How to use a Micrometer
A micrometer is a precision measuring tool that uses linear movement in order to measure the thickness, diameter, and length of an object. For this explanation, I will be using the outside style micrometer which features a "C" frame. Other types also include an inside and depth variations which do not include the "C" frame. They are available in both imperial and metric measuring systems and can also be found in vernier, mechanical digital, electronic digital, and dial measurement outputs. Below you will see photos of both the vernier and mechanical digital styles. Qualities of micrometers will vary between manufacturers, along with the accuracy as well.
Anvil: A fixed component, opposite of the spindle used as a measuring face.
Frame: A rigid construction which dictates the range of the range to which it's able to measure. The frame needs to be strong in order to help overcome any flexing and reducing expanding or contracting when exposed to different temperatures.
Spindle: A movable component across from the anvil used as the opposite measuring surface which is controlled by the rotation of the thimble.
Lock: Used to stop the rotation of the thimble and spindle to hold a measurement for reading.
Sleeve: A stationary part which has a linear scale and sometimes a vernier scale used to determine the measurement readings.
Thimble: The rotational cylinder which controls the opening and closing of the spindle, that helps determine the measurements in reference to the linear and vernier scales.
Ratchet: A mechanism with a slip release that allows the user to achieve a common pressure when measuring different objects.
When using a micrometer, always make sure the measuring faces are cleaning, along with the surface you are measuring. The accuracy of the micrometer should be checked periodically to ensure it's readings are correct by determining it's able to read a "0" value regardless of the size and also used a known measurement reference such as a gauge block. When taking a measurement of a part, the pressure applied to the surface can be done by feel or use of the ratchet. This does take some practice and experience when done by feel. There should be a mild drag on the surface, still allowing the part to move with mild resistance and not being clamped in place. The ratchet should also apply similar pressure and can be adjusted accordingly.
Once the achieved surface pressure has been applied, lock the micrometer so the thimble and spindle does not move. Using the micrometer shown in the photos, it has a 0"-1" range. Therefore our first digit of the measurement is 0.XXXX". If it were a 1"-2" micrometer, then the first digit would be 1.XXXX". In order to determine the first decimal place, using the sleeve and the edge of the thimble as the reference measurement the 4 is exposed (blue arrow). Therefore our current value is 0.4XXX". There are three lines in between each 0.1000" measurement which is 0.0250", 0.0500", and 0.0750" measurements. Each full rotation of the thimble is 0.0250". The reference edge on the thimble has also passed the 0.0750" increment line (green arrow), so far giving us a reading of 0.475X". There is a continuous linear line which runs along the sleeve, this is a zero line. The zero line is used to determine which increment on the thimble scale passes, here we have 9 (yellow arrow) which is a measurement of 0.0090". Taking our previous measurement of 0.475X", adding 0.0090", our current reading is now 0.484X". Moving over the micrometer to view the vernier scale, the best aligned line between the sleeve and thimble scale is 4 (red arrows). The 4 is our fourth decimal place, giving us a final reading of 0.4844".
Micrometers are a precise tool, therefore proper care should be taken in order to ensure they function and read correctly. Micrometers should only be used for measuring and never a clamp to hold an object in place. They should be stored in a dry place, limited from moisture which may cause rusting. It should never be dropped as this can bend the frame or damage a component affecting the accuracy. If a micrometer is dropped, have it professionally recalibrated and it may need the measuring surface grounded to achieve a square face. Usually, I like to handle my measuring equipment with clean hands so it reduces the risk of any dirt build up or particles falling inside any tight fitting components. For cleaning, they should be disassembled at certain maintenance intervals (depends on how much it is used or what environment it is exposed to) to remove any old oil or dirt that may cause sticking or harder operation. Use a light solvent, rubbing alcohol will suffice and agitate the surface with a soft brush. For lubricant, use a lightweight oil such as what is used on sewing machines, watches, or some precision tool manufacturers such as Starrett does make an instrument lubricant.