Engineering drawings have three common projection divisions. They are Axonometric, Oblique, and Perspective.
Axonometric (Isometric, Dimetric, and Trimetric)
These projections are orthographical and used to produce representative drawings of an object by rotating along one or more axes. They differ from Multi-view projections (Front, Right, Top) where there is a principal plane or axes of the object that is parallel with the projection plane. The three main types of Axonometric projections that are relevant to engineering drawings are Isometric, Dimetric, and Trimetric projections. Their differences stem from exact angle at which the view deviates from the orthogonal. These drawings don’t take into account the foreshortening effects of perspective and give a view that may appear distorted. Vertical lines, in Axonometric drawings, are used to represent height while sloping parallel edges are used for all the other sides.
Isometric projection is a method of visually representing three-dim objects in two dimensions, in which the three coordinate axes appear equally foreshortened and the angles between them are 120deg.
A dimetirc projection is created using 3 axes but only two of the three axes have equal angles. The smaller these angles are, the less we see of the top surface. The equal angle is usually around 105 deg.
A trimetric projection is created using 3 axes where each of the angles between them is different. The scale along each of the three axes and the angles among them are determined separately as dictated by the angle of viewing. Approximations in trimetric drawings are common.
Simple symmetrical flat objects and cylindrical parts, such as sleeves, shafts, rods, and studs require only two views to provide the full details of construction. In the Front view, the centerline runs through the axis of the parts as a horizontal centerline. If the plug is in a vertical position, the centerline runs through the axis as vertical centerline. The second view of the two-view drawing contains a horizontal and vertical centerline intersection at the center of the circles which make up the part in this view. The selection of views for a Two-View drawing rests largely with the designer/engineer.
Parts that are uniform in shape often require only one view to describe them adequately. This is especially true of cylindrical objects where a one-view drawing saves time and simplifies and drawing.
When a one view drawing of a cylindrical par is used, the dimension of the diameter must be preceded by the diameter symbol. The one-view drawing is also used extensively for flat(sheet metal) part. With the addition of notes to supplement the dimension on the view, the one view furnishes all the necessary information for accurately describing the part.
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