Screw catalogue

How length is measured

Screw length is measured differently depending on the head, and this is the most common source of error in quoting and goods-in. The same “10 mm” differs by a whole head thickness between a countersunk and a pan head.

One rule covers it: if the head sinks into the work, measure overall; if it stays on the surface, measure under the head.

Materials

Grades and designations are set by the standards. What follows is when each one is the right choice.

• Low carbon steelC1008 – C1022

  The general-purpose default. Lowest cost, with strength coming from cold work and the finish. Around class 4.8 without heat treatment.

• Medium carbon steelC1035 – C1045

  Heat treatable. Self-tapping, self-drilling and drywall screws need hardness to cut, so they are usually this, quenched and tempered.

• Alloy steelSCM435 / 10B21

  The high-strength materials. Classes 8.8, 10.9 and 12.9 come from here. The stronger it is, the more hydrogen embrittlement matters, so the finish needs care.

• Stainless SUS304A2

  The common stainless fastener material. Adequate for general outdoor and damp indoor use. Cannot be hardened by heat treatment, so it cannot make a self-drilling point.

• Stainless SUS316A4

  2–3% molybdenum over 304, resisting chloride pitting. Needed at the coast, in chemical plant, pools and de-icing salt; elsewhere it is money spent for nothing.

• Stainless SUS410martensitic

  Hardenable, which is why stainless self-tapping and self-drilling screws are usually this. Corrosion resistance is clearly below 304.

• Brass

  Conductive, non-magnetic, decorative. Low strength — not for loaded joints.

• Aluminium

  Light and non-magnetic. Low strength, and galvanic corrosion where it meets steel.

• Titanium

  High specific strength, corrosion resistant, non-magnetic, biocompatible. Price is the constraint.

Finishes

A finish is chosen on three things at once: corrosion performance, dimensional effect, and whether it risks hydrogen embrittlement.

• Zinc plating (trivalent)

  The common corrosion finish. Low cost, fine indoors and in general outdoor use. Hexavalent chromium is being withdrawn under regulation, so “zinc plated” now generally means trivalent.

• Hot-dip galvanisingHDG

  Much heavier coating for outdoor and corrosive service. The thread is thickened by the coating, so mating nuts are usually tapped oversize.

• Zinc flake (Dacromet / Geomet)

  Far better corrosion resistance than electroplated zinc, and non-electrolytic, so no hydrogen embrittlement risk. The usual choice for class 10.9 and above.

• Black oxide

  Almost no dimensional change, matt black appearance. Very limited corrosion protection — dry environments, or oiled.

• Nickel / chrome

  Appearance and hardness. Corrosion performance depends on thickness and undercoat.

• Phosphate

  Mostly a base for paint or oil, and used on drywall screws. Weak protection on its own.

How plating consumes thread tolerance, and the M1–M1.4 case →

Drive types

Choosing the wrong drive rarely stops a screw going in. It causes cam-out, which on an automated line becomes a batch of rejects.

• PhillipsH type

  The most widespread. It is designed to cam out above a certain torque — a safeguard in the hand-tool era, a liability on an automated line.

• PozidrivZ type

  Looks like Phillips but has four additional fine ribs in the recess. Less prone to cam-out and carries more torque. The bits are not interchangeable with Phillips.

• Slotted

  The driver slips out of the slot and marks the work, and torque transfer is poor. Outside appearance or service compatibility, not worth specifying on new designs.

• Hex socket

  Good torque transfer with inexpensive tools. The recess collects debris, and six corners concentrate load — over-torque rounds them.

• TorxTX

  Six lobes give a large contact area, almost no cam-out, high torque capacity and long bit life. The first choice for automated driving and high torque.

• External hex

  Driven with a socket; the highest torque available. Needs clearance around the head.

• Square (Robertson)

  The screw stays on the bit for one-handed work. Common in North American joinery.