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.
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