Screws

A truly ingenious invention is the screw, but the simplest ones always are. We usually think of screws first as fasteners, but actually the “cylindrical member with a continuous helical rib” appears to have started out as an irrigation device in Egypt and was only later “invented” by the Greek Archimedes in the 3rd century BC (other Greeks of the period are also given credit). Undoubtedly many Greeks would disagree as to the origins, but for the matter at hand we’ll leave the attribution of credit for the invention to the historians and concentrate on more contemporary matters. You see, the subject of screws is both complex and promising, as screws are used everywhere and aren’t likely to be replaced by more complex “high-tech” inventions anytime in the foreseeable future.

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Adapting the tool to the situation
Types
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Adapting the tool to the situation

As technology has progressed, it has also created both the demand for and the means to produce screws that are more and more adapted to precise situations. In other words, you wouldn’t use drywall screws to build fine furniture or lag bolts to install that very same drywall, so be sure to choose the right fastener to make the job easier and help assure its success. Thus, before buying screws, ask yourself the following:
  • Am I working indoors or out? If you’re outside, you may want to opt for galvanized, stainless steel or copper screws that present varying levels of corrosion resistance and compatibility with the surfaces being fastened.
  • What type of materials am I working with? There are screws for fastening everything from drywall to wood, to masonry, to metal, etc.
  • Am I working with heavy or light materials? Why go to the expense of using heavy, and consequently more expensive fasteners, when light ones will do the job. The opposite is also true; that workbench you’re building won’t stand up to tough treatment if it’s held together with one-inch screws.

    Screw sizes are first expressed in “gauge” that varies from 2 to 24 which is an expression of the hole size that the screw will make. A larger gauge number designates a larger screw. The actual screw shank diameter usually varies from 3/32 to 17/64 inches. For instance, an # 8 screw will usually have an 11/64-inch shank diameter. Screws generally measure from ¼ to 6 inches long. A third number on some packages indicates the number of threads per inch for metal screws only. As for wood the thread classifications are limited to fine, medium or coarse.

    Screws are generally stronger fasteners than nails and a screwed structure is easier to dismantle without damage than a nailed structure. There’s also a whole list of screws that fall under the category of special requirements for the aeronautic, automotive and marine industries.

    • Types

    Since screws can be custom manufactured to any size and specification, for the purpose of this article we will be outlining those types that are mass produced and readily available on most store shelves.

    Screws can be divided into two main categories, wood and metal, with sub-categories for drywall and masonry. There’s also a whole list of screws that fall under the category of special requirements for the aeronautic, automotive and marine industries.

    Wood screws

    The main difference between wood and metal screws is found in the threading of the shanks. Wood screw shanks are only partially threaded, about 75%. Wood screw heads come in three main driving configurations, slotted (single slit), Phillips (Ph) and Pozidriv (Pz) (star-shaped) and Robertson (square), and in three main shapes, flat, oval and pan (round). As well, there are also lag screws, with square or hexagonal heads that are used to fasten heavy objects to wood. With hardwood choose hi-lo threads.

    Treated wood screws are used more and more for building decks, fences and other outdoor structures. The heads are self-countersinking and drilling pilot holes for these long, thin screws isn’t usually necessary. Anti-corrosion treatments are either ceramic or zinc coatings. These screws come in green, brown and grey to match the wood colour. Although more expensive, stainless steel is also effective for outdoor use and on cedar.

    Metal screws

    Metal screws are threaded from point to head along the full length of the shank. Metal screw heads come in different driving configurations i.e., hollow six-sided (for use with an Allen key), slotted (single slit), Robertson (square) and hexagonal, and in flat and oval head shapes. Metal screws also come with self-tapping and drilling points for work in sheet metal.

    Self-tapping screws

    Self-tapping screws are short and thick and when used in sheet metal cut their own matching threads through the raw material. When large gauge screws are being used, corresponding holes must be pre-drilled to a slightly lesser diameter than that of the screw. Self-tapping screws are also used in such materials as fibreglass, plastics, resin-impregnated plywood and asbestos board.

    Masonry screws

    Masonry screws are designed to attach “other” materials to concrete. They’re made of tempered steel and the threads are large and sharp to bite into concrete. Masonry screws come in regular and self-tapping versions.

    Drywall screws

    Drywall screws are thin, tempered and very sharp. They’re threaded from point to head and are designed to embed themselves completely below the drywall surface. Threads are widely set apart for screwing into wood studs and have medium threads for screwing into metal. Drywall screw heads are always the star shaped Phillips variety.

    Particleboard screws

    Particleboard screws have large, deep, sharp threads that prevent the screw from being torn out and to cut through the board’s glue.

    Double ended studs and hanger bolts

    Double ended studs and hanger bolts are specialized combinations for such applications as table leg attachment (the former) or joining other materials such as metal to wood (the latter).
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