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
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).
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
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
Now turn the rule and look at the edge with a 16 marked
on it (Figure 4).
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 0.01 meter.
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
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 mm.
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
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
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
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.
The carpenter 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 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 center.
Figure 13 - Parts of a Carpenter's 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 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 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
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
2. The center head, when attached to the rule, bisects
a 90-degree angle. It is used for determining the center of cylindrical
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.
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 not square.
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
2. Continue the process until desired number of steps
has been laid out.
Figure 22 - Square position for steps
How To Use a Try Square
Using a try square (Figure 14) is similar to using the carpenter's
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.
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 26).
Figure 26 - Set the combination square.
2. Put the center head flush against the cylinder (Figure
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 set screw.
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
Figure 30 - Measure the angle.
4. Remove and read the measured angle on the protractor
scale (Figure 31).
Figure 31 - Verify the angle.
Using a Combination Square to Mark 90-Degree and 45-Degree
Mark a 90-degree angle (Figure 32) using the following
Figure 32 - Mark a 90-degree cut with a combination
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
Figure 33 - Mark a 45-degree cut with a combination
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
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 length.
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
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 angles.
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
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
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
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 the points.
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 numbered.
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
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 0.011 inch.
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
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
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
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
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
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
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 0.0001 inch.
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
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 numbered.
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 rust.
Store micrometers in separate containers provided by manufacturer.
Keep graduations and markings on all micrometers clean and
Do not drop any micrometer. Small nicks or scratches can
cause inaccurate measurements.
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
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 Vernier caliper.
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
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
Figure 70 - Granite surface plate.
How to Use the Surface, Depth, and Height Gauges
Follow these steps to use surface, depth, and height gauges
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 depth gauges:
Coat all metal parts of gauges with a light coat of oil to
Carefully store gauges when not in use. Use separate containers
if provided by manufacturer.
Keep graduations and markings clean and legible.
Do not drop any gauge. Small nicks and scratches can cause
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 extended position.
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
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 standard threads.
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 of thickness.
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
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
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
Figure 90 - Using a small hole gauge.
2. Expand the ball-shaped end by turning the screw at
the end of the handle.
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
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
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
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 clamp.
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 measurements.