A screw connection is a form-locked connection used for joining two parts in a detachable manner without damage. The 'locking' of the thread is ensured by matching thread geometry and dimensions on the two mating elements. Typically, one of these threaded elements is the externally threaded screw, while the other, containing the internal thread, is either a nut or one of the elememts to be joined.

The screw thread is derived using a helical path. Its creation can be imagined as a cutting tool moving along a helix with constant radius and pitch, cutting into a cylindrical body. The tool removes material along this helical path to create the thread.

The radius of the helical path determines the thread diameter (symbol: r). The axial distance moved after one complete revolution is called the lead (symbol: P).

To create an external thread, material is removed from the outer surface of a cylindrical body; for an internal thread, material is removed from the inside of a cylindrical bore.

The geometry of a thread is influenced by:

• the thread profile (shape),
• the thread lead (pitch),
• the number of thread starts.

Beyond the thread type, screws come in many varieties. Each design serves a specific purpose, making it suitable for its intended application.

Screws may differ in:

• head design,
• thread length,
• thread position on the shaft.

 Hexagon head srew 

The most commonly used type of screw is the hexagon head screw.

 T-head screw  

The T-head screw has a flattened head section that, when pressed against a flat surface, prevents the screw from rotating. Connections using T-head screws are frequently used in the fastening of cast elements.