A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for accurate measurement of components in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and digital calipers. Micrometers are usually, but not always, in the form of calipers. The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil.
Modern micrometer with a reading of 1.639 ± 0.005 mm. Assuming no zero error, this is also the measurement. (One may need to enlarge the image to read it.)
Outside, inside, and depth micrometers. The outside micrometer has a unit conversion chart between fractional and decimal inch measurements etched onto the frame
Gascoigne's Micrometer, as drawn by Robert Hooke, c. 1668
Another large micrometer in use
A screw is an externally helical threaded fastener capable of being tightened or released by a twisting force (torque) to the head. The most common uses of screws are to hold objects together and there are many forms for a variety of materials. Screws might be inserted into holes in assembled parts or a screw may form its own thread. The difference between a screw and a bolt is that the latter is designed to be tightened or released by torquing a nut.
An assortment of screws, and a US quarter for size comparison
A Brown & Sharpe single-spindle screw machine
A machine screw
Lag screw, also called a lag bolt