Technical Drawing with Engineering Graphics, 14e, Chapter 12

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THREADS, FASTENERS,
AND SPRINGS
C H A P T E R T W E LV E
OBJECTIVES
1. Define and label the parts of a screw thread.
2. Identify various screw thread forms.
3. Draw detailed, schematic, and simplified threads.
4. Define typical thread specifications.
5. Identify various fasteners and describe their use.
6. Draw various screw head types.
7. Draw springs.
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UNDERSTANDING THREADS AND
FASTENERS
The three basic applications of Screw threads are as follows:
1. To hold parts together…
2. To provide for adjustment between parts…
3. To transmit power…
Thread Used for Attachment.
(Courtesy of Arthur S. Aubry/Getty
Images, Inc.-Photodisc.)
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Screw Thread Terms
Study Page 455 of your textbook for all thread and fastener terms
Screw Thread Nomenclature
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Screw Thread Forms
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Thread Pitch
The pitch of any
thread form is the
distance parallel to
the axis between
corresponding
points on adjacent
threads.
The pitch or the number of
threads per inch can be
measured with a scale or
with a thread pitch gage.
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Thread Series
The thread series is the detail of the shape and number of threads per inch composing
different groups of fasteners.
Five series of threads were used in the old ANSI standards:
Coarse thread
Fine thread
8-pitch thread
12-pitch thread
16-pitch thread
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Right-Hand and Left-Hand Threads
Aright-hand thread is one that advances into a nut when turned clockwise,
and a left-hand thread is one that advances into a nut when turned
counterclockwise.
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Single and Multiple Threads
A single thread, as the name implies,
is composed of one ridge, and the lead
is therefore equal to the pitch.
Multiple threads are used
wherever quick motion, but not
great power, is desired, as on
ballpoint pens, toothpaste caps,
valve stems, and so on.
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American National Thread Fits
and
Metric and Unified Thread Fits
For general use, three classes of fits between mating threads (as between bolt
and nut) have been established by ANSI. These fits are produced by the
application of tolerances listed in the standard and are as follows:
• Class 1 fit
• Class 2 fit
• Class 3 fit
There are two general classes of metric thread fits:
• 6H for internal threads
• 6g for external threads
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Three Methods for Drawing Thread
1. Detailed
2. Schematic
3. Simplified
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THREAD NOTES
ASME/ANSI Y14.6, Screw Thread Representation, is a standard for
representing, specifying, and dimensioning screw threads on drawings
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EXTERNAL THREAD SYMBOLS
With Simplified representations, threaded portions are indicated by
hidden lines parallel to the axis at the approximate depth of the thread,
whether the cylinder appears rectangular or circular.
Schematic threads are indicated by alternating long and short lines.
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INTERNAL THREAD SYMBOLS
The only differences between the schematic and simplified internal thread
symbols occur in the sectional views.
In the case of blind tapped holes, the drill depth normally is drawn at least three
schematic pitches beyond the thread length
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DETAILED REPRESENTATION:
METRIC, UNIFIED, AND AMERICAN NATIONAL THREADS
The detailed representation for metric, unified, and American national
threads is the same, since the flats are disregarded.
Notice that for right-hand
threads the lines
slope upward to the right
Notice that for left-hand
threads the lines
slope upward to the left
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DETAILED REPRESENTATION
(External and Internal Square Thread):
Sometimes in assemblies the root and
crest lines may be omitted from the nut
only portion of the drawing so that it is
easier to identify the inserted screw.
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Threads in PHANTOM and Assemblies
Use phantom lines to save time when representing identical Features
Threaded shafts and springs may be shortened without using conventional
breaks but must be correctly dimensioned.
When external and internal threads
are sectioned in assembly, the V’s
are required to show the threaded
connection.
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AMERICAN NATIONAL STANDARD
PIPE THREADS
The tapered profile of the pipe thread…
American National Standard Taper Pipe Thread. (Reprinted from ASME B1.20.1-1983
(R1992), by permission of The American Society of Mechanical Engineers. All rights reserved.)
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BOLTS, STUDS, AND SCREWS
The term bolt is generally used to denote a “through bolt” that has a head
on one end, is passed through clearance holes in two or more aligned
parts, and is threaded on the other end to receive a nut to tighten and hold
the parts together.
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Screw Heads
A machine screw is similar to a slotted head cap screw but
usually smaller. A set screw is a screw, with or without a
head, that is screwed through one member and whose
special point is forced against another member to prevent
motion between the two parts.
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Do not section bolts, nuts, screws, and
similar parts when drawn in assembly
because they do not have interior detail
that needs to be shown.
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TAPPED HOLES
When an ordinary drill is used to make holes that will be tapped, it is referred to as a
tap drill. When drawing the drill point, use an angle of 30° to approximate the actual
31° slope of the drill bit. The thread length is the length of full or perfect threads. The
tap drill depth does not include the cone point of the drill.
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DRAWING STANDARD BOLTS
Bolt Proportions (Regular)
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Locknuts and Locking Devices
Many types of special nuts and devices to prevent nuts from
unscrewing are available, and some of the most common are
Shown below.
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STANDARD CAP SCREWS
Cap screws are normally finished and are used on machine tools and
other machines when accuracy and appearance are important.
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STANDARD MACHINE SCREWS
Machine screws are similar to cap screws but are usually smaller (.060"
to .750" diameter) and the threads generally go all the way to the head.
Clearance holes and counterbores should
be made slightly larger than the screws.
Typical machine screw notes are…
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STANDARD SET SCREWS
A set screw is screwed into one part so that its point bears firmly against another part. If
the point of the set screw is cupped, or if a flat is milled on the shaft, the screw will hold
much more firmly.
Set Screws. (Courtesy of Penninsula Components Inc.)
Set screws are specified as
follows…
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AMERICAN NATIONAL STANDARD
WOOD SCREWS
Wood screws with three types of heads—
flat, round, and oval—have been standardized.
(Courtesy of Michael Newman/
PhotoEdit Inc.)
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Miscellaneous Bolts and Screws
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KEYS
Keys are used to prevent movement between shafts and wheels,
couplings, cranks, and similar machine parts attached to or supported
by shafts.
Typical specifications for keys are…
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MACHINE PINS and RIVETS
Machine pins include taper pins, straight pins, dowel pins, clevis pins,
and cotter pins. For light work, taper pins can be used to fasten hubs or
collars to shafts.
Taper Pin
Note that the rectangular view of each rivet
shows the shank of the rivet with both
heads made with circular arcs, and the
circular view of each rivet is represented by
only the outside circle of the head.
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Rivet Symbols
Because many engineering structures are too large to be built in the shop, they are built in the largest
units possible and then are transported to the desired location. Trusses are common examples. The
rivets driven in the shop are called shop rivets, and those driven on the job are called field rivets.
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SPRINGS
A spring is a mechanical device designed to store energy when deflected and to return the equivalent
amount o f energy when released, ANSI Y14.13M. Springs are commonly made of spring steel, which may
be musicwire, hard-drawn wire, or oil-tempered wire. Other materials used for compression springs
include stainless steel, beryllium copper, and phosphor bronze. Urethane plastic is used in applications
where conventional springs would be affected by corrosion, vibration, or acoustic or magnetic
forces.
Steps in Detailed Representation of Spring
Springs. Norton, Robert L., Machine
Design: An Integrated Approach, 3rd,
© 2006. Printed and electronically
reproduced by permission of Pearson
Education, Inc., Upper Saddle River,
New Jersey.)
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