Projections in Engineering Drawing

As object, have three dimensions like length, width and height/ thickness. The shapes and sizes of three dimensional objects have to be represented on a sheet of drawing paper, which has only two-dimensional planes.

For obtaining the image of an object, various points on the contour of an object, are thrown forward on to a plane by means of straight lines or visual rays. The figure formed by joining various points thus obtained on the plane, is the image of the object and is called Projection.

Types of Projections in Engineering Drawing

  1. Parallel Projection
    1. Orthographic projection
      1. Multi view projection
        1. First Angle Projection
        2. Third Angle Projection
      2. Axonometric
        1. Isometric
        2. Dimetric
        3. Trimetric
    2. Oblique projection
      1. Cabinet Projection
      2. Clinographic Projection
      3. Cavalier Projection
  2. Perspective Projection
    1. Aerial
      1. One Point Perspective
      2. Two Point Perspective
      3. Three Point Perspective
    2. liner

1. Pictorial projection

It is used for easy understanding of the drawing and visualizing the object for the persons without technical knowledge. These drawings create three dimensional effect and they reveal the shape of an object, approximately, when an observer, views the object.

But for orthographic projection, persons without technical knowledge cannot understand easily, hence, trainee shall develop the ability to convert orthographic views into pictorial views.

2. Isometric projection

Isometric projection is a type of pictorial projection in which the three dimensions of solid are not only shown in one view but their actual sizes can be measured directly from it. In isometric projection, there are three principle axes such as height axis, length axis and width axis. Three axis of the object are equally inclined 1200 to each other and the three dimensions length, height and width are equally fore-shortened by using an isometric scale.

Important points for isometric drawings

  1. In isometric view, the two sides are inclined at 300 to the height axis.
  2. The length may be drawn on the right or left depending on the side view of the orthographic projection of the object.
  3. Hidden features are not to be shown in isometric views.
  4. Vertical lines will be drawn vertical, while horizontal line will be drawn at a angle 300 to horizontal.

3. Oblique projection

Pictorial projections are becoming more popular due to use of a computer in a modern drawing, dimensional object on the projection plane by one view only. This type of drawing is useful for marking an assembly of an object and provides directly a production drawing (working drawing) of the object for the manufacturing purpose.

Types of oblique projection

  1. Cavalier projection.
  2. Cabinet projection.

4. Orthographic projection

If the projections from the object are perpendicular to the projection plan, then such a projection of the object is known as Orthographic Projection. A thorough knowledge of the principles of pictorial projection is required for converting pictorial views into orthographic views.

Orthographic Projection General Procedure

  1. Determine the overall dimensions of the given object for the required orthographic views.
  2. Draw rectangles for the views using suitable scale. It is also required to keep sufficient space between the views and from border lines.
  3. Draw centre lines for circles and arcs.
  4. Draw circles and arcs of circles first, next draw straight lines for the main shapes of the object. 5 And finally draw straight lines and small curves for the minor details of the object.
  5. And finally draw straight lines and small curves for the
  6. minor details of the object.

Points to be considered for converting a pictorial view in to orthographic views

  • Dimensions which are parallel to the direction of viewing will not be seen. Edges which are parallel to
  • the direction of viewing are seen as points. Surfaces
  • which are parallel to it are seen as lines.
  • The visible edges and the intersection if the surfaces
  • are shown by object lines. But the hidden edges are
  • shown by dotted lines.
  • The centre linens of the symmetrical parts like whole
  • cylinder etc. should be clearly shown.
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