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Important things to consider when selecting screw finishes

 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:

o 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

o 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

o 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 tittings 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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