Saturday, 28 March 2020

Engineering drawing and projection


Geometric projection is the obtaining of the plane geometric shape resulting from the intersection of projection rays passing through the points of the edges of the body with the projection plane to create a clear picture of the body with its different positions in the space. The projection plane is located behind the body or in front of the body.

Depending on the location of the C projection source, the projection is classified into two basic types:
1) Central projection.
2) Parallel projection

Central projection
In this type of projection, the center of the projection rays is a specific point in the sphere, from which all the projection lines that give their intersections with the plane of the projection as it passes through the edges of the body are projected to project the object into the plane.
Central projection gives complex fees, and is rarely used in industrial engineering drawings.

Parallel projection
In this type of projection, the location of the projection beam source is an imposed point at infinity, where the central projection lines become parallel lines between them.

Parallel projection is classified according to the angle of inclination of the projection lines at the projection plane into two basic types:
Inclined projection: in which the projection lines are oblique at the projection plane.
Vertical projection: in which projection lines are perpendicular to the projection plane.

The object is positioned so that its main surfaces are parallel to the projection plane, and the projection lines are perpendicular to the projection plane and thus the resulting shape on the projection plane is known as the vertical projection. In this projection, we find that each letter of the body is perpendicular to the plane, its points applied to each other in the projection, and that each letter parallel to the plane is the same shape and length in the projection. This is the principle of projection adopted in the engineering drawing, and for the complete engineering characterization of the object with its three dimensions requires projecting the body on two or more levels perpendicular to each other.

Vertical projection on two levels
The first to use the vertical projection method on two perpendicular levels is the French scientist Mung in the late eighteenth century in an easy and simplified manner, representing the horizontal projection level K1 (the first level) and the vertical projection level K2 (the second level), the two orthogonal levels form the total vertical projection and their cross line is called Earth line X12.

The space projection levels are divided into four sections (four angles) the object to be projected is placed so that the main faces are parallel to the projection levels.

The projected body projection in the horizontal plane is called a horizontal projection, and the projected plane in the vertical plane is projected as a projection, and since the projection of the body projections takes place at one drawing level (drawing board), therefore, the projection levels should be unified in one level after the projection of the object on each of them, by stabilizing the plane Vertically and rotating the horizontal plane around the ground line at 90o angles to apply to the vertical plane (Fig. 2-3) Thus the projection levels are combined into one drawing plane.

Projection of engineering objects

The basic elements of the engineering body
Objects (mechanical parts) consist of basic geometric figures (parallel rectangles, pyramid, cylinder, cone, grouped together with geometric positions with respect to their functional performance later on. The simple and complex geometric body is distinguished by its three dimensions: length, width, height, and multiple planar faces that define The geometrical shape of the three-dimensional To get acquainted with the basic elements of the geometric model, we will study the three-dimensional rectangular model, surrounded by flat geometrical surfaces, each having only two dimensions.


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