A thread form, as relating to screws, is a spiral-shaped (helical) structure that wraps around the body of a screw. It serves a double purpose. The first is converting between rotational and linear force. Simply put, when the screw is twisted, it moves forward. This property is highly useful both because rotational force is easier to apply than directional force and because directional force can
. shatter fragile objects like wooden planks or glass. The second purpose of the screw thread is to hold the screw in place once it is attached. Nails are great for holding things together as long as the force acting upon them is lateral. Any other forces can easily pull the nail out and destroy whatever structure it was supposed to hold together.The screw's lead determines a lot of its mechanical advantage. In layman's terms, a lead is the distance a screw can travel under one rotation. A bigger lead allows the screw to be put in with fewer turns, but it also makes it far more likely to come loose and unravel. Another thing that sets screws apart is their handedness. Simply put, a screw's handedness is the direction in which it turns in order to tighten. By far, the most common handedness of screws is right. This means that the screw is turned right to tighten and left to loosen. There are some left-handed screws out ther, but they are not terribly common. Most of the time, left-handed screws are found on items where the screw's casing is constantly turned left. If these screws were not left-handed, they would unravel during the lifetime of the device. The most common place for this type of screw is in in bicycle pedals. These pedals constantly turn left, so it is smartest to have the screws be left-handed.For a calculator, check out this site:http://www.americanmachinist.com/Calculators/ThreadCutting.aspx More reference links: http://www.americanmachinist.com/Calculators/ThreadCutting.aspx http://www.engineersedge.com/fastener_thread_menu.shtml
