A screw is defined by four things at once: what its head looks like, what drives it, how its thread cuts or engages, and what point or tip it ends in. A "hex head screw" and a "hex drive screw" are not the same fastener — and telling a screw from a bolt with an identical hex head is enough of a real-world problem that ASME B18.2.1 devotes a dedicated appendix to it.
This guide covers the 12 screw and bolt types machine builders and maintenance teams actually source: hex head, socket head (cap), flat head, rounded head, thumb, shoulder, tapping, set, captive panel, carriage bolt, 12-point, and threaded rod/stud — by head, drive, thread mechanism, and material - see the full Screws and Bolts category for the complete SKU range.
Screws by Head Type
Head shape decides two things: how much torque the screw can take, and whether it sits flush, proud, or recessed in the part.
Hex head screws
A six-sided external head driven with a wrench or socket, built for high-torque assembly. Strength ranges from low-strength commercial steel up to Grade 8 and metric Class 12.9, with 18-8 stainless, brass, titanium, and even ceramic versions for corrosive or non-conductive applications. See hex head screws.
Socket head screws (cap screws)
Also called cap screws, these use an internal hex (or Torx) recess and a taller cylindrical head for high clamp load in a compact footprint — the standard choice in machine tool and fixture assembly. See socket head screws.
Flat head screws
A countersunk underside pulls the screw flush with the part surface as it tightens — used wherever a protruding head would interfere with a mating surface. See flat head screws.
Rounded head screws
The broadest family: pan, truss, button, fillister, cheese, and one-way tamper-resistant heads all fall under it, each trading off head height, bearing surface, and appearance. See rounded head screws.
Thumb screws
These replace a drive recess with a knurled head sized for finger grip — for panels and covers removed often enough that a tool becomes a nuisance. See thumb screws.
Shoulder screws
Shoulder screws aren't fasteners in the clamping sense at all.
The unthreaded shoulder is a precision-ground bearing surface for a pivot, cam, or linkage. A shorter threaded tip below it handles the actual retaining. See shoulder screws.
Screws by Drive Type
The drive is what your tool actually engages, and it's chosen independently of head shape — a rounded head screw, for example, ships with a cross-point, single-slot, star, square, or combination drive depending on the line.
Cross-point and single-slot drives remain the default for general assembly and sheet metal work.
Star and internal hex (socket) drives resist cam-out under power-tool torque, which is why they dominate socket head and structural applications.
Square drives give a firmer bit engagement for high-volume production driving.
12-point drives put twelve points of contact around the head instead of hex's six, so a wrench or socket can index onto it at half the swing angle — the reason it shows up on high-torque assembly in tight clearances, like engine and driveline hardware. LILY's 12-point screws run in alloy steel for that load range.
One-way and tri-groove drives allow installation but not removal with a standard tool — a tamper-resistance feature on public-facing equipment.
Tapping Screws: Cutting Their Own Thread
A tapping screw forms or cuts its own mating thread as it drives in, so no separate nut or tapped hole is needed.
Type A and AB threads have a coarser pitch and sharper point for sheet metal and thin plastics.
Type C is designed to cut into pre-drilled metal without the gouging a standard machine-screw thread would cause.
This is a different mechanism from a machine screw, which requires a pre-tapped hole or nut to begin with — mixing the two up is a common cause of stripped holes in the field.
Set Screws: Headless, and Built to Lock, Not Clamp
Set screws have no head at all — the entire body sits inside the bore, driven flush or below the surface — and their job is to lock a collar, pulley, or gear onto a shaft rather than clamp two parts together.
The point style is what actually does the work: a flat or nonmarring tip spreads load without denting a finished shaft, a swivel tip self-aligns against an angled surface, and an extended tip reaches across a keyway or oversized bore that a standard-length point can't bridge.
Matching point style to shaft finish is the single most common set screw sourcing error — a standard point on a hardened, polished shaft will slip under vibration.
Captive Panel Screws: Built Not to Fall Out
Captive panel screws stay attached to the panel even fully unscrewed, which matters anywhere a dropped fastener is a real problem — inside running equipment, near open circuit boards, or overhead.
Three mechanisms cover most of that need: threaded types that back out but stay retained by a spring and washer stack, quarter-turn types that release with a 90° twist instead of full unthreading, and snap-in types that press into a panel hole without a separate nut on the back side.
Carriage Bolts and Threaded Rods: Where "Screw" Becomes "Bolt"
Carriage bolts
A carriage bolt has a smooth, domed head and a square section directly beneath it that bites into wood or a square punched hole, so the bolt won't spin while a nut is tightened from the other side — no drive recess needed at all. See carriage bolts.
Threaded rods and studs
Threaded rods and studs go a step further and drop the head entirely: fully threaded stock cut to length and run with nuts on both ends, or on one end for a permanent stud-to-part connection.
The strength grades follow the same standards as hex head screws — Grade 2 and Grade 5 in inch sizes, Class 8.8 through 12.9 in metric.
The grade marking on the head or end, not the fastener's shape, is what actually determines its working load. See threaded rods and studs.
How Screw Sizes Are Written
Inch-series screws are called out as gauge or fraction, then threads per inch, then length — #8-32 x 1" means a #8-diameter screw (0.164 in.), 32 threads per inch, 1 inch long. Numbered gauges run #0 through #12, then switch to fractional sizes (1/4", 5/16", 3/8", and up). Metric screws follow nominal diameter, thread pitch, then length in millimeters — M6 x 1.0 x 20 is 6 mm in diameter, 1.0 mm thread pitch, 20 mm long.
One detail that trips up first-time buyers: length is measured differently by head style. Flat head and other countersunk screws are measured from the top of the head, since the head sits flush and counts toward the installed depth. Every other head style — hex, socket, rounded — is measured from under the head to the tip, because the head itself sits proud and doesn't factor into how far the screw reaches into the material.
Material and Finish: Matching the Environment
Material and finish decide how a screw holds up against load and corrosion together — not just one or the other.
Material | Typical use | Trade-off |
|---|---|---|
Zinc-plated / black-oxide carbon steel | General indoor assembly, structural grades (5, 8, 8.8–12.9) | Highest strength-to-cost; limited corrosion resistance |
18-8 stainless steel | Mild outdoor or washdown exposure | Good general corrosion resistance; lower strength than alloy steel |
316 / A286 stainless steel | Marine, chemical, or high-heat exposure | Best corrosion resistance in this list; highest cost |
Brass / bronze | Electrical, decorative, or galvanic-sensitive assemblies | Non-sparking, corrosion-resistant; softer, lower load rating |
Titanium | Aerospace, weight-critical designs | Excellent strength-to-weight; highest material cost |
Nylon / plastic | Electrically isolated or non-marring joints | No galvanic risk; far lower clamp load |
Galvanic corrosion note: a stainless screw driven directly into an aluminum part, in the presence of moisture, sets up galvanic corrosion that attacks the aluminum — not the screw. An isolating washer or a closer-matched material avoids it. See NASA’s Fastener Design Manual for a fuller treatment of material and corrosion selection.
How to Choose: A Four-Question Checklist
What's the load path? Clamping two parts together calls for a screw or bolt with a head; locking a component onto a shaft calls for a set screw.
How load-critical is the joint? Grade 5 or Class 8.8 covers most general machinery; reserve Grade 8 or Class 12.9 for joints where under-strength genuinely risks failure, since the higher grades cost more and are more brittle.
What's the exposure? Indoor and dry favors plated steel; any moisture, washdown, or outdoor exposure moves the decision to 18-8 or 316 stainless.
What's the access? Frequent removal by hand favors thumb screws or quarter-turn captive screws; permanent structural joints favor hex or socket head with a matched strength grade.
FAQ
Is a cap screw the same as a bolt?
Not by the technical definition, though the terms get used loosely. A screw is designed to be tightened by turning its head into a tapped hole; a bolt is designed to be tightened by turning a nut onto its thread while the bolt itself doesn't rotate. A socket head cap screw threading into a tapped block is a screw; the same fastener paired with a nut through a clearance hole is functioning as a bolt.
Do stainless steel screws lose strength compared to steel?
Yes, measurably. Standard 18-8 stainless has a minimum tensile strength around 70,000 psi, against roughly 120,000 psi for Grade 5 steel — about 40% lower. That gap is why high-load structural joints in dry environments typically stay with plated or alloy steel rather than defaulting to stainless for corrosion resistance alone.
Can I substitute a Torx drive for a Phillips drive on the same screw?
Only if the head geometry is otherwise identical and the strength rating is unchanged — the drive recess doesn't affect clamp load, but swapping drive types usually means sourcing a different SKU line rather than modifying an existing screw.
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