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 after 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 designation 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

o10-24 o3/4-10

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 number of classes of threads:

Figure 7.16

industry.

A typical screw thread with profile section and the terminology associated with its use in today's

Figure 7.16

industry.

A typical screw thread with profile section and the terminology associated with its use in today's

Figure 7.17 A typical note showing the thread designator "1/4-20x1, RHMS." It specifies the diameter, thread, length, and head.

Figure 7.18 Right- and left-handed thread screw types

RIGHT-HAND THREAD

Figure 7.18 Right- and left-handed thread screw types

• Class 1A and 1B: for work of rough commercial quality where loose fit, quick assembly, and rapid production are important and shake or play is not objectionable.

• Class 2A and 2B: the recognized standard for normal production of the great bulk of commercial bolts, nuts, and screws. Classes 2A and 2B provide a small amount of play to prevent galling and seizure in assembly and use and sufficient clearance for some plating.

• Class 3A and 3B: these have no allowance and 75 percent of the tolerance of Classes 2A and 2B. A screw and nut in this class may vary from a fit having no play to one with a small amount of play. This fit represents an exceptionally high quality of commercially threaded product and is recommended only in cases where the high cost of precision tools and continual checking are warranted.

• Class 4: this is a theoretical rather than practical class and is now obsolete.

• Class 5: for a wrench fit, used principally for studs and their mating tapped holes or a force fit requiring the application of a high torque for semipermanent assembly. It is also a selective fit if initial assembly by hand is required. It is not as yet adaptable to quantity production.

Figure 7.19A Different types of screws and screw heads.

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