All About Screws

Excerpted from “The Essential Woodworker,” by Robert Wearing.

This is one of the most common methods of joining components. It is interesting that while British use the name ‘turnscrew,’ the American term is ‘screwdriver.’ This is significant in that a screw properly arranged should not need to be driven, which implies hard work, but merely turned.

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The screw has the following parts (Fig 75): head, shank, core and thread, and provision must be made for each of these in drilling the hole, since the screw, unlike the drill, cannot remove any wood. A standard screwed job (Fig 76) is one piece screwed directly to another and Fig 77 shows a section through the joint. The top piece needs a clearance hole made with a twist drill, in which the shank is free to turn. If this is too small much more effort will be required and the screw slot may be damaged, or the work may split if the hole is near the edge. The top piece may require countersinking. The correct tool for this is the snailhead countersink, but as an alternative the rosehead bit or drill may be used (Fig 78); however this is inferior, being designed primarily for metal. A larger twist drill may be used, and this is most successful when used in a drilling machine with a depth stop.

The pieces can now be held in position with the screw dropped in place. A light tap on the screw will leave a dimple on the lower piece on which the next hole can be drilled. This is the pilot hole and its diameter is that of the screw core. The principle is that the drill makes room for the screw core and the thread bites into the sides of the hole, giving the greatest strength with the minimum effort.

The screwdriver end should be well maintained and the blade must fit the slot. If allowed to become rounded it will slip out. Too large a blade will damage the wood, too small a blade will probably slip from the slot. Handles are made in proportion to blade size in order to exert the right pressure. Long, thin electrical screwdrivers should be used with particular care to avoid stripping the thread in the wood.

Fig 80 shows alternative screw heads. The round head is commonly used to secure fittings. If these are very thin a shallow clearance hole will be required for the shank before drilling the pilot hole. The raised head is sometimes used on metal fittings, and is most commonly used in industry. It can be conveniently used by the hand craftsman in conjunction with the screwcap washer when screwing plywood. The nature of plywood and sometimes its thinness makes it difficult to countersink neatly for a screw head.

For very small screws a four-sided awl is most convenient for the pilot hole. A touch of grease on a screw allows it to go in easier and by preventing rusting makes a later withdrawal easier too. Oak eventually has a corrosive effect on iron screws so for good-quality work brass screws should always be used.


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