Chapter 4Screw Threads

Prepared By: Dishank UpadhyayMechatronics DepartmentTeamLease Skills UniversityVadodara

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

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

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.

Forms of Screw Threads

• Form is the profile shape of the thread.

Example :

“knuckle thread form”

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.

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

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

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)

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

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

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

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

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

2. Square Threads

• Types

1. Acme Threads

2. Knuckle Threads

3. Buttress Threads

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

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

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.

Threads Styles

External Thread Cutting

Internal Thread Cutting

Thread Drawing / Representation

• 3 Types of representation

1. Detailed Representation

2. Schematic Representation

3. Simplified Representation

1. Detailed Representation

• Use Slanting lines to represent crest and root.

• Roots and crests are drawn in sharp “V”s

2. Schematic Representation

• Use alternate long and short lines for representing crests and roots of the threads, respectively.

3. Simplified Representation

• Use thick continuous lines for representing crests and thin continuous lines for representing root of the threads, Respectively.

3. Simplified Representation

• Use thick continuous lines for representing crests and thin continuous lines for representing root of the threads, Respectively.

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.

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.

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.

DRAWING STEPS OF

THREADED HOLE

2. Blinded threaded holeSectional view

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.

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

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.

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

Multiple Start Threads

• Conventional Representation of Double started threads

Thank You

• Prepared by:

Dishank Upadhyay

Mechatronics Department

TeamLease Skills University.