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About projections
About projections
First angle projection is widely used throughout all parts of Europe, while the third angle projection system is used in North America.
An image of an object can be obtained by observing and referring to the observer of the visible part of the object's surface, which is called a view. The name of the view depends on which side of the object the observer is looking at.
Picture. 1.1. Perspective and parallel projections
Developing the ability to visualize and graphically express forms and spaces in three dimensions is essential in fields related to engineering and design. The design-drawing process usually begins with two-dimensional expression in the form of orthographic sketches and drawings. These multiview drawings are the graphical vocabulary that an architect/designer uses. Multiviews help us accurately examine geometric configurations, spatial relationships, and the scale and proportion of a design. Multiviews by themselves cannot, however, reveal the three-dimensional pictorial configuration of an object or a building, according to orthographic projection theory. For pictorial depth expression, closely related three-dimensional, single-view drawings, termed “paralines” and “perspectives”, are needed ( Pic.1.1).
Parallel projections are characterized by parallel lines, and perspectives are characterized by converging lines. Both projections depict volumetric shapes by combining length, width, and depth parameters, while simultaneously combining plan, height, and section into one illustration (ISBN 978-1-118-01287-1).
First and Third angle view symbols (EN ISO 128-43:2020)
First and Third angle view symbols (EN ISO 128-43:2020)
Because both types of projection are used for engineering drawings, it is necessary to be able to identify the type of projection used. The International Organization for Standardization (ISO) has recommended that one of the symbols shown in Picture 1.2 a-c is shown on all engineering drawings to indicate the type of the projection used. Each symbol is just two views of a cone with the tip cut off. The preferred location is in the lower right-hand corner of the drawing, adjacent to the title block.
a)
b)
c)
Picture 1.2: a) projections of cone; b) first angle projection symbol; c) third angle projection symbol.
Picture 1.3. The First angle orthogonal projections views
According to the ISO standard, the first-angle orthographic projection, which is used in Europe, has six principal views (Pic.1.3):
the top view is placed below the front view,
the bottom view is placed above the front view,
the left view is placed on the right of the front view,
the right view is placed on the left of the front view,
the rearview is placed at the extreme left or right, whichever is convenient.
Drag the projection names of the first and third angles onto the respective images.
View Layout
View Layout
An orthographic projection is a type of parallel projection. An object is placed in space in such a way that its front view is fixed in the vertical plane, and the top view is fixed in the horizontal plane. Object projections are perpendicular to the flat screen, hence the name "orthogonal".
Thus, a correct image of the object is obtained with the possibility of showing its initial parameters. Before making a drawing, the designer must decide how many views are needed to adequately display the part and which of the six sides of the object will provide the best main (front) view. Different factors such as how informative the view is and whether hidden lines can be avoided will influence the choice.
The front view of the drawing shows the width and height of the object. After choosing a front view, you must decide what other views are needed to adequately show the shape and characteristics of the part. A complete description of an object rarely requires more than three views.
An orthographic drawing uses each view to show the exact shape and size of an object and the relationships of all three views to each other.
An isometric drawing shows the relationships between the front, top, and right surfaces in a single view.
Picture 1.4. Orthographic projections of points
The object is positioned so the front view is captured in the vertical plane, and the top view, in the horizontal plane. The projections of the object are perpendicular to the planar screen, hence the name 'orthographic'.
Projection points on the top view are indicated with a single prime mark (A', B' and C'), on the front view, with a double prime mark (A'', B'' and C''), and on the left view, with a triple prime mark (A''', B''' and C'''). This makes it easier to tell different projections apart (see Pic.1.4).
Picture 1.5. Isometric projections of entities
Isometric projection
Isometric projection
See Picture 1.5: This form of projection shows the component in complete detail. In an isometric projection (note: "iso" means equal) the three axes are inclined towards each other at equal angles (120°). Actual dimensions are converted to the isometric scale using basic trigonometric calculations.
It is worth noting that other less conventional orthographic projections do exist, such as dimetric and trimetric projections, which separate the axes by different angles. However, the most commonly used type of orthographic projection is the isometric projection.
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