A Beginner’s Guide to Depth Micrometers

mitutoyo depth micrometer

What is a depth micrometer?

A depth micrometer (or depth mic) is a measuring tool commonly used to check precise measurements of slots, keyways, grooves and various other locations. They are a very accurate measuring device. 

Depth micrometers are commonly used to take measurements to an accuracy of .001″ or .0001″ in inches. Measurements in millimeters can be made down to .01mm or .001mm.

How to use a depth micrometer

Depth mics can be used to measure many different types of part characteristics. I will explain how to check a hole depth. 

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.  Place the tool on part over the hole. Spin the micrometer thimble until the rod extends to the bottom of the hole. Use the ratchet or friction stop if available on your tool.

depth micrometer rod
A depth micrometer rod

Note: The depth mic should be checked for accuracy whenever a rod is changed to measure a different size. It can be easy for something to get contamination in between the micrometer and the depth rod where they come together. 

Keeping things as clean as possible will help with this problem.

How to read a depth micrometer

I recommend a digital depth micrometer for ease of measurement especially if the measurer will only occasionally be taking readings with their micrometer. Unfortunately the price of a digital depth mic can be quite high so if you must use an analog micrometer then please keep reading. 

The most common varieties of depth 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 depth 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 depth micrometer with a 1-2″ rod, 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 depth micrometer with a 4-5″ rod 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″.

Formula for depth micrometer readings

Base depth micrometer rod 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 depth micrometer with a 1-2″ rod

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

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

When to use a depth micrometer

Depth micrometers while very accurate have one downfall. Depth micrometers like most standard micrometers are most commonly found in 1″ measuring range increments (3-4″, 4-5″, etc.). For a depth micrometer, this means that multiple sized rods are needed to be capable of covering the measurer’s  measurement needs. Because of this depth micrometers are commonly sold in sets. 

A 0-3″ 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 depth micrometer

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

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

Where to buy depth micrometers

Depth micrometers are available from a number of online retailers. For a more in depth guide of which depth micrometer is 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. Walmart sells depth micrometers but we do not recommend any that they currently offer.

Are cheaper depth micrometers as good as expensive ones?

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

A depth 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 depth micrometers for your application, see our Best Depth Micrometers article.

How to calibrate a 0-1" depth micrometer

  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. Spin the thimble until the depth rod is inside the tool.
  5. Place the tool on a surface plate and spin the thimble to extend the depth rod to the zero position. Use the ratchet or friction stop if available.
  6. Repeat the process by placing the depth micrometer on gage blocks and overhanging the tool to allow the depth rod to extend down to the surface plate.
  7. Check accuracy of the micrometer at various locations within the tool’s measuring range. Gage blocks which have been calibrated themselves should be use 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.
  8. Adjustments can be made at this step as needed. Different depth micrometers have different procedures for adjustment. Consult manufacturer documentation for instructions regarding the adjustment of your micrometer if needed. If adjustments are made, the calibration procedure should be started over to verify the adjustments were adequate. 
  9. Calibration results are commonly recorded in a register or database for traceability of measurement history.

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GD&T Symbols Quick Reference

A cheat sheet type reference for the most common GD&T symbols.  

See also our GD&T Font – GD&T Keyboard Shortcuts List

Symbol

Name

Description

Straightness

Straightness is how close to a straight line a feature is.

Flatness

Flatness is how flat a feature is. All points on the feature must lie within two parallel planes that are spaced the tolerance width apart.

Circularity

Often called roundness. Circularity refers to how close to a perfect circle a single location is. Circularity is at one location. This can be thought of as a single circle on a cylinder. Usually circularity would be checked at multiple locations along the cylinder. This cylinder can be the inside of a hole, the outside of a shaft or various other features.

Cylindricity

Cylindricity is the same as circularity (often called roundness) with the exception that the requirement applies across the whole surface instead of at a single location. Cylindricity works to control taper whereas circularity does not.

Parallelism

Parallelism refers to how close to 180 degrees two surfaces are.

Perpendicularity

Perpendicularity is how close to 90 degrees two features are. This can be any combination of planes or axes.

Angularity

Angularity is the same as perpendicularity with the exception that the two features are not at 90 degrees to one another but instead at a different specified angle.

Concentricity

Concentricity is how close the axes of two features run together.

True Position

True position is a theoretically exact location of a feature.

Symmetry

Symmetry is the same as concentricity but is applied to features that aren’t round. This means that the axes or centers of two features must run together.

Profile of a Line

Profile of a line controls the shape of a cross section of a feature. It can control size, form and location.

Profile of a Surface

Profile of a surface is similar to the profile of a line tolerance but it controls the entire surface instead of a single cross section.

Circular Runout

Circular runout controls the runout in a single location of a circular feature such as a cylinder.

Total Runout

Total runout controls the runout of an entire surface of a circular feature instead of at a single location. When compared to circular runout, total runout would check the entire cylinder.

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A Beginner’s Guide to Calipers – Dial and Digital

tesa dial caliper

What is a caliper?

A caliper is a measuring tool commonly used to check precise measurements in a variety of applications. The biggest strength of calipers is in their versatility. Calipers come in many forms, including digital, dial and vernier. They are commonly used to take measurements to an accuracy of .001″ or .01mm. Metric measurements can be made down to .01mm or .001mm.

Below is a list of the most common caliper uses:

  • Inside measurements – hole sizes, slot widths
  • Outside measurements – lengths, widths, diameters, thicknesses
  • Depth measurements – depth of holes, slots, step locations

How to use a caliper

Before using your caliper, check to make sure that the measuring tool and surface to be measured are free of dirt, debris, chips, etc. The body of the caliper should slide freely along the scale or bar. For an outside measurement, slide the jaws of the caliper open until they are far enough apart to be placed over the part to be measured. Now proceed to close the jaws while trying to keep the jaws perpendicular to the surface being measured. Multiple measurements should be taken to verify that the caliper has yielded the true reading. For example, if a measurement is taken where the jaws of the caliper are not perpendicular to the surface being measured then the reading obtained can be larger than the true size.

Note: Do not exert a large amount of force on the caliper in the direction of measurement. This can cause the tool to flex and distort the true measurement. It is best to place the same amount of force that is used to zero the caliper.

How to zero a digital caliper

To zero a digital caliper, close the jaws and check the reading of the caliper. If it reads all zeros great, you can stop here. Don’t be fooled into thinking that means your caliper is accurate, but it’s a start. For more info please see the how to calibrate your caliper below. If your caliper does not read all zeros then while in the close position press the zero or origin button. This button may have a different name based on the manufacturer of your caliper. If you are unsure of the proper button to use then check the manufacturer’s website. 

How to zero a dial caliper

To zero a dial caliper, close the jaws and check the reading of the caliper. If it reads all zeros great, you can stop here. Don’t be fooled into thinking that means your caliper is accurate, but it’s a start. For more info please see the how to calibrate your caliper below. If your caliper does not read all zeros then while in the close position, loosen the bezel screw nut. Spin the bezel until the caliper reads zero. Now tighten the bezel screw nut. Verify that the caliper still reads zero after tightening the screw.

How to read digital caliper

Reading a digital caliper is easy. The digital readout display clearly shows the measurement value obtained. Digital calipers are far superior to dial and vernier calipers in this regard. Digital calipers have the ability to quickly switch between metric and inch readings with fractional measurements available on some but not all digital calipers. The biggest downfall of this is the ease at which the zero setting can be changed on a digital caliper. Because of this it is best to check your zero setting at minimum each time you use the tool and if you are making a multitude of measurement, check the digital caliper occasionally during use.

If you need more help understanding the reading, see our article on Understanding Machine Shop Numbers & Values

How to read a dial caliper

Reading a dial caliper is easy, though not as easy as a digital caliper. Because costs have come down substantially in recent years, I recommend purchasing a digital caliper if possible. If a digital caliper isn’t in the cards either because of budget or because you are working with an inherited tool then keep reading. Dial calipers come in multiple varieties but most have their measurement read in the same way. Dial calipers usually have graduations along the bar that are in increments of .100″ or one hundred thousandths of an inch. Often the 1,2,3, etc whole inch increments are marked with a number and the .100″ increments will be marked with a line. To read the caliper combine all the visible whole inch and .100″ increments on the bar with the dial reading. Example: if you can see the 2 and 3 of the .100″ lines and have a reading on the dial of 46 then your caliper reading would be 2.346″.

If you need more help understanding the reading, see our article on Understanding Machine Shop Numbers & Values

Uses for a caliper

Calipers have multiple uses. They are commonly used to verify measurements in machine shops all over the world for a variety of products. Calipers are also utilized by home mechanics and businesses alike to perform specific tasks such as critical engine measurements. Calipers are a great all around measuring tool. More than any other precision measuring tool, they are capable of performing measurements on a large variety of parts. They have a larger measuring range when compared to micrometers and indicators. They are quicker to perform measurements when compared to micrometers or indicators. Go/no go gages are the only measuring tool that can take measurements faster but go/no go gages are very specialized.

What makes a good caliper

Whether digital, dial or vernier, a good digital caliper needs two things: precision and accuracy. Some adjustments can be made with most calipers to account for small errors in accuracy but nothing can be done to fix a tool that isn’t precise. A quality caliper will move smoothly without any drag. This is the telltale sign of a good tool. If your caliper ever feels like it is rubbing or dragging then it is most likely the result of damage from being dropped or contamination exposure. Unfortunately if you caliper isn’t moving smoothly there isn’t usually much that can be done besides oiling the tool and sliding back and worth. Then wipe off the oil and repeat the process over again. Make sure to consult the manufacturer’s instructions before performing this operation as calipers can vary and only use machine tool oil such as this one by Starrett.

Where to buy calipers

Calipers are available from a number of online retailers. For a more in depth guide of which calipers 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 calipers but we do not recommend any that they currently offer. 

Are cheaper calipers as good as expensive ones?

While some of the cheaper (made in China) type calipers have gotten much better than they were in years past, they are nowhere near the same quality that you will see in a tool from one of the tried and true manufacturers such as Starrett or Mitutoyo. A caliper 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. For more information on the best calipers to buy for your application see our Best Calipers article.

How to calibrate a 0-6" caliper

  1. Verify that the caliper is clean.
  2.  Visually examine the caliper for any condition that could cause errors in the calibration.
  3. Close the caliper by sliding the body of the caliper until the outside jaws are closed.
  4. Hold the caliper to a light source and visually examine for light showing between the jaws. If the jaws are not parallel, light will show between them.
  5. Check accuracy of outside jaws of the caliper at various locations within the tool’s measuring range. Gage blocks which have been calibrated themselves should be used for this operation.
  6. Check the accuracy of the inside jaws of the caliper at various locations over the measuring range. This can be done by locking a micrometer that is calibrated at a know location and checking the gap.
  7. Check the accuracy of the depth rod (if applicable) using gage blocks to set the caliper on and extending the depth rod down to the surface plate.
  8. Adjustments can be made at this step as needed. Different calipers have different procedures for adjustment. Consult manufacturer documentation for instructions regarding the adjustment of your caliper if needed.
    1. Dial calipers are most often adjusted in the zero position by loosening the dial lock and spinning the dial until the tool reads zero.
    2. Digital calipers are most often adjusted in the zero position by pressing the zero or origin button. This button can vary between manufacturers so check your instruction manual if you have one. If not, don’t worry it is usually very easy to figure out what button zeroes the caliper.
  9. After adjustments are made, the tool should be checked again to verify the adjustment worked.
  10. Calibration results are commonly recorded in a register or database for traceability of measurement history

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A Beginner’s Guide to Micrometers

types of micrometers

What is a micrometer? Basics for beginners

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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