182 Chapter 7
In mating parts the two main considerations are the surface finishing and the tolerance (especially
size and geometry). The surface finishing means the level of roughness of a surface. Its main purpose
is to control the accuracy in positioning and tightness between the mating parts. The other objective is
to reduce the friction, especially for parts that move relative to other parts.
7.4 SCREW THREADS, GEARS, AND HELICAL SPRINGS
Threads.
Machine screws are extensively used for securing parts. There are many different types and sizes of
machine screws, nuts, and bolts. Moreover, drafters use different methods to show thread on drawings.
Figure 7.16 shows a thread profile in section and a common method of drawing threads. To save time,
the drafter 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 anywhere on the drawing but most often in
the upper left corner. Figure 7.17 gives an example of a typical note showing the thread designator “1/4-
20x1, RHMS.”
The first number of the note, 1/4, is the nominal size, which is the outside diameter. The number af-
ter the first dash, 20, means that there are 20 threads per inch. The number 1 represents the length of
the screw, and the letters RHMS identify the head type (round-head machine screw). It would normally
also have letters identifying the thread series (e.g., UNC if it were a unified national coarse thread) and
a number to identify the class of thread (e.g., “3”) 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 designa-
tion are right-hand threads. Figure 7.18 shows right-hand and left-hand screws. Figure 7.19 is another
example of a screw type and the general nomenclature used to describe it.
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 unified
or national form threads, which are called national coarse or NC, and national fine, or NF, threads. NF
threads have more threads per inch of screw length.
Common NC Screw-Thread Sizes
o2-56 o1/4-20
o4-40 o3/8-16
o6-32 o1/2-13
o8-32 o5/8-11
o10-24 o3/4-10
Diameter = (N*.013) + .060 (inches)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. External threads or bolts are designated with the suffix “A,” internal or nut threads with
the suffix “B.” Figure 7.19 shows different types of screws and screw heads.
The unified and American (national) thread forms designate classifications for fit to ensure that
mated threaded parts fit to the tolerances specified. The unified-screw-thread form has specified a num-
ber of classes of threads: