Why Your Drill Bits Go Blunt So Fast, and Why the Fix Is to Slow Down and Push Harder

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The suspicion behind this question is usually that bits got cheaper. Sometimes they did. But the bigger change happened to the drill, not the bit, and it explains why the same box of bits that lasted a decade in a corded drill gets ruined in a weekend with a cordless one.

A modern drill will happily spin at two thousand revolutions a minute and keep doing it while you lean on it, with no stall, no smell, and no sound of the motor labouring. All the feedback that used to make you back off is gone. And the thing that destroys a drill bit in steel is not wear. It is heat.

A bit in metal does not wear out, it gets cooked

High speed steel is hard because of how it was heat treated. That treatment is also the thing you can undo. Take the cutting edge past the temperature it was tempered at and the hardness goes, permanently, and no amount of cooling afterwards brings it back.

That turns bit failure into a runaway rather than a slow decline. A sharp edge shears a chip off the workpiece and most of the heat leaves with the chip. A softened edge cannot shear, so it rubs instead, and rubbing puts far more heat into the tip than cutting does. More heat softens more of the edge, which rubs harder, which makes more heat. Which is why a bit that was cutting fine can be finished in about fifteen seconds of pressing on through a hole that stopped making chips. The damage is not spread over the life of the bit. Nearly all of it happens in the few seconds after the cutting stopped and you kept going.

So the corrective rule reads backwards to most people: in metal, go slower and push harder. Low speed keeps the edge temperature down. Firm feed keeps the edge biting into fresh material instead of skating on a work hardened surface, which is the other classic way to destroy a bit, particularly in stainless.

The chip is a live readout and it is free

You do not need to guess any of this, because the swarf coming out of the hole tells you what the edge is doing. Long continuous curls mean the edge is shearing properly and carrying heat away with it. Short broken flakes mean you are going too fast for the material. Powder, blue or brown discolouration on the chips, smoke or sparks mean the edge has stopped cutting and started grinding, and whatever you do in the next few seconds decides whether the bit survives.

Cutting fluid belongs in the same sentence. Its main job is not lubrication in the way oil on a hinge is, it is getting heat out of the cutting zone. Anything is better than nothing on steel, and the standard advice is to reapply often enough that it never smokes dry.

RPM is not speed, which is the part that surprises people

Here is the arithmetic that makes sense of every speed chart you have ever ignored. The number on the drill is revolutions. What the cutting edge experiences is distance travelled, and that depends on the circumference, so a bigger bit at the same setting is going dramatically faster where it matters.

At a fixed 2000 rpm, the rim of a one eighth inch bit covers roughly 65 feet a minute. A half inch bit, same drill, same dial, same trigger, covers roughly 262 feet a minute. That is four times the edge speed for no change in how you are using the tool.

Which is also why countersinks burn and chatter so readily, and why people blame the cutter. A countersink head is wide, and it has several cutting edges engaging that wide circle at once, so a 3/8 inch head at the same 2000 rpm is running its edges at roughly 196 feet a minute. A 7-piece countersink set with a bearing stop collar is the kind that helps here, because its stop collar rides on a bearing rather than spinning against the workpiece, which removes one heat source that scorches the surface at exactly the moment you reach depth. Its record lists 1/8 to 3/16 inch pilots, 3/8 and 1/2 inch heads at an 82 degree angle, 1/4 inch hex shanks and an aluminium alloy body at about 250g for the case. Honest limits: the size range is fixed, those figures are the maker's own, and an aluminium alloy body is not a hard wearing choice. Details in our review of the Naerza countersink set. The technique side we covered in how to countersink screws for a clean flush finish.

Buying your way out of heat, honestly

You cannot buy a bit that does not care about temperature, but you can buy one that gives you more room before it matters. Cobalt grade high speed steel has cobalt alloyed through the body rather than applied as a coating, and its selling point is retaining hardness at higher temperatures than plain HSS. That is the right thing to spend money on, because it is aimed at the actual failure mode.

A 19-piece M35 cobalt bit set with 135 degree split points covers both parts of the problem. The cobalt grade addresses heat, and the 135 degree split point addresses the other thing that ruins a start: it self centres instead of walking, so you are not grinding a spinning tip across the surface looking for a bite. The record lists a stepped point, a Blue Onyx oxide finish, twist flutes for chip clearing, and acrylic, steel, brass and hardened metals as its materials. One flag worth stating plainly, because it is on the record and it matters: the listing mixes M35 and M2 grade references, and M2 is ordinary high speed steel rather than cobalt. If the cobalt grade is the reason you are buying, that inconsistency is the thing to resolve before you do. Also, for all that, it is not a masonry bit and will not drill tile. Our review of the KAKILFOB cobalt set goes through what it does and does not publish.

Where the rule reverses completely

Everything above is about steel, and masonry is not a slower version of the same job. It is a different mechanism. A carbide tipped masonry bit does not shear a chip at all, it crushes and chips the material away under impact, and carbide holds its hardness at temperatures that would finish high speed steel. So the instruction flips: a 10-piece serrated masonry set carries a recommended speed on its record of 2000 RPM or higher, which is the opposite of what you would do in steel. It lists tungsten carbide cross serrated tips on high strength steel bodies, 3/16 through 1/2 inch, and 1/4 inch quick change hex shanks. The honest notes are the ones that keep the heat story intact even here: the record says carbide tips can chip if forced or overheated, and it is explicitly not for drilling reinforcing steel, which is exactly the surprise waiting inside a concrete wall. See our review of the FOCSTOL masonry set, and for the technique and which bit suits which surface, drilling into tile, stone and concrete.

Which means the honest version of "what speed should I drill at" is that the question is incomplete. Slow and heavy in steel. Fast, with hammer action, light touch, in masonry. Using either rule on the other material is how both kinds of bit get wrecked, and swapping between the two without thinking about it is the single commonest reason a mixed set dies young.

The honest limit

Nothing here needs buying. The three changes that extend bit life most are free: drop the speed when the hole gets bigger, stop the instant the chips stop coming, and put something wet on it. A cheap bit used at the right speed with a drop of oil will outlast an expensive one run dry at full trigger, every time. Cobalt and carbide widen the margin for error, they do not remove it, and a set bought to avoid learning the chip signal will fail the same way the last one did.

And one thing no bit fixes. If a hole in your wall has stopped going anywhere and the dust has turned to bright sparks, you have most likely found metal: rebar, a stud plate, or a conduit. That is a stop and work out what it is moment rather than a buy a tougher bit moment, and occasionally it is the reason to put the drill down altogether.