What screw do I use?
Put a nail in your coffin — the lid might slip off in an earthquake. Screw it down tight and your future's secured.
Screws come in a wide variety of shapes and sizes. Long ones, short ones, thin ones, fat ones, round heads, flat heads, some called Phillip, some called Robertson and others as square as they come. Like all Phillips and Robertsons, there’ll be some you like and some you don’t. It’s worth knowing the gamut so you can order exactly what you want when you need it.
You can label a screw by the slot in the top, the shape of the head, the size and thread of the shaft and/or the material it’s made of. If you have to replace an existing screw, try and find the same type. If you’re beginning a new project, I recommend you use square-recessed (Robertson) screws. They’re the easiest to use. To choose the metal type you need see here. Remember you’ll also need the correct screwdriver (see ‘Selecting the driver to fit the screw’ below ). For tips on how to use a screwdriver effectively see here. For tips on selecting the right drill bit, see 'Choosing an appropriate drill bit' in the article here.
Heads and threads
Wood screws can be used in wood and other soft materials. They have a threaded shaft tapered to a sharp point. Generally used in a pre-drilled hole. (See How to use a power drill). See illustrations below.
Machine screws, designed to hold metal, generally aren’t
tapered and have a blunt end because the hole in the metal they enter is pre-machined to the same thread size and type.
Sheet metal screws are usually short and have coarse threads designed to grab onto relatively thin metal.
Grub screws are machine screws with no head other than a
driving slot at the end of the screw thread. The head fits neatly into the machined hole and lies flush with the surface. You might find one of these attaching a car door handle to the metal shaft (which you can’t see) that goes through the door.
Countersunk screws are used for metal, plastics and wood when the surface of the screw needs to be flush with the surface of the material. The entrance to the hole needs to be tapered to
accommodate the countersunk head. This can be achieved by drilling the top of the hole with a countersink cutting bit (see also ‘Countersink bits’ in Using a power drill. Many modern self-tapping screws for soft materials create their own countersunk hole as they drill in.
Self-tapping screws have threads that tap their own holes, so no pre-drilling is necessary. They can be used only in soft materials such as wood and some plastics and aren’t suitable for hard timber or other hard materials. Nor are they suitable for use near the end of a piece of timber, as they might part the grain of the timber and cause splitting. In this situation, pre-drilling is advisable.
Self-drilling screws have self-tapping threads, but the base of the thread is formed in the shape of a drill bit. The drill head drills a hole ahead of the thread and the thread is tapered in such a way that the waste
material rises out of the hole. The waste material should be removed before final tightening. These screws are relatively expensive because of their complex form, but they can save time as drilling isn’t required.