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