Joining two pieces of wood with nails is probably the quickest and easiest way to fasten lumber together, but for those projects that require a stronger joint, or where it may become necessary to take the joint apart in the future without ruining the wood, using wood screws is the method preferred by most experienced carpernters and cabinetmakers.

Wood screws come in a large range of sizes that vary in length and diameter, as well as in the style or shape of the head. All wood screws have a sharp, awl-shaped point, and all taper, with the thickest part of the body just under the head where the screw is no longer threaded (this part is often called the shank).

Wood screws come in three popular head shapes: flat, oval and round. As a rule, flat-head screws are used when the head is to be recessed so it is flush with the surface of the wood, or when you plan to countersink or recess the head deep enough to cover it with a wood plug or some type of wood plastic. t

Oval-head screws are designed so the curve part of the head will protrude slightly above the surface of the wood when the rest of the head is recessed into a cone-shaped depression in the wood (the pressure of tightening the screw down is usually enough to accomplish th is in most kinds of wood). Because oval-head screws are more decorative than flat-head screws they are often used in place of flat-head screws on those jobs where the heads will show. They are also used with cup-shaped "finishing" washers when assembling wood parts that have to be taken apart at regular intervels.

As shown in the accompaning drawing, screw sizes are determined by length and by the diameter of the body.

The length of a wood screw is specified in inches and fractions of an inch, but its diameter is specified by guage number (the smaller the gauge number, the smaller the diameter of the screw). The most common guage sizes available in local stores range from 2 (slightly more than 1/16-inch in diameter) to 16 (about 1/4 inch in diameter). Still large sizes (up to 24) are available from large suppliers or on special order in most cases.

Note that the guage is the diamter of the unthreaded shank or body of the screw, and that the length varies according to the type of head the screw has. If the screw has a flat head, its length is its overall length; if the screw has an oval head, its length is from the tip to the beginning of the upward curve on the head; if the screw has a round head, its length is from the tip to the underside of the rounded head. In most wood screws the threads extend up to about two-thirds the total length of the screw.

Most wood screws sold in local hardware stores, lumber yards and similar retail outlets have standard slotted heads with straight slots that fit regular flat-blade screwdrivers. However, screwdrivers come in many sizes, with blades of varying thickness and width; therefore, you should select a blade that is a snug fit inside the slot of the screw. If the blade is too narrow, or if it is rounded off or tapered at the edge, it will be much more likely to slip out and cause damage to the work or to the screw head.

In addition to being the right thickness for the screw slot, the screwdriver blade should be about the same width as the screw head. If the blade is wider than the head of the screw its corners will dig into the wood as the screw is driven home. If the blade is narrower than the screw head it will be much harder to drive the screw home, and it will be more likely to slip and possibly bend the top of the blade.

Not all wood screws have straight slotted heads. Some are available with cross-slot heads; these are called Phillips-head screws. They come with the same style heads as slotted screws (flat, oval and round) and are driven in with a Phillips-type screwdrive that has a matching cross-slot type blade.

Professionals prefer cross-slot screws and drivers (in addition to Phillips-head screws there are several less easily available cross-slot screws in use) -- and with good reason. Screws with cross-slot heads provide a better grip for the blade, allowing less chance of slipping; they make it easier to apply greater torque to the screw head without damaging the screw or the driver, and they work better with fast-acting, push-pull rachet-type drivers, electric screwdrivers and screwdriving attachments for electric drills.

These advantages make it worthwhile to buy Phillips-type screws when a large project is involved. Not all local dealers stock them in all the popular sizes, but as more and more do-it-yourselfers recognize the benefits of using Phillips-type screws, availability probably will improve.

Except when using the very smallest screws in soft wood, a pilot hole should be drilled fo each screw -- to prevent splitting the wood, as well as to simplify the job of driving the screw in. When fastening a bracket or thin object to wood, a single pilot hole will often suffice. Make it about the same thickness as the root diameter. Actually, in soft wood this pilot hole should be slightly smaller than the root diameter, while in hardwood it should be at least as large or even slightly larger.

When assembling two pieces of wood, drill a larger clearance hole for the unthreaded shank of the screw through the top piece, than a smaller one equivalent to the root diameter in the base piece. This pilot hole should be deep enough to penetrate the wood a distance of one-half to two-thirds the threaded length of the screw.

If you are planning to countersink the screw head, a third hole, equal to the diameter of the head, will be needed at the surface. These holes can be drilled at one time with special combination bits that are sold in most hardware stores and tool outlets. These come in sets for use with all the holes to a controlled depth so they can also be used for countersinking flat-head screws.

When strength is important, choose the largest diameter screw you can use, but remember that you will have to drill a pilot hole first, so the diameter of the screw may be limited by the width of the material you are attaching. For example, if you are screwing a narrow molding in place, its width will restrict the size hold you can drill.