Threads and Fasteners

Fastening – What is it?

• A method of connecting or joining two or more parts together, using devices or processes.

• Examples: Bridges, towers, buildings, transportation, piping, sports, etc.

Threaded Fasteners• A mechanical fasteners used to join

together two or more parts.

• Are formed by using a tapping tool for internal threads, a die for external threads, or a machine tool for either internal or external threads.

Applications

• There are 3 major types of Threaded Fastener Applications, they are:

– Joining

– Adjusting

– Power Transmission

Adjusting• The process of locating or modifying

the position of a part.

• Examples: Drafting Compass – uses a threaded rod to adjust the radial setting.

• Calipers, micrometers, hand wrenches, etc.

Power Transmission• The process of converting or transmitting

the force exerted in one direction to force exerted in the same or a different direction.

• Screw Jacks and certain types of gears use screw threads to transmit power.

• Good example of this is a Car lift

External (male)thread

Internal (female)thread

THREAD TERMINOLOGY

A thread cut on the outside of

a cylindrical body.

A thread cut on the inside of

a cylindrical body.

Internal thread

External thread

Right-handthread

Thread that will assemble when

turned clockwise.

Left-handthread

Thread that will assemble when

turned counter-clockwise.

THREAD TERMINOLOGY

Turnbuckle use RH and LH thread at each end to double displacement.

Crest

Root

Thread angle

THREAD TERMINOLOGY

The peak edge of a thread.

The bottom of the thread cut intoa cylindrical body.

The angle between threads faces.

Internal ThreadExternal Thread

CrestRoot

Thread angle

CrestRoot

Major diameterThe largest diameter on

an internal or external thread.

Minor diameterThe smallest diameter on

an internal or external thread.

Internal ThreadExternal ThreadM

inor

dia

.

Ma

jor

dia

.

THREAD TERMINOLOGY

Min

or d

ia.

Ma

jor

dia

.

Form is the profile shape of thethread.

Thread Form

THREAD TERMINOLOGY

Example :

“knuckle thread form”

COMPARISON OF THREAD CUTTING

External Thread Internal Thread

Major Dia.

Minor Dia.

EXTERNAL THREAD CUTTING

Tools Operation

Threading Die

Die stock

Tools

Tap

Tap wrench

Operation

INTERNAL THREAD CUTTING

Pitch

DEFINITION OF SCREW THREAD TERMS

• The pitch of a thread is the distance from one point on the thread form to the corresponding point of the next thread. Pitch is measured parallel to the thread's axis or centerline. Click to see a sample appear to the right

Pitch

Notice the pitch can be measured

from crest-to-crest OR

Root-to-Root

PitchThe distance between crests of

threads.

Lead The distance a screw will advance

when turned 360o.

THREAD TERMINOLOGY

Internal ThreadExternal Thread

PitchPitch

DEFINITION OF SCREW THREAD TERMS

• The lead (pronounced "leed") is the distance that a threaded part (such as a nut) would move or travel in the horizontal direction if the screw thread was rotated 360 degrees.

• Threads are always assumed to be right handed threads unless marked with the initials LH (Left Hand) at the end of the thread note.

SINGLE AND MULTIPLE THREADS

•  Most screws have single threads. Assume single thread unless otherwise noted. Multiple threads are used where parts must screw together or apart quickly. For example, the compass adjustment bar and toothpaste tube caps have multiple threads. Notice in the picture above that:

– Single Thread - The lead is equal to the pitch. – Double Thread - The lead is twice the pitch – Triple Thread - The lead is three times the pitch.

Single and multiple threadsA single thread has a

single ridge in the form of a helix.

A double thread has two ridges started 180 apart.

A triple thread has three ridges, started 120 apart.

The Studio Dog

THREAD SERIES

• Screws of the same diameter are made with different pitches (number of threads per inch) for different uses. The various combinations of diameter and pitch have been grouped into screw-thread series.

• Coarse-thread Series (UNC or NC) In this series the pitch for each diameter is relatively large.

• Fine-thread Series (UNF or NF) In this series, the pitch for each diameter is smaller (there are more threads per inch) than in the coarse- thread series. Example automobiles

• Extra-fine-thread Series (UNEF or NEF) In this series, the pitch is even smaller than in the fine-thread series. Example airplanes and gears

CLASSES OF FITS

• Screw threads are also divided into screw thread classes based on their tolerances (amount of size different from exact size) and allowances (how loosely or tightly they fit their mating parts). You can get exactly the screw thread you need by choosing both a series and a class. In brief, the classes for Unified threads are:

• Class 1 has a large allowance (loose fit). They are used on parts that must be put together or taken apart quickly and easily.

• Class 2 is the thread standards most used for general purposes, such as for bolts, screws, nuts, and similar threaded items.

• Class 3 has stricter standards for fit and tolerance than the others. They are used where thread size must be more exact. Simply, a tighter fitting thread.

“A” Or “B”

• The letter “A” following the fit class number (1,2, or 3) denotes external threads only;

• the letter “B” following the fit class number denotes internal threads only.

READING A THREAD NOTE

Thread Note Definition of Meaning

.750 0.750 diameter (3/4”)

10 10 threads per inch

UNC Unified National Coarse thread series  

2 Class 2 (Normal Fit)

A External Thread

READING A THREAD NOTE

• You specify (call for) a particular screw thread by telling its diameter (nominal or major diameter), number of threads per inch, length of thread, initial letters of the series, and class of fit. Any thread you specify will be assumed to be both single and right hand unless you say otherwise. If you mean the thread to be left hand, include the letters “LH” after the class symbol. If you mean it to be double or triple, include "double" or "triple." Some examples using fractional sizes follow:

• 1 1/4-7UNC-1A

Means 1 ¼” diameter, 7 threads per inch, Unified National Course threads, class 1 fit, external threads.

• 7/8-14UNF-2B

Means 7/8” diameter, 14 threads per inch, Unified National Fine threads, class 2 fit, internal threads

1. 1. DetailedDetailed representation representation

2. 2. SchematicSchematic representation representation

3. 3. Simplified Simplified representationrepresentation

THREAD REPRESENTATION

External thread Internal thread

DETAILED REPRESENTATION

60oPitch

Use slanting lines to represent crest and root.

Roots and crest are drawn in sharp Vs.

Thread runout

SCHEMATIC REPRESENTATION

External thread

Pitch Crest (thin line)

Root (thick line)

Use alternate long and short lines for representing

crests and roots of the thread, respectively.

Internal thread

BOLT : Terminology

Bolt is a threaded cylinder with a head.

Hexagonal head

bolt and nutHead

thickness

Thread length

Length

Width across flat

Bolt measurements

Drawboltaxis

Draw anend view

of the head

B

Startingposition

H

Draw abolt head

B/2 B/2

30o

Length

Thread Length

Maj

or

Draw body of a bolt

BOLT : Drawing steps

NUT : Drawing steps

Draw an end viewof the nut

BH

B/2 B/2

Dash lines represent

a threaded hole are

omitted for clarity.

Schematic thread representation

• Staggered lines perpendicular to the axis of the thread represent the thread root and crests.

• Spacing between root and crest lines is any convenient size.

• Root lines are any convenient length.

Drawing detailed representation of V threads• Step 1

• 1. Establish the pitch and half-pitch at the major diameter.

2. Measure intervals the size of the pitch along the major diameter from the first two marks.

3. Add crest lines.

Drawing detailed representation of V threads

• Step 2• 1. Add the V profile for one thread, top

and bottom, locating the root diameter. 2. Add construction lines for the root

diameter.

Drawing detailed representation of V threads• Step 3

• 1. Add one side of the remaining Vs (thread profile).

2. Add the other side of the Vs, completing the thread profile.

Fasteners Quiz

Answers