them. Now look at figure 4-12. The left side shows a thread profile in section and the right side shows a common method of drawing threads. To save time, the draftsman uses symbols that are not drawn to scale. The drawing shows the dimensions of the threaded part but other information may be placed in “notes” almost any place on the drawing but most often in the upper left corner. However, in our example the note is directly above  the  drawing  and  shows  the  thread  designator “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 Coarse. The last number, 2, identifies the class of thread and  tolerance,  commonly  called  the  fit.  If  it  is  a left-hand thread, a dash and the letters LH will follow the  class  of  thread.  Threads  without  the  LH  are right-hand  threads. Specifications  necessary  for  the  manufacture  of screws include thread diameter, number of threads per inch, thread series, and class of thread The two most widely used screw-thread series are (1) Unified or Figure 4-12.—Outside threads. National Form Threads, which are called National Coarse, or NC, and (2) National Fine, or NF threads. The NF threads have more threads per inch of screw length than the NC. Classes of threads are distinguished from each other by the amount of tolerance and/or allowance specified. Classes of thread were formerly known as class of fit, a term that will probably remain in use for many years. The new term, class of thread, was established by the National Bureau of Standards in the Screw-Thread Standards for Federal Services, Handbook H-28. Figure 4-13 shows the terminology used to describe screw threads. Each of the terms is explained in the following  list: HELIX—The curve formed on any cylinder by a straight line in a plane that is wrapped around the cylinder with a forward progression. EXTERNAL THREAD—A thread on the outside of a member.   An example is the thread of a bolt. INTERNAL THREAD—A thread on the inside of a member. An example is the thread inside a nut. MAJOR DIAMETER—The largest diameter of an external or internal thread AXIS—The  center  line  running  lengthwise  through a screw. CREST—The surface of the thread corresponding to the major diameter of an external thread and the minor diameter  of  an  internal  thread. Figure 4-13.—Screw thread terminology. 4-4