Threads and Fasteners
Thread Symbols
Thread Symbols
Screw Thread Terms:
External Thread: A thread on
the outside of a member, as on
a shaft.
Internal Thread: A thread on the
inside of a member, as in a hole.
Major Diameter: The largest
diameter of a screw thread.
Minor Diameter: The smallest
diameter of a screw thread.
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.
Thread Symbols
Screw Thread Terms:
Lead: The distance a screw
thread advances axially in one
turn or revolution.
Crest: The top surface joining
the sides of two adjacent
threads.
Root: The bottom surface joining
the sides of two adjacent
threads.
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.
Thread Symbols
Screw Thread Forms: Various forms of threads are used to hold parts together, to adjust
parts with reference to each other, or to transmit power.
Thread Symbols
Right-hand and left-hand threads:
A right-hand thread is one that advances into a nut when turned clockwise (figure a).
A left-hand thread is on the advances into a nut when turned counterclockwise (figure b).
A thread is always considered to be right-handed unless specified otherwise. A left-handed
thread is always labeled (LH) on a drawing.
Thread Symbols
Single and Multiple Threads:
A single thread is composed of one ridge and the lead is equal to the pitch (figure a).
A double thread is composed of two ridges and the lead is equal to twice the pitch (figure b).
A triple thread is composed to three ridges and the lead is equal three times the pitch (figure c).
Thread Symbols
External Thread Symbols:
Simplified external thread symbols are shown in figure (a) and figure (b). The threaded portions
are indicated by hidden lines parallel to the axis at the approximate depth of the thread, whether
in section or elevation.
Schematic external thread symbols are shown in figure (c) and figure (d). Schematic threads in
elevation (figure d) are indicated by alternate long and short lines at right angles to the center line.
When the schematic form is shown in section (figure c) it is necessary to show the Vs. It is not
necessary to show the Vs to scale. To draw the Vs use the schematic thread depth and let the
pitch be determined by the 60 degree Vs.
Thread Symbols
Internal Thread Symbols:
Internal simplified and schematic thread symbols are shown below. Note that the only difference
between the schematic and simplified internal thread symbols is in the section views.
Thread Symbols
Drawing Simplified Threads
Step 1:
Draw the major diameter and locate the
thread length of full threads.
Step 2:
Draw the minor diameter by marking off the
thread depth (D) from the table.
Step 3:
From the minor diameter draw the 45
degree chamfered ends.
Thread Symbols
Drawing Schematic Threads
Step 1:
Draw the major diameter and locate the thread length of full threads.
Step 2:
Locate the minor diameter (D) from the table and draw the 45 degree
chamfered ends.
Step 3:
Draw lines to represent the crest of the threads spaced at a distance
equal to (P) from the table.
Step 4:
Draw lines to represent the root of the threads centered between the
crest lines to the minor diameter (D).
Thread Symbols
Detailed Representation: Metric, Unified, and American National Threads
The representation for metric, Unified, and American National Threads is the same since the
flats are disregarded. The steps in drawing these threads are shown below.
Thread Symbols
Thread Notes:
The thread note for Unified and American
National screw threads is shown in figure (a).
The thread note for metric screw threads is
shown in figure (b). Metric screw threads are
designated by the letter (M) metric profile
followed by the nominal size (major diameter)
and the pitch, both in millimeters.
Thread Symbols
Thread Notes:
Thread Symbols
Tapped Holes:
The bottom of a drilled hole is conical in shape formed by the point of the twist drill (figure a and
figure b). When an ordinary drill is used in connection with tapping, it is referred to as a tap drill.
On drawings an angle of 30 degrees is used to approximate the actual 31 degrees.
The portion of the drill
depth shown beyond the
threads includes several
imperfect threads
produced by the
chamfered end of the tap.
For drawing purposes, the
distance (A) may be drawn
equal to three schematic
thread pitches (figure c
and figure d)
Thread Symbols
Since the tapped thread length
contains only full threads, it is
necessary to make this length onlly
one or two pitches beyond the end
of the engaging fastener.
In simplified and schematic thread
representation, the threads are
omitted in the bottoms of tapped
holes to show the ends of the
fasteners clearly.