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SCREWS

Drilling screw holes

All screws must have pilot holes made before they can be driven home. For screws smaller than No. 6 gauge (3.5 mm) into softwood, make these with a bradawl. Drive it into the wood with its chisel point across the grain, to avoid splitting.

Screws larger than No. 6 gauge into hardwood and screws into softwood need two holes -one for the thread (the pilot hole) and one for the shank (the clearance hole). These must be made with a drill and bit.

When drilling pilot holes, mark the required depth on the drill bit with a piece of masking tape. This will tell you when to stop and cannot damage the workpiece should you overdrill.

As with nailing, where two pieces of wood are to be fixed together, screw the smaller to the larger. Drill the clearance hole right through the smaller piece so it is pulled down tight as the screw is driven home. If the clearance hole goes only part of the way through you will find it very hard to pull the top piece of wood down tight and may risk breaking or damaging the screw. Brute force should never be used-it indicates that either the thread hole or the shank hole is too small.

Driving screws

Always make sure that the tip of your screwdriver is in good condition and that it fits exactly into the slot in the screw head. A blade which is too narrow or rounded damages the slot, while too wide a blade damages the wood as the screw goes in.

As a time-saving alternative to the conventional screwdriver, a pump-action screwdriver works by converting downward movement of the sliding handle into rotation of the tip. So, simply by pushing hard, the screw is driven very quickly in or out of the wood (depending on the setting of the ratchet). Take extra care when using a pump-action screwdriver that it does not slip and cause you an injury.

Countersinking Countersinking is normally the easiest way of recessing screw heads flush with, or below the surface of the wood. The recess is made with a countersink bit after the clearance hole has been drilled, to the same depth as the countersunk screw head. Take particular care if you are countersinking with a power drill or the recess may accidentally become too large.

For some screw sizes, special bits are available to drill the thread hole, shank hole and countersink recess in one operation. Care should be taken, however, as they break easily.

Common types and uses

Countersunk screw: Used for general woodwork. The head sinks in flush with or slightly below the wood surface.

Crosshead screw: Used for general woodwork, but needs a special screwdriver which does not slip from the head.

Countersunk roundhead (raised head): Used for fixing door-handle plates and other decorative fittings with countersunk holes. The head is designed to be seen.

Roundhead screw: Used for fixing hardware fittings without countersunk holes.

Mirror (dome) screw: Used for fixing mirrors and bathroom fittings. The chromed cap threads into the screw head to hide the screw. Do not over- tighten.

Coach (or hexagon) head screw: Used for fixing heavy constructions together and heavy equipment to timbers. Tighten with a spanner.

Invisible (dowel) screw: Used for invisible joining of two pieces of timber.

Self-tapping (panel) screw: Used for fixing thin sheets of metal and plastic. Cuts its own thread as it is screwed in. Various types of head are available.

Particle (chip) board screw: Used for securing particle (chip) board and its derivatives.

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