Stainless steel screws represent an extremely versatile closure system. They are used for a diverse number of purposes by carpenters, boat builders and surgeons. Below are just a few general examples to indicate how varied these can be:
• Invisible deck fastenings
• Securing concrete
• Surgery
Of course, the same screw cannot be used for the different jobs. The screw head and alloys will be different but the basic concept is the same.
Stainless steel was first conceived by Harry Brearley in England. He formulated it in 1912 and had it patented in 1915 under the name of Staybrite. It is eco-friendly since it can be recycled so it is very applicable to the current pro-environment movements worldwide.
The usefulness of stainless steel screws is based on its holding ability and its metallic composition that prevents corrosion. The term stainless steel is a general one that includes many different combinations of elements or steels. It is a steel alloy where iron has chromium added to it which provides the non-corrosion property to stainless steel. A minimum amount of 10.5% of it is needed for this. When exposed to oxygen it will not corrode because a film of chromium oxide is protecting it. Corrosion cannot penetrate it. When stainless steel screws are used in demanding weather conditions the steel may discolour certain woods such as oak.
The screw has many different kinds of heads and it is useful to know which ones are suitable for what purpose. Corresponding screw drivers or other turning instruments are needed to turn the screw.
Screws can be turned manually or with the aid of an electric drill where the instrument has been attached to. Some screws however do not have heads but eyes such as the screw eye. These are often used on yachts but not exclusively. The opposite end of the screw too varies. For penetrating wood or other soft surfaces the screw end will have a point. Machine screws however, have a flat end since the screw will be holding pieces of metal together.
Three basic steps are required to manufacture screws:
• heading
• thread rolling
• coating
Wire is cut to the required length that corresponds to the screw being made. This is called the blank. The process called cold heading produces the head of the screw. The die in the machine determines the perforations added to the head for eventually turning the screw. The threading is done by cutting or thread rolling. A shaped die is pressed against the blank screw to cut in the threads. It is cleaned and polished by tumble finishing. For non-stainless steel screws, a final galvanic dipping is done to prevent corrosion.
The standardization of the screw threads occurred in the late 1940s when the definitions were confirmed by both the Unified Thread Standard and the ISO metric screw thread. Prior to that there was not a standardized system in use making it more difficult to locate the appropriate screws for needed use.
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