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Ⅴ. Explanation of Screws
In this section, let’s talk about screws.
1. Discovery of Screws
2. Types and Sizes of Screws
Long time ago, a man found a stick with a spiral shaped shell stuck into it. Then, he removed
the shell by twisting it. This is told the beginning of screws. Screws are used everywhere at
present. Without screws, the civilization we have now would not have existed. It is like a
miracle that this spiral shell caused the use of screws.
There are many types of screws. There are various ways of grouping screws. Commonly,
they are divided by the shape of screw head, slot, thread and industrial standard. Especially,
the industrial standard is very important because each standard has its own thread size and
numbers of thread per inch. Therefore, screws and nuts with different standards do not fit
together.
The size of the screws are often shown in a form such as “M3 X 20.” This means that
“Diameter X Length” in millimeters. The length is the part of screw which goes under the
surface. For pan headed and hex-headed screws, this length does not include the screw
head. For flat headed screws, this length includes from the screw head to the tip.
When you decide which screws to use, you have to consider materials, the shapes of the
parts that the screws will be used on, and the pressure that the parts are going to need. For
example, very big screws will not be useful for a watch because they are too heavy. Small
screws for a watch cannot be used for a car because the car will probably fall apart.
Then, if we choose suitable screws, could we turn screws as tightly as possible? When a
screw is tightened too tightly, the internal thread may be stripped, causing the screw to
become rather loose. Screws are supposed to be tight, but they should not be turned too
tightly, either. They should be tightened properly for optimum function.
Ⅴ
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