Threading is the process of creating a screw thread on a part, such as a bolt. Threading is the term used when processing the outside of a part, or the male portion. Adding threads to the inside diameter of a part to accept a threaded part is often referred to as Tapping.
There are a variety of methodologies used for threading in the manufacturing environment. Thread cutting is the process of removing material from the workpiece in order to create the thread. Thread cutting itself can be done in multiple ways. Tradition methods involve using taps and dies to remove material, often in multiple passes, to cut the desired thread. This is often done on a specialized machine called a thread cutter or screw cutter. With modern CNC machines, there are also other, fairly easy ways to cut threads.
On a CNC lathe, single point threading is a common operation. This is where a part is held by the chuck, and rotated at a preprogrammed speed. As the part rotates, the tool on the machine moves along the length of the barrel cutting the thread into the part. The thread is typically cut in 5 to 7 passes. Because the program for this process can be created quickly and efficiently using Computer Aided Machining (CAM), this process can effectively be used for small runs, or even one of custom parts.
Another way to utilize CNC machines to cut thread is on a mill, using a process called Thread Milling. In this process, the part is held stationary on the table of a CNC mill while a special thread milling tool is rotated by the spindle and moved around the part. Again, because this process can be programmed with relative ease, it can be used for low volume production runs.
For higher volume applications, thread forming or rolling is a commonly used method. In this style of threading, no material is removed from the part. The blank part is put in compression with a special tool called a thread rolling die. The tool and the part roll together under pressure, and the thread profile is actually cold formed using only pressure. This process results in no scrap and thus uses less material to from the desired part. It is also a very fast process. Because of the speed and material savings, thread rolling is very efficient for high volume applications. It is not ideal however for low volume or custom application because of the specialized tooling and machinery required.