7/30/2019 Types of Threads

1/20

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

7/30/2019 Types of Threads

2/20

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

7/30/2019 Types of Threads

3/20

THREAD TERMINOLOGY

Right-hand

thread

Thread that will assemble when

turned clockwise.

Left-hand

thread

Thread that will assemble when

turned counter-clockwise.

7/30/2019 Types of Threads

4/20

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

7/30/2019 Types of Threads

5/20

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.

7/30/2019 Types of Threads

6/20

THREAD TERMINOLOGY

PitchThe distance between crests of

threads.

Lead The distance a screw will advance

when turned 360o.

Internal ThreadExternal Thread

PitchPitch

Lead

7/30/2019 Types of Threads

7/20

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

7/30/2019 Types of Threads

8/20

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

7/30/2019 Types of Threads

9/20

THREAD TERMINOLOGY

Form is the profile shape of the

thread.

Thread Form

Example :

knuckle thread form

7/30/2019 Types of Threads

10/20

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

7/30/2019 Types of Threads

11/20

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

7/30/2019 Types of Threads

12/20

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

7/30/2019 Types of Threads

13/20

ISO (METRIC) THREAD

60 included angle

D (Internal) = 0.6143 x pitchCrest = .125 x pitch

Root = .250 x pitch

7/30/2019 Types of Threads

14/20

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

7/30/2019 Types of Threads

15/20

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

7/30/2019 Types of Threads

16/20

British Standard Whitworth

(BSW) Thread

N

PR

NPD

.1373orx.1373

.6403

orx6403.

=

=

7/30/2019 Types of Threads

17/20

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

7/30/2019 Types of Threads

18/20

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

7/30/2019 Types of Threads

19/20

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

7/30/2019 Types of Threads

20/20

Methods of Thread Cutting

On Lathe

Using Tap & Die