Threads
Threads can be generated by means of subtractive, formative or additive manufacturing methods, though the most common method for precision thread generation is by subtractive methods such as thread cutting or grinding. Different methods are used for internal and external threads being generated. Threads are used in assembly applications for various engineering systems, as fasteners and by ways of moving mechanisms linearly.
Internal threads can be made in several different ways. Tapping and thread milling is among those methods. Tapping is a method used in the manufacture of internal threads. Tapping can be performed either by a power lathe or by hand, though in industrial manufacturing using a cnc lathe or mill is the most common method used. Tapping must be performed on a previously drilled hole to a specific diameter in order for the proper thread to be generated. This method can also be used in conjunction with other methods as a way to clean up a thread. This method is called chasing. Thread milling is another form of subtractive thread manufacturing. A tapped thread will appear more jagged than a thread milled generated thread. Thread milling in general achieves a better thread quality than other methods. This is generally performed on a CNC machine tool. Commonly, CNC machine operators will drill a hole larger when tapping a thread than necessary for easier tapping, but by doing so reduces the strength of the thread.
Thread die cutting and thread grinding are two different methods of the external thread generation. Die Cutting is a method that can be used to manufacture external threads. Thread dies are either hex die nuts or round dies. Die cutters are run around an external cylinder in order to cut the male portion of the thread. Die cutting is commonly used to “chase” and clean up threads generated by other methods. Thread grinding and lapping are yet two more methods of thread generation. Thread lapping is a finishing process to thread grinding to achieve a high level of precision and surface finish. Thread grinding is used to produce higher quality threads in materials that are more difficult to machine.
Thread specifications are broken into two different types, Unified National Standard and Metric. Threads in the unified national standard are specified by major diameter, threads per inch, classification, class and finally the hand of the thread. Major diameter can either be the measurement or a number if less than one fourth of an inch. Metric threads are specified by M for metric, diameter, pitch, fit class and then the length of the thread. Thread classes for the unified national standard are divided between A classes for external threads while B classes are for internal threads. Class 2 are the most common for general applications. Class 1 is for lose allowance threads and class 3 threads are speciality application for closer tolerances. Thread Inspection can be achieved by several methods. For internal threads, thread go and no-go thread gauges are commonly used in order to inspect the thread geometry of a workpiece. They are also specified by the coarseness of threads either UNC, UNF or UNEF, which stands for Unified National Coarse, Fine and Extra-Fine respectively.
