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Guide to Understanding Projections in Engineering Drawings

Engineering drawings are a richly detailed method of representing three-dimensional things on a two-dimensional surface such as paper or a computer screen. This can be accomplished by providing a variety of views of different sides of an object in a single image or by representing all three dimensions of an object in a single image. The use of engineering drawings to represent objects culminates in construction drawings. 

Every engineer must be able to read and interpret drawings and the information included therein. We must understand the spatial relation between 3-D projections and Multiview projections. 

Let’s look at the projection system & types of projection in engineering drawings.  

What is a projection in engineering drawing? 

The goal of engineering drawings, whether freehand sketching or CAD is to represent a physical object or a visual representation of an object so that it can be communicated to others.  

A geometrically represented image (visual image or figure) of an object obtained on a surface or plane is referred to as a projection in an engineering drawing. A point, line, plane, solid, machine component, or building may be the object. 

Projections in technical drawings are created based on the observer or reader of the technical drawing. The observer is usually looking at the projection plane, also known as technical drawing paper. In addition, the projectors generate a projected view of the 3D model on technical drawing paper. Projectors are also rays that originate from the observer’s eyes or the observer himself. On the projection plane between the observer and the part, a view of the part can be created. 

Types of Projection in Engineering Drawings 

Objects can be displayed as 3-D or Multiview projections. 

3-D projections are useful because they generate an image that is similar to the image in the designer’s mind. However, 3-D projections are frequently inadequate in providing adequate details of the object, and the object is frequently distorted. 

Multiview projections on the other hand are used to compensate for the shortcomings of 3-D projections. 

Since there are different viewing angles, it is easy to depict object details. Furthermore, Multiview projections, when combined, provide a more accurate representation of the object than 3-D projections—a circular hole appears as a circle in a Multiview projection. Multiview projections, on the other hand, necessitate extensive interpretation, and the overall shape of an object is not always obvious at first glance. As a result, the best representation of an object is obtained by combining the overall image provided by 3-D projections with the details provided by Multiview projections. 

Hence, a 3-D projection immediately reveals the shape of the object, and the Multiview projection provides the detail required for an accurate description of the object. 

3-D Projections 

3-D Projections 

Most CAD software supports three types of 3-D projections: isometric, trimetric, and perspective. These 3-D projections represent all three dimensions of the object in a single planar image in all three cases. Each type of 3-D projection has advantages and disadvantages. 

  • Isometric Projection – The isometric projection has a standard orientation, making it the most common projection used in CAD drawings. The width and depth dimensions of an isometric projection are drawn at 30° above the horizontal. As a result, the three angles at the upper front corner of the object are equal to 120°.  

The three sides of the object are also equal, giving rise to the term iso (equal) metric (measure). Isometric drawings work well for objects with limited depth. When the depth is significant, an isometric drawing distorts the object. In this case, a pictorial perspective drawing is preferable. 

  • Trimetric Projection – The trimetric projection generally gives you more options when it comes to arranging your object in space. The objects’ width and depth dimensions are at right angles to the horizontal, while the three angles at the cube’s upper front corner are uneven. As a result, each of the object’s three sides has a different length when measured in the plane of the drawing, hence the name tri-metric.  

The trimetric projection in most CAD software aligns one side of the cube along a horizontal line and tilts the cube forward. 

  • Perspective Projection – In contrast to isometric or trimetric projections, where parallel lines remain parallel, a pictorial perspective, or simply perspective, projection is created so that parallel lines converge in the distance. When an object spans a long distance, such as a view of a bridge or aircraft from one end, a perspective projection is very beneficial in creating a realistic depiction of the object. 

Isometric or trimetric views, in general, are suitable representations of small, manufactured things. 

In freehand sketching, two types of pictorial sketches are commonly used: isometric and oblique.  

In relation to 3-D CAD projections, the isometric projection is very easy to draw a realistic sketch of an item using the isometric projection, it is frequently employed in freehand sketching.  

The oblique projection, on the other hand, is frequently even easier to draw. The major face of the object is parallel to the plane of the paper, and the axes in the plane of the paper are perpendicular to one another in the oblique projection. The paper’s axis is angled at an arbitrary angle to the horizontal. 

The oblique projection has the advantage of preserving the true contour of details on the object’s front face. The circle on the front face, for example, is round in oblique projection but elliptical in isometric projection. Oblique freehand drawing is typically easier than isometric sketching because of this property. 

Multiview Projections 

Multiview Projections 

Multiview projections are good for revealing the intricacies of an object, whereas 3-D projections create an immediately recognized visual representation of an object. In Multiview projections, dimensions can be easily displayed, and most details remain undistorted. 

  • Orthographic Projection – Orthographic projection is the conventional method of Multiview projection in engineering graphics. An orthographic projection is a collection of views of distinct sides of an object, such as the front, top, side, etc. For a coffee mug, for example, two orthographic projections could be utilized. The sidewall of the cup, as well as the loop that forms the handle, would be visible from the front. 

Looking down into the mug, the top view would reveal the round rim of the cup, the bottom of the inside of the mug, and the top of the handle that protrudes from the side of the mug. 

To generate an engineering drawing, the dimensions of the cup may easily be added to the projections of each side of the mug. 

Related Articles >> Four Types of Projection Methods Used in Mechanical Drawing

We hope this enables you to identify the types of projection in engineering and CAD drawings.  

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