The goal of a screw is to pull two materials together and fasten them in place using compression. Screws can be removed later with minimal damage compared to glue, rivets or welding. Screws also allow wood to expand and contract with humidity.
Screws are harder and more rigid than most nails. They are resistant to “draw out” or being pulled out of a material. The head presses down while the threads pull upward. Screws can be shorter than nails for equal structural integrity.
Screws are more accurate and easier to install in volume than before and have
become more important in various construction projects since the 1970s.
Components of a screw
Head: the top of the screw. It determines where the screw will stop
moving forward into the construction material. There are two components: shape
and drive type.
Shank: the main body of the screw.
Threads: wrap around the shank and move the screw forward. Screws can be
partially threaded or fully threaded.
Tip: pointed tips to go into wood or metal, self-drilling tips have a
split or bit tip, machine screws are squared off with no point.
Head types
Flat head: has a flat top to be flush with the material surface.
Pan head or “raised head”: sit on top of the material. Used when affixing metal or plastic to other materials. Includes truss, round, hex, button, capped and integrated washer.
Trim head: very small diameter head used for finishing work with lighter materials.
Shank The diameter of a screw is indicated by a number such as #00, #6 or #10. This number is the first number on the box. It is often followed by thread count (pitch angle) and length.
The screw gauge is a measurement of the outside thread diameter on the shank. It is an industry agreed upon scale rather than tied to any specific measurement system (metric or imperial).
A flat head screw length is measured from the top of the head to the tip for countersink purposes. All other screws are measured from the bottom of the head to the tip.
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A variety of screws include (from left to right)
zinc electroplated slotted machine , carbon steel
flat head machine screw, Galvanized hex head
lag screw/bolt, and bugle head drywall screw. |
Types of thread Threads are normally 24 or 32 per inch. This is important when working with machine screws that may need a matching nut.
Some screws are simply labeled as coarse or fine thread. Fine threads are for hardwoods and coarse threads are for softwoods and materials (sheetrock).
High thread count machine screws have more surface area than low thread count screws. While they take longer to install and remove, they hold better for the same depth.
Tips
Square: designed to be used with tapped (threaded) holes or with a nut. These are machine screws.
Pointed: pierces the material to start.
Self-starting: is cut or shaped like a drill bit to be self-tapping. Depending on the style and hardness, they can drill through an existing metal screw to repair a previous mistake.
Types of screws The materials being held together often dictates the best kind of screw for the job. Meanwhile the screw composition is determined by the environment. The driver style can be a preference but can come with added expense.
Wood: mostly flathead with a semi-threaded shank and pointed tip. Most are Phillips head in the U.S.
Drywall: flathead with bugle shape head bottom to avoid tearing drywall paper. They have a fully threaded shank and pointed tip. They are good for rough work. These are inexpensive and have a coarse thread to go into most material quickly and easily. They have thinner shanks than wood screws. They are typically coated in black paint. They are brittle and meant for soft woods. They are not structural because they break and corrode easily.
Multi-purpose (production): made with hardened steel. They have a flat head with a beveled head bottom. They also have a self-drilling point, are semi-threaded and have recessed specialized driver style. Often eliminates the need for pilot holes. These cost more but save time and frustration.
Deck: flathead with bugle shape head bottom. They have a fully threaded shank and pointed tip. They have a corrosion resistant coating.
Pocket: pan head with a semi-shank and self-drilling tip. These are designed to go into recessed areas and avoid splitting wood (fine furniture).
Machine: pan or flat head with fully threaded shank and no point. They are designed to go into tapped (threaded) or predrilled holes with a nut. Sold in threads per inch.
Sheet metal: tiny screws with a pan head, fully threaded shank and sharp point intended to pierce metal.
Masonry (cement): These require a pilot hole but can bite into concrete to pull materials tight. They are necessarily hardened, and a drill driver will break them off at moderate to high torque.
Lag screw/bolt: robust screws intended to connect heavy lumber to other materials and bear an intense load. Lag screws require a drilled hole and only have hex heads to apply enough torque.
Carriage bolt: used to fasten metal to wood or metal to metal. It has a shallow, mushroom top with a square section on the bottom of the head before the shank to make the bolt self-locking when seated into a squared recess. Often used to affix iron plates to wooden beams or to bare timber itself.
Screw Composition In addition to purpose and strength, the environment around the screw determines the best screw choice. Is the screw structural? Must it bear a heavy load? Will it be in salt water?
Carbon steel: most common.
Stainless steel: designed to avoid corrosion. Best for use near water – especially salt water. Very expensive and not as strong as deck screws.
Zinc plated: most used fastener material (machine screws and bolts).
Galvanized: alloyed steel coated to resist regular water corrosion.
Brass: does not rust, but it will corrode or tarnish if not protected with lacquer. It has no magnetism.
However, they rust as easily as steel or drywall screws when exposed to water.
Ceramic coating: exterior grade and is the second best to stainless steel for corrosion resistance.
Silicon bronze: for saltwater applications. No magnetism.
Titanium: have high strength-to-weight ratios and corrosion resistance. Titanium metal exhibits higher tensile strength at increased temperatures than many other materials. Mostly used in aerospace, military and commercial maritime applications due to expense.
Grade A (yellow electroplated zinc): like drywall screws but treated to resist the chemicals used in lumber products.
Exterior rated: resist caustic and corrosive chemicals used in pressure-treated lumber production.
Driver Styles (heads)
Slotted: the original, single slot screw turned by a flathead screwdriver. Be careful with torque because these are designed to strip to prevent overtightening.
Phillips: became popular in the 1930s and are an improvement from the slotted screw head. These are intended to use less torque to possibly be removed at some point. They are also designed to “cam-out” (slip the tool out of head slots) to prevent overtightening.
Star Drive (Torx): uses a recessed 6-pointed asterisk shape driver head. It is the preferred driver by most serious craftworkers. It ensures the driver stays connected without “cam-out.” Additionally, this style accommodates high torque. First used in an Apple computer to prevent average person from opening the machine.
Hex: same diameter and similar to star drive. The contact with the screw is at a greater radius, which allows more torque for the same force. However, some of the force is applied outward, which can damage both the tool and the screw.
Square Recess also called Robertson Screws: more common in Canada. Was an improvement over Phillips heads but not as good as star drives and has lost popularity in construction. This style of screw saved Ford about two hours of production time on each Model-T.
Other specialty heads include: Trimmit, Frearson, Pozidriv, Clutch, Hex, 12 Point, Hex Socket, Spanner, One Way, and Spline Socket.
Additional items
Washers: A washer is placed around a screw shaft between the material and the screw head or nut. The main purpose of washers is to increase the surface area of downward pressure or distribute the load over a larger area. The various kinds of washers include flat, fender, finishing, split lock, external tooth lock, internal tooth lock, square, dock and ogee.
Nuts: Nuts are most associated with machine screws and bolts. The nut applies compression pressure to the tip end of a screw or bolt shaft. They are precisely tapped pieces of metal that are designed to tighten onto a screw or bolt. They can allow the shaft to pass through or be capped to limit or cover the shaft.
Types of nuts include wing, cap, acorn, hex, heavy hex, nylon insert lock, jam, nylon insert jam lock, flange, tee, square, prevailing torque lock, coupling, slotted, castle, and K-Lock or Kep.
I’ve Got Your Six!
Mark M. Hancock, GRI, MRP, AHWD
REALTOR, New Build certified
214-862-7212 (call or text)
markhancock@judgefite.com
DFWmark.com
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