Choosing the right screws for your project can be tricky. You’ll need to consider the thickness of the material you’re screwing into, the type of head you want and the thread size. The correct screws will provide a strong hold without breaking or working themselves loose. There are many different types of screws, each with their own specific purposes and characteristics. Using the wrong screw for your work can result in them becoming damaged, weakening or breaking. Choosing the proper screw for your material and application will prevent these problems.
Screws come in a wide range of sizes, from coarse to fine, and are available in metric and imperial versions. The differences between the systems are slight, but important to know if you plan to use either system. Metric screw threads are measured in millimeters, while the imperial system measures thread diameter and pitch with inches.
Imperial screw thread sizes are based on a unified standard of inch-based measurements developed by the American, Canadian and British screw thread standards committees after World War II. The standard defines various screw thread series including Unified Coarse (UNC) and Unified Fine (UNF). Each has a unique major diameter and thread pitch that are standardized.
The numbering system for imperial screw threads is a bit confusing, as each of the four common gauges has its own distinct meaning. In general, each number corresponds to a fraction of an inch, with the lowest numbers representing the largest diameter and the highest numbers representing the smallest. For example, a #2 screw has the same diameter as a #4 screw, but is referred to as a “lower” gauge because it has a smaller overall diameter than a #8 screw.
Metric screw threads, on the other hand, are based on a decimal system that is much easier to understand. Unlike imperial screw threads, where the size of a screw is determined by its major diameter and thread spacing, metric screw thread sizes are determined by the number of peaks on a one-inch-long section of thread. Measuring the number of peaks is easy, but determining the screw’s pitch requires more precision than measuring the major diameter. This is why using a digital caliper is recommended.
Wood screws are also available in both coarse and fine threads, as well as a variety of head types. The head of a wood screw is typically rounded or square, while some have a sharp point that allows them to be driven into drywall. Screw length is also an important consideration when selecting the correct screw for your job. Screws should enter the material at least half of the material’s thickness to avoid tearing or splitting.
In addition to the aforementioned considerations, there are other factors that can affect screw selection. For example, some woods have a natural tendency to swell and shrink over time, which can change the head-bore diameter or shank-hole size of a screw. This can require you to use a slightly larger screw than would be required in another wood type. 1/4″ to mm