Debate still rages today as to who actually invented the screw. Some scholars credit the Pythagorean philosopher Archytas of Tarentum (c. 400 B.C.) for identifying the helical shape that forms the thread of the screw. But it is generally accepted that the ancient Greek engineer Archimedes is the “Father of the Screw” (c.287 B.C.). Early screws were hand-carved of wood and lacking a point, more closely resembled a bolt rather than a modern screw. Not yet used as a fastener, these early screws were used in wine, linen, and olive oil screw presses, as well as in water presses for irrigation and for pumping mines and ship bilges. These were the common applications of screws for roughly the next two thousand years.
The marvelous leap from screws for wine presses to screws for fastening may have developed in response to the limitations of nails. Hand-forged nails of the day were thick and long, and therefore could not be used for joining thin or fine materials, including metal. Also, nails could not be easily extracted without damage to the surrounding material, and their holding power was limited only to the strength of the fibers holding them in place. Somewhere along the way the metal screw was born.
By the 1500s, hand-forged metal screws could be found in metal armor, locks, and clocks but were not yet in use for common joinery applications. These early screws could be described as a nail with a threaded shank and a slot in its head for ease of removal. Both the threading and the slot were carved by hand, which led to significant differences from one screw to the next. The process was extremely laborious and time consuming, resulting in a very expensive fastener.
The late 1700s ushered in the era of mass-produced screws. This started with the development of machines that could mechanically and evenly carve the screw threads, which was a tremendous leap forward. Initially, the slot in the head of the screw still needed to be hand cut with a hack saw, but as time went on the technique was perfected and this step became part of the overall process. These first machine-made screws had blunt points, but by the late 1800s further innovations had lead to the development of mills capable of producing pointed screws.
Just as with nails, as new building materials and techniques are introduced, screw drives, heads, threads, and points have evolved to keep pace with the changing technology.
Pilot holes are used only with screws to ease driving into the material. A self-drilling screw virtually eliminates the need for a pilot point, but there are several instances where pilot holes are needed:
• When attaching to masonry.
• When working with aged hardwoods. Excessive driving resistance may be encountered around knots and tight grain. A pilot hole will prevent splitting and make driving the screw easier.
• When working close to the edge of material. Wood may split, depending on the grain of the wood. Drywall and particle board can develop distortions due to the compression of the material, resulting in reduced holding power.
A pilot hole should be equal to or just less than the root diameter of the screw thread. In hardwoods, pilot hold depth should equal screw length. In particle board and softwoods, pilot hole depth should be 2/3 of the screw length. This allows the leading two or three threads to penetrate solid material and insures adequate pull out strength.
Slot – Driven by a flat-bladed screwdriver. Earliest known style of drive.
Phillips – “+” shaped recess designed to cam out before being overdriven.
Square – Fits securely on driver bit, enabling one-handed fastening.
Combo – Accommodates either Phillips or square driver (bit).
Star – Six-sided drive easy on bits and provides better torque transfer than other drive designs.
Bugle head – Tapered, countersinking head compresses the surface of the material under it as it is driven. Ideal general-purpose fastener.
Flat head – Fits snug up against the material, unlike bugle head, which is designed to “sink” into the substrate.
Pan head – Flat bearing surface is ideal for attaching light-gauge steel to light-gauge steel (i.e., track lighting).
Trim head – For fine carpentry applications. Countersunk like finishing nails.
Washer head/modified truss – Provides a larger bearing area for working with softer materials.
Wafer head – Flat, wide head ideal for installing cement board.
Hex washer head – For H.V.A.C. (steel stove pipes and ductwork). Hex head can be driven with nut driver.
Hex washer head with neoprene washer – For steel roofing. Neoprene washer “seals” pieces together.
Coarse thread – Provides superior holding power when attaching to wood.
Fine thread – Provides superior holding power when attaching to light-gauge steel.
High-low thread – For attaching drywall to light-gauge steel studs or wood studs. Provide higher strip-out resistance.
Winged – Used with self-drilling point when attaching wood to steel or other hard material. Wings or “reamers” eliminate predrilling of the wood. Wings break off when they hit the second material to allow the threads to engage for a tight joinery.
Most screw drives, heads, and threads are consistent from one industry to the next. However, there is a more pronounced difference when it comes to screw points. Points designed for machinery, automobile assembly, and other heavy industry are not be included in this overview.
Wood Applications:
Sharp point – Standard screw point for fastening into wood and other medium to soft materials. May also be referred to as a “Type A” point.
Type 17 – Long, sharp point with flute, which creates its own pilot point as well as displaces some of wood pitch.
Spade point – Resembles a shovel or spade. Used primarily to attach cement board to wood or light-gauge steel. The spade point penetrates the board without cracking it.
Metal
Applications:
Self-piercing point – Needle point designed for use in relatively thin sections. The point literally stabs through the material, making a hole slightly smaller than the minor diameter of the threads. This allows the threads to extrude the metal for increased thread engagement and reduces the tendency to strip-out.
Self-drilling point – Point designed to drill through very thin sections too thick to be pierced. Point is either smooth (pilot point) or winged. Wings eliminate pre-drilling of wood.