A Beginner’s Guide to Micrometers

types of micrometers

What is a micrometer?

A micrometer is a measuring tool commonly used to check precise measurements in a variety of applications. Commonly referred to as mics, they are a very accurate measuring device. Micrometers are regularly used to take measurements to an accuracy of .001″, .0001″ or better in inches. Measurements in millimeters can be made down to .01mm or .001mm.

List of common micrometer types and their uses

  • Outside micrometers – measures various lengths, widths, thicknesses and diameters
  • Inside micrometers – measures hole diameters, slot widths
  • Depth micrometer – measures depth of holes, step locations
  • Thread micrometers – measures various thread characteristics
  • Crankshaft micrometer – specific measuring range for measuring crankshafts
  • Disc brake micrometer – measures thickness of brake rotors
  • Blade micrometer – measures slots, keyways and grooves

How to use a micrometer

Before using your micrometer, ensure that the measuring tool and surface to be measured are free of dirt, debris, chips, etc. The micrometer thimble should spin freely. 

The thimble should be spun until the gap between the anvil and spindle of the micrometer are far enough apart to be placed over the part to be measured. The thimble can then be rotated to close spindle on the anvil. This should be done while keeping the anvil perpendicular to the surface being measured. 

As the spindle closes on the part being measured, it can be beneficial to slightly rock the micrometer in an effort to seat the micrometer on the part. Depending on the surfaces being measured, this may not be recommended as it can damage the surface being measured. 

Multiple measurements should be taken to verify that the micrometer has yielded the true reading. For example, if a measurement is taken where the anvil and spindle of the micrometer are not perpendicular to the surface being measured then the reading obtained can be larger than the true size.

parts of a micrometer

How to read a micrometer

The most common varieties of micrometers read in increments of one thousandth of an inch (.001″) or one ten-thousandth of an inch (.0001″). The process of reading a measurement from either type is similar. 

Along the sleeve of the micrometer will be graduations similar to a ruler. The graduations at every fourth interval are most often numbered 0, 1, 2 and so forth. These numbers represent .100″ or one hundred thousandths of an inch. 

If using a 1-2″ micrometer, the graduation marked 6 would correspond to a measurement of 1.600″. The graduations between the numbers are each .025″ or twenty five thousandths of an inch. 

If we were to use a 4-5″ micrometer and obtained a measurement at the 3rd graduation after the .200″ mark, then our reading would be 4.275″. This would be the reading if the 0 on the thimble lined up exactly with the 3rd graduation after the .200 mark on the reading line. 

If instead the number ten lined up with the reading line and we could still see the 3rd graduation after the .200″ mark, then our measurement would be 4.285″. 

For micrometers that read to .0001″ we would additionally rotate the micrometer without turning the spindle to determine which numbers line up on the sleeve and thimble. If a number lines up on the thimble with the number 7 on the sleeve, our reading would now be 4.2857″. 

I realize the process can be confusing for some so use the formula and example below for guidance and give yourself a little practice measuring a known standard such as a gage block and you’ll get the hang of it.

micrometer reading example
In this example the tenths reading would be 3 or .0003"

Formula for micrometer readings

Base micrometer size + (.100″ x largest visible number) + (.025″ x graduations visible after the largest number) + (.001″ x reading from thimble) +(.0001″ x reading from sleeve for .0001″ micrometers)

Example for a 1-2″ micrometer

1.000″ + (.100″ x 4) + (.025″ x 2) + (.001″ x 3) + (.0001″ x 8) =

1.000″+ .400″ + .050″ + .003″ + .0008″ = 1.4538″

Uses for a micrometer

Micrometers have multiple uses. They are commonly used in many different forms to verify measurements in machine shops all over the world to make any and all types of products. 

Micrometers are also utilized by home mechanics and businesses alike to perform specific tasks such as measuring brake rotors, crankshaft diameters and other critical engine measurements in the automotive industry. Though calipers are more frequently used, micrometers are also used often by hobbyists to measure characteristics when reloading.

 

When to use a micrometer

Three of the most common precision measuring devices used by a hobbyist or a machine shop are calipers, micrometers and dial test indicators. In that order they start with the calipers having the largest measuring range and the lowest accuracy. 

Dial test indicator have the smallest measuring range and the greatest accuracy. Micrometers are smack dab in the middle where they provide relatively quick and accurate measurements. 

While very accurate, one downfall of the micrometer is that they are most commonly found in 1″ measuring range increments (3-4″, 4-5″, etc.). This means that multiple micrometers are needed to be capable of covering the measurer’s  measurement needs. Because of this micrometers are commonly sold in sets. 

A 0-6″ micrometer set will cover the needs of most applications while a 0-12″ set is more than most people, especially hobbyists will need.

What makes a good micrometer?

A good micrometer needs two things: precision and accuracy. Some adjustments can be made with most micrometers to account for small errors in accuracy but nothing can be done to fix a tool that isn’t precise. 

Quality micrometers will turn smoothly without any drag. This is the telltale sign of a good tool. If your micrometer ever feels like it is rubbing internally, we recommend disassembling the micrometer and cleaning per the manufacturers instructions to eliminate any possible contamination that may be causing the issue

Where to buy micrometers

Micrometers are available from a number of online retailers. For a more in depth guide of which micrometers are best for your situation, please see our reviews section. 

Some general advice, as usual for most products Amazon has a number of good options available. Harbor Freight has a couple good options that are reasonably priced and well reviewed. Home Depot and Walmart both sell micrometers but we do not recommend any that they currently offer.

Are cheaper micrometers as good as expensive ones?

While some of the cheaper (made in China) type micrometers have gotten much better than they were in years past, they are nowhere near the same quality that you will see in a micrometer from one of the tried and true manufacturers such as Starrett or Mitutoyo. 

A micrometer is the type of tool that is best to purchase once. In most cases it can be more beneficial to search for a used option on Craigslist or Facebook marketplace. Ebay can also be a good alternative. For more information on the best micrometers for your application see our Best Micrometers article.

How to adjust a micrometer

If your micrometer is in need of adjustment, most micrometers can be adjusted by using the wrench that came with your tool to spin the sleeve of the micrometer. This is usually done in the zero position. This can be especially useful for adjusting for the touch or feel of a mic when it does not include a ratchet or friction stop. 

If you no longer have a wrench or spanner for adjustment, replacement wrenches can be purchased from most manufacturers or on Amazon.

types of micrometer adjustment wrenches

How to calibrate a 0-1" micrometer

For more detailed instructions on the calibration of your micrometer, see our ultimate guide to micrometer calibration.

The short and sweet version is as follows:

  1. Verify that the micrometer is clean.
  2. Visually examine the micrometer for any condition that could cause errors in the calibration.
  3. Whenever necessary to disassemble for adjustment, use care and cleanliness to assure no damage to the internal threads of the tool.
  4. Close the micrometer by spinning thimble, use a ratchet or friction stop applicable.
  5. Hold the micrometer up to a light source and visually examine it for light showing between faces. If the faces are not parallel, light will show between them.
  6. Check the accuracy of the micrometer at various locations within the tools measuring range. Gage blocks which have been calibrated themselves should be used for this operation. Block sizes which are used should test the micrometer at different positions of the thimble and not only increments of .025″. This ensures the scale on the thimble is accurate.
  7. Adjustments can be made at this step as needed. Different micrometers have different procedures for adjustment. Consult manufacturer documentation for instructions regarding the adjustment of your micrometer if needed. Most often adjustments are made with a special wrench that will come with your micrometer. This wrench is used to spin the sleeve of the tool.
  8. After adjustments are made, the tool should be checked again to verify the adjustment worked.
  9. Calibration results are commonly recorded in a register or database for traceability of measurement history.

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