THREAD DESIGNATION: A thread is desig-
When special threads are required by the product
nated by nominal size, number of threads per inch,
designer, he will specify in detail all the thread
series symbol, and class symbol, in that order. For
characteristics and their tolerances for production
example, the designation 1/4-20 UNC-3A specifies a
information. When a standard thread is selected,
thread with the following characteristics:
however, the designer needs only to specify size,
number of threads per inch, designation of the
Nominal thread diameter = 1/4 inch
standard series and class of fit. With these speci-
Number of threads per inch = 20
fications, all other information necessary for
production can be obtained from the established
Series (unified coarse) = UNC
standard, as published. The abbreviated designations
Class = 3
for the different series are as follows:
External thread = A
Unless the designation LH (left hand) follows the
class designation, the thread is assumed to be a
Full Title of Standard Series
Abbreviation
right-hand thread. An example of the designation for
a left-hand thread is 1/4-20 UNC-3A-LH.
Unified coarse thread series
UNC
V-FORM THREADS
Unified fine thread series
UNF
The three forms of V-threads that you must know
how to machine are the V-sharp, the American National
Unified extra fine thread
UNEF
and the American Standard unified. All of these threads
series
have a 60 included angle between their sides. The
V-sharp thread has a greater depth than the others and
American National coarse
NC
the crest and root of this thread have little or no flat. The
thread series
external American Standard unified thread has slightly
less depth than the external American National thread
NF
American National fine
but is otherwise similar. The American Standard unified
thread series
thread is actually a modification of the American
National thread. This modification was made so that the
NEF
American National extra-fine
u n i f i e d series of threads, which permits
thread series
interchangeability of standard threaded fastening
devices manufactured in the United States, Canada, and
Unified constant pitch series
UN
the United Kingdom, could be included in the threading
including 4, 6, 8, 12, 16, 20,
system used in the United States.
28, and 32 threads per inch
To cut a V-form screw thread, you need to know
(1) the pitch of the thread, (2) the straight depth of the
American National Acme
NA
thread, (3) the slant depth of the thread, and (4) the
width of the flat at the root of the thread. The pitch of
thread series
a thread is the basis for calculating all other
dimensions and is equal to 1 divided by the number of
NPT
American National tapered
threads per inch. The tap drill size is equal to the
pipe thread series
thread size minus the pitch, or the thread size minus
ONE divided by the number of threads per inch.
American National straight
NPS
1
pipe thread series
Tap drill size = Thread size - n
When you feed the thread cutting tool into the
American National hose
NH
workpiece, use the slant depth to determine how far to
coupling thread series
feed the tool into the work. The point of the threading
tool must have a flat equal to the width of the flat at
American National form
NS
the root of the thread (external or internal thread, as
thread-special pitch
applicable). If the flat at the point of the tool is too
wide, the resulting thread will be too thin. If the flat
N BUTT
National buttress thread
is too narrow, the thread will be too thick.
6-48