Micrometer
This tool is used to measure sizes with great accuracy. The most commonly used micrometers can measure to one hundredth of a mm (+/-0.01mm). The micrometer is generally used for measuring external sizes although other versions are available which can measure internal sizes or the depth of holes.
How to read a micrometer scale
How to read a micrometer scale
Reading the scale can be tricky to start with but once you get the hang of it you will be able to quickly and accurately check the size of your work making sure you are producing work to tolerance. More information about micrometers can be found here. A good explanation of how to read the scale can be found at technologystudent.com.