How To Use Measuring and Layout Tools
In this book, you will learn about different types of measuring and layout
tools and and how to use them. You will also learn how to select the right
measuring and layout tools for your do-it-yourself project, read various
types of measuring tools, and provide the proper care of measuring and
layout tools to keep them in good working condition.
How To Read The Scale of a Ruler or Tape Measure
In one sense, the term "scale" means the scale of a drawing. In another
sense, it means the succession of graduations on any graduated standard
of linear measurement, such as the graduations on a steel tape or ruler.
The more common type rules and tapes are divided into fractions, inches,
and feet. Explained here are the scales on a 12-inch steel machinists rule
Figure 1 - 12-inch Machinists rule.
The rule is divided into twelve inches. The inches are further divided
into eighths, sixteenths, thirty-seconds, and sixty-fourths.
Look at the rule. There is a small numeral marked on the end of the
rule nearest the 1-inch mark. This numeral indicates the number of divisions
per inch (Figure 2).
Figure 2 - Divisions per inch
When referring to fractions, always use the reduced name. This is the
smallest numerator (top number) and denominator (bottom number). For example,
3/6 can be reduced to 1/2 by dividing both the top and bottom by 3. Generally,
fractions may be reduced to their lowest forms by repeated division by
2 or 3.
Look at the section between the "2" and the "3" on the edge marked with
an "8" for eighths (Figure 3).
Figure 3 - 1/8 -inch scale
There are eight equally spaced lines. The lengths of these lines differ
and indicate different fractions or parts of an inch.
The longest line is in the center and is equal to 4/8 - or 1/2-inch.
Each half-inch is divided in half by a slightly shorter line indicating
2/8- or 1/4-inch on the left and 6/8 - or 3/4- inch on the right.
Each 1/4 -inch is divided in half by the shortest line which indicates
1/8 -inch, and will indicate 1/8 -, 3/8 -, 5/8- and 7/8 -inch.
Now turn the rule and look at the edge with a 16 marked on it (Figure
Figure 4 - 1/16-inch scale.
There are now 16 equal divisions between each inch. Since 2/16-inch
reduces to 1/8 -inch, divide each 1/8-inch into two equal parts producing
1/16-, 3/16-, 5/16-, 7/16-, 9/16-, 11/16-, 13/16-, and 15/16-inch.
Common tapes and rules usually are not graduated smaller than sixteenths.
However, precision measurements require smaller graduations.
Look at the back of the machinists rule. Find the edge marked 32 (Figure
5) and once again look between the numbers "2" and "3."
Figure 5 - 1/32-inch scale.
To read this rule, remember:
1. Sixteen divisions (16/32) are equal to 1/2-inch.
2. Eight divisions (8/32) are equal to 1/4-inch.
3. Four divisions (4/32) are equal to 1/8-inch.
4. Two divisions (2/32) are equal to 1/16-inch.
To read 2 5/8-inches on the scale, first find the two inch mark, then
determine the number of 32nds in 5/8-inch.
To determine the number of 32nds in 5/8-inch, remember four divisions
or 4/32 are equal to 1/8-inch. If 1/8-inch is equal to 4/32-inch, then
5/8-inch is equal to 20/32-inch as shown:
If 1/8 = 4/32, then 5/8 = 20/32-inch (4 x 5 = 20)
1. Find the 20/32-inch reading on the scale as shown above.
2. Write the new fraction 2 20/32-inches.
Finally, look at the edge marked 64 (Figure 6). Each inch is now divided
into 64 equal parts.
Figure 6 - 1/64-inch scale.
To read this rule, remember:
1. Thirty-two divisions (32/64) are equal to 1/2- inch.
2. Sixteen divisions (16/64) are equal to 1/4-inch.
3. Eight divisions (8/64) are equal to 1/8-inch.
4. Four divisions (4/64) are equal to 1/16-inch.
5. Two divisions (2/64) are equal to 1/32-inch.
To read 2 3/4 -inches on this scale, first find the two inch mark. Next,
determine the number of 64ths in 3/4 -inch.
To determine the number of 64ths in 3/4 -inch, remember every sixteen
divisions or 16/64 are equal to 1/4-inch. If 1/4-inch is equal to 16/64-inch,
then 3/4-inch is equal to 48/64-inch as shown:
If 1/4 = 16/64, then 3/4 = 48/64-inch (16x3=48)
1. Locate the number 48 between the 2- and 3- inch marks on the scale.
2. Write the new fraction 2 48/64-inches.
How to Read a Metric Ruler
The metric system is based upon multiples of ten. For example, there are
10 millimeters in a centimeter and 100 centimeters in a meter.
Figure 7 - Metric rule.
The example provided will deal only with millimeters (mm).
The meter is the starting point. From that point, there are two scales
for measuring. A meter divided by 100 equals a centimeter (cm), 1/100 or
Next divide a centimeter (cm) by 10. This will equal a millimeter (mm),
1/1000 or 0.001 meter.
Now let's look at a section of the rule between 2 cm and 3 cm.
There are 10 equal divisions which are equal to 1/10 cm or 1 mm.
To measure 26 mm, first locate the longest line designated 2 cm or 20
Next count 6 additional lines to find 26 mm.
There is a table for converting from U.S. Common to metric or vice versa
in the inside back cover of this manual.
Measuring and Layout Tools
There are many types of tools used to measure and lay out projects. Measuring
tools include flat steel rules, measuring tapes, wooden folding rules,
digital measuring devices, and measuring wheels. Levels are used to check
that project components are level and/or plumb. Plumb bobs are used to
check that project components are perfectly upright. Squares are used to
mark, check, and measure components of construction projects. When you
consider which of these tools to use, keep in mind the following points:
The tool must be accurate.
The tool should be easy to use.
The tool should be durable.
Numbers on the tool must be easy to read. Black numbers on a yellow or
off white background work well.
Rules and Steel Tapes
The rule or tape is used for measuring where accuracy is not an extremely
critical factor. They can be rigid or flexible, come in various lengths,
and can be made of wood, metal, cloth, or fiberglass.
The flat steel rule (Figure 8) is the simplest measuring tool. It is usually
6 or 12 inches in length but can be longer. Steel rules can be rigid or
flexible, thin or wide. It is easier and more accurate to use a thin rule,
since it is closer to the work being measured.
Figure 8 - Steel rule.
Flat steel rules can have up to four sets of marks, two on each side
of the blade. Rules with four sets of marks are set up with divisions of
1/8 inch and 1/16 inch on one side, and divisions of 1/32 inch and 1/64
inch on the other side. The marks are longer for a division of 1/2 inch,
scaling down in length from 1/4 inch through 1/64 inch.
There are many variations of the common rule. Sometimes the graduations
are on one side only, sometimes a set of graduations is added across one
end for measuring in narrow spaces, and sometimes only the first inch is
divided into sixty-fourths, with the remaining inches divided into thirty-seconds
and sixteenths. A metal or wood folding rule may be used.
A folding rule (Figure 9) is made up of hardwood, steel, or aluminum sections,
each measuring 6 to 8 inch. The sections are connected by spring joints
that unfold for measuring distances.
Figure 9 - Folding rule.
These folding rules are usually from 2 to 6 feet long. The folding rules
cannot be relied on for extremely accurate measurements because a certain
amount of play develops at the joints after continued use.
A measuring tape (Figure 10) can come in any length from 6 to 50 feet.
The most common are 10, 16, and 25 feet. Shorter tapes usually have a curved
cross section so they roll easily but stay rigid when extended. Longer
tapes are usually flat and should be laid along a surface to avoid sagging
in the middle.
Figure 10 -Measuring tape.
A locking mechanism, such as a sliding button, keeps the tape locked
in place while a measurement is being taken. Other locking mechanisms,
such as levers and toggles, allow the tape to be retracted after measuring
by simply squeezing them. In any case, a spring mechanism in the case automatically
retracts the tape.
How to Use a Measuring Tape
Follow these steps to use a measuring tape properly:
1. Pull the tape out to the desired length.
2. Place the hook over the edge of the material you are measuring.
3. Lock the tape in place.
4. Record or mark the measurement.
5. Unhook the tape from the edge of the material.
6. Release the lock and rewind the tape.
Digital Measuring Devices
Digital measuring devices (Figure 11) are similar to conventional measuring
devices, but their digital readouts make measurement readings more precise.
They give you the ability to convert fractions to decimal or metric equivalents.
A useful function of these devices is their ability to compensate for the
size of the tape case when making measurements inside a window frame or
door jamb. Some devices have a memory function that holds a measurement;
others have a voice recorder to keep track of multiple measurements.
Figure 11 - Digital measuring device.
Carpenter's Square (Framing Square)
The Carpenter's Square, or Framing Square (Figure 12), has a large arm,
called the blade, and a small arm, called the tongue. The arms meet in
a 90-degree angle. The square is used to mark, check, and measure components
of construction projects. It has several scales etched onto the surface
for quick reference: a diagonal scale, a board foot scale, and an octagonal
scale. It has ruler increments etched on the inside and outside edges.
Figure 12 - Carpenter's (Framing) Square.
The face side contains the manufacturer's name and the inches are divided
into eighths and sixteenths (Figure 13). There are two tables down the
Figure 13 - Parts of a Carpenter's (Framing) Square.
The rafter table is used for determining the length and cut of rafters.
The octagon or eight square scale is used for cutting an octagon from
a square piece of material.
The back side contains the hundredths scale and is divided into tenths,
twelfths, and sixteenths as shown. There are two tables down the center.
The Essex board measure is used to compute the number of board feet
in a given piece of lumber.
The brace measure is used to find the exact lengths of common braces.
Common scales or inch divisions found on the carpenter's square are
listed in Table 4-2.
Table 1 - Scales and measurements of a carpenter's (framing) square
The try square (Figure 14) is an L-shaped tool used as a guide to lay out
90 degree cuts with pencil markings. It is also used to check that the
edges and ends of boards are square, and whether a board is the same depth
along its entire length. A try square has broad blades 6 to 12 inches long
set at right angles.
Figure 14 - Try square.
The combination square (Figure 15 ) is used for many purposes in woodworking
and metalworking but mainly for measuring the accuracy of a right angle.
It is made up of the following components:
1. A slotted 12-inch stainless steel rule which is graduated in eighths,
sixteenths, thirty-seconds, and sixty-fourths of an inch. It can be used
as a measuring scale by itself or with any one of the following components.
2. The center head, when attached to the rule, bisects a 90-degree angle.
It is used for determining the center of cylindrical work.
3. The protractor has a level and a revolving turret which is graduated
in degrees from 0 to 180 or 0 to 90 in either direction. It is used to
lay out and measure angles to within 1 degree.
4. The square head has a level, a scribe, and 45- and 90-degree sides.
It is used to lay out 45- and 90-degree angles and to check level. It may
also be used as a height or depth gauge.
Figure 15 - Combination square.
The sliding T-Bevel (Figure 16) is made up of a slotted blade and a solid
stock. The blade is adjustable so it can be set to measure any angle. The
T-Bevel is used for testing bevels and laying out angles.
Figure 16 - Sliding T-Bevel.
The bevel protractor (Figure 17) is made up of an adjustable blade and
a graduated dial which contains a Vernier scale. The bevel protractor is
used to establish an angle and determine its relationship to other surfaces.
The acute angle attachment is used for measuring acute angles accurately.
Figure 17 - Bevel protractor.
Rafter Angle (Speed) Square
The rafter angle square, or speed square (Figure 18) is a three-sided,
triangle shaped measuring tool. It is used to draw perpendicular lines
on boards to be cut, or to lay out angles for rafters, stairs, and other
construction projects. It has degree gradations etched onto the surface
for quick layout and cutting of lumber so you don't have to perform angle
Figure 18 - Rafter Angle Square, or Speed Square
The T-Square (Figure 19) is used to measure and cut drywall. Some table
saws come with a T-Square fence attached.
Figure 19 - T-Square.
How to Use a Carpenter's Square to Mark a Square Line
To mark a line for cutting, use the following steps:
1. Find and mark where the line will be drawn.
2. Line the square up with the bottom of the object to be marked as
shown in Figure 20.
3. Mark the line to be cut; mark an x on the material to be cut away.
4. Cut off the excess material.
Figure 20 - Using a carpenter's square.
How to Check for Square
Check that joints meet at a 90-degree angle by placing the blades of the
framing square along the two sides of the angle, as shown in Figure 21.
If both blades fit tightly, the material is square. If there is any space
between either of the arms and the side closest to it, the material is
Figure 4-21 - Checking for square.
How To Use a Carpenter's Square to Layout Steps
1. An example to properly position a square when marking cut lines for
a series of steps 9 inches by 12 inches is illustrated in Figure 22.
2. Continue the process until desired number of steps has been laid
Figure 22 - Square positions for laying out steps
How To Use a Try Square
Using a try square (Figure 14) is similar to using the carpenter's square.
1. To check a square joint, place the stock against a horizontal section
and the blade against a vertical section. Light must not be seen around
blade edge. If light is seen, the work is not square.
2. To check the end of a board, place stock on vertical edge and extend
blade over the end. Light must not be seen around blade edge. If light
is seen, the work is not square.
How To Use a Sliding T-Bevel Square
Follow these steps to use a sliding T-Bevel properly:
1. Loosen the locking nut and adjust blade to measure a desired angle
using a protractor (Figure 23). Tighten the locking nut.
Figure 23 - Set the sliding T-Bevel using a protractor.
2. The angle may now be laid out by extending the blade across the board
with the stock (Figure 24) held firmly against the edge.
Figure 24 - Transfer the angle to the material.
3. Mark with a pencil or marking crayon. Make sure the square does not
move while marking (Figure 25).
Figure 25 - Mark the angle.
How To Use a Combination Square
Follow these steps to use a combination square properly:
Using as a Center Head to Find the Diameter of a Cylinder
1. Slide center head on rule and fasten by tightening setscrew (Figure
Figure 26 - Set the combination square.
2. Put the center head flush against the cylinder (Figure 27).
Figure 27 - Press against the cylinder.
3. Mark the diameter on the cylinder (Figure 28) using a pencil or marking
crayon by drawing a straight line along the inside edge. Make sure the
square does not slip while marking.
Figure 28 - Mark the diameter.
Using as a Protractor Head to Determine an Angle
1. Slide protractor head on rule (Figure 29) and fasten by tightening
Figure 29 - Set up the combination square.
2. Loosen the protractor adjustment screws so the protractor may be
pivoted about the rule. Angle being measured is already marked.
3. Place the rule on the angle being measured (Figure 30) and pivot
the protractor head against the edge. Tighten adjustment screws.
Figure 30 - Measure the angle.
4. Remove and read the measured angle on the protractor scale (Figure
Figure 31 - Verify the angle.
Using a Combination Square to Mark 90-Degree and 45-Degree Angles
Mark a 90-degree angle (Figure 32) using the following steps.
Figure 32 - Mark a 90-degree cut with a combination square.
1. Set the blade at 90 degrees (a right angle).
2. Place the square so the head fits snugly against the edge of the
material to be marked.
3. Use the blade as a straightedge to guide the mark, starting at the
edge of the material.
Mark a 45-degree angle (Figure 33) using the following steps.
Figure 33 - Mark a 45-degree cut with a combination square.
1. Set the blade at a 45-degree angle.
2. Place the square so the head fits snugly against the edge of the
material to be marked.
3. Use the blade as a straightedge to guide the mark, starting at the
edge of the material.
Care of Squares
Observe the following guidelines when working with squares:
Avoid the following to preserve the integrity of the square, as they are
expensive to replace:
Wear gloves. The edges can be very sharp.
When you use a square as a saw guide, use a clamp to hold the square so
you can keep both hands on the saw.
Make sure squares are kept clean.
Keep the square dry to prevent rust.
Use a light coat of oil on the blade. Occasionally clean the blade's grooves
and the set screw (if there is one).
A square with a loose stock is no good. Replace the
Use squares for the appropriate purpose and in the correct way.
Prying or hammering with it.
Striking it hard enough to change the angle between the blade and the head.
Dividers are instruments used for measuring distances between two points,
transferring or comparing measurements directly from a rule, or for scribing
an arc, radius, or circle.
A spring divider (Figure 34) consists of two sharp points at the end of
straight legs, held apart by a spring and adjusted by means of a screw
and nut. The spring divider is available in sizes from 3 to 10 inches in
Figure 34 - Spring divider.
A wing-type divider (Figure 35) has a steel bar that separates the legs,
a lock nut for setting a rough measurement, and an adjustment screw for
fine adjustments. The wing-type divider is available in 6, 8, and 12-inch
lengths. Also available is a divider with one removable leg, so that a
pencil may be inserted.
Figure 35 - Wing divider.
How To Use a Divider to Scribe a Circle
1. Set the desired radius on the dividers using the appropriate graduations
on a rule (Figure 36, Panel A and B).
2. Place the point of one of the divider legs on the point to be used
as the center (Figure 36, Panel C and D).
3. Lean the dividers in the direction of movement and scribe the circle
by revolving the dividers. (Figure 36, Panel E and F).
Figure 36 - Using a divider to scribe a circle.
How To Care for Dividers
Observe the following guidelines when working with dividers:
Keep dividers clean and dry.
Protect the points against damage.
Store dividers where they will not become bent or broken.
Marking gauges are made of steel (Figure 37) or wood (Figure 38). They
consist of a graduated beam about 8 inches long on which a head slides.
The head can be fastened at any point on the beam with a thumbscrew. The
thumbscrew presses a brass shoe tightly against the beam and locks it firmly
in position. A steel pin or spur marks the wood and projects from the beam
about 1/16 inch. A marking gauge is used to mark off guidelines parallel
to an edge, end, or surface of a piece of wood. It has a sharp spur or
pin that does the marking. A marking gauge must be adjusted by setting
the head the desired distance from the spur.
Figure 37 - Steel Marking gauge.
How To Use Marking Gauges
Press the head firmly against the edge of the work to be marked (Figure
38). With a wrist motion, tip the gauge forward until the spur touches
the work. Push the gauge along the edge to mark the work, keeping the head
firmly against the work.
Figure 38 - Using a marking gauge.
Adjustable parallels (39) consist of two tapered parts fitted together.
The distance between the two outside parallel surfaces varies by moving
mating parts together or apart. This distance is then measured with a micrometer.
Adjustable parallels are used as gauges for leveling and setup work. Various
sizes are available depending on the nature of work.
Figure 39 - Adjustable parallel.
Levels (Figure 40) are tools designed to prove whether a plane or surface
is in the true vertical or true horizontal. All levels consist of a liquid-filled
glass tube or tubes supported in a frame.
Figure 40 - Parts of a level.
Master Precision Level
The master precision level (Figure 41) has a ground and graduated main
vial. The top and bottom of the level are milled and ground to make sure
both surfaces are absolutely parallel. This level is used to determine
the true horizontal with the main vial. The true vertical is determined
by using the two smaller vials.
Figure 41 - Master precision level.
The machinists level (Figure 42) has an extra large vial, increasing the
accuracy and sensitivity. Some of these levels have grooved bottoms which
fit over pipes and shafts. They are used in machine shops for leveling
work and equipment.
Figure 42 - Machinist's level.
Iron Bench Level
The iron bench level (Figure 43) is made of a special design casting which
ensures its lightness, strength, and rigidity. It is used mostly in the
construction industry. It may also be used in a machine shop.
Figure 43 - Iron bench level.
The striding level (Figure 44) is a machinists level which is mounted on
a raised base. This level is used to span existing cabling, piping, or
similar obstructions. It is extremely useful in a machine shop for checking
the true horizontal of the flatway on a lathe.
Figure 44 - Striding level
The carpenter's level (Figure 45) has three vials which are mounted horizontally,
vertically, and at a 45-degree angle. The carpenter's level is used in
construction for checking for true vertical, true horizontal, and 45-degree
Figure 45 - Carpenter's level.
The line level (Figure 46) is a single vial in a metal case with a hook
on each end for hanging on a cord. It is used to check whether two points
are level, such as two points on a floor or in an elevation. It must be
used with a tightly stretched cord.
Figure 46 - Line level.
The torpedo level (Figure 47) is a small level, generally 6 to 9 inches
in length. Its name is derived from its boat-like shape, tapered at both
ends. It is useful in small spaces where a larger level would not fit.
Figure 47 - Torpedo level.
The digital level (Figure 48) has two vials; one to check for level, the
other to check for plumb. It also includes a digital readout for:
Degrees of slope.
Inches per foot of rise and run for stairs and roofs.
Percentage of slope for drainage on decks and masonry.
Figure 48 - Digital level.
A laser level is used to level and provide reference lines for tasks such
as setting foundation levels, establishing drainage slopes, aligning plumbing
and electrical lines, and setting tile. It can be mounted on a tripod,
fixed to pipes or framing studs, or suspended from ceiling framing.
How Check a Level For Accuracy
A level may be checked for accuracy by placing it on a known level surface
and noting the position of the bubble. Reverse the level end for end. Observe
the position of the bubble. If the relative position of the bubble was
the same for both readings, the level is accurate.
How To Use a Level
The carpenter and torpedo levels are easy to use. All you need is a careful
eye to read it correctly.
1. Place the level on the object you need to check. Lay it on a horizontal
surface or hold it against a vertical surface.
2. Check the air bubble in the vial. When the bubble is centered between
the two lines on the vial, the object you are checking is level if you
are checking a horizontal surface, plumb if you are checking a vertical
surface, or at a 45-degree angle if you are checking the angle.
Care of Levels
You are not likely to have any personal injuries from using a level. However,
you can damage this sensitive instrument if you don't handle it carefully.
Observe the following guidelines when working with levels:
Replace the level if any of the vials are cracked or broken.
Keep the level clean and dry. Keep the level in its case when not in use.
Use the level properly. Avoid bending or applying excessive pressure on
the level and dropping or bumping the level.
Simple calipers (Figure 49) are used in conjunction with a scale or rule
to determine the thickness or the diameter of a surface, or the distance
between surfaces. A caliper is usually used in one of two ways. Either
the caliper is set to the dimension of the work and the dimension transferred
to a scale, or the caliper is set on a scale and the work machined until
it checks with the dimension setup on the caliper.
Figure 49 - Simple calipers.
How to Adjust Inside and Outside Calipers
To adjust a caliper to a scale dimension, hold one leg of the caliper firmly
against one end of the scale and adjust the other leg to the desired dimension.
To adjust a caliper to the work, open the legs wider than the work and
then bring them down to the work.
Outside calipers for measuring outside diameters are bow-legged; those
used for inside diameters have straight legs with the feet turned outward.
Calipers are adjusted by pulling or pushing the legs to open or close them.
Fine adjustment is made by tapping one leg lightly on a hard surface to
close them, or by turning them upside down and tapping on the joint end
to open them.
The simple outside calipers are bowlegged. Those used for inside diameters
have straight legs with feet turned outward. Calipers are adjusted by pulling
or pushing the legs to open or close them.
Transfer calipers are used for measuring chamfered grooves or flanges.
A screw attaches a small auxiliary leaf to one of the legs.
The measurement is made as with ordinary calipers. The leaf is locked
to the leg. The legs may be opened or closed as needed to clear the obstruction.
The legs are then brought back and locked to the leaf, restoring them to
the original setting.
Another type of caliper is the hermaphrodite, sometimes called the odd-leg
caliper. This caliper has one straight leg ending in a sharp point, sometimes
removable, and one bow leg. The hermaphrodite caliper is used chiefly for
locating the center of a shaft, or for locating a shoulder.
Spring Joint Calipers
Spring joint calipers have the legs joined by a strong spring hinge and
linked together by a screw and adjusting nut. For measuring chamfered cavities
(grooves) or for use over flanges, transfer calipers are available. They
are equipped with a small auxiliary leaf attached to one of the legs by
a screw. The measurement is made as with ordinary calipers; then the leaf
is locked to the leg. The legs may then be opened or closed as needed to
clear the obstruction, and brought back and locked to the leaf again, thus
restoring them to the original setting.
Slide calipers (Figure 50) can be used for measuring outside and inside
dimensions. Graduations are in inches, fractions, or millimeters. One side
of the caliper is used to measure outside and the other side is used to
measure inside dimensions. Stamped on the frame are the words "IN" and
"OUT." You use them when taking inside and outside measurements. The other
side of the caliper is used as a straight measuring rule.
Figure 50 - Slide caliper.
The trammel (Figure 51) measures distances beyond the range of calipers.
The instrument consists of a rule, rod, or beam to which trams are clamped.
The trams carry chucks. The trammel can also be used as a divider by changing
Figure 51 - Trammel.
Vernier calipers (Figure 52) work like slide calipers. The Vernier calipers
can make very accurate outside or inside measurements. A Vernier caliper
is used by loosening the two locking screws, allowing the movable jaw to
slide along the rule until desired position is obtained. The locking screw
is then retightened securing the movable jaw. Any fine adjustments to the
Vernier scale are made using adjustment control. The locking screw is then
secured and Vernier caliper is ready to read.
Figure 52 - Vernier caliper.
How to Read a Vernier Caliper
To read a Vernier caliper (Figure 53), you must be able to understand both
the steel rule and Vernier scales. The steel rule is graduated in 0.025
of an inch. Every fourth division (representing a tenth of an inch) is
Figure 53 - Reading a Vernier caliper.
The Vernier scale 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 rule. The
difference between the width of one of the 25 spaces on the Vernier scale
and one of the 24 spaces on the steel rule is a thousandth of an inch.
Read the measurement as shown in Figure 53.
Read the number of whole inches on the top scale to the left of the
Vernier zero index and record: 1.000 inch.
Read the number of tenths to the left of the Vernier zero index and
record: 0.400 inch.
Read the number of twenty-fifths between the tenths mark and the zero
index and record: 3 x .025 = .075 inch.
Read the highest line on the Vernier scale (3) that lines up with the
lines on the top scale and record (Remember 1/25 = 0.001 inch): 11/25 or
TOTAL: 1.486 inches.
Most Vernier calipers read outside on one side and inside on the other
side. If a scale isn't marked, and you want to take an inside measurement,
read the scale as you would for an outside diameter. Then add the measuring
point allowance by referring to manufacturer's instructions. An example
of the additional measurement allowance is illustrated in Table 2.
Table 2 - Additional measurement allowance
How to Reading a Metric Caliper
The steel rule is divided into centimeters (cm) (Figure 54) and the longest
lines represent 10 millimeters (mm) each. Each millimeter is divided into
quarters. The Vernier scale is divided into 25 parts and is numbered 0,
5, 10, 15, 20, and 25.
Figure 54 - Reading a metric caliper.
Care of Calipers
Observe the following guidelines when working with calipers:
Store calipers in separate containers provided.
Keep graduations and markings on all calipers clean and legible.
Do not drop any caliper. Small nicks or scratches can cause inaccurate
Protect caliper points from damage.
Micrometers (Figure 55) are instruments used to measure distances to the
nearest one thousandth of an inch. These measurements are expressed or
written as a decimal (0.0001, 0.001, 0.01), so to use them you must know
how to read and write decimals. There are four types of micrometer calipers,
commonly called micrometers or simply mikes: the outside micrometer, the
inside micrometer, the depth micrometer, and the screw thread micrometer.
Figure 55 - Common types of micrometers.
The outside micrometer (Figure 56) is used for measuring outside dimensions,
such as the outside diameter of a piece of round stock or the thickness
of a piece of flat stock, to an accuracy of 0.001 of an inch.
Figure 56 - Outside micrometer.
Inside micrometers (Figure 57) are used to measure an inside diameter to
an accuracy of 0.001 of an inch. Inside micrometers have a range of 0.500
inch, when used with 1/2-inch spacers.
Figure 57 - Outside micrometer.
Depth micrometers (Figure 58) are used to measure depths to an accuracy
of 0.001 inches.
Figure 58 - Depth micrometer.
Screw Thread Micrometers
The screw thread micrometer (shown in Figure 54) is used to determine the
pitch diameter of screws.
How To Select the Proper Micrometer
The types of micrometers commonly used are made so that the longest movement
that the micrometer spindle or rod can make is 1 inch. This movement is
called the range; for example, a 2-inch micrometer has a range of from
1 to 2 inches, and can only measure work with a thickness or diameter within
that range. A 6-inch micrometer has a range from 5 to 6 inches, and will
measure only work between 5 and 6 inches thick. The frames of micrometers,
however, are available in a wide variety of sizes, from 1 inch up to as
large as 24 inches.
It is necessary, therefore, that the mechanic 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 and find the approximate diameter of the 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
inside and 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.
How to Read a Standard Micrometer
The sleeve and thimble scales of a micrometer (Figure 59) have been enlarged
and laid out for demonstration. Reading a micrometer is only a matter of
reading the micrometer scale or counting the revolutions of the thimble
and adding any fraction of a revolution. To understand these scales, you
need to know that the threaded section on the spindle, which revolves,
has 40 threads per inch. Therefore, every time the thimble completes a
revolution, the spindle advances or recedes 1/40 inch, or 0.025 inch.
Figure 59 - Sleeve and thimble scales of a micrometer.
Note the horizontal line on the sleeve is divided into 40 equal parts
per inch. Every fourth graduation is numbered 1, 2, 3, 4, and so on, representing
0.100, 0.200, 0.300, and 0.400 inch, respectively. When you turn the thimble
so its edge is over the first sleeve line past the 0 on the thimble scale,
the spindle has opened 0.025 inch. If you turn the spindle to the second
mark, it has moved 0.025 inch plus 0.025 inch, or 0.050 inch. When the
beveled edge of the thimble stops between graduated lines on the sleeve
scale, you must use the thimble scale to complete your reading. The thimble
scale is divided into 25 equal parts; each part or mark represents 1/25
of a turn; 1/25 of 0.025 inch equals 0.001 inch.
Note that in Figure 60 every fifth line on the thimble scale is marked
5, 10, 15, and so on. The thimble scale permits you to take very accurate
readings to the thousandths of an inch.
Figure 60 - Enlarged micrometer scale.
The enlarged scale in Figure 60 can help you understand how to take
a complete micrometer reading to the nearest thousandth of an inch.
The thimble is turned far enough to expose the 7 on the sleeve scale
but not far enough to expose the first mark after the 7. Therefore, the
measurement must be between 0.700 inch and 0.725 inch. Exactly how far
between 0.700 inch and 0.725 inch must be determined from the thimble scale.
As you can see, the thimble has been turned through 12 spaces of its
scale, and the 12th graduation is lined up with the reference line on the
sleeve. When the value on the sleeve scale is added to the value on the
thimble scale that is lined up with the reference line on the sleeve scale,
the space between the anvil and spindle must be 0.712 inch (seven hundred
twelve thousandths of an inch).
How To Read a Vernier Micrometer
Many times you are required to work to exceptionally precise dimensions.
Under these conditions it is better to use a micrometer that is accurate
to ten thousandths of an inch. This degree of accuracy is obtained by the
addition of a Vernier scale.
The Vernier scale of a micrometer (Figure 61) furnishes the fine readings
between the lines on the thimble rather than requiring you to estimate
the reading. The 10 spaces on the Vernier are equivalent to 9 spaces on
the thimble. Therefore, each unit on the Vernier scale is equal to 0.0009
inch, and the difference between the sizes of the units on each scale is
Figure 61 - Vernier scale of a micrometer.
When a line on the thimble scale does not coincide with the horizontal
reference line on the sleeve, you can determine the additional spaces beyond
the readable thimble mark by finding which Vernier mark matches up with
a line on the thimble scale. Add this number, as that many ten thousandths
of an inch, to the original reading.
In Figure 62, see how the second line on the Vernier scale matches up
with a line on the thimble scale. This line means that the 0.011 mark on
the thimble scale has been advanced an additional 0.0002 beyond the horizontal
sleeve line. When you add this number to the other readings, the reading
is 0.200 + 0.075 + 0.011 + 0.0002, or 0.2862, as shown.
Figure 62 - Reading a Vernier scale.
How To Read a Metric Micrometer
The same principle is applied in reading the metric graduated micrometer
(Figure 63), but the following changes in graduations are used:
Figure 63 - Metric micrometer.
The pitch of the micrometer screw is 0.05 mm. One revolution of the
spindle advances or withdraws the screw a distance equal to 0.5 mm.
The barrel is graduated in millimeters from 0 to 25. It takes two revolutions
of the spindle to move the barrel 1 mm.
The thimble is graduated in 50 divisions with every fifth line being
Rotating the thimble from one graduation to the next moves the spindle
1/50 of 0.5 mm, or 1/100 mm. Two graduations equal 2/100 mm, and so forth.
The thimble is turned far enough to expose the 20 on the sleeve scale.
The number of lines visible between the number 20 and the thimble edge
is 2, which is equivalent to 2 mm. The line on the thimble coincides with
the long line in the barrel, 36 or 0.36 mm. When you add the measurements
together, the reading is 20 + 2 + 0.36, or 22.36 mm.
When using a Metric micrometer, remember 1 revolution is 0.5 mm. It
takes 2 revolutions to move 1 mm.
How To Care for Micrometers
Observe the following guidelines when working with micrometers:
Coat metal parts of all micrometers with a light coat of oil to prevent
Store micrometers in separate containers provided by manufacturer.
Keep graduations and markings on all micrometers clean and legible.
Do not drop any micrometer. Small nicks or scratches can cause inaccurate
Surface, Depth and Height Gauges
A surface gauge (Figure 64) is a measuring tool used to transfer measurements
to work by scribing a line, and to indicate the accuracy or parallelism
of surfaces. The surface gauge consists of a base with an adjustable spindle
to which may be clamped a scriber or an indicator.
Figure 64 - Surface gauge.
Surface gauges are made in several sizes and are classified by the length
of the spindle.
The smallest spindle is 4 inches long, the average 9 to 12 inches, and
the largest 18 inches. The scriber is fastened to the spindle with a clamp.
The bottom and the front end of the base of the surface gauge have deep
V-grooves. The grooves allow the gauge to measure from a cylindrical surface.
The base has two gauge pins. They are used against the edge of a surface
plate or slot to prevent movement or slippage.
Rule Depth Gauge
A rule depth gauge (Figure 65) measures the depth of holes, slots, counterbores,
and recesses. Some rule depth gauges can also be used to measure angles.
This measurement is done by using the angle marks located on the sliding
head. The rule depth gauge is a graduated rule with a sliding head designed
to bridge a hole or slot. The gauge holds the rule at a right angle to
the surface when taking measurements. This type has a measuring range of
0 to 5 inches. The sliding head has a clamping screw so that it may be
clamped in any position. The sliding head is flat and perpendicular to
the axis of the rule. It ranges in size from 2 to 2 5/8 inches wide and
from 1/8 to 1/4 inch thick.
Figure 65 - Rule depth gauge.
Micrometer Depth Gauge
The micrometer depth gauge (Figure 66) consists of a flat base that is
attached to the barrel of a micrometer head. These gauges have a range
from 0 to 9 inches, depending on the length of extension rod used. The
hollow micrometer screw has a 1/2- or 1-inch range. Some are provided with
a ratchet stop. The flat base ranges in size from 2 to 6 inches. Several
extension rods are supplied with this type gauge.
Figure 66 - Micrometer depth gauge.
Vernier Depth Gauge
The Vernier depth gauge (Figure 67) consists of a graduated scale either
6 or 12 inches long. It also has a sliding head similar to the one on the
The sliding head is designed to bridge holes and slots. The Vernier
depth gauge has the range of the rule depth gauge. It does not have quite
the accuracy of a micrometer depth gauge. It cannot enter holes less than
1/4 inch in diameter. However, it will enter a 1/32-inch slot. The Vernier
scale is adjustable and may be adjusted to compensate for wear.
Figure 67 - Vernier depth gauge.
Dial Depth Gauge
Dial depth gauges (Figure 68) are for rapidly checking depths of holes,
recesses, slots, scratches, and paint thickness. It should be noted that
measurements made with depth gauges should be on a longitudinal axis. The
depth gauge will give direct readings on the dial in half thousands of
an inch (0.0005 inch); press the push button down until the measuring rod
contacts the work and read the depth on the dial.
Figure 68 - Dial depth gauge.
A height gauge (Figure 69) is used in the layout of jigs and fixtures.
On a bench, it is used to check the location of holes and surfaces. It
accurately measures and marks off vertical distances from a plane surface.
Figure 69 - Height gauge.
The Vernier height gauge is a caliper with a special base to adapt it
for use on a surface plate. Height gauges are available in several sizes.
Most common are the 10-, 18-, and 24-inch gauges in English measure. The
most common metric gauges are the 25- and 46 centimeter sizes. Height gauges
are classified by the dimension they will measure above the surface plate.
Like the Vernier caliper, height gauges are graduated in divisions of 0.025
inch. Its Vernier scale is divided into 25 units for reading thousandths
of an inch.
A surface plate (Figure 70) provides a true, smooth, plane surface.
It is often used as a level base for surface and height gauges from which
to make accurate measurements. Surface plates are usually made of close
grained cast iron, are rectangular in shape, and come in a variety of sizes.
Figure 70 - Granite surface plate.
How to Use the Surface, Depth, and Height Gauges
Follow these steps to use surface, depth, and height gauges properly:
How To Use a Surface Gauge
Setting the surface gauge to transfer a 4-inch vertical measurement is
illustrated in Figure 71.
Figure 71 - Using a surface gauge.
How To Use a Rule Depth Gauge
A method of using a rule depth gauge to measure the distance from a surface
to a recessed point is illustrated in Figure 72.
Figure 72 - Using a depth rule gauge.
How To Use a Micrometer Depth Gauge
An example of measuring projection depth with micrometer depth gauge is
shown in Figure 73.
Figure 73 - Using a micrometer depth gauge.
How To Use a Vernier Depth Gauge
In Figure 74, using a Vernier depth gauge to measure the depth of a hole
from a given surface is illustrated.
Figure 74 - Using a Vernier depth gauge.
How To Use a Dial Depth Gauge
In Figure 75, measuring depths of holes, recesses, slots, scratches, and
paint thickness with a dial depth gauge is illustrated.
Figure 75 - Using a dial depth gauge.
How To Use a Height Gauge
Using a height gauge to measure a vertical distance from a plane surface
is shown in Figure 76.
Figure 76 - Using a height gauge.
Care of Surface, Height, and Depth Gauges
Observe the following guidelines when working with surface, height, and
Coat all metal parts of gauges with a light coat of oil to prevent rust.
Carefully store gauges when not in use. Use separate containers if provided
Keep graduations and markings clean and legible.
Do not drop any gauge. Small nicks and scratches can cause inaccurate measurements.
Protect all pointed gauge parts from damage.
Miscellaneous Measuring Gauges and Layout Tools
Thickness (Feeler) Gauges
Thickness (feeler) gauges (Figure 77) are made in many shapes and sizes;
usually 2 to 26 blades are grouped into one tool and graduated in thousandths
of an inch.
Figure 77 - Feeler gauge.
Most thickness blades are straight, while others are bent at the end
at 45-degree and 90-degree angles. Some thickness gauges are grouped so
that there are several short and several long blades together. Thickness
gauges are also available in single blades and in strip form for specific
measurements. For convenience, many groups of thickness gauges are equipped
with a locking screw in the case that locks the blade to be used in the
These gauges are fixed in leaf form, which permits the checking and
measuring of small openings such as contact points, narrow slots, and so
forth. They are widely used to check the flatness of parts in straightening
and grinding operations and in squaring objects with a try square.
The center gauge (Figure 78) is graduated in fourteenths, twentieths, twenty-fourths,
and thirty-seconds of an inch. The back of the center gauge has a table
giving the double depth of thread in thousandths of an inch for each pitch.
This information is useful in determining the size of tap drills. Sixty
degree angles in the shape of the gauge are used for checking Unified and
American threads as well as for older American National or U.S. Standard
threads and for checking thread cutting tools.
Figure 78 - Center gauge.
Screw Pitch Gauges
Screw pitch gauges (Figure 79) are made for checking the pitch of U.S.
Standard, Metric, National Form, V-form, and Whitworth cut threads. These
gauges are grouped in a case or handle, as are the thickness gauges. The
number of threads per inch is stamped on each blade. Some types are equipped
with blade locks. The triangular shaped gauge has 51 blades covering a
wide range of pitches, including 11 1/2 and 27 threads per inch for V-form
threads. Screw pitch gauges are used to determine the pitch of an unknown
thread. The pitch of a screw thread is the distance between the center
of one tooth to the center of the next tooth.
Figure 79 - Screw pitch gauge.
Small Hole Gauge Set
Small hole gauges (Figure 80) are adjustable, having a rounded measuring
member. A knurled screw in the end of the handle is turned to expand the
ball-shaped end in small holes and recesses. A micrometer caliper is used
to measure the ball end. Maximum measuring capacity is 1/2 inch. This set
of four or more gauges is used to check dimensions of small holes, slots,
grooves, and so forth from approximately 1/8 to 1/2 inch in diameter.
Figure 80 - Small hole gauges.
Telescoping gauges (Figure 81) are used to gauge larger holes and to measure
inside distances. These gauges are equipped with a plunger that can be
locked in the measuring position by a knurled screw in the end of the handle.
Maximum measuring capacity is 6 inches. Measurements must be calipered
on the gauge by a micrometer, as in the case of the small hole gauges.
They are also used when measurements cannot be taken with a standard micrometer.
Telescoping gauges are particularly adaptable for roughly bored work and
odd sizes and shapes of holes. Compress the plungers and lock them by turning
the handle screw.
Figure 81 - Telescoping gauges.
Thread Cutting Tool Gauges
Thread cutting tool gauges (Figure 82) are hardened steel plates with cutouts
around the perimeter. Each cutout is marked with a number that represents
the number of threads per inch.
These gauges provide a standard for thread cutting tools. They have
an enclosed angle of 29 degrees and include a 29-degree setting tool. One
gauge furnishes the correct form for square threads and the other for Acme
Figure 82 - Thread cutting tool gauges.
Fillet and Radius Gauges
The blades of fillet and radius gauges (Figure 83) are made of hard rolled
steel. The double ended blades of the gauge have a lock which holds the
blades in position. The inside and outside radii are on one blade on the
gauge. Each blade of each gauge is marked in sixty-fourths. Each gauge
has 16 blades.
Figure 83 - Fillet and radius gauges.
Drill Point Gauge
The drill point gauge (Figure 84) consists of a 6-inch hook rule with a
59 degree sliding head that slides up and down the rule. The sliding head
can be locked at any position on the rule and is graduated in thirty-seconds
of an inch. This gauge is used to check the accuracy of drill cutting edges
after grinding. It is also equipped with a 6-inch hook rule. This tool
can be used as a drill point gauge, hook rule, plain rule, and a slide
caliper for taking outside measurements.
Figure 84 - Drill point gauge.
A wire gauge (Figure 85) is circular in shape with cutouts in the outside
edge. Each cutout gauges a different size wire, from 0 to 36 of the English
Standard Wire Gauge. A separate gauge is used for American standard wire
and another for U.S. Standard sheet and plate iron and steel.
Similar gauges are also used to check the size of hot and cold rolled
steel, sheet and plate iron, and music wire.
Figure 85 - Wire gauge.
The twist drill and drill rod gauge (Figure 86) has a series of holes with
size and decimal equivalents stamped adjacent to each hole. One gauge measures
drill sizes numbers 1 to 60; the other gauge measures drill sizes 1/16
to 1/2 inch in 1/64-inch intervals. Drill gauges determine the size of
a drill and indicate the correct size of drill to use for given tap size.
Drill number and decimal size are also shown in this type gauge. Letter
size drill gauges are also available. Each drill hole is identified by
a letter instead of a number, decimal, or fraction.
Figure 86 - Drill gauge
How To Use a Thickness Gauge
Thickness (feeler) gauges are used in one of two ways: as a means for determining
a measure or a means for adjusting to a definite limit (Figure 87). A thickness
gauge is used to check piston ring gap clearance in a cylinder bore.
A long blade thickness gauge is used to determine the fit between large
mating surfaces. By combining blades it is possible to obtain a wide variation
Figure 87 - Using thickness (feeler) gauges.
How To Use a Center Gauge
The center gauge (Figure 88) is used to set thread cutting tools. Four
scales on the gauge are used for determining the number of threads per
inch. The gauge is also used to check cut threads and the scales are used
to measure threads per inch.
Figure 88 - Using a center gauge.
How to use a Screw Pitch Gauge
If the pitch of a thread is not known, it can be determined by comparing
it with the standards on the various screw pitch gauges (Figure 89).
1. Place a blade of a gauge over the threads and check to see whether
it meshes; if not, successively check each blade of the gauge against the
thread until it meshes.
2. The pitch can be read off the correct blade. The blades are made
pointed so that they can be inserted in small nuts to check inside threads
well as outside threads.
Figure 89 - Using a screw pitch gauge.
How To Use a Small Hole Gauge
The small hole gauges perform the same function as telescoping gauges (Figures
90 and 91), except that they are used in smaller work.
1. Fit the ball-shaped point into the hole or slot (Figure 90).
Figure 90 - Using a small hole gauge.
2. Expand the ball-shaped end by turning the screw at the end of the
3. Use micrometer to gauge the measurement.
How To Use a Telescoping Gauge
1. Loosen the knurled nut at the end of the handle (Figure 91).
2. Slightly tilt telescoping gauge 5 to 10 degrees and lower into object
to be measured.
3. Tighten knurled nut.
4. Remove gauge by pulling across center line as indicated by arrow.
Figure 91 - Using a telescoping gauge.
How To Use a Fillet and Radius Gauge
1. A double ended radius gauge blade is used to check the inside corner
or fillet of a machined part (Figure 92). Each blade can be locked in position
by tightening the clamp.
2. These gauges can be used in any position and at any angle for both
inside and outside radii.
Figure 92 - Using a fillet and radius gauge.
How To Use a Drill Point Gauge
The method for sharpening the cutting edges of a drill is to do one lip
at a time. Each lip must have the same length and have the same angle in
relation to the axis of the drill. Set the sliding head securely on the
rule at the mark equal to the length of the drill. Place the drill vertically
against the rule so that the drill lip contacts the 59-degree angle of
the sliding head (Figure 93). Hold up to light; correct angle is obtained
when no light is seen between gauge and drill.
Figure 93 - Using a drill point gauge.
How To Use a Wire Gauge
Determine the size of both sheet stock and wire by using a correct sheet
and plate or wire gauge (Figure 94).
Figure 94 - Using a wire gauge.
How To Use a Drill Gauge
The drill gauge is used to determine the size of a drill (Figure 95). Insert
the drill into the appropriate sized hole. A chart on the gauge indicates
the correct size of drill to use for a given tap size.
Figure 95 - Using a drill gauge.
Care of Gauges and Layout Tools
Observe the following guidelines when working with gauges and layout tools:
Exercise care when using thickness gauges to measure clearance of knives
and cutters on machines. Do not lower knife on thickness blade and then
try to remove the gauge. The blade may be shaved off if it is too tight.
Never use gauges for cleaning slots or holes. When blades are damaged or
worn they should be replaced. Blades in a case are removed by loosening
the clamp and sliding out the damaged blade. Insert new blade and tighten
Always coat metal parts of all gauges with a light film of oil when not
in use to prevent rust. Store gauges in separate containers. Do not pile
gauges on each other.
Always return blades of leaf-type gauges to case after use.
Keep graduations and markings on all gauges clean and legible.
Do not drop any gauge. Small scratches or nicks will result in inaccurate