Third angle projection

The difference between first and third angle projection is in the arrangement of views and, with reference to the illustration in Fig. 4.4, views are now positioned as follows:

View B from the left is placed on the left, View C from the right is placed on the right, View D from above is placed above, View E from below is placed underneath.

Study the rearrangement shown below in Fig. 4.4 and remember the above rules because it is vital that the principles of first and third angle projection are understood. The distinguishing symbol for this method is also shown.

If a model is made of the block in Fig. 4.1, and this can easily be cut from polystyrene foam used in packing, then a simple demonstration of first and third angle projection can be arranged by placing the block on the drawing board and moving it in the direction of the four chain dotted lines terminating in arrows in Fig. 4.5. Figure 4.5(a) shows the positioning for first angle and Fig. 4.5(b) for third angle projection. The view in each case in the direction of the large arrow will give the five views already explained.

The terms first and third angle correspond with the notation used in mathematics for the quadrants of a circle in Fig. 4.6 the block is shown pictorially in the first quadrant with three of the surfaces on which views are projected. The surfaces are known as planes and the principal view in direction of arrow A is projected on to the principal vertical plane. The view from D is projected on to a horizontal plane. View B is also projected on to a vertical plane at 90° to the principal vertical plane and the horizontal plane and this is known as an auxiliary vertical plane. Another horizontal plane can be positioned above for the projection from arrow E, also a second auxiliary vertical plane on the left for the projection of view C. Notice that the projections to each of the planes are all parallel, meeting the planes at right angles and this a feature of orthographic projection.

The intersection of the vertical and horizontal planes give a line which is the ground line GL. This line is often referred to as the XY line; this is useful in projection problems since it represents the position of the horizontal plane with reference to a front view and also the position of the vertical plane with reference to a plan view. Many examples follow in the text.

If the planes containing the three views are folded back into the plane of the drawing board, then the result is shown in Fig. 4.7 where dimensions have also been added. The draughtsman adjusts the distances between views to provide adequate spaces for the dimensions and notes.

To describe a simple object, a draughtsman does not need to draw all five views and it is customary to draw only the minimum number which completely illustrate the component. You will note in this particular case that we have omitted views which contain dotted lines in preference to those where corners and edges face the observer. Many parts do not have a definite 'front', 'top' or 'side' and the orientation is decided by the draughtsman, who selects views to give the maximum visual information.

3rd Angle Projection
(a)
Design View First Angle Projection

Fig. 4.5 (a) First angle arrangement (b) Third angle arrangement

Fig. 4.5 (a) First angle arrangement (b) Third angle arrangement

Traditionally, front views are also known as front elevations, side views are often known as side or end elevations and the views from above or beneath are referred to as plans. All of these terms are freely used in industrial drawing offices.

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Responses

  • Bayode olasunkanmi babatunde ninjay
    fist angle and third angle projection are engineering drawing because is the only drawing method/pattern in engineering field
    7 years ago
  • Marigold
    What is the difference between first and third angle drawings?
    9 years ago
  • ennio
    Why first angle projection drawings are used?
    9 years ago
  • Eliza
    What the differance between first and third angle drawings?
    8 years ago
  • Feorie
    How to draw angled industrial nozzle plan view?
    7 years ago
  • Andrea
    Why third angle projection is used in aeronautical engineering?
    2 years ago
  • kristian
    How is first and third angle projection arranged?
    2 years ago
  • amina
    Which views are arranved in 1st projection?
    2 years ago
  • tecla
    How to draw a first and third angle projection technical drawing?
    2 years ago
  • gloria
    How to simply way in third angle drawings?
    2 years ago
  • june
    What are features of third angle projection?
    2 years ago
  • Saba
    How to draw a front view ekevation of a third angle projection of a stair case?
    1 year ago
  • Anselma
    How to make parts in 3rd angle projection drawings?
    1 year ago
  • frank
    How to draw rough drawings of a component in 3rd angle projection?
    12 months ago
  • leanne
    How many principle view in third angle projection?
    11 months ago
  • Rowan
    Why third angle projection is used in industries?
    5 months ago
  • eleleta
    How to view a third angle projection blueprint?
    5 months ago
  • martina
    How to draw a third angle projected drawing?
    4 months ago
  • kisanet
    How many angels in engineering drawings?
    2 months ago

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