A screw gauge, also known as a micrometer screw gauge, is a measuring instrument used for precise measurement of small distances or thicknesses. It consists of a calibrated screw with a known pitch (the distance between adjacent threads) and a thimble with a circular scale.

The main components of a screw gauge include:

1. Frame: It provides the structure and support for the screw gauge.

2. Anvil: It is a stationary end against which the object being measured is pressed.

3. Spindle: It is a movable end attached to the screw that moves as the screw is rotated.

4. Sleeve: It houses the screw and has a linear scale marked on it.

5. Thimble: It is attached to the sleeve and has a circular scale marked on it.

6. Ratchet: It allows for consistent and controlled application of pressure when measuring.