chapter 1 screw thread

8/11/2019 Chapter 1 Screw Thread

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Screw ThreadChapter 1


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TOPICS

Introduction

Thread terminology

method of measuring and

inspection of metric V screw

thread


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Introduction


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FASTENING TYPE

1. PermanentWelding Gluing Riveting


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FASTENING TYPE

2.1 Threaded fastener

- bolts

- studs- screws

2. Temporary

2.2 Non-threaded fastener

- keys

- pin

key


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THREAD APPLICATION

1. To hold parts together.

2. To move part(s) relative to others.

Part A Part B

Part C


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THREAD APPLICATION

1. To hold parts together.

2. To move part(s) relative to others.

Wood working vise Palm fruit pressing machine


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Thread Terminology


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External (male)thread

In ternal (female)

thread

THREAD TERMINOLOGY

A thread cut on the outs ideof

a cylindrical body.

A thread cut on the ins ideof

a cylindrical body.

Internal thread

External thread


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Right-handthread

Thread that will assemblewhen

turned c lockwise.

Left-hand

thread

Thread that will assemblewhen

turned counter-c lockwise.

THREAD TERMINOLOGY

Turnbuckle use RH and LH thread ateach end to double displacement.


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Crest

Root

Thread angle

THREAD TERMINOLOGY

The peak edgeof a thread.

The bot tom of the thread cut into

a cylindrical body.

The angle between threads faces.

Internal ThreadExternal Thread

CrestRoot

Thread angle

CrestRoot


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Major d iameter

The largest diameteron

an internal or external thread.

Minor diameterThe smal lest diameteron

an internal or external thread.


Minord

ia.

Majord

ia.

THREAD TERMINOLOGY

Minord

ia.

Majord

ia.


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Pitch

The distance between crests of

threads.

LeadThe distance a screw will advance

when turned 360o.

THREAD TERMINOLOGY


Pitch

Pitch


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Form is the profile shape of the

thread.Thread Form

THREAD TERMINOLOGY

Example:

knuckle thread form


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THREAD TERMINOLOGY

Let s think together

Label the thread element

AB

FD

C

E

G

(i) External thread (ii) Internal thread


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ISO (METRIC) THREAD

External

thread

Internalthread

Center of thread assembly

60o

Pitch, P

P/4

P/8

Thread assemble occurs if and only if both (internal & external)

thread have an equal nom inal size (or diameter)and pi tch.


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MEASURING THREAD

ELEMENTS

MAJOR DIAMETER

MINOR DIAMETER

PITCH DIAMETER


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MEASURING THE MAJOR DIAMETER

Use micrometer

check to a cylindrical

standard of about the

same diameter as thescrew

anvilsof a diameter

sufficient to span two

threads


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MEASURING THE MINOR DIAMETER

Use micrometer with

a pair of vee pieces

Mean diameter value:

Dm= W2T

T

prism

W Note :

Dm - Mean diameter

W - distance between two prism

T - prism height


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MEASURING THE PITCH DIAMETER

Standard micrometer three-wire method

three wires of equal diameter

are placed in the thread

wires - hardened and lapped

to three times the accuracy of

the thread to be inspected

Wire

Distance

measured

Max. Wire

Best WireMin. Wire

BEST WIRES

- touch the thread at the

pitch diameter

- least affected by errors

from angle of thread


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Thread

Form

Max.

Wire

Min.

Wire

Best

Wire

Size range for

Best wire

ISO metric

and

Unified

1.01p 0.505p 0.557p 0.534p

0.620p

Whitworth 0.853p 0.506p 0.564p 0.535p0.593p

Three-wire Method

Wire sizes for thread m easu rement

( p = pitch of th read)


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MEASURING THE MEAN DIAMETER

BC

D

W

DE

P/2

r

A

h

E

60o

Pitch (P)

2

H

W = Distance over wires

DE= Pitch/ Effective Diameter

Dw= Wire diameter

= 600

Three-wire Method


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ISO metric and unified (a)

Whitworth (b)

W= distance wireD= diameter

d= best wire diameter

p=pitch


Three-wire Method

W=

D +3d- 1.5156p

W= D + 3.165d - 1.6 p


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THREAD TERMINOLOGY


Determine the distance of wire by using best wire sizes

Given wire for M 20 x 2.5 ISO metric thread

a) Find best wire sizes

b) Calculate the distance of wire


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Optical comparator/projector

Projector Screen

Condenser Lens

Screen rotation

knob

Surface illuminator

Screen clamp

knob

X, Y counter

Focusing wheel

Control Panel

Projector Head

Angle counter

X axis wheelY axis wheel


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Principle of optical comparator/projector

Light

Condenserlens

Shadow

Image

Mirror

Screen


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CHECK THE ANGLE OF A 60OTHREAD USING AN

OPTICAL COMPARATOR

1) Mount the correct lens in the comparator.

2) Mount the tilting work centres on the micrometer cross-slide stage.

3) Set the tilting work centres to the helix angleof the thread.

4) Set the work piece between centres.

5) Mount the vernier protractor chart and align it horizontally

on the screen.

6) Turn on the light switch.

7) Focus the lens so that a clear image appears on the screen.

8) Move the micrometer cross-slide stageuntil the thread

imageis centralized on the screen.

9) Removethe vernier protractor chart to show a

reading of 30o.10) Adjust the cross-slides until the image coincides

with the protector line.

11) Check the other side of the thread in the

same manner.

helix

angle


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THREAD TERMINOLOGY


a) State the advantage of optical comparator

b) List the disadvantage of optical comparator

chapter 1 screw thread

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