L 10-Threads-PART 1

7/26/2019 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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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


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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).


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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


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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


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


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Single and Multiple Threads


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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).



American Standard thread convention

ISO Standard thread convention

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Inch Threads


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


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


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


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


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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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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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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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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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.


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The chamfer or the undercut sizes

need not be shown in the drawing.


A comparison of metric and

inch thread sizes is given.

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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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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


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


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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Major

P/2

P

60

Root

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Detailed Representation of Square Threads


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


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


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


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


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


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

Documents

L 10-Threads-PART 1