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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, measurement, as in micrometer
Transmit motion Worms transmit motion on shafts perpendicular to each other Worm & Gear
THREAD TERMINOLOGY External (male) thread
A thread cut on the the outside of
Internal (female) thread
A thread cut on the inside of
a cylindrical body.
a cylindrical body.
External thread Internal thread
THREAD TERMINOLOGY Right-hand thread
Thread that will assemble when
Left-hand thread
Thread that will assemble when
turned clockwise.
turned counter-clockwise.
THREAD TERMINOLOGY Crest
The peak edge of a thread.
Root
The bottom of the thread cut into a cylindrical body.
Thread angle
The angle between threads faces.
External Thread
Internal Thread
Thread angle
Root
Root
Crest
Crest
THREAD TERMINOLOGY Major diameter Minor diameter
The largest diameter on an internal or external thread. The smallest diameter on an internal or external thread.
External Thread . a i d r o n i M
Internal Thread . a i d r o j a M
. a i d r o n i M
. a i d r o j a M
THREAD TERMINOLOGY The distance between crests of
Pitch
threads. The distance a screw will advance
Lead
when turned 360o. External Thread
Internal Thread
Lead
Pitch Pitch
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
Double
Start
Start
Single Lead
Double Lead
THREAD TERMINOLOGY Angle that thread makes with plane perpendicular to thread axis
Helix Angle Depth of thread Flank External Thread
Distance between crest and root measured perpendicular to axis Thread surface that connects crest with root Internal Thread Helix Angle
Helix Angle
Depth of Thread
Depth of Thread
Flank Flank
THREAD TERMINOLOGY Thread Form
Form is the profile shape of the thread. Example :
“knuckle thread form”
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
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
ISO (METRIC) THREAD Internal thread
60o
P/4
External thread Pitch, P
Center of thread assembly
The pitch, p of the thread is expressed in mm
ISO (METRIC) THREAD 60° included angle D (Internal) = 0.6143 x pitch Crest = .125 x pitch Root = .250 x pitch
METRIC COARSE THREAD Nominal 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
8.50
M12
12.00
1.75
10.11
10.00
Metric thread
Minor diameter ≈ Tap drill size In thread drawing , the following relationship is used. Minor diameter = Major diameter – Pitch
METRIC FINE THREAD Nominal size M8
M10
Major diameter
8.00
10.00
Pitch
Minor diameter
Tap drill size
0.75
7.188
7.25
1.00
6.917
7.00
0.75
9.188
9.25
1.00
8.917
9.00
1.25
8.647
8.75
Minor diameter ≈ Tap drill size Example: M8x1.0 means that the thread major diameter is 8mm and its pitch is 1.0mm
British Standard Whitworth (BSW) Thread D
R
=
=
.6403 x P or
.6403 N
.1373 .1373 x P or N
Square Thread
Being replaced by Acme thread because of difficulty in cutting it Often found on vises and jack screws Square Thread
D = .500P F = .500P C = .500P + .002
ACME Screw Thread
Replacing square thread in many cases
Used for feed screws, jacks, and vises
D = minimum .500P
= maximum .500P + 0.010
F =
.3707 P
C =
.3707 P - .0052 (for maximum depth)
ACME Thread
Worm Thread
Used to mesh worm gears and transmit motion between two shafts at right angles to each other but not in same plane Worm
