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Get to know about the Wood Screw

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The Wood Screw


Screws make it possible to have a strong neat fixing to walls, man-made board, timber and even concrete provided the correct size screw has been used for the job. When the screw is turned clockwise, the threaded section is pulled into the material, making the fixing very strong. Where nails hold by friction, screws form a stronger mechanical fixing.

Types of Screws
There are three types of wood screws for general use, the traditional slotted head with tapered shank Fig 1. The more modern Pozidrive, with the parallel shank Fig 2. There is also the phillips cross head, which is very similar to the pozidrive, but close examination shows that they are different (Pozidrive Fig 3, Phillips Fig 4). There is also a difference between Phillips and Pozidrive screwdrivers.

Fig 1 Fig 2

Fig 3

Fig 4

Countersunk Screws
Countersinking is the easiest method of recessing screw heads flush with, or below, the surface of the wood or hardware. The recess is made with a countersink bit after the pilot has been drilled. Care must be taken if countersinking with a power drill, as the recess may accidentally become too large.

Roundhead Screws
Roundhead screws are used for assemblies that may be dismantled, such as pipe box covers, fix hardware or for fixing thin sheet material that cannot be countersunk.

Raised Head Screws
Raised head screws are a combination of the other two types, and are used for fine finished work, for fixing decorative hardware, often in conjunction with screw cups.

What Material Screws Are Made Of
Screws are commonly made of steel or brass. Both types are available with protective or decorative finishes such as zinc or chromium plating. Steel screws are stronger than brass, but are liable to corrosion if exposed to the elements. In this situation use plated steel screws, which should be painted over to protect any damage to the plating caused during installation. For non-painted exposed situations use brass. Brass screws are softer than steel ones, and  should pilot and shank hole should always be used to avoid putting pressure on them. If the wood is hard, first use a steel pilot screw of the same size to form the thread shape, then replace with the brass screw.
Black lacquered (Black Japan) versions are also produced, to match black hinges and latches.

Screw Lengths and Gauges
Screw sizes are specified by the length and the gauge (the diameter of the shank). General-purpose screws range from 6mm to 100mm, although specialist sizes may be smaller or larger, up to 150mm. The gauge size is from 4 to 12. The length of screw is measured from different points according to the head shape

Using Screws
When two equal thicknesses of wood are being fixed together, use a screw length that falls just short of the combined thickness. For very thick wood sections, it is better to counter bore the head rather than use a very long screw; aim to have about two thirds of the screw's length in the lower member. The screw length should be about three times the thickness of the timber it is fixing in place. The thicker the screw, the greater the grip. Screws are sold according to length and gauge (shank diameter). The length is defined as the distance between the tip of the screw and the surface of the wood. The greater the gauge number, the thicker the screw.

Pilot Holes
Screws need pilot holes when been used with wood, otherwise there is a strong possibility of the wood splitting. Two pilot holes are required, one for the thread, and one for the shank (particularly important when using hardwood). Use a twist drill bit the same size as the shank for the first hole, and a smaller bit the size of the core of the screw where the threads are for the second. When drilling pilot holes, mark the required depth of the drill bit with a piece of masking tape. This will tell you when to stop and cannot damage the work piece should you over drill.
Fig 5 Fig 6 Fig 7

As with nailing, where two pieces of wood are to be fixed together, screw the smaller to the larger. Drill the shank hole right through the smaller piece so it is pulled down tight as the screw is driven home. If the shank hole goes only part of the way through you will find it very hard to pull the top piece of wood down tight and may risk breaking or damaging the screw. Fig 5 shows the screw to be used, Fig 6 shows the countersink and the two pilot holes, and Fig 7 shows the same screw driven home.
For very small screws such as No 6 gauge or less, a bradawl should be used. 



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