid, and no pausing once started. The combination of cobalt substrate plus correct technique handles 304 and 316 stainless cleanly. Cheap "cobalt" drill bits on eBay or budget retailers frequently fail because the cobalt content is below the marked grade or absent entirely.

Why does my drill bit keep burning out in stainless steel?

Almost always one of: running too fast (stainless wants slow speed — about one-third of mild steel RPM); pecking (lifting and restarting creates a work-hardened zone); insufficient cutting fluid (stainless needs continuous fluid); light feed pressure (rubbing instead of cutting work-hardens the material); or using HSS instead of cobalt. The mechanism is stainless steel's work-hardening behaviour — under heat and friction the surface gets harder, and if your bit can't cut through the hardened layer faster than new layers form, it rubs and burns. Fix: cobalt M35 or M42, slow speed, firm feed, continuous fluid, no pausing.

Is M42 cobalt better than carbide for hardened steel?

Up to about 45 HRC: M42 cobalt is the right choice. Cobalt is much tougher than carbide and survives the interrupted cuts and slight misalignment that hand drilling creates. Above 45 HRC: solid carbide takes over because cobalt softens at the cutting temperatures generated. The crossover point depends on how hard the material is and how production-grade your setup is — for occasional hand-drill work, M42 cobalt can stretch up to 50 HRC if you go slow with fluid; for production work, carbide above 45 HRC.

Can cobalt drill bits be resharpened?

Yes — cobalt is just alloyed HSS, so it sharpens on the same equipment as standard HSS drill bits. A drill bit sharpener (Drill Doctor, Tradesman) handles cobalt fine; a bench grinder with the right wheel and a steady hand also works. The cutting edge geometry matters: 135° split-point is the modern standard for stainless and metal drilling, much better than the older 118° tip. Cobalt holds a sharp edge well after regrinding. The regrindability is part of the cost story — a $25 cobalt bit with two regrinds at $5 each delivers $35 total cutting capacity.

Are cobalt drill bits the same as cobalt-coated drill bits?

No — and the distinction matters. Genuine cobalt drill bits have cobalt alloyed into the steel itself (5% for M35, 8% for M42). The cobalt is part of the steel and stays there even after resharpening. "Cobalt-coated" or "cobalt-finish" drill bits are HSS with a thin surface treatment — the coating wears off in normal use and is gone after the first regrind. Marketing language sometimes blurs this distinction; check for the M35 or M42 grade marking, and the brand reputation, to verify genuine cobalt.

What cutting fluid should I use for drilling stainless with cobalt?

For stainless steel: a sulphurised cutting oil is the standard recommendation — Trefolex, Tap Magic, Rocol RTD, or similar dedicated thread-cutting and stainless drilling fluids. Even general-purpose soluble oil or a few drops of motor oil is better than nothing. For hardened steel: same — sulphurised oil. For cast iron: nothing — drill dry, fluid creates abrasive paste. For mild steel: optional, but soluble oil extends tool life. For aluminium: WD-40 or kerosene work well; never sulphurised oil (stains aluminium). See our Cutting Fluids Guide for the full breakdown.

Why are some cobalt drill bits gold-coloured and others silver?

The colour difference is mostly cosmetic. Bright silver/grey is uncoated genuine cobalt — the natural colour of HSS-Co. Gold-tinted cobalt bits have a thin TiN (titanium nitride) coating over the cobalt substrate, intended to add slightly more wear resistance. The TiN coating wears off the cutting edge in normal use, after which the bit performs identically to uncoated cobalt. The colour is not a reliable indicator of cobalt grade — check the manufacturer's marking (M35, M42) for the actual grade. Cheap drill bits sometimes use gold colouring to imply cobalt content that doesn't exist.

Can cobalt drill bits drill through hardened bolts?

Yes — that's exactly what they're designed for. Grade 8.8 high-tensile bolts run about 30 HRC; grade 10.9 about 35 HRC; grade 12.9 about 40 HRC. M35 cobalt handles 8.8 cleanly; M42 cobalt handles up to 10.9 reliably and 12.9 with care. Above grade 12.9, you're approaching solid carbide territory. Technique matters: slow speed, cutting fluid, firm feed, no pecking. For broken bolt extraction specifically (snapped studs in tapped holes), see our broken tap removal guide — same principles apply for studs.

What is HSS-PM and is it worth the extra cost over M42?

HSS-PM stands for high-speed steel — powder metallurgy. The steel is produced from atomised powder rather than conventional ingot casting, giving a more uniform grain structure and higher toughness at the same hardness. Common designations include ASP 2030, T15, M48-PM. HSS-PM holds an edge as well as M42 cobalt with better toughness, particularly in interrupted cuts and shock-loaded applications. The cost premium is significant — typically 3–5× M2 HSS — and most workshops will never need it. Specialist territory for production-volume hard milling and drilling.

How fast should I run a cobalt drill bit?

Significantly slower than you'd run HSS in mild steel. For 304 stainless with a 6 mm cobalt bit: 800–1,170 RPM. For 10 mm: 480–700 RPM. For 12 mm in 316 stainless: 320–480 RPM. For hardened steel: lower again — about half those numbers. The general rule for cobalt: about one-third the speed of mild steel for stainless, half the speed for hardened material, and same speed as HSS for mild steel and aluminium. See our Cutting Speeds and Feeds Chart for the full reference table.

Are budget cobalt drill bit sets worth buying?

Generally no. The Practical Machinist thread "Where can you get a REAL M42 cobalt drill?" runs to many pages of buyers reporting that budget cobalt sets test out as standard HSS or low-cobalt M35 sold as M42. The cost saving is real (~30–50% off premium prices) but tool life is often half or less of premium cobalt — wiping out the saving on the first major job. A premium M35 from Sutton, Bordo, Tivoly or Dormer at $15–25 is a smarter spend than a $40 unbranded "M42" set with 13 sizes that may or may not actually be cobalt. Quality variance in cheap drill bit sets is huge; brand reputation is the only reliable check.

What is a "split point" cobalt drill bit and why does it matter?

A split-point drill bit has a small secondary cutting edge ground into the chisel point at the tip — converting the chisel from a wedge that pushes material aside into a cutting edge that cuts material away. The split point is sharper, starts cleanly without wandering, and reduces feed pressure required. For stainless steel and hardened material drilling, 135° split-point geometry is the standard recommendation — it cuts cleanly with less heat generation than the older 118° tip. Most premium cobalt drill bits come split-point as standard. If you're choosing between split-point and standard 118° cobalt for stainless work, choose split-point.