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Projection in Mechanical Drawing

A mechanical drawing of a machine part or a combination of parts forming a complete mechanism is usually made up of two or more separate views. Each view represents a different side of the object drawn. The number of separate views on the drawing depends on the number of views actually needed to clearly show the general shape of the piece and dimensions including the lengths and widths of different parts. It also includes the shape as seen from different sides, and whatever information related to the form and dimensions is needed for reproducing the mechanical device that the drawing represents.

Perspective drawings that show objects just as they appear to the eye are useful, especially when the idea is to show the general shape of what part is depicted. The use of these types of drawings in books as a means of illustrating various objects is common. In mechanical work, and especially wherever machinery is constructed, drawings are used which are quite different in appearance from the ordinary perspective drawings. A perspective drawing may show two or three sides of an object on one view, the same as a photograph. While a mechanical drawing does not look like a perspective drawing in appearance, it does show the form of the object and usually much more clearly and accurately than a perspective drawing, provided that the mechanical drawing is understood.

Mechanical drawings are based on a type of drawing known as "orthographic projection." In order to illustrate this method of drawing, imagine that some object is held in the hand on the same level as the eyes and is turned so that the front side, top side, and end are each seen successively. These different views will then correspond to the different views of the same object as illustrated by a mechanical drawing made according to the orthographic projection method.

These mechanical drawings that have different views showing different sides of a machine part or other mechanical device show the length, breadth, and thickness of various portions of the piece. This is helpful in mechanical drafting because the main purpose of most mechanical drawings is to show mechanical devices form and function clearly and accurately that they may readily be reproduced in steel, plastic, or whatever material is to be used. It shouldn’t be inferred from this however, that drawings are made so accurately that the pattern maker, machinist, and toolmaker can measure them in order to determine the required dimensions at various places. This method of determining the dimensions is unnecessary, because an important part of the drafters work is to show on the drawing all necessary dimensions, either in feet, inches, or fractional parts of an inch, depending upon the size of the work and the degree of accuracy required.

About the Author


Tim Davis has created a very complete course on learning to draw mechanical drawings in a CAD program at http://mechanicaldrawing.us. You will also find some woodworking plans on this site that are free to download.


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