Machine Drawings, Continued
To save time, use symbols that are not drawn to scale. Show the dimensions
on the threaded part and place other information on a note in the upper-left
corner of the drawing. In our example, the note shows the thread designation
of 1/4-20 UNC-2.
The first number of the note (1/4) is the nominal size, which is the outside
diameter. The number after the first dash (20) means there are 20 threads per
inch. The letters UNC identify the thread series as Unified National Course.
The last number identifies the class of thread and tolerance commonly called
the fit. If it is a left-hand thread, a dash and the letters LH follow the class
of thread. Threads without the LH are right-hand threads.
Specifications for the manufacture of screws include thread diameter, number
of threads per inch, thread series, and class of thread. The two most
common screw thread series are the Unified or National Form Threads,
which are called National Course or NC and National Fine (NF threads). NF
threads have more threads per inch of screw length than NC.
The amount of tolerance and/or specified allowance distinguishes between
classes of threads. Formerly known as class of fit, a new term, class of
thread is now in use by the National Bureau of Standards in the Screw-
Thread Standards for Federal Services, Handbook H-28.
Figure 4-36 show thread designation for an external thread.
Figure 4-36.External thread designations.
Continued on next page