This document discusses different types of screw threads, their purposes, and terminology. It describes threads in terms of pitch, lead, thread form, and various thread standards. It also covers thread representation, types of threaded fasteners like bolts and screws, nuts, and thread gauging. The overall purpose of threads is to hold parts together, adjust parts relative to each other, and transmit power or motion between parts.
2. To hold parts together
To adjust parts with reference to each
other
To transmit power
3. PITCH: The distance from a point on a screw
thread to a corresponding point on the next thread
measured parallel to the axis. The pitch P is equal
to 1 divided by the number of threads per inch.
LEAD: The distance a screw thread advances in
one turn.
Thread Form: The cross section of thread cut by
a plane containing the axis.
4. 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
Notice the
pitch can be
measured
from crest-to-
crest OR
Root-to-Root
5. 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.
In triple threads, the lead is three
times the pitch.
In double threads, the lead is
equal to twice the pitch.
In single threads, the lead is equal
to the pitch.
6. What term refers to the distance a
threaded part would move parallel to
the axis in one rotation?
Lead
7.
8. British Standard Whitworth Thread (B.S.W)
British Standard Fine Thread (B.S.F)
British Standard Pipe Thread (B.S.P)
British Association Thread (B.A)
Square Thread
Buttress Thread
Acme Thread
Knuckle Thread
10. Example of where a Buttress Thread would
be used
11. Example of where a Square Thread would
be used
12. British Standard Whitworth Thread (B.S.W)
A general standard
British Standard Fine Thread (B.S.F)
A standard developed for accurate threads
British Standard Pipe Thread (B.S.P)
A standard developed for pipe work
Not used as commonly as the Metric standard
13.
14. Plug Gauges
For checking internal
threads
Ring Gauges
For checking external
threads
15. Threads are always assumed to be right
handed threads unless marked with the initials
LH (Left Hand) at the end of the thread note.
20. 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.
21.
22.
23. Thread series refers to standard number of
threads per inch set for different diameters.
They include:
› UNC: A course-thread series
› UNF: A fine-thread series
› UNEF: An extra-fine thread series
and constant pitch threads, with a constant
number of threads per inch:
› 8-thread series
› 12-thread series
› 16-thread series
24. 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.
25. Thread Class refers to sets of standard
tolerances and allowances
Classes 1A and 1B
› Loose fit for ease of assembly
Classes 2A and 2B
› Ordinary good grade of commercial products, like
machine screws and fasteners
Classes 3A and 3B
› Snug fit, used for high-grade commercial products
26. Grade numbers reflect the size of tolerance in
that grade
Grade 4 tolerances are smaller than grade 6
tolerance
› Grade 6: Medium-quality length of engagement
› Grades higher than 6: Course quality and/or long
length of engagement
› Grades lower than 6: Fine quality and/or short length
of engagement
27.
28. Does the term UNF refer to a grade, class
or series?
Series
29. 1/2” - 13 UNC - 2A
external thread
(B means internal)
Class of fit
(1 is loosest tolerance, 3 is tightest)
Thread Series
UNC (Unified Coarse)
UNF (Unified Fine)
Pitch (threads/inch)
Nominal Diameter
(also shown as decimal or screw #)
30. The screw is enormously useful in when
pressure is needed to seal the lid onto a jar,
a top on a bottle, hold a light bulb in its
socket or hold two pieces of metal together.
*(Note: Ball screws are used in some
machines to greatly reduce friction.)
31. There are three methods of representing
screw threads on drawings:
1. Detailed
2. Simplified
3. Schematic
43. Step 1
1. Establish the pitch and half-pitch at the
major diameter.
2. Measure intervals the size of the pitch
along the major diameter from the first
two marks.
3. Add crest
lines.
44. Step 2
1. Add the V profile for one thread, top
and bottom, locating the root diameter.
2. Add construction lines for the root
diameter.
45. Step 3
1. Add one side of the remaining Vs
(thread profile).
2. Add the other side of the Vs, completing
the thread profile.
46. Step 4
Add the root lines to complete the detailed
representation of the threads.
47. Staggered lines perpendicular to the axis
of the thread represent the thread root
and crests.
Spacing between root and crest lines is
any convenient size.
Root lines are any convenient length.
48. Machine screw
› Used in tapped hole
or with a nut
Cap screw
Passes through a
clearance hole in one
part and screws into
a tapped hole in a
second part
49. Bolt
› Passes through a
clearance hole
and threads into a
nut
Stud
A shaft threaded at both
ends used in assemblies
50.
51.
52. What is the term for a shaft that is
threaded on both ends?
Stud
62. Summary
There are many types of threads for
different applications
These come in various sizes with a set
standard tolerance
Metric sizes are more commonly used
today
Mechanical bolts, screws and rivets
come in various forms and sizes for
different purposes
Editor's Notes
Acme thread used extensively for transmitting motion on a lathe.
Buttress thread used for when pressure acts in one direction only such as a quick acting vice.
Square thread used for moving heavy loads such as lifting jacks.
If you had to repair an old machine regarding a thread. Its more than likely to be manufactured to either BSW or BSP standard.
Multi start lead screws
When a double thread is formed it starts and finishes at opposite ends. 180 degrees. When a triple thread is formed the start and finishing point are equally spaced. 120 degrees
Resources: Paper, pencil, rubber, 60 degrees tri square, ruler
Draw a parallel line 150mm long 20mm from bottom of page. Draw another parallel line 50mm from the previous line 100mm long. M42, 4mm pitch, scale 2:1 (width 84, 8mm pitch) Half pitch 4mm. Add crest lines.
To reduce friction so parts last longer
High friction means it will ware quite easy
Less friction, less ware