HSS Twist Drill Bit Set

Product Overview

A twist drill is the default hole-making tool and the one every other drilling operation is measured against. Two helical flutes run up the body; where they meet the end face, two cutting lips shear the material and the flutes carry the chips out. It sounds simple, and for a century of general engineering it has been good enough that most of the detail gets ignored — right up to the point where a hole comes out oversize, a bit snaps in stainless, or a 13 mm drill refuses to start where you marked it.

This set is built around three deliberate choices.

• Solid, fully ground M2. The flutes are ground from a solid bar rather than rolled into shape, which is what keeps the point symmetric and the body concentric. An asymmetric point is the number one cause of oversize holes: if one lip is longer than the other, the hole comes out roughly twice that error oversize and the thrust all goes onto one lip.
• A ground point, not a stamped one. 118° conventional as the general-purpose grind, with a 135° split point offered from 2 mm up. Below about 2 mm the web is too thin to split without leaving the centre fragile.
• 1/4 inch hex shank. Sized for the quick-change collets and impact drivers that most site work now runs on. See the caveat in the shank section — hex is not a licence to put a twist drill in a rotary hammer.

Application coverage follows the material spec: alloy and non-alloy steel, cast iron and sintered iron, non-ferrous metals, sheet metal and plastics. That is the honest range of an uncoated M2 jobber drill, and it is wide enough that most shops never need anything else for through-holes.

What Is in the Set: the Diameter Ladder

A drill set is only as good as its ladder. Two questions decide whether a set gets used or sits in the case: does it contain the clearance sizes you actually need, and can you find the size you want without squinting at a flute?

The 25-piece metric ladder

1.0 – 13.0 mm in 0.5 mm steps is the standard industrial configuration and the one we recommend as a default:

1.0 · 1.5 · 2.0 · 2.5 · 3.0 · 3.5 · 4.0 · 4.5 · 5.0 · 5.5 · 6.0 · 6.5 · 7.0 · 7.5 · 8.0 · 8.5 · 9.0 · 9.5 · 10.0 · 10.5 · 11.0 · 11.5 · 12.0 · 12.5 · 13.0

The reason 0.5 mm steps matter is that clearance holes are not round numbers. A metric bolt needs a hole about 0.4 to 1.0 mm over its nominal size, and the ISO 273 medium series is what most fabrication uses:

Notice that the medium column lands on half-millimetres almost every time — 4.5, 5.5, 6.6, 9.0, 11.0, 13.5. A set in whole-millimetre steps forces you to either ream or accept slop, and a set in 1 mm steps for the small sizes leaves you without a pilot for M3. That is the whole argument for the 0.5 mm ladder.

The inch-and-metric trap

If your market runs imperial fasteners, a metric set will bite you, and it is worth knowing exactly where:

The rule: for a clearance hole, rounding up by less than 0.2 mm is harmless. For anything that locates — a dowel, a shoulder bolt, a reamed fit, a threaded hole you intend to tap — buy the exact size. And if you are pre-drilling for a reamer, the standard allowance is about 0.2 mm under finished size up to 12 mm (10 mm reamer → 9.8 mm drill).

Reading the case, not the flute

The cheapest failure in a drill set is a case with blank slots. Sizes stamped into a flute become unreadable the first time the bit gets hot and oily, and then every size change becomes a guess against a caliper. An indexed case with the size printed next to each slot keeps the set complete — and a set with one missing size is a set that stops being used.

Anatomy of a Twist Drill

Six features decide how a drill behaves, and every one of them has a characteristic failure. Knowing which feature has failed tells you whether to regrind or bin the bit.

The Point: 118° Against 135°

Point angle is the included angle between the two cutting lips, measured across the tip. It is the single most quoted number on a drill and the most misread one, because it gets asked to do two jobs at once: it sets how the drill cuts, and it is used as shorthand for whether the drill has a split point. Those are separate things.

The practical rule

If you drill mainly mild steel, cast iron, aluminium and plastics, fit 118° and use a centre punch. If stainless or alloy steel is more than an occasional job, fit a 135° split point and stop carrying a centre punch. For a mixed shop the split point is the safer default even though it costs more, because the failure mode it prevents — a bit walking off your mark — is the one that scraps the workpiece.

And one thing the table makes plain: a 135° point is not automatically a split point. Plenty of cheap 135° drills are ground conventional with no split at all, which gives you the worst of both — more thrust than 118° and no self-centring. If the listing does not say “split point”, assume it does not have one.

The Chisel Edge, and the Thrust Paradox

Here is the question that confuses buyers, and the answer that resolves it: does a 135° drill need more pressure or less?

Both answers are correct, for different drills. Increasing the point angle on a conventional grind lengthens the chisel edge and puts more of the work into extrusion, so thrust goes up. Splitting the point removes the chisel edge entirely and replaces it with cutting edges that have positive effective rake, so thrust goes down — by a quarter to a half in published comparisons. Most of the contradiction you will read online comes from articles that treat “135°” and “split point” as the same thing.

How much of your push is being wasted

The chisel edge runs at a large negative rake angle — effectively a tiny flat chisel being pushed into the work. Material under it is not sheared into a chip, it is plastically displaced. The specific pressure in that zone runs several times the specific cutting force of the lips, because there is no rake face to speak of.

The experimental evidence is unusually direct. In the classic University of Illinois twist-drill tests, opening a pilot hole equal in diameter to the web — so the chisel edge had nothing left to extrude — reduced measured thrust by 59 percent while torque was barely affected until the pilot exceeded about 15 percent of the drill diameter. That is the purest available demonstration that the chisel edge is a thrust device, not a cutting device.

Why you should care about thrust specifically

• Small drills break from thrust, not torque. Below about 3 mm the limiting failure is column buckling under axial load. Halving thrust roughly doubles the practical feed you can apply before the bit snaps.
• Extrusion is heat without a chip. Work done displacing material at the centre becomes temperature and nothing else. It is the reason a drill can be smoking hot while producing almost no swarf.
• Thrust is what makes a drill walk. Until the lips bite, the drill is a column with a blunt end being pushed against a surface. Reduce the blunt end and you reduce the walking.
• Thrust is what a worn drill spends. As the chisel edge rounds off it behaves like a progressively wider chisel, so thrust climbs while the holes get worse. Rising feed pressure on a familiar material is a sharpening signal, not a reason to push harder.

This is also why web thickness grows towards the shank. The drill needs torsional strength, so the manufacturer thickens the web along the body — which means a drill re-ground repeatedly into a stub ends up with a thicker web at the point and therefore higher thrust than when new. It is a real, if slow, form of degradation.

Web-Thinning Families: X, N, R and S

“Split point” is the umbrella term; the specific grind matters. Four families cover almost all shop work, and they are not interchangeable.

Standardise on X for a general-purpose set and N where hole position is critical. Keep R in mind for cast iron, where the abrasive, short-chip material rewards a strong tip over minimum thrust, and S for small diameters where the web is too thin for a conventional split.

One limit applies to all of them: there is a floor on how far you can thin the web. The web is what stops the drill splitting up the flutes under cutting load. Over-thin a web on a drill that sees high feed or intermittent cut and the point opens up. The usual target is a remaining chisel length of about 2–6 percent of diameter, depending on the material group in the next section.

Point Geometry by ISO Material Group

Rather than memorising materials one at a time, use the ISO workpiece groups. They map cleanly onto the three numbers you actually set: point angle, lip relief, and how far to thin the web.

Two entries deserve a word. Group N at 90–110°: a sharp point in aluminium produces a clean entry with less burr, and a generous lip relief of 12–16° is what stops the bit grabbing and screwing itself into soft material. Plastics at 60–90° with reduced rake: the aim is to scrape rather than bite. A sharp 118° point in acrylic or polycarbonate will grab on breakthrough and crack the sheet; a flatter point with little or no positive rake cuts cleanly. That is the same anti-grab logic a step drill uses in thin sheet, one reason a step drill set is worth having alongside this one.

On coolant: for group P water-soluble flood is standard, and peck for anything past about 3×D. Group M wants coolant delivered to the point under pressure rather than dribbled on top — starving it is how you work-harden the surface you are trying to cut. Group K is usually run dry with air, because cast iron swarf plus water makes a grinding paste. Group N wants flood or mist, mainly to stop built-up edge on the lips.

Helix Angle: Type N, H and W

The helix angle is the angle of the flute relative to the drill axis, and it is doing two things at once: it sets the rake angle at the cutting lip, and it sets how fast chips climb out. Standard type N is 30°.

The thin-sheet point is worth stating plainly, because it is where twist drills get a bad name. A steep helix acts like a screw thread. While the point is buried in thick material the flutes are supported and it does not matter. The instant the point breaks through thin sheet there is almost nothing left for the flutes to grip, so they bite and pull the drill through, dragging the sheet up the helix — the classic out-of-round triangular hole and a bent panel. A slow-helix bit, a backing board, and a falling feed pressure at breakthrough all help; a step drill solves it structurally. We cover that comparison on the single step drill bit page.

For deep holes — anything past about 5×D — helix angle alone will not save you. You need either a parabolic flute with genuinely more chip space, or a peck cycle, or both. A jobber drill is specified at 5×D for exactly this reason.

The HSS Grade Ladder

“HSS” on a drill means very little on its own. Five grades are in common use and the gap between the cheapest and the best is not marginal — in continuous steel drilling, a true M2 drill typically lasts several times as long as a low-alloy substitute, and in stainless the gap is wider.

Red hardness is the property that decides tool life, not room-temperature hardness. A cutting edge in steel runs above 500 °C within seconds. What matters is whether the steel is still hard at that temperature. M2 holds; 4341 and 4241 start to soften, and once the edge softens it deforms faster than it wears, which is why cheap drills die abruptly rather than gradually.

Cobalt is not free. M35 typically costs 15–25 percent more than M2, so the argument is cost per hole, not cost per bit: if M2 gives 50 holes in stainless and M35 gives 150, the expensive bit is the cheap one. Equally, M42 is harder and less forgiving — it punishes side load and deflection, so it belongs in a drill press or a CNC machine rather than in a hand-held drill.

Two buying cautions. First, the grade label only tells part of the story: carbide distribution, cleanliness of the steel and the heat treatment cycle vary enormously between mills, so ask for the mill certificate when the grade is contractual. Second, M2 is the grade most often substituted in export quotes — a set sold as M2 and made from 4341 looks identical and fails on the first stainless job.

Incidentally, this is why M42 is the standard tooth material for a quality bi-metal hole saw set: the teeth take far more heat per tooth than a drill lip does, and only a high-cobalt HSS survives it.

Solid, Rolled and Ground

The original description says the body is “the production of solid metal”, which points at the right idea — but there are three distinct ways a drill blank becomes a drill, and they are not equivalent.

The measurable consequence is runout. A rolled drill that is not concentric cuts with one lip doing most of the work: the hole comes out oversize, thrust is unbalanced, and the drill wears on one side. That is precisely the failure mode the chisel-edge discussion above predicts, and it is why a ground drill is cheaper per hole in anything harder than free-cutting mild steel.

On heat treatment, the sequence that produces those hardness figures is a high-temperature austenitising quench followed by multiple tempers — typically three, in the 540–570 °C band, to convert retained austenite and develop secondary hardness. It is also the reason overheating in service is unrecoverable: if you draw the temper out at the cutting edge, no sharpening brings the hardness back. See the failure section.

Shanks, and the Impact-Driver Caveat

Shank choice is a chuck question, and getting it wrong costs you either slipping or runout.

DIN Length Standards and Depth

Length is a standard, not a preference. Four DIN families cover straight-shank drills, and the one you want is decided by hole depth — because a longer drill flexes more and clears chips worse.

Jobber is the general-purpose length and the DIN 338 default: a flute of roughly 8–10 × D, rated in practice to about 5 × D of hole depth because beyond that the chips cannot get out before they pack. Stub (DIN 1897) is the one to specify when you want stiffness and accuracy — a stub drill deflects far less, which is why it is the usual answer to a walking problem on a drill press. Long and extra-long are for reach, and you pay for it in deflection and in chip evacuation.

The practical rule for a set: buy jobber, and add a stub set for positional accuracy if you do production work. A jobber drill with 60 mm of stick-out is a cantilever; the same diameter in stub with 25 mm of stick-out is a very different tool in the same steel.

Speed and Feed: the Two Numbers

Drilling has two independent settings and most people only think about one. Speed (cutting speed at the periphery) governs heat and therefore tool life. Feed (advance per revolution) governs chip thickness, and chip thickness is what carries the heat away.

Step 1: cutting speed from the material

These are HSS values in metres per minute. They assume sharp drills, adequate coolant and reasonable rigidity.

Step 2: turn speed into rpm

n = V × 1000 / (π × D)  —  with V in m/min and D in mm.

A 10 mm drill in mild steel at 25 m/min: 25 × 1000 / (3.142 × 10) ≈ 800 rpm. The same drill in 304 stainless at 10 m/min: about 320 rpm. Note how violent the change is — going from mild steel to stainless costs you 60 percent of your spindle speed, and trying to keep the mild-steel speed is the most common way a drill is killed in stainless.

Step 3: feed per revolution

Feed is where most hand drilling goes wrong, in the direction of too light. Too light a feed does not cut a chip at all — it rubs, work-hardens the surface and burns the lip. Two continuous curling chips, one per flute, is what correct looks like.

Feed rate in mm/min is simply feed per revolution × rpm, so the 10 mm example works out at about 105 mm/min in mild steel. On a hand-held drill nobody measures that — you read the chips instead. Dust or tiny flakes means you are rubbing, not cutting. Increase feed, or reduce speed if the drill is bogging.

Four Faces of Bit Failure

Every drill death falls into one of four patterns, and each has a different verdict: regrind, or retire.

One more signal worth a mention because it is cheap to check: monitor how hard you have to push. Axial thrust scales with wear, so a drill that needs noticeably more feed pressure in the same material is telling you it is dull — before the holes start going wrong.

Three Ways to Stop a Bit Walking

Walking is the drill skating across the surface before the lips bite. It is a chisel-edge problem at root: the blunt centre has to build up enough force to start extruding before the lips are loaded symmetrically, and until then the drill is free to move. Four fixes, in ascending order of how much they cost.

1. Centre punch. A dimple gives the chisel edge somewhere to sit, so the lips load symmetrically from the first revolution. It costs nothing and it is still the right answer when hole position matters. Put the punch exactly on the mark — a punch mark that is off by 0.3 mm gives you a hole that is off by 0.3 mm, and a drill will happily follow a bad punch mark.
2. Pilot hole. Drill a small hole first, then open it out. The experimental evidence is that a pilot equal to the web diameter removes essentially the entire chisel-edge contribution; in practice a pilot around a third of the finish diameter is enough to stop walking and it also reduces the torque the finish drill sees. Two caveats: the pilot drill must be stiff and short, and on a very hard surface you may need to spot-face first.
3. Drill bushing or jig. A hardened bushed plate sitting just above the work constrains the drill physically, so even a walking point cannot go anywhere. This is the production answer when you are drilling the same hole in fifty parts, and it also fixes the deflection problem at the same time. A drill press with the table square and the work clamped gets you part of the way for free.
4. Fix the geometry instead. A split point removes the cause rather than the symptom: the centre cuts, so there is no blunt feature to skate and no need for a punch, a pilot or a bushing on ordinary work. It is more expensive per bit and it is why the split point is the recommended specification for anything beyond casual use.

Two supporting habits. Minimise stick-out: chuck the drill as short as the job allows, because deflection at the point is what turns a small centring error into a badly placed hole. And square the spindle to the work — even a split point will skid on a steeply angled entry, and a drill entering at an angle loads one lip and walks regardless of its grind.

Resharpening: the Four Checks

A twist drill is one of the few cutting tools worth sharpening by hand, but only if you hold four things. Get any one wrong and you will produce a drill that is sharp and useless.

1. Point angle, correct for the material. 118° for general work, 130–140° for stainless. Check against a drill point gauge rather than by eye — the eye is remarkably bad at this.
2. Both lips the same length. This is the one that matters most. Unequal lips put all the thrust on one lip, cut the hole roughly twice the error oversize, and make the drill walk. Measure, do not eyeball.
3. Relief behind the lip. The heel must sit lower than the cutting edge, typically 10–14° at the margin. Too little relief and the heel rubs so the drill will not cut however hard you push; too much and the lip becomes thin and chips.
4. The chisel edge stays on the axis. After grinding, the chisel edge must be centred and, ideally, shortened. If it is off-centre the drill walks; if it is left full width you have regained sharpness but not the thrust benefit. This is the step a split-point grind adds, and it needs a jig or a dedicated sharpener to reproduce.

On the wheel itself: HSS is a steel, so the correct abrasive is CBN or aluminium oxide — never diamond. Diamond and steel react at grinding temperature and the diamond graphitises, so a diamond wheel is both the wrong tool for HSS and an expensive way to destroy a wheel. Our resin-bonded CBN grinding wheels are built for exactly this job. If you are sharpening carbide instead, that is the reverse case, and it is covered on the carbide-tipped Forstner page.

Be realistic about the economics. Hand sharpening restores the lips but rarely reproduces a split point accurately. If your drills are specified with a split point and you sharpen them freehand, you are quietly converting them back to conventional points and losing the thrust and centring benefit. Either invest in a sharpener with a split-point capability, or treat sharpened drills as general-purpose and keep a dedicated split-point set for work that needs it.

Full Specifications

Applications

Fabrication and steelwork

Clearance holes for bolted connections, holes in plate, angle and box section, and the pilot holes that precede tapping or reaming. This is the core use, and the ISO 273 ladder in the set section is the reference for hole size.

Maintenance and repair

Drilling out broken fasteners, opening a damaged hole to the next size, adding fixings to existing structures. The 0.5 mm ladder matters here because repair work rarely uses a size you planned for.

Electrical and mechanical installation

Holes in enclosures, backplates, trunking, brackets and mounting rails — generally in thin sheet, where the helix-angle advice above applies. For repeated knockout-sized holes in enclosures, a step drill is the better tool; the two sets complement each other rather than competing.

Automotive and vehicle work

Brackets, panels, chassis plate and bodywork repairs. Mild steel and aluminium are both well within M2’s range; stainless exhaust and trim is where an M35 cobalt set earns its premium.

Plastics and sheet material

Point angle and relief change for plastics — 60–90° with reduced rake — so if plastics are a major share of the work, specify that grind rather than accepting the standard 118°.

Trade, wholesale and private label

The highest-volume configuration in the catalogue. We build to the piece count, diameter ladder, shank type, finish and packaging your market expects; see the OEM section.

How to Drill with a Twist Bit

1. Mark, then punch. Cross-centre the mark and punch it, unless you are using a split point on non-critical work.
2. Clamp the work. Never hold sheet or small parts by hand. A drill that snags will spin the workpiece, and thin sheet will cut a glove before it cuts skin.
3. Pick the speed from the material and the diameter. V from the cutting-speed table, then n = V × 1000 / (π × D). Stainless is roughly a third of the mild-steel speed for the same diameter.
4. Chuck short. Take the minimum stick-out the job allows. It is the cheapest accuracy you will ever buy.
5. Start square, at low speed. Let the point find the punch mark before you apply full feed and full speed.
6. Feed hard enough to make a chip. Two continuous curls, one per flute. Dust means you are rubbing, and rubbing is what burns a lip.
7. Peck past about 3×D. Withdraw fully to clear the flutes; packed chips are how bits snap in deep holes. Retract more often in stainless and in material that produces long stringy swarf.
8. Coolant to the point, not onto the swarf. In steel, flood or a generous stream of soluble oil. In aluminium, prevent built-up edge. In cast iron, air and run dry.
9. Ease off at breakthrough. Reduce feed as the point breaks through so the bit does not snatch — the full method is in the auger bit set guide, and the principle is identical for a twist drill in steel.
10. Clear chips with a brush, not a hand. Swarf leaves the flute hot and sharp.
11. Deburr and check. A twist drill leaves a burr on both faces. Break the edge with a countersink or a deburring tool before assembly.
12. Read the drill before you put it away. Chipped corner, blue lip, rounded centre, bent shank — four checks, ten seconds, and you will not discover a dead bit halfway through the next hole.

Safety

Frequently Asked Questions

OEM & Wholesale

Huzhou Gu’s Imp. & Exp. Co., Ltd. manufactures and exports the GUSHI range of drilling, cutting and surface-preparation tooling from Zhejiang, China, supplying importers, distributors, tool brands and private-label programmes in more than 20 countries.

For this set we can customise:

• Set configuration: piece count, diameter ladder (metric, imperial, wire gauge or letter), and case layout. 13, 19, 25, 29 and 170-piece sets are standard production options.
• Grade: M2 as standard; M35 cobalt for a stainless-capable premium version; 4341 for price-led volume programmes where the application supports it.
• Point geometry: 118° or 135°, conical or split point, X or N web-thinning form, and material-specific relief angles.
• Finish: bright as standard, plus TiN, black oxide, amber and steam-treated — see the Ti-coated set, black oxide, Ti-coated and amber finish ranges.
• Shank: 1/4 inch hex, straight, three-flat or reduced, matched to the chuck type your market uses; impact-rated torsion-zone versions on request.
• Branding and packaging: laser-marked sizes and logos, printed sleeves, colour boxes, blister cards, blow-moulded and metal index cases.
• Order terms: MOQ 100 sets standard and 500 sets custom; 5–10 days standard lead time and 15–30 days custom; T/T, L/C, D/A, D/P and PayPal; loading from Shanghai or Ningbo.