For wood screws that do not take a mating nut, like those used to fasten trim or molding, they are denominated with industry numeric sizes and do not include a thread count.
When a screw is described as 3/8-16, this means the screw has a major diameter of 3/8 inch and 16 threads per inch. Knowing these two measurements helps you select the correct screw for your project.
Length
The length of a screw is an important measurement to keep in mind when selecting a fastener for a project. Choosing a fastener that is too short can result in the screw stripping out of the substrate, while a fastener that is too long could cause damage to materials beyond the surface being joined.
The overall length of a screw is defined by the distance from the head to the tip or blunt end of the screw. For hex-, pan-, button-, and truss-head screws, the length is measured from directly under the head. For countersunk screws, the length is measured from the flat top of the screw head.
While determining the correct diameter, length, and gauge is easy, finding the right type of wood screw for a specific project can be challenging. This is especially true when working with plans, instructions, or specs that reference certain types and sizes of screws frequently. Fortunately, there are some general guidelines that can help.
Diameter
The diameter of a screw is the maximum width at which the threads reach. It is determined by the lead (/’li:d/) and pitch of the screw. The lead is the distance along the screw’s axis covered by one complete rotation of its threads, and the pitch is the distance between the crests of adjacent threads at the same point.
Screws come in a wide variety of sizes to accommodate a number of construction tasks. The wrong size can split wood or affect the soundness of a structure, so it’s important to understand how screws are measured.
Most imperial system screws list the gauge first followed by a length measurement, like “10 x 2”. Sometimes the threads per inch, or TPI, is included between the two measurements. For example, the Senco Duraspin washer-head screw has a #8 gauge and 8 TPI.
Threads
The threads on screw heads create the gripping surface for fasteners. They can be coarse or fine, which allows for precise adjustment when turning a bolt or nut. They can also be straight or angled. A straight thread is more easily cut with a wrench but offers less friction than an angled one.
Screws come in different thread standards, depending on the country and industry. Some use the Unified Thread Standard (UTS) while others, such as the automotive industry in the United States, have fully switched over to ISO metric threads.
When determining screw size, you must take into account both the diameter and the thread pitch or number of peaks on a single inch of length. The first number of a screw’s gauge is the major diameter, while the second number indicates the number of threads per inch. This is the distance from crest to crest. Both UTS and ISO metric screws have pre-determined, standardized thread pitches.
Material
The material used to make screws has a big impact on their strength, durability and appearance. Screws are available in a variety of materials, including stainless steel, brass and aluminum, as well as different coating options.
Choosing the correct screw depends on a number of factors, including the thickness of the material being screwed into, the job at hand and the intended use. For example, screws used in wood are typically made of low-carbon steel, while machine, sheet metal and drywall screws might be made of medium or high-carbon steel, or even a different material altogether such as aluminium.
Another consideration is the screw’s tip, which can be either sharp or blunt. A sharp tip requires less force to drive into the material and produces a smaller hole, while a blunt tip reduces the chance of splitting the material. Screws may also have a threaded shank or helix.5/16 to mm
