Screws are used in a variety of applications from woodworking to metal roof installations. But choosing the right screw can be tricky, especially if you are not familiar with the three basic measurements for screws: gauge, length and threads per inch (TPI). The wrong size of screw will not only split wood, but may also impact the structural integrity of a project. The following are some general guidelines for selecting the correct type of screw for your job:
Screw diameter, or screw size, is indicated by the first number of a screw sizing chart. For inches, the diameter is measured at the widest point of the threaded part of the screw, called the major diameter. For metric screws, the diameter is given in millimeters and is preceded by the letter M.
The second number, which is often missing from screw sizing charts, indicates the number of threads in an inch of the screw. The more threads a screw has, the closer together they are and the higher the number of TPI. For wood screws, a number between 6 and 18 TPI is usually recommended.
Third, the length of a screw is indicated by a third number that follows the gauge and TPI numbers, and is generally abbreviated in a chart as either the head diameter or shank diameter. For instance, a screw that is labeled as 6 x 1 1/2 has a head diameter of 1 1/2″ and a shank diameter of 1″.
Many screw manufacturers also list the length of a screw from the top of the head to the tip of the shaft. Screws with round heads measure the length from below the head, while flat headed screws measure the length from the top of the head to the screw tip.
Lastly, screw heads come in a variety of shapes and configurations. They can be slotted, Phillips, square or Robinson and can be round, oval, or flat. Knowing what type of screwdriver you will be using to drive the screw can help determine what head style is best for your project. The most common types of screwheads are slotted and Phillips, though other options are available as well. Screws also have varying thread configurations to suit different needs. For instance, finish screws have finer threads than standard wood screws, while self-tapping screws have coarser threads to make driving them into place easier. Taking the time to understand these fundamental qualities can help you choose the perfect screw for your next project. screw size chart