Screws are generally made from low to medium carbon steel wire, but other tough and inexpensive metals may be substituted, such as stainless steel, brass, nickel alloys, or aluminum alloy. Quality of the metal used is of utmost importance in order to avoid cracking. If a finish is applied to the screw, it must be of a compatible makeup. Steel may be coated or plated with zinc, cadmium, nickel, or chromium for extra protection.
On a single thread screw, the lead and pitch are identical, lead is twice the pitch on a double thread model, and three times as much on a triple thread. The pitch of a screw is the distance between two threads (or grooves) from the same point on each thread. It is also more commonly known as the number of threads per inch or centimeter. The lead of the screw measures how far it is driven in for each revolution.
Machining is only used on unique designs or with screws too small to be made any other way. The machining process is exact, but too time consuming, wasteful, and expensive. The bulk of all screws are mass manufactured using the thread rolling method, and that is the procedure described in further detail.
- 1 Wire is fed from a mechanical coil through a prestraightening machine. The straightened wire flows directly into a machine that automatically cuts the wire at a designated length and die cuts the head of the screw blank into a preprogrammed shape. The heading machine utilizes either an open or closed die that either requires one punch or two punches to create the screw head. The closed (or solid) die creates a more accurate screw blank. On average, the cold heading machine produces 100 to 550 screw blanks per minute.
- 2 Once cold headed, the screw blanks are automatically fed to the thread-cutting dies from a vibrating hopper. The hopper guides the screw blanks down a chute to the dies, while making sure they are in the correct feed position.
- 3 The blank is then cut using one of three techniques. In the reciprocating die, two flat dies are used to cut the screw thread. One die is stationary, while the other moves in a reciprocating manner, and the screw blank is rolled between the two. When a centerless cylindrical die is used, the screw blank is rolled between two to three round dies in order to create the finished thread. The final method of thread rolling is the planetary rotary die process. It holds the screw blank stationary, while several die-cutting machines roll around the blank.
- All three methods create higher quality screws than the machine-cut variety. This is because the thread is not literally cut into the blank during the thread-rolling process, rather it is impressed into the blank. Thus, no metal material is lost, and weakness in the metal is avoided. The threads are also more precisely positioned. The more productive of the thread-rolling techniques is by far the planetary rotary die, which creates screws at a speed of 60 to 2,000 parts per minute.
The National Screw Thread Commission established a standard for screw threads in 1928 for interchangeability. This was followed by an international Declaration of Accord in 1948, adopting a Unified Screw Thread system. The standards focus on three main elements: the number of threads per inch, the designated pitch and shape of the thread, and designated diameter sizes. In 1966, the International Standards Organization (ISO) suggested a universal restriction on threads to ISO metric and inch size ranges with coarse and fine pitches. Compliance with the ISO suggested standards has been global.