Threads and Fasteners

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Threads and Fasteners

• Stuarts Draft High School
• Technology Education Department
• Technical Drawing

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DEFINITION OF SCREW THREAD TERMS
Pitch

• 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

Notice the pitch can be measured from
crest-to-crest OR
Pitch
Root-to-Root
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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.

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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.

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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. These series are listed in
  ANSI B1.1. Each is denoted by letter symbols, as
  follows

• Coarse-thread Series (UNC or NC) In this series
  the pitch for each diameter is relatively large.
  This series is used for screws, bolts, and nuts
  produced in quantity, and also for fastening in
  general.
• 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. This series is used where a finer thread
  is needed, as in making automobiles and
  airplanes.
• Extra-fine-thread Series (UNEF or NEF) In this
  series, the pitch is even smaller than in the
  fine-thread series. This series is used where the
  thread depth must be very small, as on aircraft
  gear or thin-walled tubes.

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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.

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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.

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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

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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 5/8-18UNEF-3B-LH means . 1 5/8 diameter, 18
  threads per inch, Unified National Extra-Fine
  threads, class 3 fit, internal, left hand threads.

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Drilled and Tapped Holes

• You specify tapped (threaded) holes by a note
  giving the diameter of the tap drill (21/64")
  depth of hole (1 3/8) thread information (½
  diameter, Unified National Course threads, Class
  2) and length of thread (1"), as

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Drawing Screw-threads

• A detailed representation approximates the real
  look of threads (Fig. 10-10). For this kind of
  drawing, you do not have to draw the pitch
  exactly to scale. Instead, just make it about the
  right size for the drawing. Draw the helixes as
  straight lines. Draw the threads as sharp Vs. In
  general, you do not usually use the detailed
  representation in working drawings, except where
  you need it for clearness. You also do not
  usually use it if the screw is less than 25 mm (1
  in.) in diameter.

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Drawing Screw-threads

• A schematic representation shows the threads with
  symbols rather than as they really look. For this
  kind of drawing, you leave out the Vs (Fig.
  10-11).You also need not draw the pitch to scale.
  Just make it about the right size for the
  drawing. Then draw the crest and root lines
  accordingly. Space them by eye to make them look
  good. These lines can be at right angles to the
  axis or slanted to show the helix angle (see Fig.
  10-17). The American National Standards calls for
  crest lines to be thin and root lines to be
  thick. However, you may make all your lines the
  same thickness to save time. This is especially
  so on regular pencil working drawings.

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Drawing Screw-threads

• A simplified representation is much like a
  schematic representation. In this case, however,
  you draw the crest and root lines as dotted
  lines, except where either of them would show as
  a visible solid line (Fig. 10-12 at A, C, F, G,
  H, I, and J). You save time by using the
  simplified representation because it leaves out
  useless details.