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THREADS IN BOTH SYSTEMS
Thread forms and designations have been the subject of
many long and arduous battles through the years.
Standardization in the inch series has come through
many channels, but the present unified thread form could
be considered to be the standard for many threaded
products, particularly high strength ones such as socket
head cap screws, etc. In common usage in U.S.A.,
Canada and United Kingdom are the Unified National
Radius Coarse series, designated UNRC, Unified National
Radius Fine series, designated UNRF, and several special
series of various types, designated UNS. This thread,
UNRC or UNRF, is designated by specifying the diameter
and threads per inch along with the suffix indicating the
thread series, such as 1/4 - 28 UNRF. For threads in Metric
units, a similar approach is used, but with some slight
variations. A diameter and pitch are used to designate
the series, as in the Inch system, with modifications as
follows: For coarse threads, only the prefix M and the
diameter are necessary, but for fine threads, the pitch is
shown as a suffix. For example, M16 is a coarse thread
designation representing a diameter of 16 mm with a
pitch of 2 mm understood. A similar fine thread part
would be M16 x 1.5 or 16 mm diameter with a pitch of
1.5 mm.
For someone who has been using the Inch system, there
are a couple of differences that can be a little confusing.
In the Inch series, while we refer to threads per inch as
pitch; actually the number of threads is 1/pitch. Fine
threads are referenced by a larger number than coarse
threads because they"fit" more threads per inch.
In Metric series, the diameters are in millimeters, but the
pitch is really the pitch. Consequently the coarse thread
has the large number. The most common metric thread
is the coarse thread and falls generally between the inch
coarse and fine series for a comparable diameter.
Also to be considered in defining threads is the tolerance
and class of fit to which they are made. The International
Standards Organization (ISO) metric system provides
for this designation by adding letters and numbers in a
certain sequence to the callout. For instance, a thread
designated as M5 x 0.8 4g6g would define a thread of
5 mm diameter, 0.8 mm pitch, with a pitch diameter
tolerance grade 6 and allowance "g". These toleranc
and fields are defined as shown below, similar to the
Federal Standard H28 handbook, which defines all of the
dimensions and tolerances for a thread in the inch series.
The callout above is similar to a designation class 3A fit,
and has a like connotation.
Example of thread tolerance positions and magnitudes.
Comparision 5/16 UNC and M8. Medium tolerance grades (R)C Pitch diameter
