Hand Threading and Reaming

SUBJECT OUTCOME

3.2  Demonstrate an understanding of screw-threads.

    
 

Thread Terminology

External Thread

A external thread is defined as a ridged uniform section in the form of a helix on the external surface of a cylinder. Examples are bolts.

   Internal Thread

A internal thread is defined as a ridged uniform section in the form of a helix on the internal surface of a cylindrical hole. Examples are nuts.
 

 
 

  • The pitch diameter (often called the effective diameter) of a parallel thread is the diameter of the imaginary co-axial cylinder which intersects the surface of the thread in such a manner that the intercept on a generator of the cylinder, between the points where it meets the opposite flanks of a thread groove, is equal to half the nominal pitch of the thread.

  • The major diameter of a thread is the diameter of the imaginary co-axial cylinder that just touches the crest of an external thread or the root of an internal thread.

  • The minor diameter is the diameter of an imaginary cylinder that just touches the roots of an external thread and (or) the crests of an internal thread.

  • The crest of a thread is the prominent part of a thread, whether internal or external.

  • The root is the bottom of the groove between the two flanking surfaces of the thread whether internal or external.

  • The flanks of a thread are the straight sides that connect the crest and the root.

  • The thread angle of a thread is the angle between the flanks, measured in an axial plane section.

  • The pitch of a thread is the distance, measured parallel to its axis, between corresponding points on adjacent surfaces, in the same axial plane.

  • The terms lead angle or helix angle are the same. The helix angle is the angle between the helix of the thread and a line parallel to the axis of rotation

  • The lead is the axial advance of a helix or screw during one complete turn (360°) The lead for a screw thread is the axial travel for a single revolution

Types of thread and use

Use
  • General fasteners
  • Bolts and nuts

METRIC V THREAD

Use
  • General fasteners
  • Bolts and nuts
  • Pipe fittings

WHITWORTH THREAD

Use
  • Bench vices
  • Machine vices
  • Jacks
  • Building props
  • Where great force must be resisted in one direction only

 

 

BUTTRESS THREAD

Use
  • Ideal for power transmission
  • Cross and compound slide on lathe
  • Table moment on milling machine

 

 

 

SQUARE THREAD

Use
  • Where backlash and play must be reduced
  • Lead screw on lathe for screw thread cutting

 

 

ACME THREAD

Left and Right thread
The handedness of a screw refers to the direction in which the helical thread wraps around the screw shaft - right-hand threads run in a clockwise direction, whereas left-hand threads run in a anti-clockwise direction.  Most thread is right hand thread, but in some cases such as in gas installations left hand thread is used. It is also used in turnbuckles where one side is right side thread and the other side is left hand thread.

 

Multi Start Thread
What is lead of Multi start thread?

A multi-start thread consists of two or more intertwined threads running parallel to one another. Intertwining threads allow the lead distance of a thread to be increased without changing its pitch.

What are multi start threads used for?

Multiple-start, or multiple-groove, threads are used for getting a high lead per revolution with a shallow thread depth. 

The following is an example of a two start thread with a pitch of 1.5 mm. 

Lead  = Pitch x No of starts
          = 1.5 x 2
          = 3 mm