Screw Manufacturing Process
Screws fit in the category of threaded fasteners, such as studs, bolts, woods screws and automotive cap crews. The groove on screws can run tapered, parallel, straight and left- or right-handed. Two types of screws exist: wood and machine. Screws consist of metal material; machine screws have a consistent diameter and require nuts. A wood screw has a tapered shape and grasps the wood as it enters.
Generally, manufacturers fabricate screws from low to medium carbon steel wire rods. Sometimes, they use inexpensive and more durable materials, such as brass, aluminum alloy or nickel alloys. The quality of the material determines whether a screw breaks or cracks. Coating on the screws must have compatibility with the material used to manufacture the screw. For example, steel screws should have zinc, nickel or chromium plating material.
For single thread screws, the lead and the pitch have the same attributes. The lead is twice the pitch on a double thread screw and three times the pitch on a triple thread. The pitch refers to the measurement between two grooves from the same point on each thread, or the number of threads per inch. The lead measures the penetration distance for each revolution of the screw.
The machine method works for small screws or screws with unique designs. A device called a mechanical coil feeds straightened wire into a mechanism called a cold head machine, which cuts a piece of wire to a certain length This part of the process also preforms the screw heads on screw blanks. Screws blanks go into a die, called a head-slotting machine; the machinery fastens the blanks into channels positioned around the perimeter of a wheel on the head-slotting machine. As the wheel revolves, a circular cutting device slots the screws.
Manufacturers use one of three thread rolling techniques for cutting blanks screws. A vibrating hopper feeds screw blanks into thread cutting dies. The hopper directs the screw blanks down a channel into dies. The channel positions the screw blanks so that the blanks feed into the dies properly. One method rolls the blank screw between a stationary and reciprocating die. The centerless cylindrical technique rolls the blank between two or three dies, which create the screw’s finished thread. The the planetary rotary method positions the blank screws in a stationary manner as multiple die-rolling machines rotate around the blank screws. The three thread-rolling manufacturing method creates a superior product, compared with the machine technique, because the processes do not cut the thread into the blanks but presses the thread. The machining process weakens screws because of the material loss. The thread-rolling method has more exact thread locations.