Choosing Screw That’s Just Right for the Job : Stronger than nails, screws give you many ways to fasten things and take them apart.

Screws are handy helpers around the home workshop.

They provide a stronger bond than nails and, unlike glue, allow the fastened parts to be taken apart.

Here are some tips on selecting screws:

--Screws are measured by both length and diameter. If possible, choose screws long enough so that at least two-thirds of their length will lodge in the base material to which you are fastening it.

Screw diameter is expressed as a gauge number, generally running from gauge 0 (about 116th of an inch) to gauge 24 (about a 38th of an inch). Screws should be thick enough to hold pieces firmly together. If you feel a screw is too thick and may split the wood, try it out on a scrap piece of the same wood.

--Use flathead screws when a smooth surface is desired and roundhead or ovalhead screws if easy disassembly is important. To create an unblemished finish, countersink flathead screws below the surface by countersinking the hole, and cover the heads with wood filler or plugs.

Easiest to Drive In

--The crossed-slot design of Phillips screws make them the easiest to drive in because the slots hold the screwdriver more firmly. However, be sure to use a Phillips screwdriver of the proper slze.

--When using a power screwdriver, Phillips tips are the best choice; even if a straight tip fits a screw perfectly it is likely to slip out of the slot and damage the surface.

In addition to flathead and Phillips screws there are many other special-purpose type screws. For example:

--Oval head screws have heads whose lower portion is countersunk and whose top is rounded. Oval head screws are easier to remove and more decorative than flathead screws.

--Roundhead screws are often used where the fastened piece is too thin to permit countersinking. They’re also used on parts that may require a washer.

Can’t Be Removed

--One-way screws are designed to prevent theft. They can be driven in with a screwdriver, but can’t be removed. (The head will hold a screwdriver blade when turned in a clockwise direction, but not when turned counterclockwise.)

--Dowel screws have two threaded ends and no head. They can be used in wood joiners to make simple end-to-end joints.

--Hanger screws or bolts have one end threaded like a screw and the other end threaded like a bolt to accept a square or hex nut.

Some tips on using screws:

--Lag bolts are actually heavy-duty screws made in sizes up to 6 inches long. The head end is square to allow the bolt to be driven in with a wrench. To get a screw as large as gauge 6 started into softwood such as pine or spruce, make a shallow dent into the surface with a nail or awl. Larger screws in softwood and all screws in hardwood such as oak, maple or birch, require pilot holes.

When drilling a pilot hole, select a drill bit the same diameter as the core of the screw, which is the shaft of the screw without threads. Line up the pieces being fastened and drill into both pieces of wood to slightly less than the depth which the screw will penetrate. Mark the drill bit with tape to prevent drilling too deep, or use an adjustable drill-stop collar.

To keep screws secured in end grain, drill a hole on an edge, going straight through the board 3/4 of an inch to 1 inch from the end grain. Insert a dowel that’s been coated with wood glue into the hole. The screws will anchor themselves to the dowel for a lasting job.

For information on the source for this article write Reader’s Digest, P.O. Box 700, Pleasantville, N.Y. 10570. TYPES OF SCREWS * Flathead screws are used in applications where the head must be flush with the surface or below it. * In oval head screws, the lower portion of the head is countersunk and the top is rounded. * Roundhead screws are used when the fastened piece is too thin to permit countersinking or when a washer is required. * Phillips head screws have crossed slots to prevent screwdriver slip-out. A Phillips head screwdriver is required. * One-way screws are designed to prevent theft. If anyone attempts to remove the screw, the screwdriver slips out of the slot. * Dowel screws are used for end-to-end joints and similar applications unsuited to conventional screws. * Hanger bolts are threaded on one end for driving into wood: the other end is threaded to accept a square nut. * Lag bolts are heavy-duty screws with square heads that are driven in with a wrench. they are usually made in sizes up to six inches long. SOURCE: Reader’s Digest