Download - Chapter 4 Screw Threads
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Chapter 4Screw Threads
Prepared By: Dishank UpadhyayMechatronics DepartmentTeamLease Skills UniversityVadodara
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Objectives
• Thread Terminology• Form of Screw Threads• Types of Threads• Thread Style & Cutting Operation• Conventional Representation
– External Threads (With/Without Section)– Internal Threads (With/Without Section)
• Multiple Start Thread
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Thread Terminology
External (male)
thread
A thread cut on
the outside of
a cylindrical
body.
Internal (female)
threadA thread cut on
the inside of
a cylindrical body.
Internal
thread
External
thread
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Thread TerminologyRight-hand
thread
Left Hand
Thread
Thread that will assemble when turned counter-clockwise.
Turnbuckle use RH and LH thread at each
end to double displacement.
Thread that will assemble when turned clockwise.
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Forms of Screw Threads
• Form is the profile shape of the thread.
Example :
“knuckle thread form”
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Forms of Screw Threads
• 2 main types of screw threads1. Triangular or V Thread2. Square Thread- Other forms are either modified forms of square threads or combination of the two forms.
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1. Triangular or V Threads
- Types of Threads
1. Unified Threads
2. Metric Threads
3. Withworth Threads
4. British Standard Fine (BSF) & British Standard Pipe (BSP)
5. Sellers Threads
6. British Association (B.A.) Threads
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Unified Threads
• External Threads(Screw) Varies slightly in shape from the Internal Threads (Inside a nut).
• Angle of thread : 60°
• Roots : Rounded
• Crests: Cut Parallel to the axis of Screw
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Metric Threads
• BIS (Bureau of Indian Standards) designed screws in Metric System. And as designated it as Metric Screw Threads.
• In this System
– Pitch : Fixed (Instead of No. of Threads)
– Metric Thread designated by “M” followed by nominal diameter
– E.g. M 20 (20 is nominal diameter)
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Withworth Threads
• Also known as a BSW (British Standard Withworth)
– Angle : 55°
– Theoritical Depth = D = 0.96 P
– 1/6 D is rounded off at top & bottom
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British Standard Fine (BSF) & British Standard Pipe (BSP)
• Same witheworth profile
• Pitches are finer
• Depth are smaller
• Uses:
– BSF : Automobile & Aircraft work
– BSP : Gas, Steam & Water pipe
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Sellers Threads
• Standard form in USA
• Thread Angle : 60°
• Cutoff = D/8
• Crests and Roots teeth are flat
• Theoretical depth D = 0.866P
• Actual Depth = 0.649P
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British Association (B.A.) Threads
• Angle: 47.5°
• Round off = 0.236D
• Actual depth = 0.6P
• Th.depth D = 1.136P
• Act. Depth d = 0.6 P
• Use: Small Instruments sccrews
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2. Square Threads
• Flanks & Sides normal to the axis.
• Use:– Power transmission
– To obtain larger axial movement of the nut or the screw per revolution
– Lead screw of lathe machine
– Screw jacks
– Vices
• Pitch of the thread is normally larger than the triangular threads
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2. Square Threads
• Types
1. Acme Threads
2. Knuckle Threads
3. Buttress Threads
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Acme Threads
• Modified from Square thread• Stronger and easier to cut• Thread Angle : 29°• Depth d= 0.5P + 0.25 mm• Thickness at crest = 0.3707P• Uses
• Lead screw of lathe
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Knuckle Threads
• Made by rounding off the corner of the square threads
• Comprises of semi circles of radius R = 0.25P
• Depth d = 0.5P• Wear resistant and used in
rough usage• Uses:
– Coupler of railway carriages– Electric bulb
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Buttress Threads
• Combination of triangular and square threads
• One flank perpendicular to the thread axis
• Thread angle is = 45°
• D = P
• d = 0.75 P
• Use:– Screw of Bench-vice.
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Threads Styles
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External Thread Cutting
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Internal Thread Cutting
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Thread Drawing / Representation
• 3 Types of representation
1. Detailed Representation
2. Schematic Representation
3. Simplified Representation
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1. Detailed Representation
• Use Slanting lines to represent crest and root.
• Roots and crests are drawn in sharp “V”s
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2. Schematic Representation
• Use alternate long and short lines for representing crests and roots of the threads, respectively.
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3. Simplified Representation
• Use thick continuous lines for representing crests and thin continuous lines for representing root of the threads, Respectively.
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3. Simplified Representation
• Use thick continuous lines for representing crests and thin continuous lines for representing root of the threads, Respectively.
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Thread Length
DRAWING STEPS OF
EXTERNAL THREAD
Starting
position
Draw
thread
axis
Ma
jor
dia
.M
ino
r d
ia.
Draw
45o Chamfer
Draw line making
30o with thread axisDraw a circle thatrepresents a crest.
Draw an arc thatrepresents a root.
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Internal Thread
1. Through threaded hole
DRAWING STEPS OF
THREADED HOLE
Draw
thread
axis
Ma
jor
dia
.M
ino
r d
ia.
Draw a root
Draw a crestSectional view
Draw a circle thatrepresents a crest.
Draw an arc thatrepresents a root.
root
crest
Section lines are drawn into the crest of a thread.
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Min
or
dia
.
DRAWING STEPS OF
THREADED HOLE
2. Blinded threaded hole
Draw
thread
axis
Ma
jor
dia
.
Hole depth
Thread depthDraw a circle thatrepresents a crest.
Draw an arc thatrepresents a root.
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DRAWING STEPS OF
THREADED HOLE
2. Blinded threaded holeSectional view
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DIMENSIONING EXTERNAL THREAD
M10 ×1.5
×1.0 Fine thread
Coarse thread
xxThread
length
Use local note to specify :- thread form, nominal size,
pitch (if it is a fine thread)
Use typical method to specify :- thread length.
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3. Thread form
4. Nominal size
5. Pitch
1. Tap drill size
2. Drill depth
6. Thread depth
8.50 Drill, 20 Deep,
M10 Tapped, 15 Deep
DIMENSIONING THREADED HOLE
Use local note to
specify
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Multiple Start Threads• Single start threads P = L (Lead)
• W.k.t Depth of thread is depending on the pitch,
• Greater the lead = Greater will be the depth of thread and smaller will be the core diameter.
• Fig2. shows large depth and smaller core dia. Which makes screw weak so multiple cutting of thread is required.
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Multiple Start Threads• Multiple cutting: two or more
threads having the same pitch, with increasing lead, run parallel to each other.
• Slope of thread is changed
• S = 0.5L
• Relation between pitch and lead is
• Double start thread : P = 2L
• Triple start thread : P = 3L
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Multiple Start Threads
• Conventional Representation of Double started threads
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Thank You
• Prepared by:
Dishank Upadhyay
Mechatronics Department
TeamLease Skills University.