Like the screw, the bolt occupies an integral position in both industrial and everyday life. In fact, bolts and screws are used more than any other type of mechanical fastener, and they can be found in nearly every simple or complex machine. Although there is no absolute distinction, the difference between screws and bolts can be broadly defined as one of thread size and tapering. Bolts are generally larger and do not have tapered ends. In standard usage, a fastener that is torqued with a nut is usually considered a bolt.
1- Cold Forging
This process is also known as Cold Heading. It is the basic operation in most bolt and screw plants to give their heads. The material is squeezed into a cavity of a mold, normally at room temperature, and the resultant part is a nail, a bolt or a screw with the head. The tooling is relatively simple and cheaper, moreover it offers other benefits such as better surface finish, improved dimensional stability and efficient use of material.
Basically, it is a removal process where jobs of desired size and shape are produced by gradually removing the excess material in the form of scrap with the help of sharp cutting edges or tools. Screw blanks go into a die, called a head-slotting machine, then the machinery fastens them 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.
Standards and types
Fastener thread standards are commonly judged by ISO standards, as laid out in ISO 68-1, ISO 261, ISO 262, and ISO 965-1.
There is a huge range of screws and bolts and depending of their uses and shapes we found several classifications. One of them is the following:
Covered by the standard DIN 97 and under various heads (flat, oval, countersunk, Phillips head, Allen type head, Torx, head).
These are used to fix brackets with a high weight. Covered by standard DIN 571.
Or thread-forming screws feature threads that are designed to tap their own holes. This type is not suitable for harder materials. DIN 7970, ISO 1478, BS 4174.
Machine screws have finer threads than wood screws. They are designed to be used in conjunction with a nut or tapped hole. DIN 7985, ISO 7045, DIN 965, DIN 966, DIN 84, DIN 85, DIN 933, DIN 931, DIN 912, …
Sheet Metal Screws
These screws usually are short and have coarse threads that are designed to grab onto relatively thin sheet metal. DIN 7981, ISO 7049, DIN 7976, DIN 7983, DIN 7971, DIN 7504, …
Although, screws are usually made in steel, they can be made in stainless steel, brass, titanium, bronze, zinc, nylon, … Screws also can have various finishes such as zinc plating, black oxide or non-stick coating, HDG (hot dip galvanized).