1 SECTION Fixing Devices SCREWS 8 MODULE 3 SHEET 14 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 Screws have the added advantage that they can be withdrawn very easily and replaced if needed.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 8 SECTION MODULE 3 SHEET 15 Fixing Devices SCREWS 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) 1 SECTION Fixing Devices SCREWS 8 MODULE 3 SHEET 16 Basic types and head shapes Standard countersunk screws Standard slotted round head Sizes and gauges are the same as per steel screws. 8 SECTION MODULE 3 SHEET 17 Fixing Devices SCREWS 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 1 SECTION Fixing Devices SCREWS 8 MODULE 3 SHEET 18 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.