Post on 04-Apr-2018
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Threads
What is thread?
Helical ridge of uniform section formed on inside or
outside of cylinder or cone
Thread Applications:
Used to fix temporarily one part to another
Provide accurate measurement, as in micrometer
Transmit motionWorms transmit motion on
shafts perpendicular to each
other
Worm & Gear
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THREAD TERMINOLOGY
External (male)
thread
Internal (female)
thread
A thread cut on the outside of
a cylindrical body.
A thread cut on the inside of
a cylindrical body.
Internal thread
External thread
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THREAD TERMINOLOGY
Right-hand
thread
Thread that will assemble when
turned clockwise.
Left-hand
thread
Thread that will assemble when
turned counter-clockwise.
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THREAD TERMINOLOGY
Crest
Root
Thread angle
Thepeak edge of a thread.
The bottom of the thread cut into
a cylindrical body.
The angle between threads faces.
Internal ThreadExternal Thread
CrestRoot
Thread angle
CrestRoot
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THREAD TERMINOLOGY
Major diameterThe largest diameteron
an internal or external thread.
Minor diameterThe smallest diameteron
an internal or external thread.
Internal ThreadExternal Thread
Minordia.
Majordia.
Minordia.
Majordia.
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THREAD TERMINOLOGY
PitchThe distance between crests of
threads.
Lead The distance a screw will advance
when turned 360o.
Internal ThreadExternal Thread
PitchPitch
Lead
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THREAD TERMINOLOGY
The lead and the pitch of a single lead thread
are the same.
On double lead threads, the lead is twice the pitch.
A double lead thread has two start points.
Single Lead Double Lead
Single
Start
Double
Start
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THREAD TERMINOLOGY
Helix AngleAngle that thread makes with plane
perpendicular to thread axis
Depth of threadDistance between crest and root
measured perpendicular to axis
FlankThread surface that connects crest
with rootInternal ThreadExternal Thread
Depth of Thread
Helix Angle
Flank
Helix Angle
Depth of Thread
Flank
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THREAD TERMINOLOGY
Form is the profile shape of the
thread.
Thread Form
Example :
knuckle thread form
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Thread Forms
According to use and shape, threads are
classified as follows:
V Thread
Square Thread
Acme Thread
Buttress Thread
Knuckle Thread Worm Thread
Power Thread
Used for power
transmission
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V Thread
These are classified as follows:
Metric
International Standard
British Standard Whitworth (BSW) British Standard Fine (BSF)
British Standard Pipe (BSP)
American National or Seller Thread
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ISO (METRIC) THREAD
External
thread
Internal
thread
Center of thread assembly
60o
Pitch, P
P/4
The pitch, p of the thread is expressed in mm
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ISO (METRIC) THREAD
60 included angle
D (Internal) = 0.6143 x pitchCrest = .125 x pitch
Root = .250 x pitch
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METRIC COARSE THREADNominal
size
Major
diameter
Pitch Minor
diameter
Tap drill size
M6 6.00 1.00 4.92 5.00
M8 8.00 1.25 6.65 6.75
M10 10.00 1.50 8.38 8.50M12 12.00 1.75 10.11 10.00
Minordiameter = Majordiameter Pitch
Minor diameter Tap drill sizeIn thread drawing, the following relationship is used.Metric thread
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METRIC FINE THREAD
Nominal
size
Major
diameter
Pitch Minor
diameter
Tap drill size
M8 8.000.75 7.188 7.25
1.00 6.917 7.00
M10 10.000.75 9.188 9.251.00 8.917 9.00
1.25 8.647 8.75
Example: M8x1.0 means that the thread major diameter is
8mm and its pitch is 1.0mm
Minor diameter Tap drill size
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British Standard Whitworth
(BSW) Thread
N
PR
NPD
.1373orx.1373
.6403
orx6403.
=
=
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Square Thread
D = .500P
F = .500P
C = .500P + .002
Being replaced by Acme thread because of
difficulty in cutting it Often found on vises and jack screws
Square Thread
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ACME Screw Thread
Replacing square thread in many cases
Used for feed screws, jacks, and vises
D = minimum .500P
= maximum .500P+ 0.010
F= .3707P
C= .3707P - .0052(for maximum depth)
ACME
Thread
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Worm Thread
Used to mesh worm gears and transmit
motion between two shafts at right angles to
each other but not in same plane
D = .6866P
F = .335P
C = .310P
Worm
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Methods of Thread Cutting
On Lathe
Using Tap & Die