Threads


What is a thread?

  • A helical groove formed on the outside or the inside of a cylinder.
  • Derived from the inclined plane.

What are threads used for?

  • Fastening one part to another part.
  • Adjustment.
  • Measurement.
  • Transmission of power

What are some common thread types?

  • ‘V’ thread: 60º for a unified or metric thread
  • Square thread, Modified Square Thread 5º
  • Acme: 29º for a standard ACME Thread
  • Buttress: 33, 45 AND 50 degree thread angles are common.
  • Rolled Threads: For screw shels of electric sockets and lamp bases.

The Unified Thread Form

American Standard B1.1-1949 complied with the agreement between the USA, Canada and England for the development of an interchangeable thread form.  This form is ‘V’ shaped and has a 60 degree thread angle.

Unified thread series (‘V’ thread)

  • UNC – Unified National Coarse
  • UNF – Unified National Fine
  • UNS – Unified National Special
  • UNEF – Unified National Extra Fine
  • UNR – Unified National Radius

Unified Thread Form

Unified Thread Form

Unified Thread Form

Size Details

Size Details

How are threads designated?

1/4 (Nomimal Size) – 20 (Threads per inch) UNC (Thread series)

When would I use a UNF thread?

  • On thin-walled material.
  • When resistance to loosening is important.
  • For fine adjustment.
  • For short lengths of engagement.
  • To increase the shear strength of the bolt.

When would I use a UNC thread?

  • On ductile, soft or brittle materials.  The thread bites deeply into the bolt.  Resists stripping.
  • When fast assembly is required.
  • Definition:  Ductile materials are able to withstand sudden impact and can be drawn out into thin wire.  Copper is a good example of a ductile metal;  lead is not.  Lead can withstand sudden impact but cannot easily be drawn into wire.

To calculate pitch from TPI

  • To find the pitch of a thread when the TPI is known just divide one by the TPI.
  • Pitch = 1 / TPI

Class of fit

  • Threads, when assembled, may have a loose or a tight fit.  The degree of looseness or tightness is referred to as the class of fit.
  • There are three basic classes of fit: 1 2 & 3 – Class 3 is the tightest fit, class 1 is the loosest.

Example

  • 7/8 – 9 UNC 1A has considerable clearance between the nut and bolt.
  • 7/8 – 9 UNC 3A has very little clearance between the nut and bolt.
  • The letter ‘A’ means that this is an external thread.
  • The letter ‘B’ would indicate an internal thread.
  • If a class of fit is not specified then class 2 is assumed.

What is the lead of a thread?

  • The lead of a thread is the distance a nut would move along the thread in one complete revolution.
  • The pitch of a thread is the distance from one thread to the next thread.  Therefore, the lead of a thread is equal to the pitch multiplied by the number of starts.
  • The lead of a single start thread is equal to the pitch.

Formula for “Lead”

  • Lead = Number of starts x pitch

Find the lead of the following thread: 

Find the lead of the following thread:

Find the lead of the following thread:

What about metric threads?

  • Metric threads have a 60 degree thread angle, the same as a Unified thread.
  • Metric threads are designated by their nominal outside diameter and the distance from one thread to the next.  This distance is known as the pitch of the thread.  For example:
Metric threads

Metric threads

 When Choosing a Metric Thread

  • Where possible, choose threads from the Standard M Profile Screw “Coarse Thread  Series”.  These coarse threads are the preferred choice when selecting metric threads for general purpose fastening

The lead of the following thread: 

  • M20 x L5 – P2.5 – 6g  (TWO START)
  • Pitch = 2.5 mm, Lead = # starts x pitch = 5.0 mm

Designation for a “Rounded Root”

  • M42 x 4.5 – 6g – 0.63R:  A rounded root is desirable on any metric thread and should be at least 0.125 x P
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