Screw Threads
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Screw Thread Uses
Fasteners
Adjusters
Transfer Power
Mechanical Advantage
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Screw Thread Uses
• Fastener – (bolts, nuts, and screws) used to
hold parts together
• Movement – (lathe lead screw) transmits
twisting power to straight line motion
• Mechanical advantage – (vise and clamp
screws) increases force to hold items
• Adjustments – allows parts to be moved
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Simple Definition of a Thread
A screw thread is a wedge wrapped around a
cylinder.
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Coarse Thread
Metric
Square Thread
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External and Internal Threads
External Threads
are on a bolt.
Internal Threads
are in a nut.
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Right-Handed Bolt
• All bolts and nuts are
right handed unless
marked.
• Right-handed threads
tighten by turning
clockwise.
• Left-handed threads
tighten by turning
counter-clockwise.
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Thread Size
Threads are sized by the diameter of the bolt
and the number of threads per inch.
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Threads per Inch
1 2 3 4 5 6 7 8 9 10
A ¾” – 10 bolt has
10 threads per inch.
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Standard Thread Sizes
Coarse Threads
• ¼” – 20
• ³∕₈” – 16
• ⁷∕₁₆” – 14
• ½” – 13
• ⁹∕₁₆” – 12
• ⅝” – 11
• ¾” – 10
Fine Threads
• ¼” – 28
• ³∕₈” – 24
• ⁷∕₁₆” – 20
• ½” – 20
• ⁹∕₁₆” – 18
• ⅝” – 18
• ¾” – 16
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Metric Threads
• Metric threads are sized by diameter and
thread pitch in millimeters.
• The coarse metric threads use only the
diameter size (example, M10).
• The fine metric threads will have the diameter
and pitch size stated (example, M10 X 1.25).
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Standard Metric Sizes
Coarse Threads
M6.3 X 1
M8 X 1.25
M10 X 1.50
M12 X 1.75
M14 X 2
M16 X 2
M20 X 2.5
Fine Threads
M6.3 X 0.75
M8 X 1
M10 X 0.75
M12 X 1
M14 X 1.5
M16 X 1.5
M20 X 1
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Types of Threads
Standard threads are
V-style with 60° angles.
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Types of Threads
Square Threads are
square in shape, with
the same width and
depth.
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Types of Threads
ACME Threads are like square threads, but with
a 29° angle on the sides; they’re used to transfer
power.
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Types of Threads
Pipe Threads are tapered; the back is shallower
than the front of the thread.
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Basic Thread Terminology
•
•
•
•
•
A. Major Diameter
B. Pitch Diameter
C. Minor Diameter
D. Crest
E. Root
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Basic Thread Terminology
• A. Major Diameter- the largest diameter of a
bolt at the crest, or the diameter of the roots
of a nut.
• B. Pitch Diameter- the diameter halfway
between the major diameter and minor
diameter.
• C. Minor Diameter- the small diameter of a
bolt at the root, or the diameter of the crest of
a nut.
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Basic Thread Terminology
• D. Crest- the top surface where the two sides
of the threads meet.
• E. Root- the bottom surface where the two
sides of the threads meet.
Crest
Root
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Basic Thread Terminology
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Basic Thread Terminology
• Pitch- the distance between thread crests of
the thread.
• Height- the measurement from the root of the
thread to the crest of the thread.
• Slope- the angle of the side of the thread.
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Assessment
• Quiz
• Cut 10 different sizes of internal and external
screw threads using a set of tap and die, or
lathe and tap.
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Reference
• Oberg, E. (2012). Machinery’s handbook, 29th.
South Norwalk, CT: Industrial Press, Inc.
• Threads and Threading: pages1802-2121
• US Standard Screw Thread: page 1806
• Definitions: pages 1808-1813
• Metric: pages 1878-1920
• Acme: pages 1921-1944
• Square: page 1944
• Pipe: pages 1956-1972
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