Best drill bits for metal: cobalt and HSS sets picked by alloy, point and shank

01What should a drill bit for metal have?

A drill bit for metal should be made of a steel that stays hard when hot, have a point that starts without wandering, and hold in the chuck without spinning. The alloy matters most, because the heat at the cutting edge is what ends a bit's life in steel. Each requirement below is stated, then backed with the published figure.

Alloy: HSS for mild steel, cobalt for stainless

High-speed steel (HSS) bits drill metal, hardwood and most other materials, and they replaced carbon-steel bits for that reason. Cobalt bits are a different alloy, not a coating: cobalt is part of the steel itself, while black oxide and titanium nitride are thin surface layers on ordinary HSS. That distinction is the most common buying mistake on this topic, since a gold-coloured coated bit can look like a premium metal bit without the alloy behind it. Bosch sells its M42 cobalt sets for drilling stainless steel where very high drilling temperatures occur, and Milwaukee lists its RED HELIX cobalt set for stainless steel, cast iron and black pipe.

M35 or M42: what do the cobalt grades mean?

The two grades on cobalt bit packaging name how much cobalt is in the steel. M35, also sold as HSS-E, is M2 high-speed steel with 5% cobalt added, and M42 is a molybdenum high-speed steel with 8% cobalt. Cobalt helps the edge keep its hardness at the high temperatures stainless steel generates, at the cost of being more brittle than standard HSS. In this group, Bosch labels its sets M42; Milwaukee and DeWalt say cobalt without naming a grade.

Because M35 is also called HSS-E, a metric set marked HSS-E is the same grade as an inch set marked M35. A US example of how a full M35 set is laid out: IRWIN's M-35 cobalt set runs 1/16 to 1/2 by 1/64 in steps, with split points on every bit, per IRWIN's listing.

Point: split point to stop walking

The common general-purpose twist bit has a 118 degrees point that works in wood, metal and plastic. Starrett describes 135 degrees split-point HSS drills as self-centring, easier to start and less prone to wander, with less feed pressure needed. Ordinary twist bits tend to skate on a flat metal surface, which is countered with a small spotting drill or a split point. Every pick on this page has a split or self-centring tip under its maker's own name: PILOT POINT on DeWalt, QUAD EDGE on Milwaukee, and a no-skate tip on Bosch.

Shank: flats so the bit does not spin

Bits with flats ground on the shank are made to resist slipping in the chuck, which matters in metal where the cutting load is high. All six picks have three-flat shanks, although Bosch's spec reads three-flat on 3/16 in and larger, so its smallest bits are round. A chuck's size names the largest shank it accepts, so sets that top out at 1/2 in fit a chuck of that size.

02What rules a metal drill bit set out before you compare alloys?

A set is ruled out if its shanks do not fit your drill's holder, if its sizes do not cover your holes, or if its maker does not list the metal you are drilling. These three checks apply to US fractional sets and metric sets alike.

• Holder. A drill/driver or hammer drill with a jawed chuck takes round and three-flat shanks. An impact driver takes only a 1/4 in hex quick-change holder, so it needs impact-rated bits with a hex shank; Milwaukee makes impact-rated twist bits with a 1/4 in hex shank for that reason. None of the six picks has a hex shank.
• Size range. Five of the six sets list 1/16 to 1/2 in in fractional sizes; DeWalt's page for the DWA1184 does not state its range. Metric holes for European fixings need a metric set; our drill bit size chart lists the nearest fractional match for each metric size.
• Listed material. Only cobalt sets in this group carry a maker's statement for stainless steel. DeWalt lists its titanium-nitride and black-oxide sets for metal, wood and plastic in general, not for stainless.

04How do the cobalt sets differ from the coated HSS sets?

The cobalt sets are a harder alloy that their makers list for stainless steel, while the coated sets are ordinary HSS with a surface finish that their makers list for general metal, wood and plastic. That split decides most purchases before brand comes into it.

Bosch CO21B vs CO14B: is the bigger set worth it?

Both Bosch sets share the M42 cobalt alloy, the no-skate tip and the 1/16 to 1/2 in range. The difference is depth: the CO21B lists two of several small sizes where the CO14B has one. Spares of the small sizes suit anyone drilling stainless regularly, while the smaller set covers occasional jobs.

Milwaukee 48-89-2332 vs DeWalt DWA1240: four-edge tip or pilot point?

Both are cobalt sets with three-flat shanks and the same size range. Milwaukee uses a four-edge QUAD EDGE tip and a flute whose angle changes along its length (variable helix 35 to 15 degrees), and its set runs to 29 pieces. DeWalt uses a PILOT POINT split tip and names angle iron and I-beams among its uses. Neither maker states the cobalt grade, unlike Bosch's M42 label. Choose Milwaukee for cast iron and pipe work, which it names; choose DeWalt where structural steel is the main job.

DeWalt DW1354 vs DWA1184: which coating?

The DW1354 carries a titanium nitride coating and a PILOT POINT tip, and the DWA1184 is black-oxide HSS with a 135 degree split point. DeWalt lists both for metal, wood and plastic, and neither for stainless. Both are general-purpose sets for mild steel rather than metal specialists. Where the coating starts to count is in harder steel drilled dry: a 2008 Tribology in Industry lab study of 7 mm M2 HSS drills in a 12% chromium steel got 29 vs 3 holes from a TiN-coated drill versus an uncoated one, while all three drills it tested lasted the full run in mild 1015 steel. It is an older study and among the few open-access comparisons, and black oxide was not part of it, so read it as a case for the TiN set on tougher steel rather than a ranking of these two DeWalt sets.

05What speed should you drill metal at?

Drill metal much slower than wood, and slower again as the bit gets larger or the metal gets harder. Starrett's chart gives about 1167 rpm for a mid-size metric HSS drill in mild steel, with lower figures for stainless at every diameter; Engineering ToolBox gives about 1000 rpm for small twist drills in steel. Those two sources use different size bands, so read the table for your bit size rather than mixing them.

Starrett and Milwaukee both recommend lubricant when drilling metal: it reduces friction wear, extends the life of the bit, keeps the edge sharp and lowers the risk of breakage. Milwaukee's accessory catalogue also ties the advice to depth: it calls for cutting fluid on all but shallow cuts with its steel-plate hole cutters and recommends a drill press for thicker plate. Lab work puts a size on the coolant effect: a 2005 COBEM study of 10 mm TiAlN-coated M42 cobalt drills in high-strength low-alloy steel recorded drill life 52% longer on average with flood semi-synthetic coolant than drilling dry, and raising cutting speed cut drill life sharply. That is an older industrial study, still the clearest open-access figure of its kind, and a squirt of cutting oil from a bottle is not the same as flood coolant, but the direction is the same. The top speeds of the drill/drivers in the chuck table above sit above these figures for most bit sizes, so use the low gear and the variable trigger on larger bits.

06Can these bits drill hardened steel?

None of the six makers lists hardened steel among its bit's uses, so treat them as stainless and hard-alloy bits rather than hardened-steel bits. Bosch names "hard and abrasive materials" and Milwaukee names "hard metals" such as stainless and cast iron, but neither publishes a hardness limit. The sources behind this page do not give a tested method for drilling through-hardened tool steel, and none of the picks is solid carbide.

What the sources do say is that carbide bits are extremely hard and hold an edge longest but are much more brittle than steel. For a part you know is hardened, look for a bit whose maker names hardened steel and follow that maker's speed and feed guidance; pushing harder with a cobalt bit is not a substitute.

07When does a metal drill bit fail?

A metal drill bit fails when it overheats, when it wanders off the mark, or when it is used in the wrong tool. Each has a clear cause and a fix that does not need a more expensive bit.

• Overheating. Without lubricant the edge wears faster, according to Starrett. Hold to the speeds above for your diameter and add cutting oil.
• Walking. A bit that skates on bare steel leaves a scarred surface and an off-centre hole. A split point helps, and a small pilot hole helps more. A center punch struck with a hammer marks the centre so the drill does not wander as it starts, as McMaster-Carr's listing for a Starrett center punch describes; our guide to stopping a drill bit wandering goes through the causes.
• Snapping. Cobalt is more brittle than plain HSS, so a bit can snap when the drill tilts in the hole. Keep the drill square to the work.
• Wrong holder. A round or three-flat shank is not impact-rated, and a hex-shank impact bit is the tool for an impact driver.
• Large holes in sheet. For large holes in sheet, a step bit, which drills a range of sizes with one bit, or a hole saw is the usual tool; the step bit picks cover electrical and sheet-metal sizes.

08What if you need to cut metal rather than drill it?

Cutting metal needs a saw blade or wheel, not a drill bit, and the tooth pitch rules are similar: fine teeth for thin metal, coarser teeth for thick. Reciprocating saw blades for thin and medium sheet run about 18–24 TPI, and those for thick metal about 8–14 TPI. Jigsaw blades for sheet and light section are compared in our guide to the metal-cutting jigsaw blades.

Angle grinders cut metal with thin abrasive cut-off wheels, which cut only with their rim and must never be used for side grinding. A cutting wheel needs the closed cutting guard, not the open guard used for depressed-centre grinding discs. The wheel types are explained in Type 1, Type 27 and Type 41 wheels, and the flap discs for metal finishing handle deburring after the cut.

Sanding and blending metal on a grinder, with fibre discs or flap discs, is a separate job from drilling; the guide to grinder discs for metal and the section on fibre discs and backing pads cover it. For a full list of comparisons, browse the buyer guides, or use the tool accessory lookup to see what your drill's chuck accepts.

09Questions readers ask