The subject of drawing is very important in all lines of industrial work. A photograph, picture or perspective drawing shows how a thing will appear, but does not give dimensions nor show the detail of how it is made. A workman who is going to execute a piece of work in the shop must know the exact size and shape of every part. In order that the man who is designing the work may give correct ideas to him the science of mechanical drawing has been developed.
When you first look at a mechanical drawing it seems very complicated; it is not expected that you should be able to tell immediately just how it is constructed. It requires considerable thought and study to understand a mechanical drawing well enough to undertake the work. In fact, you must use your imagination a great deal, but after you have studied a few mechanical drawings they will soon become quite clear. There are just a few things which you need to know about the subject of drawing in order to interpret mechanical drawings correctly, because there are certain recognized and established ways of representing certain ideas. These established ways are called conventions and you should acquaint yourself with these conventions so you will understand exactly what they mean.
Very heavy lines are also used for border lines.
In looking at an object there are of course a great many parts and lines which you are unable to see from any one view. In a picture these would not be presented at all, but in mechanical drawing the hidden parts are represented by dotted lines. At first they may have a tendency to confuse you just a little, but if you will remember that dotted lines always represent parts which lie back of the parts represented by the heavy line, you will soon learn to understand mechanical drawings.
One of the most important things about a mechanical drawing is the fact that it gives dimensions, that is, it tells the exact size of every part. In order that you may understand perfectly the point from which the measurements are taken, broken lines are used with little arrow heads at each end to show you where the dimensions start and end. To illustrate, if you see a broken line with a figure 12" somewhere in the line, that means that in the finished article it is 12 inches from the point represented by one arrow head to the other.
Circles and curves are usually indicated in mechanical drawings by having the diameter (marked D) or the radius (marked R) given. The point where the compass should rest when the circle is drawn is also indicated. In measuring distances between circles the measurements are taken from the center of one to the center of the other.
Mechanical drawings are usually drawn to some definite scale, because it is not often practical to make a drawing the same size as the object, unless the object is very small. By drawing to scale, we mean that the drawing is a certain fractional part of the size of the complete object. For illustration, 1" is sometimes used to represent 1 ft. or 1/2" or 1/4" for a foot. Of course, if the drawing is for some very large piece of construction, such as a house or a bridge, small fractions of an inch will be used to represent a foot; if the drawing deals with some smaller article, as a chair or footstool, 1", 2", 4", or even 6", may be used to represent a foot. It must be remembered, however, that the dimensions given on the drawings always refer to the sizes of the completed article and not to the size of the drawing.