In Engineering drawing the orthographic projection is an illustration technique in which up to six pictures of an object are may be produced. Each projection plane is parallel to one of the coordinate axes of the object. The views are positioned relative to each other. The appearances of views may be thought of as being projected onto planes that form a 6-sided box around the object, and are positioned relative to each other.
Modern orthographic projection is derived from Gaspard Monge's descriptive geometry. Monge's original formulation uses two planes only, and obtains the top and front views only. The addition of a third plane to show a side view (either left or right) developed later.
On engineering drawings, the projection angle is denoted by an international symbol consisting of a truncated cone. The symbols define whether a projection is either first angle or third Angle.
ISO standards state that both systems of projection are equally acceptable but they should never be mixed on the same drawing. The projection symbol should be added to the completed drawing to indicate which system has been used.
In first-angle projection, the projectors originate as if radiated from a viewer's eyeballs and shoot through the 3D object to project a 2D image onto the plane behind it. The 3D object is projected into 2D "paper" space as if you were looking at a radiograph of the object: the top view is under the front view, the right view is at the left of the front view. First-angle projection is the ISO standard and is used primarily in Europe and other countries that use ISO standards.
In third-angle projection, the projectors originate as if radiated from the 3D object itself and shoot away from the 3D object to project a 2D image onto the plane in front of it. The views of the 3D object are like the panels of a box that envelopes the object, and the panels pivot as they open up flat into the plane of the drawing. Thus the left view is placed on the left and the top view on the top; and the features closest to the front of the 3D object will appear closest to the front view in the drawing. Third-angle projection is primarily used in the United States and Canada, where it is the default projection system according to ASME standard ASME Y14.3M.
Choice of views
The most informative view of an object shall be used as the front or principal view. Generally, this view show the part in the functioning position. Parts which can be used in any position should preferably be drawn in the main position of manufacturing or mounting.
When other views (including sections) are needed, these shall be selected according to the following principles:
- to limit the number of views and sections to the minimum necessary and sufficient to fully delineate the object without ambiguity.
- to avoid the need for hidden outline and edges.
- to avoid the unnecessary repetition of detail.
The true size and shape of any feature in an engineering drawing can only be known when the Line of Sight is perpendicular to the plane being referenced. when an object contains some sort of inclined plane, none of the six principal views can show the true size and shape. Using the auxiliary view allows an inclined plane to be projected in their true size and shape.
Partial views may be used where complete views would not improve the information to be given. The partial view shall be cut off by a continuous thin freehand line or straight line with zigzag.
A detail view can be used when part of a view has detail that cannot be clear dimensioned due to drawing scale or complexity. To establish a detail view, a thick phantom line circle is placed around the area to be enlarged.
Provided that the presentation is unambiguous, it is permitted to give a local view instead a complete view for symmetrical items. The local view should be drawn in third angle projection, regardless of arrangement used for general execution of the drawing.
Break Views are typically used to display parts that are very long in one dimension on a drawing in such a way that you can see both ends or other important features. Broken views make it possible to display the drawing view in a larger scale on a smaller size drawing sheet.
Views of symmetrical parts
To save time and space, symetrical objects may be drawn as a fraction of the whole. The line of symmetry is identified at its ends by two thin short parallel lines drawn at right angles to it.
Simplified representation of repetitive features
In all cases, the number and kind of repetitive features should be defined by dimensioning or by a note.
Alternate Position View
Alternate Position Views indicate the range of motion of an assembly component by showing it in different positions. This is another view type that doesn’t create a new view but alters an existing view.
The isometric projection shows the object from angles in which the scales along each axis of the object are equal. "Isometric" comes from the Greek for "same measure". One of the things that makes isometric drawings so attractive is the ease with which 60 degree angles can be constructed with only a compass and straightedge