Screws provide more holding power than nails and are much quicker to install. However, the wide variety of screw types and sizes can be confusing for novice woodworkers, especially when plans, instructions or specs reference specific kinds and sizes of screws. In order to select the right fastener for a project, you must understand four primary factors: driver type (flathead, Phillips head or square drive), length, shank diameter and threads per inch.
Most commonly, wood screws come in three different sizes: a gauge number, a shaft length in inches and a thread count per inch. In addition, many manufacturers designate the screw with a tolerance class, which describes how tightly the screw fits into the hole. For example, a size 1 screw fits very loosely, while a size 3 screw fit tighter. Finally, some screw sizes are also designated as left-handed, which indicates that the threads run in reverse.
The gauge number on a screw is the first number listed on its packaging, and it corresponds to a fraction of an inch in diameter. The second number is the shaft length in inches. The third number on a screw is the thread count per inch, and it specifies how close together the screw’s threads are. Screws with coarse threads have more threads per inch than those with finer threads.
In the United States, there are two systems for identifying screw sizes: the imperial system and the metric system. The imperial system uses a combination of numbers to describe the screw’s diameter and length, while the metric system specifies the screw’s diameter and pitch.
For instance, a screw with a gauge of 6 and a shaft length of 1 1/2 inches has a diameter of 7/64 inch, and its threads have a pitch of 2 mm. A screw with a gauge of 10 has a diameter of 5 mm and a pitch of 1.5 mm.
Screws are also categorized by the material into which they will be used. Drywall screws, for example, are distinguished by a curve on the junction of the head and shaft that prevents them from tearing through drywall. Other screws may feature a special finish or have a particular thread pattern that is suited to certain tasks, such as woodworking or metalworking.
The best way to determine the correct screw size for your project is to measure one of the existing fasteners in the area where it will be installed and then use a sizing chart to find the equivalent metric or imperial measurement. It is also helpful to have a screw set with a range of sizes, so that you can try out different fasteners and choose the best one. If you’re not sure which size to select, start with the lowest gauge number and work your way up until you find a screw that fits snugly but does not become stripped or damaged. Then, you can move on to the next step. If you have trouble finding the proper screw, consult a hardware store employee for help. screw size chart