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Fixing Devices

SCREWS
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Screws

Wood screws
Wood screws, like nails, are used to secure timber joints and to resist any lateral movement.

They have an advantage over nails in that they have a greater holding power against
withdrawal and they pull the joint together much better.

Screws have the added advantage that they can be withdrawn very easily and replaced
if needed.

Screw heads are made in three shapes:
• Countersunk head.
• Round head.
• Raised countersunk head.

The countersunk is used where there must be no projections. The raised countersunk has
a better appearance. The round head is used where the part to be joined is too thin for
countersinking, such as a metal bracket.

The size of a screw is the diameter of the shank which is known by a gauge number, the
larger the number the larger the shank diameter. Numbers 4, 6, 8 and 10 are the most
popular sizes whilst number 12 and 14 are use for larger types of work.

The length of the thread is usually twice the length of the shank though there are screws
with the threads running the whole length of the screw. Screws are made from different
metals such as steel and brass, but there are many types which, have different finishes and
are made of metals such as Japanned steel, nickel chrome and galvanised steel.




        Countersunk    Round      Raised countersunk
           head         head            head
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    Purchasing screws (identification labels)
    Wood screws are usually sold in boxes of 100 or 200, although they can be bought in
    smaller quantities.

    On the end of each box there is a label that, by its colour and screw head silhouette, allows for
    quick identification of type, size, gauge, length and the metal from which the screw is made.

    A green label indicates bright steel and a yellow label brass.

    Green coloured label (base metal steel)


    Drive method                                             Quantity per box
    (e.g. slotted)

    Silhouette
    Head style
                                                             Green coloured label
    Head style
    (e.g. Countersunk)

                                                             Length in mms




    Yellow coloured label (brass)
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Basic types and head shapes




  Standard countersunk screws




Standard slotted round head

Sizes and gauges are the same as per steel screws.
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    Basic types and head shapes
    There are screws which employ an alternative driving method. These screws have the
    same basic head shapes but instead of the drive method being slotted, it is star shaped.

    There are two distinct types of these star shaped heads:

    a) The Pozidrive type head.         b) The Phllilps type head.




    Pozidrive type screws
    These screws are manufactured in the same sizes and gauges as standard slotted head
    screws. However, there is a new generation of screws developed for use with portable
    electric screwdrivers and which have the same head design, but the shank is threaded
    along its total length and the tip of the screw has a sharp, hard point for easy insertion
    without the need for drilling a pilot hole. These screws are known as Twinfast screws.




                        Twinfast type
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Phillips type screw
These are similar in shape and design to the Pozidrive type screw. They are manufactured
in the same sizes and gauges, and they also have a ‘Twinfast” version for use with
electric screwdrivers.




                     Twinfast version

Preparation of timber to receive screws
When preparing timber to receive these screws it must be drilled at a size smaller than
the threaded portion of the screw, to allow the thread to grip the timber. This is called
the pilot hole or thread hole. A clearance hole and countersink hole are then prepared to
accommodate the screw. When fully in place, the screw head should be just below the
surface of the timber.

				
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