Chapter 4 : Metrology

Overview

The U.S. Secret Service is tasked with combating the industry of counterfeiting money. One of the strategies utilized in identifying fake money is through the practice of visual identification. Through the process of inspecting authentic paper bills, the agent develops a keen eye for specific details. This process is repeated time and again. The agent’s skills become so refined that when a counterfeit bill is encountered, the agent can spot the fake immediately. The experienced machinist becomes conditioned in the same manner. A major skill mastered by all machinists is measuring and evaluating dimensions or the size of a feature called out on a print. It is not uncommon for a machinist to look at a feature and be able to give you a very close approximation of its size. This skill is often referred to as using the “eyecrometer” to measure.

One of the main criteria for manufacturing a part is the ability to measure the finished part. Engineers develop parts that perform under specific tolerances. Unlike a U.S. Secret Service agent, the machinist must possess the ability to use an abundant selection of inspection equipment and record those results for further verification.

After this chapter, you will begin your journey toward mastering the art of precision measurement, beginning with a basic understanding of measurement principles.

 

 

4.1 Decimal Numbers

As machinists, we live in a world ruled by decimals. This is true whether we are using the metric or inch unit of measure. Whole numbers are relatively easy as they are the complete quantity of units we are measuring such as an inch or a millimeter. When we manufacture parts with precision tolerances, we must practice principles that support this effort. Precision is a term you will hear quite a bit in the manufacturing industry and it refers to the process of being exact or accurate. You are entering into a trade which has its own language that is based on numbers and accuracy. Accuracy is how close a measured feature is to the intended dimension. Read this chapter as often as necessary to master this language.

4.1.1 Decimal Theory

Decimal place refers to how many digits after the decimal the machinist needs to take into consideration when measuring a dimension. They are directly related to the level of resolution required by the design engineer. Resolution is the smallest increment of measurement in a numerical value or measurement. In general, the more decimal places a number has, the finer the resolution, and the higher tolerance translates to less deviation from the stated value.

When using decimal places to express a value, each digit to the right of the decimal point represents a fraction of a whole unit based on powers of 10. For example:

One decimal place: .1 represents one-tenth (1/10) of a unit. There are ten of these .1’s in an inch. .1” is a little smaller than an 1/8th of an inch or .125”

Two decimal places: .01 represents one-hundredth (1/100) of a unit. There are 100 of these .01’s in an inch. This is about 3 sheets of copier paper.

Three decimal places: .001 represents one-thousandth (1/1000) of a unit. This is why we call these thousandths. And you are now in the base language of the machinist. A thousandth is about the size of a human hair.

Four decimal places: .0001 represents one ten-thousandths 1/10,000) of a unit. This resolution is rather hard to get your head around because we run out of objects to describe it. We need powerful magnification to see things this small. Because heat can cause a part to grow by tenths (.0001”), micrometers have plastic covers on them to isolate the heat from your hand, which would affect the dimension. A micrometer is a gage that reads small distances.

4.1.2Talk Like a Machinist

Traditional counting is based on the power of ten. Zero through nine repeat every ten numbers. Because of this, tradition reads decimal places as divisions by ten, and the first decimal place is the tenth’s position and is read as tenths of an inch. This convention works just fine for many occupations, such as the weather reporter on the local news. If it rained .1” last night, they would report “a tenth” of an inch of rain fell last night. And for weather reporting, this works fine because the weather is not an exact practice. In fact, it may just be the only job where you can be wrong 50% of the time (National Oceanic and Atmospheric Administration, 2024) and still keep your job.

A problem arises when machinists work with .001” (one thou) tolerances most of the time. The practice of working with such high tolerance has created a machinist vocabulary when referring to decimal numbers. The language developed as the base 1000 system, in which all numbers are expressed as if they were in thousands or three decimal places, i.e., .001”.

How a machinist reads these inch numbers is as follows:

.1= one hundred thousandths (one hundred thou)

.2= two hundred thou

.01= Ten thousandths (ten thou)

.03= thirty thou

.001= one thousandths

.125= a hundred and twenty five thousandths

.0001= one ten-thousandths (one tenth)

.3751= three hundred and seventy five thou and one tenth

.3755= three hundred and seventy five thou and five tenths

Notice we save the term “tenths” for four decimal places, where traditionally it would be used for the first decimal place. Tenths are outside the intent of this book and entry level machinists, so we will stick to the thousandths as the highest resolution for our projects.

4.2 Measuring Equipment and Decimals

What piece of measuring equipment should you use? The answer is based on a few details, such as:

• What type of dimension is it? Length, depth, radius, diameter, Inside Dimension, Outside Dimension…
• Do I need a gage or direct measurement instrument?
• Are these space limitations? Measuring an o-ring groove, for example.
• And finally, what is the resolution of the dimension? (.1, .01, .001, …)

Measuring equipment is designed to be used for specific purposes and, in some cases, has a singular purpose. One of these instances is when measuring threads to ensure they comply with an engineer drawing. For example, a ½-13 UNC 2B thread gage can only verify that specific class of fit thread and requires many different gages to verify different sizes and classes of fit. Threads and classifications will be discussed later.

On the other hand, the slide caliper can be used for four different measuring types and is good for three decimal places in an experienced hand. A slide caliper is a measuring tool used to measure inside, outside and depth dimensions utilizing a set of measuring surfaces attached to two different mechanisms which have a sliding relationship to one another.

A gage is a specific type of measuring equipment that is primarily used to measure a single quantity or attribute. The figure above is of a radius gage. This radius gage is used to measure .260” radius.

Due to their singular function, gages typically come in sets, which can make this type of measuring equipment more expensive to purchase due to the singular purpose of the tool. Gages can be used as a Go/No-Go gage. A Go/No-Go gage will Go into a properly sized feature, and a No-Go gage will not go into a feature of the correct size. It may seem strange that, as machinists, we do not always know what size a dimension is. As machinists, we are responsible for verifying that dimensions are within tolerance. The use of a properly set-up Go/No-Go gage provides us with this information. We will discuss more gages and how this is performed later in this chapter.

The figure above has a drawer full of assembled Go/No-Go Pin gages designed to gage holes. Many Go/No-Go gages are locally assembled for specific dimensions, where the operator will ensure the Go gage fits the feature and the No-Go gage will not. Notice this practice does not provide the specific dimension of a feature but only provides an IN tolerance or OUT of tolerance feedback. If I need to verify a tap drill hole for a ½-13 UNC 2B thread, a machinist can use a Go/No-Go pin gage. If the tap drill is 27/64ths (.4219), and let’s assume there is a three decimal tolerance of ∓.005” (.427-.417), we can set up a .417 Go and .428 No-Go gage pin set to verify this hole feature.

The above figure is an example of needing a specific piece of inspection equipment to measure a dimension. When measuring small grooves, a blade micrometer is the only tool that will fit, and it has a resolution for four decimal places.

Direct read measuring equipment provides numerical values or readings that represent a measured quantity.

Direct read measuring equipment has the added value of measuring multiple dimensional values, which makes these tools more versatile than gages. In the example above, we know the diameter of the bolt is .616”.

The number of decimal places in the dimension is the last concern for selecting the correct measuring equipment.

Remember the rule of 10X. The 10X rule requires measuring equipment to be ten times more accurate than the dimension being measured. Remember, ten times means one additional decimal place. If you must measure a two place decimal, then you may use a caliper because a caliper measures to the thousandths (0.001”) or three decimal places.

In the above figure, all the dimensions are two decimal places. This will require measuring equipment capable of accurately measuring to three decimals, such as a slide caliper.

The above dial caliper is capable of a .001” resolution, as indicated by the information outlined in the red box.

The display has a legend, which explains the resolution of the caliper. The common legend used is two arrows pointing inward toward each other that touch two vertical lines (see red box). The vertical lines represent the resolution lines (tic lines) around the dial, which is meant to represent that every line equals 0.001”. This dial is rather self-evident that the finest resolution is 0.001” which means it can be used for any dimension of two decimal places or less.

AUTHOR’S TIP

2.Press the ABS/INC button. This will establish a temporary zero. Note: the “INC” indicator is visible

3.Move the measuring device to the second shelf. The indicator now displays the difference between the two surfaces. This is the dimension needed to satisfy the inspection requirement.

4.Return the instrument back to the origin zero by pressing and holding the INC button until the indicator extinguishes.

In the next scenario, we need to measure the depth of a blind hole. Remember, a blind hole does not extend through the part. In this example, the hole has a shelf near the bottom of the hole, and we are going to measure the depth of the hole from the top of the hole to the depth of that shelf.

The tools necessary for this measurement are the digital caliper and an inspection pin of the correct diameter. In this case a .383” pin was the largest pin that would contact the shelf within the hole.

1. The first step is to close the caliper over the length of the pin. This pin measures just over 2.00” in length. I would recommend wiggling the pin while applying closing force on the caliper to ensure a good measurement.

2. While holding the caliper tight against the pin, press the INC button to establish the incremental zero. In this case, the incremental zero is really 2.00”.

Holding the caliper against the pin while ensuring the end of the caliper is resting on the flat surface of the part is a critical technique to ensure the caliper remains perpendicular during the measuring process.

3. Using the height measuring feature of the caliper, measure the remaining height of the pin when the pin is placed in the hole and contacting the shelf.

4.While the caliper is being held in the above manner, read the display on the caliper. This is the depth of the hole from the top of the part to the end of the pin that is resting on the shelf. In our example, the depth of the hole from the surface to the top of the shoulder is .747”.

4.3.3 Outside Micrometers

Micrometer is a term used extensively in the precision machining industry, and it refers to a certain type of measuring equipment that is capable of extreme accuracy, usually in the tenths of a thousandth of an inch (.0001”). There are various types of micrometers designed to measure just as many types of dimensions, but we will discuss the most common types in this chapter.

After the slide caliper, the Outside Dimension (O.D.) micrometer, or just an “outside mic” is the next most common measuring tool. This tool allows the operator to measure dimension resolutions to ten thousandths (.0001) of an inch. Since the volume of precision measurements are in the thousandths range, mastery of this tool is paramount.

The most common micrometer or mic, is the 0-1”. With this micrometer, the spindle closes until it touches the anvil. This is the zero position and all operators must close the mic and verify zero before use.

Key features and components of a micrometer include:

Frame: The frame is the main body of the micrometer. It typically consists of a C-shaped or U-shaped structure that houses the other components. It is common to have the frame clad with plastic to prevent the operator’s hands from transferring body heat into the measuring device, which may lead to inaccurate measurements.

Anvil and Spindle: The anvil and spindle are the two opposing, flat, and parallel surfaces between which the object being measured is placed. The anvil is stationary, while the spindle moves when measurements are taken. To preserve the flat relationship between the two mating faces, all micrometers should be stowed with a slight gap between the faces to account for thermal growth, which may damage the faces.

Sleeve: The sleeve is the cylindrical portion of the frame that contains the barrel and the thimble. It usually has a graduated scale, often marked in millimeters (mm) or inches (in) to provide a coarse measurement.

Thimble: The thimble is located at the end of the barrel and can be rotated to move the spindle incrementally. It has a circular scale, typically divided into 25 or 50 divisions, representing one full rotation of the spindle. The thimble is turned to open and close the micrometer. A ratchet stop may be attached to the back of the thimble, which allows the operator to gently close the micrometer onto the part and twist the ratchet while seating the micrometer with delicate wiggling to seat the faces against the measured surface. On some models, the thimble has a clutch under it which serves a similar purpose of the ratchet stop; however, the ratchet stop will click while turning and the clutch thimble will slip, preventing the operator from over-torquing the micrometer, which will contribute to inaccurate measurements.

Spindle Lock: As the name implies, the spindle lock, when applied, prevents the spindle from turning. This can be used to maintain the position of the micrometer faces while the operator reads the measurement. On a digital micrometer, it is incumbent for the operator to lock the spindle before storing the micrometer. This action prevents the digital micrometer’s “turn to wake” function from energizing the circuits when the tool gets bumped, which results in dead batteries.

The operation of an outside micrometer involves placing the object to be measured between the anvil and the spindle. The thimble is then rotated, causing the spindle to move toward the object until it makes contact. The measurement is read by observing the positions of the graduations on the barrel and thimble scales or reading the digital display. The barrel scale provides the coarse measurement in millimeters or inches, while the thimble scale provides the finer measurement in micrometers or thousandths of an inch.

4.3.4 Operation of a Micrometer

4.3.4.1 Preoperational Inspection

Before using a micrometer to perform a measurement, the operator must ensure the micrometer is ready to perform the task accurately. This is done by performing a preoperational inspection of the micrometer. There are many factors that influence whether a micrometer can accurately measure a dimension. First, remove the micrometer from the protection of the storage case and visually inspect the components for dents, scratches, misalignment or any signs of abuse. In shops that utilize calibration programs, there should be a calibration sticker on the equipment with a date for when the next inspection is due. Verify the inspection date is still valid.

After the visual inspection, the faces of the spindle and anvil must be cleaned to remove surface contaminants that accumulate during normal use. The surfaces are wiped with a clean piece of paper.

Using a clean piece of paper, gently close the micrometer onto the paper and give the thimble clutch a couple of turns. Pull the paper free from between the jaws, transferring the contaminants from the faces onto the slip of paper. Repeat as necessary.

4.3.4.2 Zero the Indicator

Once the faces of the spindle and anvil are clean, gently close the micrometer until the faces touch (0-1” micrometer). Give the thimble a couple of turns, simulating the action used to perform an accurate measurement, and verify that the display reads zero. DO NOT press the “ ZERO/ABS” button to zero the micrometer, as this can lead to inaccurate measurements.

If the micrometer indicates a good zero after performing these steps, you may start performing measurements.

If a good zero can not be obtained using the above methods, the micrometer should be removed from service and repaired/calibrated by qualified individuals.

Absolute Zero/Origin: On quality equipment, the absolute “zero” is set by the metrology lab when calibrating the micrometer and is referred to as the “origin”. Understanding that the origin is the absolute zero is critical to proper use of a micrometer. As an operator, you must ensure the micrometer references the absolute zero before making a measurement.

On this Mitutoyo brand micrometer, the origin button is on the face of the dial. This button is only to be used by the metrology lab. Never press the origin button as this will void the calibration of the micrometer.

The “ZERO/ABS” feature is used by the operator to switch between incremental and absolute mode. This feature allows the operator to set an incremental zero, which is useful for making comparative measurements, when an absolute measurement is not necessary.

Part of the preoperational inspection of a digital micrometer is to verify the micrometer is NOT in the incremental mode. Notice the figure above. The “INC” displayed denotes that the micrometer is NOT using the origin (“ABS” or Absolute) zero but rather an incremental zero. To set the micrometer back to absolute mode on this micrometer, the “ZERO/ABS” button is pressed and held until the “INC” flag is extinguished.

Micrometers generally only have one inch of travel. On a 0-1” micrometer, the spindle and anvil can touch, allowing a zero/origin to be verified. On larger micrometers, such as the 1-2” micrometer above, we need to use an inspection standard to verify the equipment zero. An inspection standard is a piece of equipment with a calibrated size that is used to check other pieces of inspection equipment.

4.3.4.3 Measuring with a micrometer.

After the micrometer passes a thorough preoperational inspection, you are now able to perform accurate measurements.

To perform an accurate measurement with a micrometer, you need to open the micrometer larger than the dimension to be measured. This is done by rotating the thimble counter-clockwise. Before closing the micrometer on the object, ensure the area is free of oil and debris. Any contamination on the measured surface will add error to the measurement. For high tolerance dimensions, less than .001”, allow the part to cool before measuring, as the excess heat will cause the part to grow and throw off an accurate measurement.

The shape of the micrometer frame rests in the hand naturally, with the back nestled in the palm of your hand. The fingertips support the open side of the micrometer and twist the thimble, which moves the spindle either in or out. This is a good time to mention the pressure you should feel in your fingertips. Measuring is a delicate process that utilizes feedback from the pads of the fingers to indicate how much pressure you are applying.

With the micrometer open enough to allow a close fit over the part, place the anvil against one side of the part to be measured.

Turn the spindle closed (clockwise) using the thimble until the spindle face nears the part. Slowly turn the thimble until the spindle face gently touches the part.

Once both faces of the micrometer are in contact with the part, gently wiggle the micrometer while turning the thimble or ratchet if applicable. This action helps to seat the faces securely against the part eliminating any angular differences between the faces and the part. You can now take a reading. Pressing the hold button will lock the reading, preventing the display from changing due to operator movement. The brake may also be applied at this point to perform the same purpose as the hold button.

Micrometer Accessories

For some jobs, there are never enough hands. That’s when a micrometer stand comes in handy. The mic stand is a clamping device which has an adjustable hinge to present the mic at a convenient angle for reading.

The figure above has a mechanical digital disc micrometer clamped in a mic stand. This is what passed for digital before electronic versions became widespread.

4.3.4.4 Reading a vernier micrometer

The vernier scale has an advantage due to its basic design, which does not require the use of batteries (which always seem to lose charge at inconvenient times). Vernier measuring equipment is more difficult to read compared to digital tools, which require only reading the display. They are included here because you will find them in industry right alongside their digital counterparts.

4.3.4.5 Reading in thousandths of an inch, .001

Starrett is a prominent U.S. manufacturer of precision measuring equipment, and they provide online access to their catalogs.

Below are detailed instructions on how to read the micrometer in thousandths of a inch, from the Starrett Corporation’s Operations Procedures for micrometer operation (2014):

The pitch of the screw thread on the spindle is 40 threads per inch. One revolution of the thimble advances the spindle face toward or away from the anvil face precisely 1/40″ or .025 inches. The reading line on the sleeve is divided into 40 equal parts by vertical lines that correspond to the number of threads on the spindle. Therefore, each vertical line designates 1/40″ or .025 inches. Lines vary in length for easy reading. Every fourth line, which is longer than the others, designates a hundred thousandths. For example: the line marked “1” represents .100″ and the line marked “2” represents .200″, etc. The beveled edge of the thimble is divided into 25 equal parts with each line representing .001″ and every line numbered consecutively. Rotating the thimble from one of these lines to the next moves the spindle longitudinally 1/25 of .025″, or .001″. Rotating two divisions represents .002″, etc. Twenty-five divisions indicate a complete revolution of .025″ or 1/40 of an inch. To read the micrometer in thousandths, multiply the number of vertical divisions visible on the sleeve by .025″, and to this add the number of thousandths indicated by the line on the thimble that coincides with the reading line on the sleeve. (p. 22)

4.3.4.6 Reading in the ten-thousandths of an inch, .0001

Below are detailed instructions on how to read the micrometer in the ten-thousandths of a inch, from the Starrett Corporation’s Operations Procedures for micrometer operation (2014):

Micrometers graduated in ten-thousandths of an inch read like micrometers graduated in thousandths, except that an additional reading in ten-thousandths is obtained from a vernier scale on the sleeve. The vernier consists of ten divisions on the sleeve, which occupy the same space as nine divisions on the thimble (Fig. B). Therefore, the difference between the width of one of the ten spaces on the vernier and one of the nine spaces on the thimble is one-tenth of a division on the thimble, or one ten-thousandth (.0001″). To read a ten-thousandths micrometer, first obtain the thousandths reading, then see which of the lines on the vernier coincides with a line on the thimble. If it is the line marked “1” on the sleeve, add one ten thousandth, if it is the line marked “2”, add two ten-thousandths, etc. (p. 22)

4.3.4.7 Reading in hundredths of a millimeter, 0.01mm

Below are detailed instructions on how to read the micrometer in the hundredths of a millimeter, from the Starrett Corporation’s Operations Procedures for micrometer operation (2014):

The screw head pitch is one-half millimeter (0.5mm). One revolution of the thimble advances the spindle face toward or away from the anvil face precisely 0.5mm. The reading line on the sleeve is graduated above the line in millimeters (1.0mm) with every fifth millimeter being numbered. Each millimeter is also divided in half (0.5mm) below the reading line. Two revolutions of the thimble advance the spindle 1.0mm. The beveled edge of the thimble is divided into fifty equal parts, with each line representing 0.01mm and every fifth line being numbered. Rotating the thimble from one of these lines to the next moves the spindle longitudinally 0.01mm; rotating two divisions represents 0.02mm, etc. To read the micrometer, add the number of millimeters and half-millimeters visible on the sleeve to the number of hundredths of a millimeter indicated by the thimble graduation indicated by the reading line. (p. 23)

4.3.4.8 Reading in thousandths of a millimeter, 0.001mm

Below are detailed instructions on how to read the micrometer in the thousandths of a millimeter, from the Starrett Corporation’s Operations Procedures for micrometer operation (2014):

Reading a 0.001mm micrometer is exactly like reading a 0.002mm micrometer except that there are ten divisions on the vernier occupying the same space as nine divisions on the thimble (Fig. B). Therefore, the difference between the width of one of the spaces on the vernier and one of the nine spaces on the thimble is one-tenth of a division on the thimble, or one-thousandth (0.001mm). First obtain the hundredth of a millimeter (0.01mm) reading. Next, see which of the lines on the vernier coincides with a line on the thimble. If it is the first line add 0.001mm to the reading, if it is the second line add 0.002mm, etc. Only every second vernier line is numbered on a 0.001mm reading tool because of space congestion. (p. 23)

4.3.5 Blade Micrometers

The blade micrometer is primarily used to measure grooves. Grooves are common in industry to create space for an o-ring or other sealing device to create a seal between a piston and a cylinder bore. A bore is a female cylindrical feature.

As noted in the pictures above, the only difference is that the blades have replaced the flat anvil and spindle faces of traditional O.D. micrometers. The blade micrometer above makes use of a small diameter knob on the end of the thimble. Its purpose is to allow the operator the ability to rapidly advance the thimble when making large movements on the scale. Once the micrometer nears the final dimension, the operator would transition to the larger thimble knob for the delicate final adjustments.

4.3.6 Disc Micrometer

The disc micrometer is another O.D. micrometer designed for more specific purposes. The discs protrude out beyond the range of the standard anvil/spindle faced micrometers. This design allows for precision measurement provided by the micrometer’s resolution for tighter toleranced dimensions.

Above: the disc micrometer is used to measure the diameter of a stud with a resolution of .0001”.

4.3.7 Thread Micrometer

Thread micrometers measure the pitch diameter on external threads. They come with a selection of anvils to allow the measuring of different thread pitches.

A thread micrometer set comes with a mic body which determines the maximum major diameter allowed and a selection of paired conical and V-style anvils dependent upon pitch of the thread. Note the range displayed near the top set of conical anvils.

The anvils cover both Unified Standard (U) and Metric (M) threads. In the photo above, the anvils used to measure a ½-13 UNC thread are missing from the case because they are installed in the micrometer.

Two anvils are required to measure a thread, one conical (cone shaped) and one V-style.

The photo above provides a better view of the anvils for a thread mic. They are precision ground and provide a tight fit in the micrometer body.

4.3.7.1 Thread Micrometer Operation

Start by selecting the correct set of anvils for the corresponding pitch of the thread. For a ½-13 UNC thread, select the 13-9 anvils.

Open the micrometer to allow room to install the anvils.

The thread micrometer differs due to the various anvils which need to be installed. The stationary anvil socket is adjustable to allow the operator to set zero with all the different anvils.

First the spindle is turned until the zero is reached. Apply the spindle lock to prevent movement.

Second, loosen the stationary anvil’s socket and turn the knob until the two anvils touch.

Apply the stationary anvil brake and loosen the spindle brake to open and measure threads.

Insert the anvils into the mic with the conical anvil in the spindle and the V-anvil in the base.

Gently close the micrometer over the thread to be measured.

A close up view of the anvils and bolt threads. Because of the cylindrical nature of a bolt, you will need to rock the bolt back and forth “feeling” for the peak of the measurement while adjusting the anvil until you feel the faintest rub against the anvils.

You may read the thread pitch diameter as you would any ordinary O.D. vernier micrometer. Starting on the barrel, we can see the .400” is exposed. Next record the .025” value from the barrels. For our final thousandth position we see the 9 on the thimble has not reached the horizontal line on the barrel so we may add that to the accumulated value. Our thread pitch diameter for the 1⁄2-13 UNC thread is .434”. This micrometer does not have the .0001 ” position so we may estimate that with our eyecrometer and call it .0005” if necessary.

Table 4.1

Thread Information of Class, Allowance, Major Diameter and Pitch Diameter

Note. The Machinery’s Handbook (Jones et al., 2004) states the pitch diameter for a ½-13 UNC 2A thread is .4435 to .4485” (p. 1740).

4.3.8 Inside Micrometers

Inside micrometers are used in various applications where accurate measurements of internal dimensions are essential. Common uses include measuring the internal diameter of holes, cylinders, and bores in machine parts, engine components, bearings, and other objects. Inside micrometers provide precise and repeatable measurements, ensuring that components fit correctly and meet specified tolerances. They are valuable tools for quality control, manufacturing, and engineering tasks where precision is critical.

4.3.9 Depth Micrometers

Depth micrometers are just one method to measure the depth of a dimension. Their size permits measuring in areas where larger devices can not fit, such as in a CNC mill or lathe. The set above is common in that you have the body of the mic, a selection of varying length anvils and a small spanner for adjusting the barrel when calibration is needed.

An important feature to distinguish on a depth mic is the direction of scale graduations. When zeroed on a flat surface, the thimble is extended. As the thimble is turned in, the numbers increase. This is the opposite of standard micrometers and may take a couple of tries to truly understand.

The first step of measuring with a depth micrometer is to verify the zero. Place the micrometer on a known good flat surface, such as a surface plate (this unfinished surface in the photo is not optimum), and turn the thimble down until it gently contacts the flat surface. Notice the zero is off by about .0005”. Before adjusting the barrel with the spanner to set a good zero, try this again on a surface plate.

Place the micrometer spindle over the depth to be measured and turn the thimble down until the anvil gently contacts the measured surface. Read the feature depth on the micrometer scale using techniques from a standard micrometer vernier scale. Remember that the graduations on the depth micrometer increase in value with advancing turns.

The scale above indicates the counterbore depth at .530”. The thimble has covered the .525” markings but NOT the .550 markings. Record the .525 from the barrel and then add the .005” from the thimble. If this micrometer has .0001” indications, we would add .004” then read the tenth’s scale to the last digit.

4.4 Height Gages

Height gages of various styles are used throughout the machining industry. They are usually found on top of granite surface plates. They are referred to as a gage, but they are capable of performing direct read measuring too. This is just one reason they are so versatile. They also come in varying heights to suit the needs of the task.

Height gages are a common measuring tool in all machine shops. The versatility of a height gage makes it the primary tool on most surface plates. There are digital, vernier and dial type height gages; however, with the low cost of digital measuring equipment, digital is more common.

When placed on a surface plate, the height gage can reference the surface plate for a datum as well as points on the part such as counterbores, thru holes, blind holes and stepped features. The versatility comes from the many accessory attachments that can be mounted on a height gage. The attachment point is very basic, permitting a simple clamping arrangement.

4.4.1 Height Gage Attachments

The above figure shows the common clamping system on most height gages. This simple design is one of the key factors why the height gage is so popular. A thumb screw clamps the attachment inside the clamping sleeve. Simply loosen the thumb screw to change the attachments.

4.4.2 Scribe

The scribe attachment can be used to actually scribe a surface for layout purposes or used as a flat bottom surface for performing length measurements. A gentle touch must be used when measuring height with a scribe attachment as the operator can lift the height gage by applying too much force which will prevent accurate measurement.

The figure above illustrates lifting from applying too much force to the height control knob. Accurate height gage operation requires a delicate touch. Not only touching off on the surface plate to register the datum zero but also when touching the surface that is to be measured. To do otherwise would add inaccuracy to the process.

To perform a height measurement perform the following steps:

Zero the scribe against the surface plate datum by gently touching the plate with the scribe and then pressing the “ZERO/ABS” button.

Raise the scribe and gently touch the top datum surface with the scribe.

Read the height measurement from the digital readout for the dimension.

4.4.3 Depth Gage Attachment

The figure above has a depth gage attachment which is being used to measure the depth of a counterbore. The depth probe is gently touched off against the datum surface of the object and the indicator is zeroed.

The depth gage is moved to the bottom of a counterbore and the depth is read on the height gage display.

4.4.4 Test Indicator

A test indicator can be affixed to the height gage using an indicator bar and clamp. The figure above demonstrates the dovetail style clamp attaching to the test indicator case but there are additional bar styles for popular test indicator models.

Utilizing a test indicator on a height gage allows the operator to perform extremely accurate measurements due to the fine resolution of the test indicator coupled with that of a height gage. However, the approach angle of the test indicator needs to be less than 20° to prevent cosine error. An angle greater than 20° can add ten thousandths to a measurement. The test indicator contact points may be adjusted up and down by applying pressure to the pointer against the friction fit of the threaded swivel.

A test indicator has a bezel (which the numbers are attached to) that can be rotated to suit the operator. The figure above has the zero at the top, away from the contact point.

The indicator above is in the no load position meaning the pointer is not contacting any surface. Notice the needle rests at -.007” from zero. The purpose of this feature is to allow the operator to “preload” the indicator which allows a plus and minus swing of the dial during use. To preload the indicator simply touch the indicator’s contact point against the surface to be measured and turn the height gage dial until the test indicator indicates zero. Now, when you sweep the point across the surface to be measured, you can find the high spot by watching the indicator needle.

In the figure above, the indicator dial is rotated with the zero at the bottom. The advantage of one over the other is the needle’s ability to swing in both the negative and positive directions. With the dial’s zero on the bottom, you are permitted more range for sweeping surfaces and testing for a high spot. With the dial’s zero on the top, the needle has very little travel in the negative direction and can prevent an accurate reading.

Once the test indicator is zeroed against the datum surface, zero the height gage without moving the height adjustment. You now have the datum set, and you are ready to measure.

Once the test indicator has been zeroed utilizing the adjusting knob on the height gage, you may zero the height gage indicator by pressing “ZERO/ABS”. We have now coupled the indicator accuracy of a height gage with a test indicator. This coupling allows for very accurate measuring due to the ability to sweep a dimension datum and find the highest point from which to measure a dimension.

After setting a zero datum on the surface plate, the test indicator is moved to the top of an inspection pin placed in the counterbore of the vice jaw being measured.

The indicator is swept back and forth by moving the height gage while watching the test indicator. When the test indicator measures the highest reading, stop the sweep.

Raise or lower the height gage until the test indicator reads the same zero that was set against the surface plate datum.

When all the steps have been performed accurately, the indicator on the height indicator will display the height to the top of the pin. Since the inspection pins are precision ground for accuracy, subtracting the radius of the pin will provide the distance from the surface plate to the center of the counterbore.

4.4.5 Cleaning a Height Gage

Figure 4.94

Cleaning a Height Gage

This is a good time to mention how to clean a height gage to remove excessive resistance caused by a dirty piece of equipment. The height gage suffers many of the same influences as the slide calipers. Contamination from operators’ hands and general shop air collects on the surfaces causing excess friction experienced from moving the sliding height adjuster. Many of the same techniques used to clean calipers are the same here.

Moisten a lint free cloth with alcohol and wipe the beam surfaces. Contamination collects here from the oil on the operator’s hands. Unfortunately, the beam is such a prominent feature of the height gage that operators use it as a handle when transporting the device.

A height gage should never be transported by grabbing the beam in order to prevent contamination and damage to the beam. Always grasp the height gage by the base to transport or move. A stabilizing hand can be rested against the beam to prevent tipping while transporting.

The image above shows how much contamination can accumulate on a height gage beam. This example is not very bad because this height gage resides in a metrology lab where parts have usually been cleaned before measuring, and the operator’s hands are not as dirty. This digital indicator has an encoder strip under the blue strip on the beam. Since the measuring component in the slide assembly reads the surface of the encoder strip, excess moisture must be removed from this surface before use to prevent indicator malfunctions.

4.5 Measuring diameters

The ability to accurately measure inside diameters is a skill that requires time to master. Here we will discuss common tools used to measure inside diameter dimensions. We will discuss direct read equipment as well as gages.

4.6 Snap gages

Snap gages are relatively inexpensive tools which are used to measure the I.D. of parts. They are transfer devices which require the use of a micrometer to acquire the final dimension.

Snap gages come in sets which allow measuring of various bore sizes. A bore is a cylindrical feature created to enlarge a drilled hole. This gage is used to record the diameter of a hole and then transfer that recording to a direct reading tool such as an O.D. micrometer.

Snap gages have two collapsible anvils, which can be locked in place by rotating a knob on the end of the handle.

A snap gage is installed into a bore to be measured in such a manner as to have the anvils of the snap gage extended to a diameter larger than the bore.

Collapse the anvils and place in the bore at an angle as pictured above.

Turn the lock. This applies pressure to the anvils, restricting their movement.

3. Swing the snap gage so the arms compress as they pass through the bore diameter.

4. Take the snap gage to a micrometer and gently measure over the anvils.

4.7 Bore Gages

Bore gages are used to measure bore diameter in comparison to the inspection standard. In order to use a bore gage, the operator must install the required anvils and/or shims to create a length near the bore diameter dimension.

Bore gages come with replaceable anvils, which the operator must replace to fit the desired bore.

The above figure shows the correct anvil installed on a bore gage to measure a 1” bore.

After installing the correct anvil on the bore gage that relates to the bore being measured, the bore gage is installed into a special inspection standard called a ring gage. A ring gage is a cylindrical device used to measure the outside diameter of features. Ring gages are precision ground to tight tolerance.

With the anvils inside the standard, the dial is rotated until the needle rests on the zero. The bore gage is now set to the stand diameter.

With the bore gage setup correctly, it is transferred to the bore to be measured.

Reading the dial indicates what size the bore is compared to the standard used to set up the gage. In this example, the indicator is showing a negative .0009”. Add this dimension to the standard diameter and you have the measured bore size.

4.8 Pin Gages

Pin gages are precision ground cylindrical gages used to measure holes and bores. Pin gages come in sets. The figure above is a set of minus gages from .501 to .625 in .001” increments.

Minus gages are ground .0002” undersized. The purpose of this is to allow the pin to fit into a cylindrical feature of the nominal size printed on the pin. Nominal refers to the intended size dimension without any tolerance applied. If a .500” pin was ground to .500” it would not fit into a .500” feature.

Gage pins come in minus and plus sizes. The tolerance to which they have been ground defines how much they are undersized or oversized.

The two most common tolerance sizes of gage pins are Z and ZZ. Z tolerance is +.0000 -.0001” for minus pins and +.0001 – .0000 for plus pins. ZZ tolerance is +.0000 -.0002” for minus pins and +.0002 -.0000 for plus pins.

Pin gages are great for Go/No-Go gaging a hole feature. The figure above is a drawer of Go/No-Go pins gages set up for operators to check out and use in production. The pin holders have a green and red cap that has a precision collet beneath it to hold a pin. If you are measuring a dimension of a .375” hole with a tolerance of +.000 -.005”, you would place a .375” plus pin in the red end and a .370” minus pin in the green end. During production, the Go/No-Go pin is quickly used to gage a hole to ensure it is within tolerance.

Gage pins are plain or coated with a black oxide finish, which will show wear, alerting the operator that the pin needs to be replaced. Replacement pins can be ordered individually to replace worn or missing pins. The above .375” pin has been used extensively and needs to be replaced.

A tighter tolerance pin is an X or XX tolerance which is .00004” and .00001” respectively. They are kept in a metrology lab until they are needed for measurement. The figure above is an X tolerance pin set for .2500” ∓ .001” in .0001” increments. This set consists of a nominal pin, in this case .2500” and ten minus pins in .0001” increments, as well as ten plus pins in .0001” graduations. They are only used for very tight tolerance holes, such as in an aerospace press fit situation.

4.9 Gage Blocks

Gage blocks are precision ground tool steel blocks that may be combined or used individually for gaging dimensions. Quality gage blocks are precision ground to tolerances in the millionths of an inch and have such a fine surface finish that they stick together when properly assembled. This process is called wringing and includes a twisting motion as the blocks are slid together.

4.10 Radius Gages

Radius gages are used to gage machined radii on parts. These sets come in inch as well as metric. Inch sizes come in decimal or fractional. It is very common to use all three sets to gage a tight tolerance radius due to the larger resolution.

The figure above is an example of a radius gage. This one is .500”. There are five radius examples on each gage. This allows the gage to measure both inside and outside radii.

4.11 Thread Plug Gages

Thread plug gages are used to measure I.D. threads. Machine shops have a large inventory of thread gages due to the volume of threads available and the required class of fit.

Thread gages come in the Go/No-Go configuration with the No-Go plug using a pitch diameter larger than the class of fit allows, and the Go size set to the minimum thread pitch diameter for the class of fit. The thread gage above is for a ½ X 13 UNC 2B thread with a pitch diameter of .4500 for the Go end and a .4565” pitch diameter for the No-Go end.

4.12 Indicators

The two most common indicators you will use are the dial and test indicators. As illustrated in the figure above, they look quite different and they function a bit differently, however they both behave as a gage because they are not direct read indicators such as a caliper or micrometer.

4.13 Dial Indicators

Dial drop indicators are similar in that they all have a dial with two needles. The smaller needle will record how many complete revolutions the large needle has made, and the large needle indicates how many smaller graduations have been made. In the inch indicator above, the large needle indicates the number of .001” of arm travel. The drop arm slides up and down within the indicator, which is why it’s called “drop”. The amount of travel will vary with the model of indicator, however the one inch is most common.

In the example above, the dial indicator is being used to measure the depth of a counterbore. The indicator was “zeroed” on the top of the part at .700”.

Due to the nature of the indicator, the indicator is at zero travel when the arm is fully extended. The operator has decided to load the dial indicator with .700” of travel in the drop arm.

With the top surface datum recorded at .700”, the indicator is dropped into the counterbore to rest against the counterbore shoulder.

Once the drop arm rests on the bottom of the counterbore, you can take a reading of the indicator dial. Since the small indicator is below the two, it is reading in the .100” range. Add the large needle reading of .069” and we have a current reading of .169”. Subtract that number from the starting zero of .700” and we have a counterbore depth of .531”. This method is a quick way to measure a feature when performing several specific dimension measurements, such as during a production run.

Dial indicators have a ⅜” stem, which can be used to hold the indicator in many different styles of holders. The figure above is a NOGA brand indicator stand.

The back of most dial indicators have a lug for attaching mounting hardware, such as this 5/16” post.

The figure above illustrates how the back lug on a test indicator can mount on a magnetic base. In this instance, a spacer shim had to be added to fit the stand (see silver inside clamp).

4.14 Test Indicator

Test indicators vary in several characteristics from drop dial indicators, the first being the resolution of the indicator. The most popular test indicators have a resolution of .0005” however there are .001” graduations as well as .0001” if necessary. This means that every line is a half a thou, .0005”. Resolution like this makes reading fine variations easier than a .001 graduation dial.

The dial above has .0005” resolution and a dial that only makes one revolution, or .030” travel. Due to this limited travel, test indicators are only used for very fine measurements.

The above figures illustrate how the indicator contact points can be moved to intercept the datum surface at the correct angle. The top example is the angle desired to eliminate the cosine error induced by having too steep an angle on the contact tip. A cosine error is induced by the contact point not being set to the correct sweep angle in relation to the contact arm swivel point.

There is no industry standard to how a test indicator mounts, however, the dovetail feature allows for test equipment to be mounted with very little interference from the mount. Above, a test indicator has been mounted onto a height gage using a dovetail accessory.

 

 

References

Jones, F. D., Ryffel, H. H., Oberg, E., McCauley, C. J., & Heald, R. M. (2004). Machinery’s handbook (27th ed.). Industrial Press.

National Oceanic and Atmospheric Administration. (2024, April 30). How reliable are weather forecasts? SciJinks. National Environmental Satellite, Data, and Information Service. Retrieved May 7, 2024, from https://scijinks.gov/forecast-reliability/

Starrett. (2014). Precision tools (Catalog 33) [Catalog]. L. S. Starrett. https://www.starrett.com/resources/catalogs