Screws are made of a variety of materials, including mild steel, brass, copper and aluminium. They can be black japanned, galvanized or plated with nickel, tin, zinc or chromium. Standard screws have what is known as a single start thread, but some have a twin start head, which reduces the number of turns need to drive it home. These types of screws are also hardened, making them suitable for use with power tools.
Screws are measured by their length: the distance between the tip and the part of the head that will lie flush with the surface of the wood. A second measurement is the gauge – the diameter of the screw’s shank. Screw gauges are numbered from 1 to 20 – the thicker the screw the higher the gauge number.
Screws for woodworking come with a range of head shapes. Those with countersunk heads are used where the screw head is to be recessed flush with the work surface or just below it. Round-headed screws are used with sheet materials that are too thin to take countersunk screws. Often these are used for metal fittings and can have decorative heads, which remain on view; bath-room mirrors are often attached to walls with domed-head screws, which have a threaded hole in the centre of the head into which a decorative dome can be screwed.
There are also special ‘tamper-proof’ screws known as clutch heads. These can be tightened but don’t offer any grip if you try to unscrew them. All screws – apart from clutch-head and domed-head ones – are available as cross-slot or as single-slot screws. Where heavy sections of timber are being joined, coach screws (sometimes called coach bolts) are often used. These are heavy-duty screws with bolt heads, which are screwed into a start hole with a spanner. Chipboard screws, on the other hand, are specially designed with a double spiral thread and taper only at the end to give them a strong hold in this relatively weak material.
Special connectors called ‘knock-down’ fittings are strong and easy to fit. These are useful for joining chipboard sections to construct cabinets. The plastic joins pull the boards together when screws are fitted through the joints. The plastic join is fitted to one board with two screws, then screwed through again to join the second board. This system avoids having screw or nail heads on view and means that the piece can be disassembled in future.
Dowel joints use small wooden pegs or dowels in place of hand-cut timber joints. The joint can be any configuration: ‘L’-shaped, ‘T’ shaped, or ‘X’-shaped. Two pieces are butted or mitered together with a series of carefully aligned drill holes to accommodate the dowel. Normally these pegs are made of stout wood and manufactured in short lengths in three main diameters: 6mm, 8mm, and 10mm (1/4in, 5/16in and 3/8in) with flutings or grooves along their lengths. The grooves allow glue to disperse evenly; without them, the dowel would compress the glue in the bottom of the drill hole.