A screw threader is a tool or piece of equipment that can shape threads on to a screw or within a nut. Threads are helical, or spiral, structures that wrap the outer or inner face of a cylinder. These threads fasten separate elements together using rotational force, which prevents them from slipping apart in the absence of rotation. External threads are called male threads and internal threads are called female threads. They can be produced by hand using tap and die screw threaders, or machined by lathes into cut or rolled threads.
The process of crafting male threads is different from that of female threads. One type, the tap and die model, can be used to illustrate the difference. A tap is essentially a T-shaped lever that permits rotation by hand with a downward arm that is sharpened for better penetration of a material.
By placing the tap on a surface, applying downward pressure, and turning, the device drills a hole with counterpart screw threads. This process calls for proper alignment and is aided with the use of oil. Rather than rotating the tap continuously, the process requires short turns and reversals in order to remove shards of the material. A tap may also be fashioned into a bit for use in a power drill.
Typically, a die tool functions in essentially the same way. Instead of a central arm, however, it features a straight bar with a hole in the middle to contain a button die. This is a small button-shaped disk that slides over a shaft and etches threads directly into the shaft.
As industrial equipment, a screw threader removes material from a bolt blank by carving into it. This is accomplished using a cutting die or machine such as a lathe to cut threads into a cylinder. This automated process extracts material from the grain flow.
In contrast, rolled threads get their name by being rolled into a cylinder between two reciprocating serrated dies. This type of die, however, reshapes the bolt material itself into the required threads. The process works in a way similar to squeezing a balloon in the middle; excess material gets forced outside of the diameter of one's fingers. Threads from the rolled type of screw threader are generally more uniform, featuring smoother ridges, superior thread strength, and resistance to fatigue and shear.
Various sizes and shapes of additional screw threader tools create thread styles with attention to internal and external thread diameters, thread pitch, which is the distance between threads, and number of threads per inch or millimeter. Special ring dies ensure optimal tolerances by restricting these dimensions. Additional screw threader tools include screw extractors, machine pipe and bolt threading dies, tap wrenches, and bolt threader button dies.