THREADS AND FASTENERS - Goodheart-Willcox

This sample chapter is for review purposes only. Copyright © The Goodheart-Willcox Co., Inc. All rights reserved.
A
B
C
D
E
F
G
UNLESS OTHERWISE SPECIFIED
DIMENSIONS ARE IN INCHES
TOLERANCES ON
DECIMALS
DO NOT SCALE DRAWING
I 2
3 4 5
DRAWN DATE
CHECK
DESIGN
STRESS
MATERIALS
PRODUCTION
SUPERVISOR
After completing this unit, you will be able to:
Define terms related to screw threads and
fasteners.
Identify screw thread forms.
List pipe thread forms.
Explain how thread types are represented and
specified on prints.
A screw thread or thread is a ridge of a particular
shape, often a V, which follows a helical
path around the surface of a cylindrical surface.
A helix is like a barber pole, candy cane, or can
be compared to wrapping a rope around a cylinder.
By definition, a helix is formed by a point
progressing uniformly along a path with a constant
obliquing angle to the axis of a cone or
cylinder. Threads are formed by hand, ground or
rolled on a lathe, and formed using thread cutting
machines.
Threads are a very important feature of
many industrial fasteners and machine parts.
Threads such as the American National or the
Unified Standard (V-type) are used for bolts,
screws, and nuts; and for fastening machine
parts together. Other thread forms are designed
for special purposes, such as transmitting power
along an axis, as in the lead screw of a machine
lathe. Pipe threads are also standardized and
designed for particular functions.
Thread Terms and Definitions
There are several terms associated with
threads. Refer to Figure 11-1 as you read the
following definitions.
UNIT 11
THREADS AND
FASTENERS
• An external thread is a thread on the outside
of a shaft. The threads on a bolt are
external threads.
• An internal thread is a thread on the
inside of a hole. The threads in a nut are
internal threads.
• The crest is the top of a ridge or “point.”
• The root is the bottom or bottomland
between two ridges or “points.”
• The major diameter is the largest diameter
of a thread. For an external thread,
this is the diameter between crests.
• The minor diameter is the smallest diameter
of a thread. For an external thread, this
is the diameter between roots.
• The pitch diameter is an imaginary
diameter where the width of the thread
ridge is equal to the width of the groove
between ridges. This is important for
designing the fit of threaded parts.
• The pitch is the distance from one point
on a screw thread to the same point on
the next thread. American inch threads
are specified by number of threads per
inch. If there are 16 threads per inch, the
pitch is 1/16″.
• The lead is how far a thread advances
when rotated one complete revolution.
For most threads, the lead and pitch are
equivalent. For multiple threads, the lead
is double the pitch for double threads or
triple the pitch for triple threads.
Multiple threads are covered later in
this unit.
131
132 Print Reading for Industry
MAJOR
DIA.
MINOR
DIA.
THREAD ANGLE
Screw Thread Forms
The thread form describes the shape of the
ridge or groove which forms the threads. See
Figure 11-2. Most thread forms used in industry
today are based on the approved American
Standards. Unified Screw Thread Standard ASA
B1.1 is the recognized straight thread standard
in the United States for screws, bolts, nuts, and
other threaded parts. These threads are called
Unified Standard, or Unified, and are also commonly
used in Great Britain and Canada.
Unified threads have the general appearance of
ROOT
CREST AXIS
PITCH DIA.
EXTERNAL THREAD INTERNAL THREAD
Figure 11-1.
There are several terms associated with threads.
.16P
P = PITCH
.66P
PITCH
PITCH
P
2
55°
PITCH
ROOT
CREST
a “sharp V.” Metric threads are also based on a
sharp V form. Unified threads are similar to the
older American National thread form, but are
easier to manufacture. This is due to the flat or
rounded crests and roots.
There are various uses for threads other than
for fasteners. Thread forms in use for purposes
other than fasteners include square, Acme, buttress,
and Whitworth. A common example of
knuckle threads is the threads found on a common
incandescent lightbulb.
PITCH
.5P
.125P 60°
PITCH
WHITWORTH UNIFIED ACME
45°
PITCH
P
2
BUTTRESS SQUARE KNUCKLE
P
2
PITCH
P
2
Figure 11-2.
The thread form describes the shape of the ridge or groove which forms the threads.
.5P
P
2
.5P
.37P

Thread Representation
A true projection view of a screw thread is
not used due to the helical shape. Instead,
threads are represented on a print in conventional
ways. The three conventional methods in
which threads may be represented on drawings
are the detailed, schematic, and simplified methods.
See Figure 11-3.
The detailed convention most closely represents
screw threads as they actually appear. This
convention is sometimes used to show the
geometry of a thread form as a portion of a
greatly enlarged detail on a drawing. A rule of
thumb is to draw detailed threads if the major
diameter appears larger than 1″ on the drawing.
The schematic convention and simplified
convention are representations of the thread and
are easier to draw. These conventions are recommended
when the major diameter is less than 1″
on the drawing. Both conventions are commonly
used to save drafting time. The simplified
convention is the easiest convention to use with
most CAD systems.
EXTERNAL THREADS
REGULAR VIEWS SECTION VIEWS
DETAILED REPRESENTATION
SCHEMATIC REPRESENTATION
SIMPLIFIED REPRESENTATION
Unit 11 Threads and Fasteners 133
Thread Series
There are four series of Unified screw
threads—coarse, fine, extra fine, and constantpitch.
See Figure 11-4 and Figure 11-5. Each
series has its own designation. There are also
three special thread series designated UNS, NS,
and UN. These series cover special combinations
of diameter, pitch, and length of engagement.
The Unified coarse thread series is designated
UNC. It is used for nuts, bolts, screws, and
general uses where a fine thread is not required.
The Unified fine thread series is designated
UNF. It is used where the length of the threaded
engagement is short and where a small lead
angle is desired.
The Unified extra fine series is designated
UNEF. It is used for very short lengths of thread
engagement and for thin-wall tubes, nuts, ferrules,
and couplings.
The Unified constant-pitch series is designated
UN with the number of threads per inch
INTERNAL THREADS
Figure 11-3.
The three conventional methods for representing threads are detailed, schematic, and simplified.
134 Print Reading for Industry
NORMAL
DIAMETER
BASIC
DIAMETER
COARSE
NC & UNC
THDS
PER
IN
TAP
DRILL
DIA
FINE
NF & UNF
THDS
PER
IN
TAP
DRILL
DIA
EXTRA FINE
NEF & UNEF
THDS TAP
PER DRILL
IN DIA NORMAL
0 .060 80 .0469
1 .073 64 No.53 72 No.53
2 .086 56 No.50 64 No.50
3 .099 48 No.47 56 No.45
4 .112 40 No.43 48 No.42
5 .125 40 No.38 44 No.37
6 .138 32 No.36 40 No.33
8 .164 32 No.29 36 No.29
10 .190 24 No.25 32 No.21
12 .216 24 No.16 28 No.14 32 No.13
1/4 .250 20 No.7 28 No.3 32 .2189
5/16 .3125 18 F 24 I 32 .2813
3/8 .375 16 .3125 24 Q 32 .3438
7/16 .4375 14 U 20 .3906 28 .4062
1/2 .500 13 .4219 20 .4531 28 .4688
9/16 .5625 12 .4844 18 .5156 24 .5156
5/8 .625 11 .5313 18 .5781 24 .5781
11/16 .6875 ... ... ... ... 24 .6406
3/4 .750 10 .6563 16 .6875 20 .7031
13/16 .8125 ... ... ... ... 20 .7656
7/8 .875 9 .7656 14 .8125 20 .8281
15/16 .9375 ... ... ... ... 20 .8906
Figure 11-4.
Thread specifications for NC, UNC, NF, UNF, NEF, and UNEF threads.
NORMAL
DIAMETER
8 PITCH
8N & 8UN
THDS
PER
IN
TAP
DRILL
DIA
12 PITCH
12N & 12UNF
THDS
PER
IN
TAP
DRILL
DIA
16 PITCH
16N & 16UN
THDS
PER
IN
TAP
DRILL
.500 ... ... 12 .422 ... ...
.563 ... ... 12 .484 ... ...
.625 ... ... 12 .547 ... ...
.688 ... ... 12 .609 ... ...
.750 ... ... 12 .672 16 .688
.813 ... ... 12 .734 16 .750
.875 ... ... 12 .797 16 .813
.934 ... ... 12 .869 16 .875
1.000 8 .875 12 .922 16 .938
1.063 ... ... 12 .984 16 1.000
1.125 8 1.000 12 1.047 16 1.065
1.188 ... ... 12 1.109 16 1.125
1.250 8 1.125 12 1.72 16 1.188
1.313 ... ... 12 1.234 16 1.250
1.375 8 1.250 12 1.297 16 1.313
1.434 ... ... 12 1.359 16 1.375
1.500 8 1.375 12 1.422 16 1.438
1.563 ... ... ... ... 16 1.500
1.625 8 1.500 12 1.547 16 1.563
1.688 ... ... ... ... 16 1.625
1.750 8 1.625 12 1.672 16 1.688
1.813 ... ... ... ... 16 1.750
1.875 8 1.750 12 1.797 16 1.813
1.934 ... ... ... ... 16 1.875
2.000 8 1.875 12 1.922 16 1.938
Figure 11-5.
Thread specifications for constant pitch threads.
DIA NORMAL
DIAMETER
BASIC
DIAMETER
COARSE
NC & UNC
THDS
PER
IN
TAP
DRILL
DIA
FINE
NF & UNF
THDS
PER
IN
TAP
DRILL
DIA
EXTRA FINE
NEF & UNEF
THDS
PER
IN
TAP
DRILL
DIA
1 1.000 8 .875 12 .922 20 .953
1–1/16 1.063 ... ... ... ... 18 1.000
1–1/8 1.125 7 .904 12 1.046 18 1.070
1–3/16 1.188 ... ... ... ... 18 1.141
1–1/4 1.250 7 1.109 12 1.172 18 1.188
1–5/16 1.313 ... ... ... ... 18 1.266
1–3/8 1.375 6 1.219 12 1.297 18 1.313
1–7/16 1.438 ... ... ... ... 18 1.375
1–1/2 1.500 6 1.344 12 1.422 18 1.438
1–9/16 1.563 ... ... ... ... 18 1.500
1–5/8 1.625 ... ... ... ... 18 1.563
1–11/16 1.688 ... ... ... ... 18 1.625
1–3/4 1.750 5 1.563 ... ... ... ...
2 2.000 4.5 1.781 ... ... ... ...
2–1/4 2.250 4.5 2.031 ... ... ... ...
2–1/2 2.500 4 2.250 ... ... ... ...
2–3/4 2.750 4 2.500 ... ... ... ...
3 3.000 4 2.750 ... ... ... ...
3–1/4 3.250 4 ... ... ... ... ...
3–1/2 3.500 4 ... ... ... ... ...
3–3/4 3.750 4 ... ... ... ... ...
4 4.000 4 ... ... ... ... ...
DIAMETER
8 PITCH
8N & 8UN
THDS
PER
IN
TAP
DRILL
DIA
12 PITCH
12N & 12UNF
THDS
PER
IN
TAP
DRILL
DIA
16 PITCH
16N & 16UN
THDS
PER
IN
TAP
DRILL
DIA
2.063 ... ... ... ... 16 2.000
2.125 ... ... 12 2.047 16 2.063
2.188 ... ... ... ... 16 2.125
2.250 8 2.125 12 2.172 16 2.188
2.313 ... ... ... ... 16 2.250
2.375 ... ... 12 2.297 16 2.313
2.438 ... ... ... ... 16 2.375
2.500 8 2.375 12 2.422 16 2.438
2.625 ... ... 12 2.547 16 2.563
2.750 8 2.625 12 2.717 16 2.688
2.875 ... ... 12 ... 16 ...
3.000 8 2.875 12 ... 16 ...
3.125 ... ... 12 ... 16 ...
3.250 8 ... 12 ... 16 ...
3.375 ... ... 12 ... 16 ...
3.500 8 ... 12 ... 16 ...
3.625 ... ... 12 ... 16 ...
3.750 8 ... ... ... 16 ...
3.875 ... ... 12 ... 16 ...
4.000 8 ... ... ... 16 ...
4.250 8 ... 12 ... 16 ...
4.500 8 ... ... ... 16 ...
4.750 8 ... 12 ... 16 ...
5.000 8 ... ... ... 16 ...
5.250 8 ... 12 ... 16 ...

preceding the designation, such as 8UN. This series
of threads is for special purposes, such as highpressure
applications. Constant-pitch threads are
also used for large diameters where the other
thread series do not meet the requirements.
Thread Classes
After classification by form and series,
threads are further classified by manufacturing
tolerance. These classes are 1A, 2A, and 3A for
external threads and 1B, 2B, and 3B for internal
threads. On some older drawings, classes 2 and
3 may appear without a letter designation.
Classes 1A and 1B replace the older
American Standard class 1. They are used in
applications requiring minimum binding and
allow for frequent and quick assembly or disassembly
of parts.
Classes 2A and 2B are threads with tighter
tolerances. They are used for general purposes
such as for nuts, bolts, screws, and normal applications
by mass production industries.
Class 3A and 3B are threads with very stringent
and close tolerances. They are used for
applications in industries requiring tighter tolerances
than the preceding classes of 1A and 1B, or
2A and 2B.
Multiple Threads
Threads can have two or three ridges running
side-by-side. These are called double threads or
triple threads. It may help to think of rope
wrapped around a pole. If one rope is wrapped, it
is like a single thread. If two ropes are wrapped,
they are like double threads. Three ropes
wrapped around the pole are like triple threads.
Simply looking at the thread generally will
not indicate if it is single, double, or triple.
However, a close inspection should reveal a
greater slope to double and triple threads than
that found on single threads. Due to the greater
slope angle of the thread, the “holding power”
of a double or triple thread is less.
Remember, the lead of a double thread is
twice the pitch. One revolution of the threaded
Unit 11 Threads and Fasteners 135
cylinder advances two pitches because there are
two ridges. Likewise, a triple thread advances
three pitches.
Left-Hand Threads
Threads are most commonly created so that
clockwise revolutions advance the threaded
parts together. Common nuts and bolts tighten
with clockwise turns. These threads are called
right-hand threads. However, some applications
require threads which tighten with counterclockwise
movement. These threads are referred
to as left-hand or reverse threads.
Left-hand threads are usually found on
items where right-hand threads may produce
unwanted loosening, perhaps due to motion.
Examples include bicycle pedal threads, threads
on a flush handle, arbor nut on a table saw
blade, and similar applications. Left-hand
threads are indicated in a print using a callout.
Detailed representations should also reflect a
slope angle opposite that of right-hand threads.
See Figure 11-6.
CLOCKWISE
RIGHT–HAND THREAD
LEFT–HAND THREAD
Figure 11-6.
Right-hand and left-hand threads.
COUNTERCLOCKWISE
136 Print Reading for Industry
Specification of Screw Threads
A screw thread is specified on a drawing
using a standard note with a leader and an
arrow pointing to the thread. The note contains
specific information for the specific thread. The
note in Figure 11-7 is interpreted as follows.
A) Nominal size (major diameter or screw
number)
B) Number of threads per inch
C) Thread form
D) Thread class number or symbol
E) Left-hand (LH), double threads (DOUBLE)
Threads are right-hand and single lead
unless otherwise specified. The letters LH after
the class symbol specify the thread as left hand.
The word DOUBLE indicates double thread. The
word TRIPLE indicates triple thread. Single
threads and right-hand threads do not require
RH or SINGLE to be specified.
Schematic and simplified thread representations
are not impacted by the thread form, the
pitch, left-hand, or multiple thread status. The
schematic spacing is usually just a factor of the
major diameter, and nothing more.
MAJOR DIAMETER IN INCHES
NUMBER OF
THREADS PER INCH
THREAD
FORM
THREAD
CLASS
1/2 – 13 UNC 2A
Figure 11-7.
A screw thread is specified in a local note.
Other specifications for threads may be
given. These additional specifications may
include thread length, hole size (for internal
threads), or chamfer. See Figure 11-8.
If the tolerance of the thread pitch diameter
is given, it is placed at the end of the specification
for the thread as follows.
3
− 10UNC − 2B PD 0.6850 TO 0.6927
4
The specification for a constant-pitch series
thread with a tolerance for the pitch diameter
would be listed as follows.
1
− 8UN − 3A PD 2.1688 TO 2.1611
4 2
Metric Threads
Metric threads are graphically represented
in the same manner as Unified and American
National Standard. However, the format of the
associated note is different. In a note for metric
threads, the letter M is followed by the diameter
and pitch. The M designates the thread as a
metric series. Thread designation is for a
coarse thread unless otherwise noted. Metric
A B C D E
1 – 5 ACME – 2A LH DOUBLE

17/32 – 1.25
5/8 X 82°
5/8 – 11UNC – 2B .75
Figure 11-8.
Additional specifications for a thread may
include thread length, hole size, or chamfer.
fine threads are designated by listing the pitch
as a suffix.
For example, in Figure 11-9, M8 is a coarse
thread designation representing a nominal
M8
Figure 11-9.
Metric threads are specified slightly different than other threads.
Unit 11 Threads and Fasteners 137
thread diameter of 8mm with a standardized
pitch of 1.25mm understood. A fine thread designation
for the part is M8×1.0. This indicates an
8mm diameter with a pitch of 1.0mm. The most
common metric thread is coarse. Metric coarse
threads are generally between Unified coarse
and fine series for a comparable diameter.
The specification for metric series threads
does not indicate the number of threads per unit
of length, such as threads per millimeter. Instead,
the pitch is indicated in the specification. For
example, M8×1.0 designates the pitch is 1mm.
The tolerance and class of fit for metric
threads are designated by adding numbers and
letters in a certain sequence to the callout. The
thread designation in Figure 11-10 calls for a
20mm diameter fine thread with a pitch of
.75mm. In addition, the thread has a pitch diameter
tolerance of grade 6 with an allowance “h”
and a crest diameter tolerance grade 6 with an
allowance “g.” See Unit 13 for a discussion of
these tolerancing symbols.
Pipe Threads
The three forms of pipe threads used in
industry are regular, aeronautical, and dryseal
M8 X 1.0
138 Print Reading for Industry
METRIC THREAD
DESIGNATION
M20X1.5–6h6g
MAJOR
DIAMETER
Figure 11-10.
The tolerance class is added as a suffix to the thread note.
pipe threads. The regular pipe thread is the standard
for the plumbing trade. The aeronautical
pipe thread is the standard in the aerospace
industry. Regular and aeronautical pipe thread
forms must be filled with a lute or sealer to prevent
leakage in the joint. The dryseal pipe thread
is the standard for automotive, refrigeration,
and hydraulic tube/pipe fittings. Dryseal pipe
threads do not allow leakage, even without the
use of sealer. This is due to the metal-to-metal
contact at the crest and root of the threads.
Representation and Specification
Straight and taper pipe threads are represented
graphically in a similar manner as other
screw threads. However, pipe threads are shown
tapered at an angle of approximately 3° to the
A B C D
1/4 – 18 DRYSEAL NPTF
Figure 11-11.
Pipe threads are drawn similar to normal threads,
but at a slight angle to the centerline axis.
PITCH
(FINE THREAD)
PITCH DIAMETER TOLERANCE
6 = GRADE
g = POSITION (ALLOWANCE)
CREST DIAMETER TOLERANCE
6 = GRADE
g = POSITION (ALLOWANCE)
TOLERANCE CLASS
centerline axis. See Figure 11-11.
The specifications for American Standard
Pipe Threads are listed in sequence of the nominal
size, number of threads per inch, and the symbols
for the thread series and form. For example,
the thread specification 1/2-14NPT designates a
1/2″ nominal size, 14 threads per inch, and an
American Standard taper pipe thread.
Pipe Thread Symbols
The following symbols are used to designate
American Standard pipe threads.
• NPT. American Standard taper pipe
thread.
• NPTR. American Standard taper pipe
thread for railing joints.
• NPSC. American Standard pipe thread
for couplings.
• NPSM. American Standard pipe thread
for free-fitting mechanical joints.
• NPSL. American Standard pipe thread
for loose-fitting mechanical joints with
locknuts.
• NPSH. American Standard pipe thread
for hose couplings.
Symbols for dryseal pipe threads are designated

as follows:
• NPTF. Dryseal American Standard pipe
thread.
• PTF-SAE SHORT. Dryseal SAE short
taper pipe thread.
• NPSF. Dryseal American Standard fuel
internal straight pipe thread.
• NPSI. Dryseal American Standard intermediate
internal straight pipe thread.
A typical specification for dryseal pipe
thread is shown in Figure 11-11. This specification
includes:
A) Nominal size
B) Number of threads per inch
C) Form (dryseal)
D) Symbol
Unit 11 Threads and Fasteners 139
Print Reading Activities
In the activities in this unit, you will read
some actual industry prints. Many companies
have their own drafting standards. As such, some
items shown on industry prints may not exactly
conform to the standards taught in this text.
However, an important aspect of print reading is
being able to correctly interpret all prints, including
prints drawn to older or company-specific
standards. Additional advanced print activities
for this unit, APR11-1 and APR11-2, are located at
the back of the book.
140 Print Reading for Industry
Print Reading Activity 11-1
External Threads
Refer to the print in PR11-1 and answer the
following questions.
1. What is the name of the part?
2. What is the drawing number?
3. As given in the title block, what is the
scale of the drawing?
4. What material is required?
5. What is the maximum allowed finished
length?
6. What size is the hole drilled through
the piece?
7. Interpret the thread specification for A.
.750
10
UNC
2A
8. Give the dimensions for the groove on
the left-hand side of the part.
9. Interpret the thread specification for B.
1
8
UNC
2A
1.25 LG
10. What does UNC stand for?
1. ______________________________________
2. ______________________________________
3. ______________________________________
4. ______________________________________
5. ______________________________________
6. ______________________________________
7.
______________________________________
______________________________________
______________________________________
______________________________________
8. ______________________________________
9.
______________________________________
______________________________________
______________________________________
______________________________________
______________________________________
10. ______________________________________

B
A
PR11-1.
Print supplied by Rockwell Manufacturing Company.
Unit 11 Threads and Fasteners 141
142 Print Reading for Industry
A
PR11-2.
Print supplied by Johnson and Towers, Inc.

Print Reading Activity 11-2
Internal Thread
Refer to the print PR11-2 and answer the following
questions.
1. What is the name of the part?
2. What is the number of the part?
3. What material is specified?
4. How many counterbored holes (C-BORE)
are there on the part?
5. What is the overall outside diameter of
the part?
6. How many threaded holes are there on
this part?
7. What is the diameter of the bolt circle
that serves as a reference for the location
of the threaded holes?
8. What are the location dimensions for the
hole at A?
9. What is the size of the large center
“through” hole?
10. What type of thread representation is
used in this drawing?
Unit 11 Threads And Fasteners 143
1. ______________________________________
2. ______________________________________
3. ______________________________________
4. ______________________________________
5. ______________________________________
6. ______________________________________
7. ______________________________________
8. ______________________________________
9. ______________________________________
10. ______________________________________
144 Print Reading for Industry
Threads Activity 11-3
Thread Interpretation
For numbers 1–9, fill in the blanks for the
sample threads shown in the drawings A–D. Use
the colored lines as guidelines for lettering. Also,
answer A, B, and C for number 10.
1 1
2 – 8 UNC – 2A
FULL SCALE
HALF SCALE
A B C D
.625 – 18UNF–2A LH TRIPLE
LETTER THE INFORMATION FOR FIGURES A THROUGH D IN THE SPACES BELOW:
A B C D
1. NO THREADS/INCH
2. PITCH OF THREAD
3. THREAD FORM
4. RH/LH?
5. SINGLE OR WHICH?
6. LEAD OF THREAD
7. MAJOR DIAMETER
8. INTERNAL/EXTERNAL?
9. HOW REPRESENTED?
M12 X 1.75
10. FOR THE METRIC THREAD:
A. WHAT IS THE MAJOR DIAMETER?
B. WHAT IS THE PITCH?
C. HOW IS IT REPRESENTED?