Ballooning/Numbering A Blueprint

What is a ballooned or numbered blueprint?

ballooned drawing example

Commonly referred to by many different names including ballooned drawing, bubble drawing, numbered print, etc. A numbered drawing or blueprint is a way to identify individual attributes of a part or assembly as depicted in an engineering drawing. The numbers and balloons or bubbles are ordinarily done in red ink or a red font as seen above.

What are ballooned drawings used for?

Numbered drawings are a common component in the inspection process. They can be a part of your in house inspection procedure or a requirement which you provide to your customers. Ballooned drawings are also frequently used as part of a first article inspection report package. They allow the reader to connect an individual measurement to its location on the blueprint. This is especially handy when multiple attributes with the same nominal values are present.

How to number a drawing

Numbering or ballooning a blueprint is a process that has some flexibility to the order in which attributes are labeled. The most important aspect is that all attributes are assigned a number including all notes and general tolerances as needed. When in doubt, number it. Where to start is a matter of preference but make sure that your numbering sequence is easy to follow. Generally the person numbering the drawing will start in the top left view and work their way clockwise assigning numbers to attributes in that particular view. This process is repeated for all views present on the drawing in a top to bottom, left to right manner similar to the way you read a book. Some users will list attributes in the notes first such as material, but this is simply a matter of preference. Just make sure to number all the relevant attributes in the notes. If you work in a logical manner, it will be much easier for the customer or reader to follow along.

Example of a fully numbered drawing

What to include in a numbered drawing

Assign a number to every attribute on the print including all notes and applicable general tolerances. Some notes may include more than one attribute in a single note which requires a numbered attribute.

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Datums – All About

What is a datum?

A datum is a reference point for measurements. A datum is a theoretically perfect feature. It is often a main surface of a part called a plane but can also be a point such as the center of a diameter or an axis such as the center of a cylinder.

How are datums used?

Datums are used to orient measurements. In the case of something simple such as a perpendicularity specification, the datum is the surface against which the toleranced feature is checked. For a positional tolerance, features are located relative to a datum.

Symbol for a datum

Datum Blueprint GD&T Symbol a in a box with arrow

Datum names

Datum names are identified with a capital letter. The most important reference feature is usually identified with a capital A. In order of importance datums will be assigned consecutive letters (B, C, etc).

How is a datum used in a feature control frame?

feature control frame description with parts identified

Datums are placed at the end of a feature control frame.

Primary datums

primary datum identifier

A primary datum is the main locating surface used for alignment.

Think of a box set on a table. In this case the bottom of the box is the primary datum.

The table would be used to simulate the primary datum. In this example measurements could be taken from the table to other features of the box to verify they meet the required dimensions.

The primary datum controls one axis of freedom. The box can move back and forth or spin in place on the table but it can not turn end over end. This is similar to the way a granite surface plate is used in machine shops all over the world.

Secondary datums

secondary datum identifier

A secondary datum is the secondary alignment feature. In the box on a table example, if the table and the box were pushed up against a wall then the wall would be the secondary datum.

The box would contact the wall at a minimum of two points. Contacting the wall would constrain the movement in another axis.

The table controls end over end movement. The wall stops the box from spinning. It is however still able to move back and forth along the wall.

Tertiary datums

tertiary datum identifier
A tertiary datum is the third alignment feature. In the box on a table example, if the box on a table which is now against a wall was pushed into a corner then the new wall would be the tertiary datum. This new datum works to control the last remaining axis of movement.

What is a datum target?

A datum target is a specified location that is used to measure a part. It identifies the point(s) from which measurements will be taken.

The use of datum targets or points provides a level of control over the datum surface. Some surfaces are incapable of being used like a normal datum.

A very large datum surface or a surface that has a lot of variation in its form are two common reasons why datum targets are used.

Datum targets are also used to mimic real world use such as contact points in an assembly.

Datum target symbol

Datum Target Blueprint GD&T Symbol circle with diameter 3 in top and A1 in bottom half

The top half of the datum target symbol is often empty. The upper half lists the area of the datum target when specified. The lower half of the datum target symbol lists the datum target name. 

Is a datum real or theoretical?

Both really. The real datum is the actual surface referenced. This surface is imperfect and will have variation to it.

No surface is perfectly true.

In practice though a datum is used as a theoretically perfect surface.

A good example would be placing a datum surface down on a surface plate which is known to be very flat. The imperfect actual surface of the part will come to rest on the high points sitting on the surface plate. Measurements can be taken from the surface plate as if it was the datum surface itself.

Examples

true position callout

True position callout referencing datums A & B

perpendicularity callout example with feature control frame

Perpendicularity callout referencing datum A

circular runout callout

Circular runout referencing datum A

Datum dimensioning vs chain dimensioning

Chain dimensioning is the process of dimensioning features of one another in a row. Datum dimensioning is used when features are referenced from a common point. Take a look at the differences in the examples below.

datum dimensioning blueprint example
Datum Dimensioning
chain dimensioning blueprint example
Chain Dimensioning

A disadvantage of chain dimensioning is that the tolerances stack up. This means that the location of the far right hole can actually vary by as much as +/- 0.040”.

The example which uses datum dimensioning maintains the +/- 0.010” tolerance for each location. Chain dimensioning can be a perfectly acceptable way to tolerance your parts, just make sure you have taken into account the additional tolerance that can apply to the feature.

Plural of datum

Nothing too strange. More than one datum would be multiple datums.

Want to learn more?

GD&T is a complicated subject and understanding it correctly can be the difference between a perfect part and scrap.

The best way to learn GD&T is from experienced teachers who can break down the material into manageable pieces.

Luckily, we know someone.

And MachinistGuides.com readers get an exclusive discount on training!

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Feature Control Frames – All About

What is a feature control frame?

A key component of geometric dimensioning and tolerancing (commonly referred to as GD&T). On engineering blueprints, the feature control frame consists of a symbol to identify the type of tolerance, the amount of tolerance and reference datums if applicable.

How to read a feature control frame

A feature control frame is read from left to right. It reads “Type of control” of “Tolerance” to Datum. It should be noted that if a diameter symbol is present before “Tolerance” then it indicates the shape of the tolerance zone is cylindrical.

Examples

true position callout

True position of 0.2 to datums A and B

perpendicularity callout example with feature control frame

Perpendicularity of 0.001 to datum A

cylindricity callout

Cylindricity of 0.001

circular runout callout

Circular runout of 0.010 to datum A

Composite feature control frame

A composite feature control frame controls both a pattern on a part and the location of individual items in the pattern. 

The upper section of a composite feature control frame specifies the tolerance for the pattern to the overall part. 

The lower section specifies the tolerance for individual features to the pattern. In the example of a bolt hole circle, the upper section controls the tolerance for the location of the bolt hole circle on the part. The lower section would control how closely the individual holes must follow the pattern.

composite feature control frame

Feature control frame symbols

gd&t symbols
gd&t symbols

For more information see our GD&T Symbols Quick Reference

Basic dimensions

Basic Dimension Blueprint GD&T Symbol dimension in a box

Basic dimensions are identified by a rectangular frame around the dimension. 

They are dimensions that are theoretically exact. They do not have a tolerance themselves (general blueprint tolerances do not apply). 

Instead they are controlled by another characteristic. This is often seen with positional tolerances such as the true position of a hole. The hole location will be specified as basic dimensions. 

A true position tolerance will then be assigned to the hole which will control how far off the nominal location the hole can be.

Want to learn more?

GD&T is a complicated subject and understanding it correctly can be the difference between a perfect part and scrap.

The best way to learn GD&T is from experienced teachers who can break down the material into manageable pieces.

Luckily, we know someone.

And MachinistGuides.com readers get an exclusive discount on training!

Related Articles

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.

Related articles

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.

Want to learn more?

GD&T is a complicated subject and understanding it correctly can be the difference between a perfect part and scrap.

The best way to learn GD&T is from experienced teachers who can break down the material into manageable pieces.

Luckily, we know someone.

And MachinistGuides.com readers get an exclusive discount on training!

Related Articles

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 precision measuring tool

They are often used in the machine shop trade, as well other industries including manufacturing, automotive and construction industries. 

Micrometers are commonly called simply “mics”

Mics are a very accurate measuring device. Micrometers are regularly used to take measurements with an accuracy of .0001″ or better in inches. Measurements in millimeters can be made down to .01mm or .001mm.

List of common micrometer types and their uses

There are a ton of different micrometer types available

There are many different examples of micrometers that are used in specific industries. We even listed some that are used by engine builders and mechanics below.

  • 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

parts of a micrometer

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

The micrometer thimble should spin freely. No hangs up or anything similar.

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.

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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