L 10-Threads-PART 1
Transcript of L 10-Threads-PART 1
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MECH 313 EngineeringDrawing & Design
Lecture 10 PART 1
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Outline
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Threaded fasteners are the most common and most used
parts in assemblies
Parts with counterbores, spotfaces are a direct result of
designers using these fasteners
In this lecture, the emphasis will be on how the threaded
fasteners are going to be represented in engineering drawing
Types of threaded fasteners available, and related
components that will be used in an assembly will be looked at
Fastening devices are important in construction of
manufacturing products, machines and many other devices
Overview
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Simplified Thread Representation Permanent Fasteners
Rivets and Welds
Removable Fasteners
Bolts, screws, studs, pins, keys
Due to requirements like industrial progress and need for
automated assembly tools, fasteners became standardized
over time
A favorite cost-reducing method, standardization, not only
cuts the cost of parts but reduces paperwork and simplifies
inventory and quality control.
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A screw thread is a ridge of uniform section in the form of a helix on
the external or internal surface of a cylinder (Fig. 10-1-2).
The lead L is the distance the threaded part would move parallel to
the axis during one complete rotation in relation to a fixed mating part
(the distance a screw would enter a threaded hole in one turn).
Simplified Thread RepresentationScrew Threads
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Simplified Thread RepresentationScrew Threads
0
1
2
3
4
57 6
01234
5
76
8
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The helix of a square thread is shown inFig. 10-1-3.
Lead screw on a lathe, (mainly used to
transmit motion or power
Simplified Thread RepresentationScrew Threads
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The pitch of a thread P is the distance from a point on the thread formto the corresponding point on the next form, measured parallel to the
axis (Fig. 10-1-4).
Simplified Thread RepresentationScrew Threads
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Shows some common thread forms
ISO Metric is the most common of all
Inch & Metric have same proportion
Thread Forms
Simplified Thread Representation
Knuckle thread is rolled or cast
(used in light bulbs and sockets)
Square and Acme threads are
used for transmitting power
Buttress thread takes pressure on
one side ( to the axis) 9
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In Engineering drawing true representation
of threads are seldom done
There are three types of conventions to
represent threads
Simplified representation is used, if it gives
all the necessary information
Detailed representation is done while
dimensioning enlarged views, layouts
Schematic is the same as detailed but easy
to draw in case of manual drafting
Thread Representation
Simplified Thread Representation
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If it is not stated in the drawing, it is
always assumed to be right handthread
A bolt threaded into a tapered hole
should be turned clockwise
Right and Left Hand Threads
Simplified Thread Representation
Some special cases (where the
torque may loosen the fastener) may
require Left hand threads
If Left hand threads are necessary it
is indicated in the drawing by the
letters LH after the thread
designation 11
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If it is not stated in the drawing, it is always
assumed to be single thread
Single thread has a single ridge in the form of
helix and lead is equal to the pitch
Double thread has 2 ridges in the form of 2
helices starting 180apart and the lead is equalto twice the pitch
Triple thread has 3 ridges in the form of 3 helices
starting 120apart and the lead is equal to three
times of the pitch
Multiple threads are required when small rotation
must gives faster movement (Example
mechanism for opening and closing
windows
Single and Multiple Threads
Simplified Thread Representation
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Single and Multiple Threads
Simplified Thread Representation
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Thread crests, except in hidden
views, are represented by a thickoutline and the thread roots by a
thin broken line (Fig. 10-1-10).
Simplified Thread Representation
Simplified Thread Representation
American Standard thread convention
ISO Standard thread convention
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Inch Threads
Simplified Thread Representation
In North America still design is based on
inch threads
Here the pitch is equal to the 1/No of
Threads per inch
Based on the No of threads per inch for a
set of diameters coarse threaded series or
fine threaded series are available
Thread Class Thread class basically differs in allowance and tolerance in size in
each class
For external threads the classes are 1A, 2A, and 3A
For internal threads the classes are 1B, 2B, and 3B
Each class have specific characteristic and used in different situations15
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Thread Class
Simplified Thread Representation
Classes 1A and 1B
These classes produce the loosest fit, that is, the greatest amount ofplay (free motion) in assembly. Such as for stove bolts and other
rough bolts and nuts.
Classes 2A and 2B
These classes are designed for the ordinary good grade of
commercial products, such as machine screws and fasteners, and for
most interchangeable parts.
Classes 3A and 3B
These classes are intended for exceptionally high-grade commercial
products, where a particularly close or snug fit is essential and the
high cost of precision tools and machines is warranted.
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Thread Designation
Simplified Thread Representation
In drawing it is designated in the following order; with 3-4 decimal places -
no of TPI form and series class of fit (number 1,2, or 3 and letter a,or b)
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Metric Threads
Simplified Thread Representation
Based on the diameter-pitch combination metric threads are
grouped into coarse thread and fine thread seriesCoarse-Thread Series
This series is intended for use in general engineering work and
commercial applications.
Fine-Thread Series
For general used when thread finer than coarse thread is
desirable.
In comparison with a coarse-thread screw, the fine-thread screw
is stronger in both tensile and torsional strength and is less likely
to loosen under vibration.
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Thread Grades and Classes
Simplified Thread Representation
A fit of a screw is the amount of clearance when internal and external
threads are assembled
The number of the tolerance grades reflects the size of the tolerance.
For example, grade 4 tolerances are smaller than grade 6 tolerances,
and grade 8 tolerances are larger than grade 6 tolerances.
Grade 6 tolerances should be used for medium-quality length-of-
engagement applications.
The tolerance grades below grade 6 are intended for applicationsinvolving fine quality and/or short lengths of engagement.
Tolerance grades above grade 6 are intended for coarse quality
and/or long lengths of engagement.
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Simplified Thread Representation
In addition to the tolerance grade, a positional tolerance is
required This controls the MMC limits of the pitch and crest diameters of
the internal and external threads
For external threads:
Tolerance position e (large allowance)
Tolerance position g (small allowance)
Tolerance position h (no allowance)
For internal threads:
Tolerance position G (small allowance)
Tolerance position H (no allowance)
Thread Grades and Classes
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Simplified Thread Representation
ISO metric threads are defined in the
drawing by nominal size and pitch, bothexpress in millimeter
For example, a 10-mm diameter, 1.25-pitch,
fine-thread series is expressed as M10 X
1.25.
A 10-mm diameter, 1.5 pitch, coarse-thread
series is expressed as M10; the pitch is not
shown unless the length of thread is
required.
If the latter thread were 25 mm long and this
information was required on the drawing,
the thread callout would be M10 X 1.5 X 25.
ISO Metric Screw Thread Designation
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Simplified Thread Representation
ISO Metric Screw Thread Designation
A complete designation for an ISO metric
screw thread comprises, in addition to thebasic designation, an identification for the
tolerance class.
The tolerance class designation is
separated from the basic designation by adash and includes the symbol for the pitch
diameter tolerance followed immediately by
the symbol for crest diameter tolerance.
Each of these symbols consists of a
numeral indicating the grade tolerance
followed by a letter representing the
tolerance position (uppercase - internal
threads and lower case - external threads) 22
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Simplified Thread Representation
For external threads, the length of thread may be
given as a dimension on the drawing. The length given is to be the minimum length of
full thread.
For threaded holes that go all the way through
the part, the term THRU is sometimes added to
the note.
If no depth is given, the hole is assumed to go all
the way through.
For threaded holes that do not go all the way
through, the depth is given in the note, for
example, MI2 X 1.75 X 20 DEEP. The depth
given is the minimum depth of full thread.
ISO Metric Screw Thread Designation
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Simplified Thread Representation
The chamfer or the undercut sizes
need not be shown in the drawing.
ISO Metric Screw Thread Designation
A comparison of metric and
inch thread sizes is given.
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Simplified Thread Representation
Pipe Threads
Pipe is universally used in inch
size.
Nominal diameter and wall
thickness is given
While calling for size of the
thread note is used that is
similar to screw threads
When calling a pipe for a thread
on a metric drawing the
abbreviation IN follows the pipe
size
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Simplified Thread Representation
Pipe Threads
Example 1
4 x 8NPT
Example 2
4 x 8NPS
Where 4 = nominal diameter of pipe, in inches
8 = number of threads per inchN = American StandardP = PipeS = Straight pipe threadT = Taper pipe thread
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Detailed Thread Representation
Detailed Thread Representation
Close approximation of how
thread looks in real
The form is simplified by helices
as straight lines and truncated
or rounded crests as Vs
Detailed Representation of V
Threads
The threads are represented by
sharp Vs
The pitch is generally
approximated and not drawn to
scale
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Detailed Representation of V Thread
Step 1
Establish the pitch and half pitch, add crest lines
Step 2
Locate the root dia and add the V profile (60)
Step 3
Add one side of remaining Vs, and then the
other side, to complete thread profile
Detailed Thread Representation
Step 4
Complete the root lines
and the detailed
representation is done
Fig 10-2-1 Detailed Representation of Threads 28
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Detailed Thread Representation
Major
P/2
P
60
Root
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Detailed Representation of Square Threads
Detailed Thread Representation
Note that the depth of the square thread
is one half of the pitch (P/2)
Step 1 - Establish spaces equal to P /2
along the diameter and add construction
lines to locate the depth (root dia.) of
thread
Step 2 - Add the crest lines
Step 3 - Add the root lines, as shown
Step 4 - Internal square thread is shownin section.
Note the reverse direction of the crest
and root lines.
Fig 10-2-2 Detailed Representation of square Threads30
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Fig 10-2-2 Detailed Representation of Acme Threads
Detailed Representation of Acme Threads
Detailed Thread Representation
Note that the depth of the acme thread
is one half of the pitch (P/2)
Step 1 Locate root dia and outside dia
Step 2 Locate the Pitch dia midway
between the two
Step 3 locate P/2 spaces on the pitchdia and draw root and crest lines, as
shown (F shows enlarged view)
Step 4 Sectional views of an internal
acme thread is shown. It is optional toshow root and crest lines beyond cutting
plane
Note the reverse direction of the crest
and root lines. 31
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Threaded Assemblies
Detailed Thread Representation
It is often desirable to show
threaded assembly drawings indetailed form, that is, in
presentation or catalog
drawings
Hidden lines are omitted to
improve clarity of these
drawings
One type of thread
representation is used in one
drawing
If required, all three methods
can be used in one drawing 32
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Schematic Thread Representation
Detailed Thread Representation
The root and crest lines are
perpendicular to the axis of the
thread
The spacing between the root
and crest lines are drawn to
convenient size
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Fastener Selection
Common Threaded Fasteners
Fastener selection is done at
design stage keeping in the
interests of designer, production,
and purchase personnel.
Designer optimum performance
Production Ease of assembly
Purchase minimize initial and stocking
cost
It is important to determine the objectives and functions of the fastener
This can be done in consultation with fastener suppliers
The key things are the load that the screw should withstand and the type of
load (tension, shear, impact shock or vibration)
Once this is determined, then the size, strength, head shape and thread
type can be selected 34
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Fastener Definitions
Common Threaded Fasteners
Machine Screws
Machine screws have either fine or
coarse threads and are available in avariety of heads. They may be used in
tapped holes as shown, or with nuts
Cap Screws
A cap screw is threaded fastener that
joins two or more parts by passing
through a clearance hole in one part and
screwing into a tapped hole in the other
A cap screw is tightened or released by
torquing the head. Cap screw sizes start
at .25 in. (6 mm) in diameter and are
available in five basic types of head. 35
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Fastener Definitions
Common Threaded Fasteners
Captive Screws
Captive screws remain attached to the panel or parent material
even when the mating part is disengaged.
They are used to meet military requirements, to prevent
screws from being lost, to speed assembly and
disassembly operations, and to prevent damage
from loose screws falling into moving parts or electrical circuits.
Bolts
A bolt is a threaded fastener that passes through clearance
holes in assembled parts and threads into a nut
Studs
Studs are shafts threaded at both ends, and they are used in
assemblies. The exposed end is threaded over with a nut36