Screws are often the best way of fastening things together. The other two common methods nailing and gluing are often quicker and require less preparatory work but both produce a permanent joint which cannot readily be dismantled again, and neither method of fixing is likely to produce the strength and abuse resistance that screwed joints have.
About screws
Choosing the right screw for the job can be quite complicated as there are variations in materials, plated finishes, head styles, sizes and thread types.
In addition to different head styles there are two basic head slot styles - the traditional slotted-head style and the so-called cross-head style. Unfortunately, there are three cross-head styles which can be difficult to identify. The most familiar one - Pozidriv is found mainly on wood screws. The more recent Supadriv head is similar and is said to give more grip between the screw and screwdriver, though in practice with wood screws the difference is not significant. The third style, known as Phillips, is found mainly on machine and self-tapping screws fitted to products manufactured on the Continent and in the Far East.
Traditional slotted-head screws are turned by means of a single slot into which a screwdriver blade should fit snugly. Because the slot size varies with the screw, a range of screwdrivers is needed to cope with all the different sizes of screws likely to be used. In practice, a screwdriver of the wrong size is often used and, if it is too small, the blade can easily slip or ride out of the slot.
Cross-head screws have the advantage that they are easier to start turning than conventional screws and are easier to turn when it is impossible to make sure that the screwdriver blade and the screw head are exactly in line. Another advantage of the Pozidriv system is that only three screwdriver points are needed to cover all sizes of screw. (No 2 size is the most common.) Pozidriv screwdrivers can also be used for Supadriv screws but Phillips screws need their own driver.
Cross-head screws do have disadvantages if their slots become filled with paint (as often happens on door hinges) you will have great difficulty in cleaning them out sufficiently well to get the screwdriver blade in, whereas a conventional slotted-head can easily be cleaned with a trimming knife or hacksaw blade.
Most screws designed for use in wood have a thread which extends approximately 60 per cent of the way from the tip to the head, leaving a portion of blank slightly thicker shank. Any screws found which have threads extending over their full length should be looked at closely. They may be cither self-tapping screws designed for fastening metal or Twinfast screws designed for use in chipboard but useful for general woodworking.
The most recent development in cross-head screws is the Supascrew. This has a thinner shank than normal screws (so is less likely to split the wood) and is case-hardened, making it less likely to be damaged. The twin thread has a steeper pitch (which makes the screw quicker to put in) and a sharper point than normal. Supascrews are ideal for chipboard, but useful for other types of wood. A slotted version - Mastascrew -is also available.
Types of head
The two basic head shapes are round-head and countersunk and both are available with cross-head or slotted drives.
Round-head screws are used mainly for holding thin materials which cannot take countersunk screws. This generally means fixing metal things to wood in positions where the shape and appearance of the head is hidden or docs not matter. With this head you need to drill only a clearance hole and a pilot hole.
Countersunk screws are shaped so that their heads can be sunk into the surface of the top material you are fixing. They are less obtrusive than round-head screws and must be used where a bulky head cannot be tolerated such as fixing hinges. Besides needing to drill a clearance hole and a pilot hole, you may have to make a tapered hole for the head with a countersink bit particularly when fixing metal or hardwoods. With soft woods, tightening the screw to pull the head into the wood may be enough.
A third type of screw, known as a raised countersunk, is a sort of combination of the other two. They are used for fixing door handles and other visible hardware. The heads do not stick out too far but still keep the end of the screwdriver away from the hardware.
Countersunk screws of both types can be used with cup washers to protect the wood under the head or to avoid having to make a countersunk hole.
Screw caps - in brown or white - can be used to cover the heads of screws. They fit either in a cross-head recess or in a counterbored hole in which the screw is recessed.
Materials and finish
The most widely used material for screws is mild steel. It is quite a weak metal and rusts easily.
When using screws which are likely to get damp, choose those which are plated with a corrosion-resistant finish such as cadmium, sheradized or bright zinc plated. You could also use screws made of a non-rusting metal like aluminum alloy, brass or stainless steel. These last three types are expensive; aluminum alloy and brass screws are particularly weak and sometimes break when screwing them into hardwood the secret is to put in a mild steel screw first.
Supascrcws and Mastascrcws combine extra strength with bright zinc plating.
There are a few other important things to consider when selecting screw finishes:
when fixing metal things, beware of chemical interaction between the metal and the screw finish. Interaction between steel and aluminum alloy is particularly common and the resulting corrosion may make the screws jam. Wherever possible choose screws with the same finish as the metal being fixed untreated mild steel screws will react chemically with some hardwoods - particularly afrormosia, idigbo and oak -causing discoloration of the wood. Use brass or stainless steel screws instead occasionally the appearance of the screw head is of paramount importance. Brass and steel screws plated with chrome or nickel (both shiny) and Pozidriv and Supadriv screws with a bright zinc finish are all available. Chrome-plated screws with domed heads are available for fixing mirror and bathroom fittings the domed head is actually separate from the screw head and has to be screwed into a tiny threaded hole in the screw head once the screw is fitted.
Screw sizes
In addition to length (measured from the point of the screw to the surface the screw is driven into, not always the overall length), the diameter or gauge needs to be specified too. The gauge indicates the diameter of the screw and the bigger numbers indicate bigger sizes.
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