Basic types of wood screws

Wood screws are classified by the type of drive, the shape of head, its length and whether it is designed for wood or metal; this page refers to wood screws only.

Types of drive. The two basic drive designs are single slot and crosshead, crossheads are normally either 'Philips' or 'Pozidrive', these require specific types of screwdriver although a Philips driver can be used on Pozidrive screws. It is always important to use the correct size of screw driver to ensure that the workpiece and screw are not damaged.

Head shapes.

Countersunk heads can be concealed below the woods surface; raised heads are countersunk with a slight domed head; round heads rest on the surface.

Threading on the shank is designed specifically for wood; wood threads have a tapered screw while sheet-metal screws have mainly a parallel thread. Wood type screws are also normally used for securing into wall plugs. Screws for chipboard usually have 2 threads the full length of the shank.

Screw sizes.

Screws are sized by gauge number and length. The gauge number was thought up by someone with a weird mind, if the gauge is not known, simply measure the diameter of the head in sixteenths of an inch, take away one and double - that is the gauge number.

Example:

• if the head is a quarter of an inch, that equates to 4 sixteenths;
• take away 1 equals 3;
• double that equals 6, so it is a 6 gauge screw.

The length of screw is taken from the surface of the material to the point of the screw.