Descriptive geometry and mechanical drawing

Descriptive geometry and mechanical drawing

Descriptive geometry

Is a mathematical-graphical procedure that has for its purpose the visualization of structures and their exact representation in drawings. After analysic of any structure, each element is shown in the drawing in its exact geometrical relation to the other elements.

The basic methods of descriptive geometry are the projection method and the direct method. There are two general types of views, perspective and orthographic. A perspective view is observed from a fixed station point, or point of view, by means of converging rays of light that meet at the eye of the observer. An orthogographic view of an object is observed in a chosen direction by means of paralel rays of light.

Mechanical drawing

Language is a defined as the expression of thought. But if we attempt to describe in words the appearance and details of a machine, or bridge, or building, we find it not only difficult but in most cases impossible. Here we must use another language, the universal graphic language of drawing.

A writen description of a new machine part would have to be very long to tell about it and even then might be misunderstood. A picture of it would serve the purpose much better, but the picture would not show the exact method of construction. It would gove only the external appearence without telling what was inside. It would be impossible to construct a locomotive or an airplane from either a word description or a picture.

Fortunately, another form of description has been developed by which the exact shape of every detail of any structure may be defined accurately and quickly. This method consist of the making of a series of views arranged according to a definite system, with figures added to tell the sizes. This is know as "mechanical drawing" and it forms so important a part of all undustrial an mechanical work that it is called the "language of industry".

Shape description

There are two things that a deigner, invertor or builder must be able to do: 

• first, he must be able to visualize what an object looks like without actually having the objects; 

• second, he must be able to describe it so that it could be built. 

His problem then is how to represent solid objects on a sheet of paper in such a manner as to tell the exact shape. This is done drawing a system of views of object as seen from different positions..

A picture of an hydraulic jack for an automobile (fig. 1) shows this tool as it ordimarily appears to us, but it does not show the true shapes of the parts. The top of the cylinder appears as an ellipse, although we know it really is circular. If we look down at the jack from above, we obtain a view showing the exact shape of the cylinder, and the outline of the other parts as seen above. This is called a top view or plan. This view deos not tell us the height of the jack, so it is necessary to take another view from a postition directly in front view or side viewm to show the height, is added. Often, as in this case, both the front and side views, in addition to the top view, are needed to describe the object (fig. 2). The three views taken together completely define the shapes of all visible parts of the jack and their exact relations to each other. 

Sometimes a left-side view describes the object or construction more clearly than the tight-side view and in such cases it should be used. It is sometimes desirable or necessary to show the rear view or the bottom view of an object. Views can then be projected to all six faces or planes of an object.

Sections 

We know that the parts of an object that cannot be seen are repesented by hidden lines composed of short dashes.

This method is satisfactory where the object is solid or the interior simple. There are many cases, especially where there is considerable interior detail or where several pieces are shown together, in which the hidden lines become confusing or hard to read. This diffictly is avoided by using a sectional view. A sectional view is obtained by supposing the piece to be cut apart by an imaginary cutting plane, and the front part removed, thus exposing the interior.