Threads

 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

THREAD TERMINOLOGY External (male) thread 

A thread cut on 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 clockwise..

turned counter-clockwise counter-clockwise..

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

THREAD TERMINOLOGY Crest 

The peak 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 diameter on an internal or external thread. The smallest diameter on 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 Thread Form

Form is the profile shape of the t he thread.

Example : “knuckle thread form”

COMPARISON OF THREAD CUTTING External Thread

Internal Thread

Minor Dia.

Major Dia.

EXTERNAL THREAD CUTTING Tools Threading Die

Die stock

Operation

INTERNAL THREAD CUTTING Tools

Tap

Tap wrench

Operation

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-tocrest OR

Pitch

Root-to-Root

THREAD TERMINOLOGY Pitch Lead

The distance between crests of  threads. The distance a screw will advance when turned 360o.

External Thread

Internal Thread

Pitch Pitch

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 LH (Left Hand) at the end of the thread note.

SINGLE AND MULTIPLE THREADS 

•

Most screws have single single threads. Assume single thread unless 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 multiple threads. Notice in the picture above that: 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 threads  A 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.



°   °  

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

• the letter “B” 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

THREAD REPRESENTATION 1. Detailed representation 2. Schematic representation 3. Simplified representation

DETAILED REPRESENTATION Use slanting lines to represent crest and root. Roots and crest are drawn in sharp Vs.

External thread Thread runout

Pitch

60o

Internal thread

SCHEMATIC REPRESENTATION Use alternate long  long and and short  short lines lines for representing crests and roots of the thread, respectively.

External thread

Pitch

Root (thick line) Crest (thin line)

Internal thread

BOLT : Terminology Bolt is Bolt  is a threaded cylinder with a head. Thread length

Length

Hexagonal head bolt and nut

Width across flat

Head thickness

Bolt measurements

BOLT : Drawing steps Draw an end view of the head

Draw bolt axis

Draw a bolt head 30o

Draw body of a bolt

       φ

B/2

B

H

  r   o    j   a    M

B/2

Length Thread Length

Starting position

NUT : Drawing steps

Draw an end view of the nut

B/2

B

B/2

H

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 of V threads • representation 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 of V threads • representation 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

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