3.Precision Measuring Tools

a.General. Micrometer calipers (figure 35) are probably the most often used precision instruments in a machine shop. There are many different types, each having been designed to permit measurement of surfaces for various applications and configurations of workpieces. The degree of accuracy obtainable from a micrometer also varies. The most common graduations on the micrometer are from one thousandth (0.001) of an inch to one ten thousandths (0.0001) of an inch. The measurement is usually expressed as a decimal. There are three types of micrometers which are commonly used: The outside micrometer, the inside micrometer, and the depth micrometer.


 b.Outside Micrometer.


(1) General. An outside micrometer (figure 36 ) often called a micrometer caliper, or mike, is used to measure the thickness or the outside diameter of various parts. They are available in sizes ranging from 1 inch to about 96 inches, in steps of 1 inch; or in sets graduated to read in units of the metric system, from 0 to 25 millimeters by hundredths of a millimeter. However, in most shops, standard sets up to 24 inches are more common. The larger sizes normally come as a set with interchangeable anvils which provide a range of several inches. The anvils have an adjusting nut and a locking nut to permit setting the micrometer with a micrometer standard. Regardless of the degree of accuracy designed into the micrometer, the skill applied by each individual is the primary factor in determining the accuracy and reliability of measurements. Training and practice will result in proficiency when using this tool.

(2)  The types of micrometer commonly used are made so that the longest movement possible between the anvil and the spindle is 1 inch. This movement is called the “range”. The frames of the micrometer, however, are available in a wide variety of sizes, from 1 inch up to as large as 24 inches. The range of a 1 inch micrometer is from 0 to 1 inch. In other words, it can be used to measure work where the part to be measured is 1 inch or less. A 2 inch micrometer will only measure work between 1 and 2 inches thick. A 6 inch micrometer has a range of from 5 to 6 inches, and will only measure work between 5 and 6 inches thick. It is necessary, therefore, that the machinist first find the approximate size of the work to the nearest inch, and then select a micrometer that will fit it. For example, to find the exact diameter of a piece of round stock use a rule and­first find the approximate diameter of that stock. If it is found to be approximately 3 1/4 inches, a micrometer with a 3 to 4 inch range would be required to measure the exact diameter. Similarly, with the inside and the depth micrometers, rods of suitable lengths must be fitted into the tool to get the approximate dimension within an inch, after which the exact measurement is read by turning the thimble. The size of a micrometer indicates the size of the largest work it will measure.

(3)  On some outside micrometers, the frame can be smaller, so that the range of the micrometer is only 0 to 1/2 inch or 0 to 13 millimeters; or it can be larger, so that the range is 23 to 24 inches. The head has a constant range of 0 to 1 inch. The shape of the frame may be varied to adapt to the physical requirements of some types of work. For example:

(a)   The frame back of the anvil may be cut away to allow the anvil to be placed in a narrow slot.

(b)   The frame may have a deep throat to permit it to reach into the center sections of a piece of sheet metal (sheet metal or paper gage).

(c)   The frame may be in the form of a base so that the gage can be used as a bench micrometer.

(d)   The frame may have a wooden handle and may be of extra­heavy construction for use in steel mills for gaging hot sheet metal.

(4) The spindle and anvil may vary in design to accommodate special physical requirements. For example:

(a)   The spindle and anvil may be chamfered so that the micrometer gage can slide on and off the work easily, as when gaging hot metal.

(b)   The ball­shaped anvil is convenient in measuring the thickness of a pipe section of small diameter.

(c)   The V­shaped anvil is necessary on the screw thread micrometer caliper to mesh properly with the screw thread. The spindle of the screw thread micrometer is cone­shaped. This micrometer measures the pitch diameter of the thread.

(d)   The interchangeable anvils of various lengths make it possible to reduce the range of the micrometer. A micrometer having a range of 5 to 6 inches can be changed to one having a 4 to 5, or a 3 to 4 inch range by inserting a special anvil of the proper length.

(5)  Design of Micrometers. The micrometer makes use of the relation of the circular movement of a screw to its axial movement. The amount of axial movement of a screw per revolution depends on the thread, and is known as the lead. If a circular nut, on a screw, has its circumference divided into 25 equal spaces and if the nut advances axially 1/40th of an inch for each revolution, each space will represent 1/25 X 1/40, or 1/1000 of an inch. In the micrometer, the nut is stationary and the screw moves forward axially a distance proportionate to the amount it is turned. The screw on a micrometer has 40 threads per inch, and the thimble has its circumference divided into 25 parts, so 1 division on the thimble represents an advancement of 1/1000 of an inch axially.

(6)  Construction.

(a)   The steel frame is U­shaped, and one end holds the stationary anvil. The stationary anvil is a hardened button, either pressed or screwed into the frame.

(b)   The steel spindle is actually the unthreaded part of the screw. It is the spindle that advances or retracts to open or close the open side of the U­frame. The spindle bearing is a plain bearing and is part of the frame.

(c) The hollow barrel extends from this bearing, and on the side of the barrel is the micrometer scale, which is graduated in tenths of an inch, then divided into subdivisions of 0.025 of an inch. The end of the barrel supports the nut which engages the screw. This nut is slotted and its outer surface has a taper thread and a nut which makes it possible to adjust the diameter of the slotted nut, within limits, to compensate for wear.

(d)   The thimble is attached to the screw and is a sleeve that fits over the barrel. The front edge of the thimble carries a scale divided into 25 parts. This scale indicates parts of a revolution, where the scale on the barrel indicates the number of revolutions. The thimble is connected to the screw through a sleeve that permits it to be slipped in relation to the screw for the purpose of adjustment. The inner sleeve is sweated to the screw. The outer sleeve is clamped to the inner one by the thimble cap. Loosening the cap makes it possible to slip one in relation to the other.

(e)   On top of the thimble cap there may be a ratchet. This device consists of an overriding clutch held together by a spring in such a way that when the spindle is brought up against the work, the clutch will slip when the correct measuring pressure is reached. The purpose of the ratchet is to eliminate any difference in personal touch, and so reduce the possibility of error due to a difference in measuring pressure. Not all micrometers have ratchets.

(f) A clamp ring or locknut is located in the center of the spindle bearing on those micrometers equipped with it. This clamping makes it possible to lock the spindle in any position to preserve a setting.

(7) Use of Micrometers.

(a) Reading a Standard Micrometer.


  1. Reading a micrometer (figure 37 ) is only a matter of reading the micrometer scale or counting the revolutions of the thimble and adding to this any fraction of a revolution. The micrometer screw has 40 threads per inch. This means that one complete revolution of the micrometer screw (1) , view A, moves the spindle (2) away from or toward the anvil (3) exactly 1/40 or 0.025 inch.
  2. The lines on the barrel, view B, (4) conform to the pitch of the micrometer screw (1), each line indicating 0.025 of an inch, and each fourth line being numbered 1, 2, 3, and so forth.
  3. The beveled edge of the thimble is graduated into 25 equal parts. Each line indicates 0.001 of an inch, and one complete and exact revolution of the micrometer screw and the thimble moves the spindle away from or toward the anvil exactly 0.025 of an inch. Every fifth line on the thimble is numbered to read a measurement in thousandths of an inch.
  4. To read a measurement on the standard micrometer use figure 37, view C, and perform the following:
  1. Read the highest figure visible on the barrel (5)
2 = 0.200 in.
  1.  Number of lines visible between the No.2 and
    the thimble edge (6)
1 = 0.025 in.
  1. The line of the thimble that coincides with or
    has passed the revolution or long line in the barrel (7)
16 = 0.016 in.
  Total = 0.241 in.

(b) Reading a Vernier Micrometer.


  1. Read the highest figure visible on the barrel (5)
2 = 0.200 in.
  1. Number of lines visible between the No.2 and
    the thimble edge (4)
3 = 0.075 in.
  1. The line of the thimble that coincides with or
    has passed the revolution or long line in the barrel (5)
11 = 0.011 in.
  1. The line on the vernier scale that coincides with a line on the thimble (6)
2 =  0.0002 in.
  Total = 0.2862 in.

(c) Reading a Metric Micrometer. The same principle is applied in reading the metric graduated micrometer, but the following changes in graduations are used (see Figure 39):


  1. Read the highest figure visible on the barrel (1)
20 = 20.0mm
  1. Number of lines visible between the No.20 and
    the thimble edge (2)
2 = 2.0mm.
  1. The line of the thimble that coincides with or
    has passed the revolution or long line in the barrel (3)
36 = 0.36mm
  Total = 22.36mm


Remember that one complete revolution of the thimble is 0.5mm. It takes two complete revolutions to advance it to 1mm.

(d) Measuring With The Micrometer.


  1. When checking a small part with the use of a micrometer (figure 40, view A, hold the part in one hand. Hold the micrometer in the other hand so that the thimble rests between the thumb and the forefinger. The third finger is then in a position to hold the frame against the palm of the hand. The frame is supported in this manner making it easy to guide the work over the anvil. The thumb and the forefinger are in position to turn the thimble either directly or through the ratchet, bringing the spindle down against the work.
  2. On larger work, it is necessary to have the work stationary and positioned to permit access to the micrometer. The proper method of holding a micrometer when holding a part too large to be held in one hand is shown in figure 40, view B, on the previous page.The frame is held by one hand to position it and locate it square to the measured surface. The other hand operates the thimble either directly or through the ratchet. A large flat part should be checked in several places to determine the amount of variation.
  3. To measure a shaft as shown in figure 40, view C, the frame is held by one hand while the thimble is operated by the other. In measuring a cylindrical part with a micrometer, it is necessary to “feel” the setting to be sure that the spindle is on the diameter, and also to check the diameter in several places to determine the out­of­roundness.
  4. For measuring very large diameters, micrometer calipers are made in various sizes up to 168 inches. A pulley is being checked (figure 41) with a micrometer whose range has been reduced by a special anvil which has been screwed into the frame. A set of different length anvils permits the use of the micrometer over a wide range of sizes; yet the spindle only moves 1 inch. This micrometer has been lightened in weight by the I­section construction and by boring holes in the frame.


c.Inside Micrometer.

(1)   General. An inside micrometer (figure 42 ) is used to measure inside diameters or between parallel surfaces. They are available in sizes ranging from 0.200 inch to about 107 inches. However, the average inside micrometer set has a range that extends from 2 to 10 inches. The various steps in covering this range are obtained by means of extension rods. The minimum dimension that can be checked is determined by the length of the unit with its shortest anvil in place and the screw set to zero. It consists of an ordinary micrometer head, except that the outer end of the sleeve carries a contact point attached to a measuring rod. The micrometer set may also contain a collar for splitting the inch step between the rods. The collar, which is 1/2 inch long, extends the rod another 1/2 inch so that the range of each step can be made to overlap the next. The range of the micrometer screw itself is very short when compared to its measuring range. The smallest models have a 1/4 inch screw, and the largest has only a 1 inch screw.


(2)   Extension Rods. The individual interchangeable extension rods that are assembled to the micrometer head vary in size by 1 inch. A small sleeve or bushing, which is 0.500 inch long, is used with these rods in most inside micrometer sets to provide the complete range of sizes. Using the inside micrometer is slightly more difficult than using the outside micrometer, primarily because there is more chance of not getting the same “feel” or measurement each time the surface is checked.

(3) Using an Inside Micrometer.

(a) The correct way to measure an inside diameter is to hold the micrometer in place with one hand and “feel” for the maximum possible setting of the micrometer by rocking the extension rod from left to right and in and out of the hole. The micrometer is adjusted to a slightly larger measurement after each series of rocking movements until no rocking from left to right is possible and a very slight drag is felt on the in and out rocking movement. There are no specific guidelines on the number of positions within a hole that should be measured. When checking for a taper, the measurements should be made as far apart as possible within the hole. When checking for roundness or concentricity of a hole, several measurements should be taken at different angular positions in the same area of the hole. A reading can be taken directly from the inside micrometer head, or an outside micrometer may be used to measure the inside micrometer.

(b) The normal procedure in using an inside micrometer is to set it across a diameter or between the inside surfaces, remove it, then read the dimension. For this reason, the thimble on an inside micrometer is much stiffer than the one used on an outside micrometer and it holds the dimensions well. It helps at times to verify the reading of an inside micrometer by measuring it with an outside micrometer.


(4) Transferring Measurements. To transfer a measurement from an inside to an outside micrometer, the following steps can be followed:

Step 1. After setting the inside micrometer or an inside caliper to the work, hold the outside micrometer with one hand and the inside micrometer with the other hand.
Step 2. Turn the thimble of the outside micrometer with the thumb and the forefinger until you feel the inside tool legs lightly contact the anvil and the spindle of the outside micrometer. Hold the tips of the inside tool legs parallel to the axis of the outside micrometer spindle.
Step 3.The outside micrometer will be accurately set when the inside tool will just pass between the anvil and the spindle of the outside micrometer on its own weight.

d.Depth Micrometer. A depth micrometer is used to measure the depth of holes, slots, counterbores, recesses, and the distance from a surface to some recessed part. This type of micrometer is read exactly opposite to the method used to read an outside micrometer. The zero is located toward the closed end of the thimble. The measurement is read in reverse and increases in amount (depth) as the thimble moves toward the base of the instrument. The extension rods come either round or flat (blade like) to permit measuring a narrow, deep recess or groove.

e.Thread Micrometers.

(1)   Thread micrometers are used to measure the pitch diameter of threads. They are graduated and read in the same manner as ordinary micrometers. However, the anvil and spindle are ground to the shape of a thread.

(2)   Thread micrometers are used to measure the depth of threads that have an included angle of 60°. The measurement obtained represents the pitch diameter of the thread. They are available in sizes that measure pitch diameters up to 2 inches. Each micrometer has a given range of the number of threads per inch that can be measured correctly.

f. Ball Micrometer. This type of micrometer has a rounded anvil and a flat spindle. It can be used to check the wall thickness of cylinders, sleeves, rings, and other parts that have a hole bored in a piece of material. The rounded anvil is placed inside the hole and the spindle is brought in contact with the outside diameter. Ball attachments are available to fit over the anvil of regular outside micrometers. When these attachments are used, compensation for the diameter of the ball must be added to the regular reading.

g.Blade Micrometer. A blade micrometer has an anvil and a spindle that are thin and flat. The spindle does not rotate. This micrometer is especially useful in measuring the depth of narrow grooves, such as an O­ring seat, on outside diameters.

h.     Groove Micrometers.A groove micrometer looks like an inside micrometer with two flat disks. The distance between the disks increases as the thimble on the micrometer is turned. It is used to measure the width of grooves or recesses on either the outside or the inside diameter. The width of an internal O­ring groove is an excellent example.

i.     Care of Micrometers. The micrometer is one of the most used, and often one of the most abused, precision instruments in the shops. Careful observation of the do's and don'ts listed below will enable machinists to take better care of the micrometers they use.

(1)   Always stop the workpiece before taking a measurement. Do not measure moving parts because the micrometer may get caught in the rotating workpiece and be severely damaged.

(2)   Always open a micrometer by holding the frame with one hand and turning the knurled sleeve or thimble with the other hand. Never open a micrometer by twirling the frame, because such practice will put unnecessary strain on the instrument and cause excessive wear of the threads.

(3)   Apply only moderate force to the knurled thimble when taking a measurement. Always use the friction slip ratchet if there is one on the instrument. Too much pressure on the knurled sleeve will not only result in an inaccurate reading, but may also cause the frame to spring, forcing the measuring surfaces out of line.

(4)When a micrometer is not in use, place it where it is not likely to be dropped. Dropping a micrometer can cause the frame to spring; if dropped, the instrument should be checked for accuracy.

(5) Before a micrometer is returned to storage, the spindle should be backed away from the anvil. Wipe all exterior surfaces with a clean, soft cloth, and coat the surfaces with a light oil. Do not reset the measuring surfaces to close contact because the projecting film of oil will be squeezed out.

j.Maintenance of Micrometers. A micrometer should be checked for zero setting (and adjusted when necessary) as a matter of routine to ensure that reliable readings are being obtained. To do this, proceed as follows:

(1)   Wipe the measuring faces with a clean soft cloth, making sure that they are perfectly clean. Use the same moderate force that is ordinarily used when a measurement is taken and bring the spindle into contact with the anvil. The reading should be zero; if it is not, the micrometer needs further checking.

(2)   If the reading is more than zero, examine the edges of the measuring faces for burrs. Should burrs be present, remove them with a small slip of oilstone. Clean the surfaces again, then recheck the micrometer setting for zero.

(3)   If the reading is less than zero, or if a zero reading is not obtained after making the correction described in (2) above, the relation between the spindle and the thimble will have to be adjusted.The method for setting zero differs considerably between the makes of micrometers. Some makes have a thimble cap which locks the thimble to the spindle; some have a rotatable sleeve on the barrel that can be unlocked; and some have an adjustable anvil.

(4)   Methods For Setting Zero.

(a)   To adjust the thimble­cap type, back the spindle away from the anvil, release the cap with, the small spanner wrench provided for that purpose, and bring the spindle in contact with the anvil. Hold the spindle firmly with one hand and rotate the thimble to zero with the other. After zero relation has been established, rotate the spindle counterclockwise to open the micrometer, then tighten the thimble cap. After tightening the cap, check the zero setting again to be sure that the thimble­spindle relation was not disturbed while the cap was being tightened.

(b)   To adjust the rotable­sleeve type, unlock the barrel sleeve with the small spanner wrench provided for that purpose. Bring the spindle into contact with the anvil and rotate the sleeve into alignment with the zero mark on the thimble. After alignment is made, back the spindle away from the anvil, and retighten the barrel sleeve locking nut. Recheck for zero setting to be sure that the thimble spindle relation was not disturbed while the locking nut was being tightened.

(c)   To set zero on the adjustable­anvil type, bring the thimble to a zero reading. Lock the spindle, if a spindle lock is available, and loosen the anvil lock screw. After the lock screw is loosened, bring the anvil in contact with the spindle making sure that the thimble is still set on zero. Tighten the anvil setscrew lock nut slightly, unlock the spindle, and back the spindle away from the anvil; then lock the anvil setscrew firmly. After locking the setscrew, check the micrometer for zero setting to make sure that the anvil was not moved out of position while the set screw was being tightened.

(d)   The zero check and methods of adjustment apply directly to micrometers that will measure to zero. The procedures for larger micrometers is essentially the same, except that a standard must be placed between the anvil and the spindle in order to get a zero measuring reference. For example, a 2 inch micrometer is furnished with a 1 inch standard. To check for zero setting, place the standard between the spindle and the anvil and measure the standard. If zero is not indicated, the micrometer needs adjusting.

(e) All micrometers should be disassembled periodically for cleaning and lubrication of internal parts. When this is performed, each part should be cleaned in a noncorrosive solvent, completely dried, then given a light coat of watchmaker's oil or a similar light oil.

k. Vernier Calipers.


(1) General. This type of caliper (figure 44) uses the vernier scale. The vernier scale consists of a short auxiliary scale usually having one more graduation in the same length as the longer main scale. The vernier caliper consists of an L­shaped frame, the end of which is a fixed jaw; the long arm of the L is inscribed with the main true scale or fixed scale. The sliding jaw carries the vernier scale on either side. The scale on the front side is for outside measurements; the scale on the back is for inside measurements. On some vernier calipers, the metric system of measurement is placed on the back side of the caliper in lieu of a scale used for inside measurements. In such cases, add the thickness of the nibs to the reading when making inside measurements. The tips of the jaws are formed so that inside measurements can be taken. The thickness of the measuring points is automatically compensated for on the inside scale. The length of the jaws will range from 1 1/4 inches to 3 1/2 inches, and the minimum inside measurement with the smallest caliper is 1/4 inch or 6 millimeters. Vernier calipers are made in standard sizes of 6, 12, 24, 36, and 48 inches, and 150, 300, 600, and 900 millimeters. The jaws of all vernier calipers, except the larger sizes, have two center points, which are particularly useful in setting the dividers to the exact dimensions. 

(2) Reading a Vernier Caliper.


(a) To read a vernier caliper (figure 45) , one must be able to understand both the steel rule and the vernier scales. The steel rule (1) is graduated in 0.025 of an inch. Every fourth division (2) (representing a tenth of an inch) is numbered. 

(b)    The vernier scale (3) is divided into 25 parts and numbered 0, 5, 10, 15, 20, and 25. These 25 parts are equal to 24 parts on the steel or main rule (1). The difference between the width of one of the 25 spaces on the vernier scale and one of the 24 spaces on the main rule (1) is 1/1000 (0.001) of an inch.

(c) Read the measurement in figure 45 on the previous page as illustrated below:

Read the number of whole inches on the top scale (1) to the left
of the vernier zero index (4) and record
1.000 inch
Read the number of tenths (5) to the left of the vernier zero
index (4) and record
0.400 inch
Read the number of twenty-fifths (6) between the tenths mark (5)
and the zero index (4) and record
3 X .025 = .075 inch
Read the highest line on the vernier scale (3) which lines up with a line
on the top scale (7) and record. (Remember 1/25 = .001 inch)
11/25 or 0.011 inch


1.486 inches

(d) Most vernier calipers read OUTSIDE on one side and INSIDE on the other side. If a scale isn't marked, and an inside measurement is to be taken, read the scale as you would for an outside diameter. Then add the measuring point allowance by referring to manufacturer's instructions or the following table.

(e) Reading a Metric Caliper (figure 46 ). The same principle is applied in reading the metric graduated vernier; however, the following differences should be noted:


1 The steel rule or main scale (1) is divided into centimeters (cm) (2) and the longest lines represent 10 millimeters each instead of inches. Each millimeter is divided into quarters or fourths.

2 The metric vernier scale (3) is divided into 25 equal parts and is numbered 0, 5, 10, 20, and 25.

  1. Read the total number of millimeters (4) to the left of the vernier zero index (5) and record
  1. Read the number of quarters (6) between the millimeter mark and the zero index and record
  1. Read the highest line on the vernier scale (3) which lines up with a line on the scale (7) and record
  Total = 32.43mm

(3)    Use and Application. The vernier caliper has a wide range of measurement applications, and the shape of the measuring jaws and their position with respect to the scale makes this tool more adaptable than a micrometer. However, the vernier caliper does not have the accuracy of a micrometer. In any 1 inch of its length, a vernier caliper should be accurate within 0.001 of an inch. In any 12 inches, it should be accurate within 0.002 of an inch. Inaccuracy increases about 0.001 of an inch for every additional 12 inches. The accuracy of measurements made with a vernier caliper is dependent on the user's ability to feel the measurement. Because the jaws are long, and there is the possibility of play in the adjustable jaw, especially if an excessive measuring pressure is used, it is necessary that one develop an ability to handle the vernier caliper. This touch may be acquired by measuring such known standards as gage blocks and plug gages. There are various applications of the vernier caliper. In figure 47, view A, , the machinist is checking the outside diameter of a part. One hand is holding the stationary jaw to locate it, while the other hand operates the adjusting nut and moves the sliding jaw to the workpiece. The same procedure is used in view B in checking the inside dimension.


(4)    Care.

(a)   The accuracy of the vernier caliper depends on the condition of fit of the sliding jaw, and on the wear and distortion in the measuring surfaces. The fit of the sliding jaw should be such that it can be moved easily and still not have any play. It may be adjusted by removing the gib in the sliding jaw assembly and bending it. The function of the gib is to hold the adjusting jaw against the inside surface of the blade with just the right amount of pressure to give it the proper friction.

(b)   Wear on the jaws of the vernier caliper is mostly at the tips where most of the measurements are made. A certain amount of this wear may be taken up by adjusting the vernier scale itself. This scale is mounted with screws in elongated holes which permit it to be adjusted slightly to compensate for wear and distortion. When the error exceeds 0.0002 of an inch, either in parallelism or flatness, the caliper should be returned to the manufacturer for reconditioning. Wear on the jaws can best be checked by visual means and by using measuring rolls or rings of known dimensions.

(c)   When the machinist is finished using the verniers for the day, he should coat all metal parts with a light coat of oil to prevent them from rusting. Calipers should be stored in separate containers that are provided for them. The graduations and markings on all calipers should be kept clean and legible.

(d)   Care should be taken to prevent dropping any caliper, as small nicks or scratches can cause inaccurate measurements. Also protect the vernier caliper points from damage.

(e) Vernier gages also require careful handling and proper maintenance if they are to remain accurate. The following instructions apply to vernier in general:

l. Universal Vernier Bevel Protractor.

(1)  General. The universal vernier bevel protractor (figure 48 ) is used to lay out or measure angles on a workpiece to very close tolerances. It reads to 5 minutes or 1/20° and can be used completely through 360°. The protractor is made up of an adjustable blade (1) and a graduated dial (2) which contains a vernier scale. The bevel protractor is used to establish an angle and determine its relationship to the other surfaces. The acute angle attachment (3) is used for measuring acute angles separately. The tool can be laid flat upon paper or on the workpiece. The dial is held rigidly in position and the blade can be moved back and forth and clamped independently of the dial. Interpreting the reading on the protractor is similar to the method used on the vernier caliper.


(2)  Reading the Protractor Vernier Scale. (a) General. The protractor vernier scale indicates every five minutes (5") or 1/20°. Each space on the vernier scale is 5 minutes less than two spaces on the main scale. When the zero on the vernier stale exactly coincides with a graduation on the main scale, the reading is in exact degrees, as shown in figure 49, view A, .When the zero of the vernier scale does not exactly coincide with a graduation on the main scale, check the graduations on the vernier and locate one that lines up with a graduation on the main scale.


(b) Once this graduation is located it will indicate the number of 12ths of a degree in units of 5 minutes that is to be added to the whole degree reading. Example: Figure 49, view B, shows the zero on the vernier between 12° and 13° on the main scale. Counting to the right from the 0 on the main scale, the 0 on the vernier has moved 12 whole degrees. Reading in the same direction (to the right), note that the 10th line of the vernier scale coincides exactly with a line on the main scale. The tenth line of the vernier indicates 50 minutes (50") , since each line indicates 5 minutes.

Now add 50 minutes to the 12° and the final reading is 12° and 50 minutes. Since the spaces, both on the main scale and the vernier scale, are numbered both to the right and to the left from zero, any angle can be measured. The readings can be taken either to the right or to the left, according to the direction in which the zero on the main scale is moved.

(3) Use and Application. The bevel protractor is used to measure the angular clearance on a ring gear (figure 50, view A, ). The bevel protractor can be used to establish an angle and determine its relationship to other surfaces as shown in figure 50, view B. The acute angle attachment is connected to the slotted extension of the dial and is used to accurately measure acute angles as shown in figure 50, view C.


m. Gear Tooth Vernier Caliper.


(1) A gear tooth vernier (figure 51, views A and B, on page 80) is used to measure the chordal thickness of a gear tooth on the pitch circle and the distance from the top of the tooth to the pitch chord (chordal addendum) at the same time. The vernier scale on this tool is read in the same way as other verniers, except that graduations on the main scale are 0.020 inch apart instead of 0.025 of an inch.


(2) In order to understand measurements made with the gear tooth vernier, the machinist should know gear tooth terminology. The following terms (figure 52) are used to describe gear and gear teeth (symbols in parentheses are standard gear nomenclature symbols).

(a) Outside Circle (OC): The circle formed by the tops of the gear teeth.

(b) Outside Diameter (OD): The diameter the gear blank is machined to; the overall diameter of the gear.

(c)    Pitch Circle (PC): (1) Contact point of the mating gears, basis of all tooth dimensions. (2) Imaginary circle one addendum distance down the tooth.

(d) Pitch Diameter (PD): (1) The diameter of the pitch circle. (2) In parallel shaft gears, the pitch diameter can be determined directly from the center to center distance and the number of teeth.

(e)  Root Circle (RC): The circle formed by the bottoms of the gear teeth.

(f)   Root Diameter (RD): The distance from one side of the root circle to the opposite side passing through the center of the gear.

(g)   Addendum (ADD): The height of that part of the tooth extending outside the pitch circle.

(h)    Circular Pitch (CP): The distance from a point on one tooth to a corresponding point on the next tooth is measured on the pitch circle.

(i)  Circular Thickness (CT): (1) One­half of the circular pitch. (2) The length of the arc between the two sides of a gear tooth on the pitch circle.

(j)  Clearance (CL): The space between the top of the tooth of one gear and the bottom of the tooth of the mating gear.

(k) Dedendum (DED): (1) The depth of the tooth inside of the pitch circle. (2) The radial distance between the root circle and the pitch circle.

(l)  Whole Depth (WD): The radial depth between the circle that bounds the top of the gear teeth and that which bounds the bottom.

(m) Working Depth (WKD): (1) The whole depth minus the clearance. (2) The depth of the engagement of the two mating gears; the sum of their addendums.

(n)   Chordal Thickness (tc): (1) The thickness of the tooth measured at the pitch circle. (2) The section of the tooth that is measured to see if the gear is cut correctly.

(o)   Chord Addendum (ac): The distance from the top of a gear tooth to a chord (pitch chord) subtending (to extend under) the intersections of the tooth thickness arc and the sides of the tooth (used for setting the gear tooth vernier calipers for measuring tooth thickness).

(p)   Diametral Pitch (DP): (1) The most important calculation, it regulates the tooth size. (2) The ratio of the number of inches of the pitch diameter. (3) The number of teeth that will go into each inch of the pitch diameter evenly.

(q)   Number of Teeth (NT): The actual number of teeth of the gear.

(r) Backlash (B): The difference between the tooth thickness and the tooth space of the engaged gear teeth at the pitch circle.

(3) Use and Application. When selecting gear cutters for a gear having 24 teeth, a number 5 cutter would be used. The reason for this is that this particular cutter will cut all gears containing from 21 to 25 teeth. However, for measuring these teeth, certain values must be known. To check the dimensional accuracy of gear teeth, a gear tooth vernier caliper is used (figure 51, view B,) . The vertical scale is adjusted to the Chordal Addendum (ac), and the horizontal scale is used for finding the Chordal Thickness(tc). Before calculating the chordal addendum and performing any measurements, the tooth thickness (t) and addendum (a) must first be determined. The following formulas must be applied:

(a) The formula for the addendum is

AD =


(b) The formula for the chordal thickness is

tc = PD sin


4. Conclusion

Measuring workpieces and determining the correct amount of material needed to be removed is one of the tasks performed by the machinist. The machine, with the use of a machine tool, can only remove the correct amount of stock as determined and programmed by the operator. Therefore, as a machinist, it is essential that one becomes proficient in the use and care of precision measuring tools and gages in order to machine parts and components to the correct degree of accuracy. Undersized or oversized work is not acceptable in most machine shops and waste amounts to money lost. It is hoped that the information in this lesson will enlighten you on the proper use and care of the various measuring tools and gages.