How to Choose Right Screws For Carpentry Projects
To be able to pick exactly the screw you need for a particular purpose, it helps to know what each part of the screw does.
The thread is the spiral that actually pulls the screw into the wood and holds it there. Most have the same profile, but chipboard screws combat the material's crumbly quality with their shallower spiral; some screws have a double thread, which means the screw won't wander off-centre and can be driven more quickly. Most wood screws have about two-fifths of their length unthreaded, forming the shank, but chipboard screws are threaded all the way up to head for better grip.
Screw heads come in three main shapes. Countersunk is the commonest. The name describes how the screw head fits into the surface of what you're fixing - into a hole with sloping sides. This hole is made in wood with a special countersink bit; many metal fittings such as hinges have their screw holes already countersunk.
The raised countersunk head looks more handsome, and is often used with exposed metal fittings. The round head is used for fixing metal fittings without a countersink to wood.
On wood screws the Pozidriv recess has now given way to the similar-looking Supadriv type. Each has its own screwdriver shape, but you can use a Pozidriver for both.
Screws for special purposes include the clutch-head screw, which can't be undone once driven. The coach screw, used for heavy framing work, has a square head and is tightened with a spanner. The mirror screw is inserted in the usual way; then a chrome-plated dome is screwed into the head, making it a decorative feature.
Sizes and materials
How big is a screw?
Its length ranges from 6 to 150mm (1/4 to 6in). The gauge - the diameter of the shank - has a number from 0 (the smallest) to 32; 4, 6, 8, and l2 are the commonest. Remember that you can have the same screw length in different gauges and the same gauge in different lengths.
And what are screws made of?
Steel is the commonest and cheapest material, but isn't very good-looking and rusts easily. Luckily there are several alternatives. Steel itself comes with various coatings, from nickel plate to black japanning. Of other metals, brass (available plain or chromium-plated) is fairly corrosion proof but weak. Aluminum (also weak), stainless steel and silicon bronze are virtually corrosion-free. Stainless steel is the strongest of the three, but is expensive.
Remember when buying screws to give all the relevant details - length, gauge number, head type, material, recess type and finish.
