§1A thread where none exists
A self-threading screw is driven into a plain hole and generates its own mating thread on the way in — trading a tapping operation for a harder screw and a carefully sized pilot hole.
The machine screws page drew the line: a machine screw runs into a thread that already exists, cut by a tap or formed in a nut. A self-threading screw abolishes that step. It is case-hardened so that its thread is harder than the sheet, casting or plastic it enters, and as it is driven its thread creates the internal one — by displacing material, by cutting it, or, with a drill point, by first making the hole as well (§2–3). The economics are the point: in assembly work every tapped hole costs a drilling, a tapping and an inspection, and a self-threading screw collapses those into one driving operation. The price is paid in engineering care elsewhere — the pilot hole must be sized correctly, the torque window managed (§5), and the joint’s strength is now set by a thread in material that is often only a millimetre or two thick (§4).
Contents§2Forming against cutting
The two classic families differ in what happens to the displaced material — a forming screw pushes it aside, a cutting screw removes it — and each behaviour suits a different parent.
A thread-forming screw has a plain tapered point and simply extrudes the parent material into the shape of its thread, the way thread rolling does on the threads pages: nothing is removed, no chips are made, and the displaced metal is cold-worked and flows tightly around the screw’s flanks. That gives it the stronger, tighter thread — work-hardened, chip-free (which matters greatly inside electrical and instrument enclosures), and gripping with enough residual elasticity to resist vibrating loose. Its cost is driving torque: forcing metal to flow takes effort, and in thick or hard parents the torque to drive approaches the torque to strip (§5). A thread-cutting screw solves exactly that: one or more flutes are ground into its point (the hero), giving it cutting edges and somewhere for chips to go, so it drives at much lower torque and suits thicker sections, castings and brittle materials that would crack rather than flow. The trade is a slightly weaker thread — material has been removed, not compacted — and chips that must be tolerated or cleaned. The rule of thumb: form where the parent is thin or ductile; cut where it is thick, hard or brittle.
| Family | Makes its thread by | Chips | Driving torque | Best parent |
|---|---|---|---|---|
| Thread-forming | displacing metal — cold-forms the flanks | none | highest | thin, ductile sheet and light alloys |
| Thread-cutting | cutting — flutes give edges and chip room | yes | lower | thick sections, castings, brittle parents |
| Self-drilling | drilling its own hole, then forming | drilling swarf | moderate | steel sheet and sections, stacked work |
| Screws for plastics | forming, with a narrow flank angle | none | low | thermoplastic bosses (cutting types for thermosets) |
| The pilot hole is part of the system in every row but the third: forming and cutting screws enter a punched or drilled pilot whose size is specified for the screw and the sheet, while the drill point makes its own — which is why the §5 torque window is managed by pilot size in the first two families and by point-length selection in the third. | ||||
§3The self-drilling point
The self-drilling screw goes one step further and eliminates the pilot hole too — its point is a genuine twist-drill tip, so a single driving operation drills, threads and fastens.
Look at the point of one (the hero) and it is unmistakably a small drill: a fluted, angled tip below an unthreaded section, then the thread proper. Driven with a power tool, the point drills through the sheet, the thread follows into the fresh hole and forms its mating thread, and the head seats — hole-making, threading and fastening in one pass, which is why these screws dominate steel-framing, roofing and cladding work where thousands of fastenings are made into sheet and light sections. Two details govern whether they work. The point length must exceed the total thickness being drilled: the thread must not engage until the drilling is finished, because a thread that starts pulling while the point is still cutting advances the screw faster than the drill can remove metal, and either the point snaps or the thread strips. Longer points suit thicker steel, and the point series is chosen to match the stack. And the speed must stay moderate: a small drill point run at very high speed burns rather than cuts — the same speed logic as the drilling pages, at fastener scale. Where the sheet is very thin, the plain forming screw of §2 into a punched hole remains the better tool; the drill point needs enough metal to drill.
Contents§4What thin sheet can hold
The whole joint hangs on a thread engaged in a millimetre or two of sheet — so the honest question is how much force that thread can carry before it strips, and the arithmetic is short.
The engaged thread can be treated exactly as the inserts page treats a nut: a cylinder of threads whose shear area is about 60% of π d t. Take a #8 screw — 0.060 + 0.013 × 8 = 0.164 in = 4.17 mm, using the machine-screw gauge formula — in mild steel sheet shearing at 240 N/mm². In 1.0 mm sheet: F = 0.6 × π × 4.17 × 1.0 × 240 = 1.89 kN. In 1.5 mm: 2.83 kN. In 2.0 mm: 3.77 kN — exactly double the 1.0 mm figure, because the relation is linear in t: double the sheet, double the hold. Run it backwards and the formula sizes the sheet: to hold 2 kN on this screw needs t = 1.06 mm. Two practical extensions follow. Where the sheet is too thin, an extruded (flanged) hole — the sheet-metal pages’ trick of drawing a collar out of the sheet — raises the engaged length to two or three times the raw thickness without adding material. And the numbers explain the trade the whole page rests on: a screw thread in 1 mm of sheet holds a couple of kilonewtons, not the tens of kilonewtons of the bolted joints elsewhere in this section — self-threading fastening is a light-duty, many-fastener technique, and its joints are designed accordingly.
§5The drive–strip window
Every self-threading joint lives between two torques — the torque needed to drive the screw and the torque that strips the new thread — and production reliability depends on the gap between them.
Driving torque is what it costs to form or cut the thread and seat the head; stripping torque is where the freshly made internal thread shears and the joint is destroyed. The assembly tool must be set between them, and since screws, sheets and holes all vary, the window has to be wide: the working rule is that stripping torque should be at least twice driving torque. Everything about the joint moves those two numbers. A pilot hole too small raises driving torque steeply (more metal to displace) and closes the window from below; too large leaves shallow thread engagement and drops stripping torque from above — which is why the pilot size specified for a given screw and sheet is not advisory. Thicker sheet raises stripping torque (§4’s linear rule) and widens the window; a forming screw in hard sheet raises driving torque and narrows it, which is when the cutting screw of §2 earns its place. On the line, power drivers with torque clutches are set inside the window — high enough to seat every screw, safely below the weakest plausible strip — and a joint whose window is too narrow to set a clutch reliably is redesigned, not driven more carefully. It is the torque-and-tension page’s scatter problem in miniature, with the same answer: control the friction and geometry, or the torque number means little.
Contents§6Screws in plastics
Plastics take self-threading screws well — but on their own terms: the thread form, the boss around the hole and the re-assembly technique all change.
The materials pages divide plastics in two, and the screw follows the split. Thermoplastics flow, so they suit thread-forming screws — but a metal-pitch 60° thread stresses the plastic too much, so screws made for the job use a narrower flank angle, around 30–45°, and a coarser pitch: the slimmer thread penetrates deeper for the same displaced volume, cutting the hoop stress that splits plastic bosses while raising pull-out. Thermosets are brittle and will crack rather than flow, so they take thread-cutting screws, which remove material gently instead of wedging it aside. Around the screw, the boss is part of the fastening: its outside diameter is made about twice the screw diameter so the hoop stress has wall to react against, and the engagement about two to two and a half diameters — the soft-parent logic of the inserts page again, in plastic. And re-assembly has its own discipline: driving a screw back into a used hole must not cut a second thread across the first, which halves the strip torque. The technique is to turn the screw backwards until it is felt to drop into the existing thread start, then drive forward — a small habit that preserves the joint through many service cycles. Where the joint must survive indefinitely many, the answer stops being a self-threading screw at all and becomes a moulded-in or press-in insert.
Contents§7Quick reference
The working core of the page on one card rack.
Families
form: displace, no chips
cut: flutes, chips, less torque
Drill point
drills + threads in one pass
point longer than the stack
Sheet holding
F = 0.6 π d t τ — linear in t
#8 in 1 mm steel → 1.89 kN
Torque window
strip ≥ 2 × drive
pilot size is not advisory
Plastics
narrow flank · boss ≈ 2d
reverse first when re-driving