THREADS Machined, Fastener,

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THREADSMachined, Fastener, Associated Topics

• Mr. Bader

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Threaded Hole Specifications

• Typical specification for thread looks like this
  .250-20 UNC-2B
• First figure represents major thread diameter
• First figure may be expressed three ways
• Decimally
• Fractionally
• Wire gauge

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Threaded Hole Specifications

• Typical specification for thread looks like this
  .250-20 UNC-2B
• Second set of numbers represents the number of
  threads per inch
• Third set of alphanumerics represents the thread
  form and the series
• Last letter is used to represent A for external
  threads or B for internal

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Threaded Hole Specifications

• If thread is left-hand-designation is followed by
  abbreviation LH
• Right hand thread is assumed if the abbreviation
  LH is omitted
• Depth must be specified if it is a blind hole
• Through hole is assumed if the depth is omitted
• Drill tap size is included with the thread
  specification
• If drill size is omitted-use a tap drill size
  chart

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Unified Thread Form

• Unified thread forms are most commonly used for
  fasteners
• Compromise between American National and British
  Whitworth

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Unified Thread Form

• Unified are mechanically interchangeable with
  American national threads of same diameter and
  same pitch

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Thread Series

• Groups of diameter-pitch combinations
• Distinguishing specification is number of threads
  applied to a specific diameter
• Three standard series
• Coarse-abbreviated UNC
• Fine-abbreviated UNF
• Extra fine-abbreviated UNEF

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Thread Series

• Coarse series-abbreviated UNC
• Most commonly used in bulk production of bolts,
  screws, nuts and other general engineering
  applications
• Applicapable for rapid assembly or disassembly
• Used where slight corrosion is possible

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Thread Series

• Fine series-abbreviated UNF
• Suitable for production of bolts, screws, and
  nuts where a coarse thread is not desirable
• Used where length of engagement is short-smaller
  lead angle to start thread
• Used where wall thickness demands a fine pitch

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Thread Series

• Extra-fine series-abbreviated UNEF
• Used where finer pitches are desirable
• Use where short lengths of engagement are
  critical for function
• Use in thin-walled tubes, nuts, ferrules or
  couplings

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Classes of Fit

• Three standard classes of fit
• Class 1 is loosest-applications requiring most
  clearance between mating threads
• Class 2 is standard for most fastener
  applications
• Class 3 is closest fit between mating
  threads-used for more critical applications
• When class of fit is omitted-assume Class 2
• Omit A or B if thread is visibly external or
  internal

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Threaded Hole Illustrations

• Geometry on part print includes dashed circle
  around solid circle
• Dashed circle indicates major thread diameter
• Solid circle indicates minor thread diameter

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Threaded Hole Illustrations

• When hole is chamfered-dashed circle may be
  replaced by another solid line representing
  chamfer
• Side view will show two pairs of dashed lines
  representing both major and minor thread
  diameters
• Sectioned side views represent threads schematic
  method or more commonly used simplified method

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Thread Quiz
1. What tap drill diameter is recommended for a
7/16-14 UNC threaded hole?
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Thread Quiz
2. What tap drill diameter is recommended for a
¼-28 UNF threaded hole?
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Thread Quiz
3. What tap drill diameter is recommended for a
5/16-32 UNEF threaded hole?
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Thread Quiz
4. What is the major thread diameter of a NO. 4
Unified thread?
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Thread Quiz
5. What numbered thread size is closest to the
fraction 3/16 (.1875)?
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Thread Quiz
6. What thread requires a 5/16 diameter tap drill?
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Thread Quiz
7. What is the pitch (decimally) of a 1 UNEF
thread?
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Thread Quiz
8. How thick would a threaded member have to be
to provide 3 full threads using a 9/16 UNC thread?
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Thread Quiz
9. How far would a threaded member advance with 3
revolutions on a shaft threaded 1 UNF?
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Thread Quiz
10. How much difference in diameter is there
between a letter I drill and a number 1 drill?
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Thread Quiz
11. What is the closest numbered drill to 1/8
inch diameter
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Thread Quiz
12. What is the closest numbered drill to 3/16
inch diameter?
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Thread Quiz
13. Is a No.12 drill larger or smaller than a No.
10 drill?
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Thread Quiz
14. Is a No. 12 thread larger or smaller than a
No. 10 thread?
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Thread Quiz
15. Which has the largest major diameter a No. 6
thread or a No. 6 drill?
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Constant Pitch Series

• Pitch remains the same as the diameter varies
• Designated as UN-4,6,8,12,16,20,28, or 32 threads
  per inch
• Used where threads in coarse, fine and extra-fine
  do not meet the design requirements
• For pitches that are non-standard, the
  abbreviation UNS (Unified National Special) is
  used

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Constant Pitch Series

• Preference given to 8-, 12-, and 16-thread series
• 8-thread series (8UN) is used as a substitute
  for coarse thread for diameters larger than one
  inch
• 12-thread series (12UN) is used as a continuation
  of fine threads for diameters larger than 1 ½
  inches
• 16-thread series (16UN) is used as a continuation
  of extra-fine thread for diameters larger than 1
  11/16 inches

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Constant Pitch Series
1. Arrange for print appearance format Right
hand thread, .625 major thread diameter, Unified
thread form, internal threads, coarse series,
class 2 fit.
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Constant Pitch Series
2. Arrange for print appearance format Unified
thread form, left-hand thread, class 3 fit, fine
series, internal thread, .438 major diameter,
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Constant Pitch Series
3. Arrange for print appearance format 32 TPI,
Unified form, internal threads, right-hand, .216
major diameter, class 2.
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Constant Pitch Series
4. Arrange for print appearance format Left-hand
thread, 8 threads per inch, class 1 fit, 1.125
major diameter, internal threads.
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Constant Pitch Series
5. Arrange for print appearance format Class 3
fit, internal threads, 40 TPI, right-hand,
Unified form, .250 major diameter.
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1. How many tapped holes does the part contain?
Revolving Support Brkt
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2. What thread series is specified for the tapped
hole?
Revolving Support Brkt
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3. How much deeper is the tap drill than the
depth of the blind holes?
Revolving Support Brkt
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4. How many full threads will each of the .250-20
UNC tapped holes contain?
Revolving Support Brkt
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5. What type of section was drawn for the
left-hand view?
Revolving Support Brkt
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6. What would be the overall thickness before
machining? (Includes finish allowance.)
Revolving Support Brkt
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7. What is the diameter of the annular groove
inside the hole?
Revolving Support Brkt
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8. If the bored hole is cored into the casting,
what size would it be before boring?
Revolving Support Brkt
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9. How much wall thickness remains after
machining the annular groove?
Revolving Support Brkt
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10. How much tolerance can accumulate on the
overall height dimension?
Revolving Support Brkt
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Determine the dimension for the letter
A
Revolving Support Brkt
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Determine the dimension for the letter
B
Revolving Support Brkt
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Determine the dimension for the letter
C
Revolving Support Brkt
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Determine the dimension for the letter
D
Revolving Support Brkt
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Determine the dimension for the letter
E
Revolving Support Brkt
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Determine the dimension for the letter
F
Revolving Support Brkt
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Determine the dimension for the letter
G
Revolving Support Brkt
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Determine the dimension for the letter
H
Revolving Support Brkt
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Determine the dimension for the letter
I
Revolving Support Brkt
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Determine the dimension for the letter
J
Revolving Support Brkt
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External Threads

• Illustrated in much the same manner as internal
  threads
• Simplified
• Schematic
• Detailed
• Simplified method is more common
• Specified on longitudinal view rather than
  circular view

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External Threads

• External threads are chamfered to minor diameter
  to assist in assembly
• External threads are undercut or necked to minor
  diameter to permit assembly bolt shoulder to
  assembled par without interference

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External Threads

• Specifications follow the same format as internal
  threads
• The only difference is that the letter A follows
  the class of fit and indicates that this is an
  external thread rather than an internal thread

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Thread Forms

• Unified National thread form is most common

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Thread Forms

• Threads used to transmit power
• Square-bench vise
• Acme-lead screw of a machine
• Buttress-transmits power in one direction only

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Thread Forms

• Knuckle thread used with light bulbs
• V-sharp-used where increased friction from full
  thread face is useful-e.g. brass pipe work

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Other Thread Forms
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Tabulated Dimensions

• Combine similar objects on the same drawing
• Substitute letters in place of dimensions
• Letters appear in an accompanying table
• Table assigns values to letters for individual
  parts

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Tabulated Dimensions

• Threaded fasteners are good example of using
  tabulated dimensions
• Example shown combines five different hex head
  cap screws on the same drawing

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Multiple Threads

• Most threads are single lead threads-one
  revolution equals 360
• One revolution results in advance equal to one
  thread or the pitch

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Multiple Threads

• Applications that require a faster rate of
  advancement need multiple threads
• Specification could be double of triple lead
  threads
• Appears at end of thread specification

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Multiple Threads

• Thread may be assumed to be a single lead unless
  otherwise specified
• Double lead thread will advance two threads per
  revolution (twice the pitch)
• Triple lead thread will advance three threads per
  revolution (three times the pitch)

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Multiple Threads

• Multiple threads can be seen as having more than
  one thread beginning at the start of the threaded
  portion of the shaft or at the entry of the hole
• Drawing makes no attempt to illustrate multiple
  threads differently than single lead threads

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Pipe Threads

• Pipe thread designations begin with the nominal
  pipe size
• Pipe size is smaller than the major thread
  diameter
• Pipe is specified by ID
• Threads are on the OD

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Pipe Threads

• NPT-National pipe, tapered
• NPS-National pipe, straight
• Only one size pitch is available for each pipe
  size-whether tapered or straight
• Illustrated as straight threads on a drawing

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Dryseal Pipe Threads

• Dryseal pipe threads are based on USA (American)
  pipe thread
• Designed to seal pressure tight joints without
  using sealing compounds

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Dryseal Pipe Threads

• Dryseal pipe abbreviations
• NPTF-Dryseal USA Standard Taper Pipe Thread
• NPSF-Dryseal USA Standard Fuel Internal Straight
  Pipe Thread
• NPSI-Dryseal USA Standard Intermediate Internal
  Straight Pipe Thread