Threads and Threading

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Threads and Threading
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Types
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1. Translation threads

• square
• acme
• buttress

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2. American National thread form

• WWII - US military equipment did not interchange
  with equipment made in Britain and Canada

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3. Unified thread form

• After WWII countries agreed on need for
  interchangeability
• Essentially the same as the American National
  thread form except Unified has a rounded root and
  either a rounded or flat crest
• Is interchangeable with the American National
  thread form
• Has 60 deg thread angle
• Two major thread series - UNC and UNF

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4. Metric threads
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5. Right hand vs. left hand
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Screw thread Nomenclature (external)
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Major Dia largest dia
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Minor Dia smallest dia
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Pitch Dia imaginary point where width of groove
and thread are equal
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Root bottom surface connecting 2 sides of a
thread
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Crest top
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Pitch linear distance from corresponding points
on a thread
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Depth Distance from crest to root perpendicular
to axis of thread on one side
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Flank Sides of a thread that connect crest to
root
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Helix Angle

• Distance of movement
  
  
  
  compared to each revolution
• The Lead of the helix

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Lead Distance of 1 revolution (lead pitch on a
single lead thread)
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Thread angle included angle between flanks of
thread
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Unified Screw thread designation (5 components)
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1/2-13-UNC-2A

• 1/2 major dia
• 13 threads per inch
• UNC Unified National Course
• 2 Class of fit
• A External Thread form (B internal)

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Classes of fit

• Class 1 - largest mfg tolerances, used for ease
  of assembly
• Class 2 - used on largest percentage of threaded
  fasteners
• Class 3 - smallest mfg tolerances, threads will
  be tight when assembled

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Methods of manufacturing threads

• taps and dies
• lathe
• milling
• grinding - used when material cannot be machined
• rolling - most common

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Thread measuring instruments-regardless of
method, pitch dia is always measured or compared

• 1. mating part - simplest, no measurement
  involved
• 2. comparator micrometer - does not measure
  pitch, only compares to a known standard
• 3. thread micrometer - each micrometer measures a
  range of TPI
• (8 - 13), (14 - 20), (22 - 30), (32 - 40)

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Measuring instruments (cont.)

• 4. three wire system (most accurate)
• 5. go / no go thread gages - used in production
  where quick gaging is necessary
• 6. optical comparator - light beam shows a
  profile of the thread for checking thread form,
  helix angle, and depth (external threads)

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Taps and Dies

• Taps - create internal threads
• Dies - create external threads
• Usually made of high speed steel

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Standard set consists of

• Taper tap - used for starting a tapped thread
  square with the hole
• Plug tap - most common
• Bottoming tap - to produce threads almost to the
  bottom of a blind hole

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Types of taps

• interrupted thread taps - used for tough
  materials - alternate teeth reduce friction
• spiral pointed (gun) - chips are forced ahead of
  the tap
• spiral fluted - helical flutes to draw chips out
  of the tap
• thread forming taps - fluteless taps that do not
  cut, they displace the material to form the
  threads - ductile materials
• Tapered pipe taps

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Tapping procedures

• by hand with a tap wrench
• by machine

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Drilling the proper hole diameter

• tap drill size
• selected from a chart
• hole should be reamed before tapping
• Tapping problems (Table B-3)

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Types of dies

• Round split adjustable or (button) - allow for
  small adjustments in size
• 2 piece split die. - blanks are placed in cap
  with guide

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Hand threading procedures

• always start the die on the leading (throat) side
• use lathe, drill press, or mill to start the die
  squarely
• use lubricant
• chamfer the end of the rod
• reverse the die (or tap) after each full turn to
  clear chips