Nearly all woodworking projects require some form of mechanical reinforcement, and for the structural joinery, this usually means wood screws, often in combination with wood glue. Screws are the primary method for connecting wood to wood, and when done correctly in combination with glue, screwed joints can be nearly indestructible. Screws are also the primary means for attaching metal accessories, such as hinges, locks, and other hardware.
Although many different types of screws are used to fasten wood, many are applicable mostly to construction work, and a relatively small number of types are commonly used in fine woodworking projects. When professionals speak of "wood screws," they are often using the term to refer those types of screws common to woodworking projects, not those used in construction work, drywall assembly, or for deck- or fence-building.
All wood screws have sharp, aggressive threads and a wedge-shaped shank designed cut and bore into wood, but within this overall definition there are many differences in types and styles. Wood screws can be categorized in several ways: by drive type (the means by which the head of the screw is forced into the wood), by head shape, by gauge (thickness of the screw shank), by length, and by the type of metal used in the screw.
The first form of classification for woodworking screws is according to the shape of the slots used to drive the screws into the wood. There are several different forms:
- Slotted: This is the traditional screw style with a straight slot across the head of the screw. It is a classic design, but screwdrivers can have a tendency to slip out of the slot, particularly when driving screws with a drill bit. Unless they are being used for period authenticity, slotted screws are no longer used much by woodworkers, who favor other drive types that are less likely to slip when driving.
- Phillips (cross-head): This style was developed in the1930s to correct the slipping problem that occurred with slotted screw heads. The crossed slots allow screwdrivers to grip better when driving the screws. It is a favorite style among woodworkers, although newer drive types are gradually gaining favor.
- Square-head: This relatively new type is designed to grip better and resist stripping better than traditional flathead and Phillips-type screws. Another variation has a hex-shaped recessed drive opening. The screwdriver or drill-driver bit is shaped to match the recess in the screw head.
- Star-drive: The heads on these screws have slots configured in a star shape, with six or eight sides, like an exaggerated Phillips head. These screws are most often used for structural connections, where considerable force is needed to drive the screws. There are several variations of this type, with drive shapes unique to the manufacturer. Torx is the best-known brand; others include, Pozidrive, and Polydrive, each with a slightly different drive shape.
There are many other drive shapes to be found in wood screws, ranging from multiple variations on the Phillips-head to various polygon-shaped drives. Some even combine drive types, such as the PoziSquare, which can be driven with a star-shaped driver bit, a square-shaped driver bit, or a special bit that combines both shapes. However, most of these are more often used in construction screws and are less common in fine woodworking.
Wood screws are also categorized according to the shape of their heads. There are several types of wood screw heads, but three styles are by far the most common:
- Flat-head screws fit into tapered recesses, such as the holes in hinges, and will lie flush with the surface when properly driven. The tapered shape of the head makes them self-countersinking—they drive themselves virtually flush with the surface of the wood. Or, they can be driven into counterbored pilot holes that allow the screw heads to lie below the surface of the wood. These have the most holding power of any of the standard woodworking screws.
- Round-head screws have a rounded top that has a flat underside, rather than the tapered shape found in flat-head screws. Round-head screws are typically used for affixing objects to wood.
- Oval-head screws (also called pan-head) are a combination of the previous two, with a head that is tapered slightly on the bottom but with a slightly rounded top. This allows the screw to partly embed itself in the wood, while the head remains slightly above the surface for a more decorative look than is possible with a round-head screw.
There are also other head shapes for specialty uses, including:
- Cheese-head screws (also known fillister head) have heads with a cylindrical shape. The name "cheese-head" is said to derive from the resemblance to flat cheese wheels. A variation known as the fillister-head screw has a slightly rounded top to the screw head.
- Truss-head screws (sometimes called “mushroom head” screws) have an oval-shaped head, but one that is flatter and larger than that on a traditional oval-head screw. These work well for applications such as mounting drawer slides, where you need a screw with good holding power but one that doesn't protrude too far.
- Washer-head screws resemble round-head screws with small washers fused into the heads. The extra surface area on the underside of the head keeps the screws from sinking too deeply, which can be helpful when using power drivers. There are several variations on this design, going by names such as "wafer head" or "super-washer-head."
Wood screws are rated by a numerical gauge, which indicates their shank diameter. The larger the gauge, the thicker the screw. The full range of screw gauges is from #2 to #24, but most home centers carry wood screws in gauges ranging from about 5, which has a shank slightly thicker than 1/8 inch, up to a 14-gauge, which has a shank about 1/4 inch in diameter. In the United States, wood screws larger than 12-gauge are typically listed by their imperial sizes, beginning at 1/4 inch.
The last major differentiation between screws is in the length of the screw. Most lumberyards and home centers stock screws in lengths from 1/2 inch up to 4 inches, depending on the gauge.
Woodworking screws are all generally made of steel, but not all steel is created equal.
- Steel: The standard shiny woodworking screws with which most everyone are familiar are made from standard forged steel. These are not particularly moisture-resistant, however, so are not the best choice for outdoor projects, such as deck or patio furniture.
- Stainless steel: Most types of standard woodworking screws are also available in stainless steel versions. Although more expensive, they have good resistance to rust, making them a good choice for outdoor furniture projects.
- Hardened steel: Hardened-steel is often used for screws known as "construction screws" or "multipurpose screws." They have largely replaced other types of utility screws for making hidden structural connections, and are especially good for furniture. These screws are hard enough to penetrate the hardest materials without drilling pilot holes, and they rarely break. Common brand names include GRK Fasteners, SPAX, PowerPro, and Saber Drive. If zinc-plated or treated with another coating, they have good resistance to rust and can be used outdoors.
- Brass: Brass woodworking screws are relatively soft compared to steel, but they are a good choice where the screw heads will be exposed. They are often used to attach door hinges and other hardware, as the yellow metal is more attractive than shiny, silver-colored screws.
Hardwood is quite brittle and can be susceptible to cracking if you drive the screws into the wood without first drilling countersunk pilot holes. If you want to hide the heads of wood screws entirely by covering them with wood plugs or caps, the pilot hole can be countersunk so that the screw head lies entirely below the surface of the wood.
Also, choose a screw that is as long as possible without poking through the back side of the receiving piece of stock. This will help ensure a strong connection without defacing the receiving piece.