Major Diameter: The largest diameter of the thread
Minor Diameter: The smallest diameter of the thread
Pitch: Distance between two threads (mm) or no of
threads per inch.
• Pitch Diameter
The diameter of an imaginary cylinder passing
through the threads to make equal the widths of
the threads and the widths of the spaces cut by the
cylinder.
• Lead
The distance a screw thread advances axially in
one turn.
• Angle of thread
The angle included between the sides of the thread
measured in a plane through the axis of the screw
Crest: The top surface joining the two sides of the threads.
Root: The bottom surface joining the sides of two adjacent
threads.
Side: The surface of the thread that connects the crest with the
root.
• Axis of screw: The longitudinal center line
through the screw.
• Depth of thread: The distance between the
crest and the root of the thread measured
normal to the axis.
• Form of thread: The cross section of thread
cut by a plane containing the axis.
13.3 SCREW THREADS FORM
• Sharp V threads called United States Standard
threads. Used on brass pipe works.
• American National Threads replaces the Sharp
V for general use.
• Unified thread is the standard agreed upon by
the USA, Canada, Britain in 1948 and has
replaced American National form.
• Square Thread are to transmission of power.
13.3 SCREW THREADS FORM (Continue)
• ACME Thread replaces the Square threads
since it is difficult to manufacture.
• Whitworth thread (British standard) has been
replaced by the Unified thread.
• Knuckle thread is used in electrical bulb and
other electrical applications.
• Buttress Thread is designed to transmit power
in one direction.
13.4 THREAD PITCH
Thread Symbol
• Three method of representing thread on
drawing.
• Schematic, use for smaller diameter, about
25mm (1 inch)
• Detailed, for diameter more than 25 mm (1
inch)
• Simplified, commonly used.
13.20 Metric and Unified Threads fit
• Metric threads fit
• Two class of fit, one for general purpose, 6H
for internal threads and 6g for external threads,
therefore default class of fit is 6H/6g if not
specified.
• Second class for closer fit, 6H for internal
threads and 5g6g for external threads.
Unified threads fit
• A represents for external threads and B
represents for internal threads.
• Number 1, 2 and 3 represents the class of fit.
• 2A/2B is general class and equivalent to 6H/6g
• 3A/3B for close fit.
• 1A/1B for Rapid assembly.
Bolt: Through bolt, head on one end
and passes through clearance holes and
nut on other side.
Cap Screw: Similar to a bolt, has
greater length, use without a nut. Other
member has thread and acts as a nut.
Stud: A steel rod threaded on both
ends. Screwed in one member and nut
on other member.
• MACHINE SCREW:
is similar to the slotted head cap screws but in
general is smaller. Can be used with or without
nut.
• SET SCREW:
is a screw with or without a head that is
screwed through one member and whose
special point is forced against another member
to prevent relative motion between the two
parts.
• Clearance hole: A simple hole without thread
used to insert a bolt.
• Tapped hole: A hole with internal thread. Read
13.24 page # 379 for more detail.
• There is clearance between clearance hole and
the bolt or screw. For standard, see page # 379.
•
•
•
•
.75-10 UNC-2A x 2.50 HEXAGON CAP SCREW
M8 X 1.25-40 HEX CAP SCR
5/8-11 UNC-2B SQURE NUT
M8 X 1.25 HEX NUT
Key
A key seat in a shaft and a key way in
the hub or surrounding part.
•
•
•
•
¼ x 1 ½ SQ KEY
No. 204 WOODRUFF KEY
¼ x 1/16 x 1 ½ FLAT KEY
No. 10 P & W KEY
Rivets