1. Orthogonal Projection
Chapter Six
Orthogonal Projection

2. Purpose This chapter provides an overview of how to:
understand the principles of third angle orthogonal projection
produce a detailed orthogonal drawing of a component, including all information necessary for its manufacture

3. Orthagonal and third-angle projection
Orthogonal projection is a method of drawing an object so that a number of plane views can be obtained and may include dimensions of length, breadth and depth.
Dihedral angle – where vertical and horizontal planes intersect at 90°; refer to Figure 6.2(a) p.147.
The third dihedral angle is commonly known as third-angle projection and is recommended by standards Australia for orthogonal projection.

4. Third-angle projection
In third-angle orthogonal projection the views of each side are drawn relative to each other.
Figure 6.2 (f)

5. Third-angle projection
Figure 6.1, p.146, shows how the third-angle three dimensional viewing box is unfolded to give the particular orientation of the six possible two dimensional views.
Drawings in third-angle projection can be identified by the standard symbol illustrated in Figure 6.2, p.147, or by the words ‘third-angle projection’ included in the title block.

6. Third-angle projection
There are six (6) possible views; however, only enough views are drawn to sufficiently indicate the shape and dimensions of the object, usually three (3).
The front view is always indicated and other views are drawn adjacent to it; examples of three, two and one view drawings are shown in Figures 6.3 (a), (b) and (c), p.148.
Other views (i.e. section, auxiliary, partial and revolved views) may also be included for greater clarity.

7. Third-angle projection
Orthogonal views have a standard relationship to each other and may be projected horizontally to the side and rear views or vertically to the top and bottom views from the front view (normally drawn first). Refer to Figure 6.4, p.148.

8. Third-angle projection
There are three methods of projecting between the top and side views:
Figure 6.5(a), p.149, uses a 45° set square.
Figure 6.5(b) uses a compass.
Figure 6.5(c) combines horizontal and vertical projection lines from a 45° line.
In each case distances are constant; however, the distances may be varied by moving the projection plane to one side (Figure 6.5(d)).

9. Production of a mechanical drawing
The production of mechanical drawings can be divided into five (5) stages:
Drawing of borderline and location of views on the drawing sheet
Light construction of views
Lining in of views
Dimensioning and insertion of subtitles and notes
Drawing of title block, parts list and revision tables.

10. Production of a mechanical drawing
To give a drawing a balanced appearance, once the number of views has been decided, their layout in the available drawing space (space left after title block, revision table and parts list has been accounted for) has to be calculated; that is add up all horizontal values and subtract from horizontal value of drawing space and do the same for vertical values.
It is best to create a rough free-hand sketch taking into account all the parameters mentioned; see Figure 6.8, p.150.

11. Production of a mechanical drawing
Using the measurements from your rough sketch to identify a starting point for your front view and lightly construct it; from this view you lightly project your other views and any hidden detail required; see Fig 6.9, p.151.
Commence systematically lining in all views; that is, start horizontal lines from the top working down, then vertical lines from the left side working across. Projection lines may be left if they are light enough or erased if required. Refer to Figure 6.10, p.151.

12. Dimensioning and insertion of subtitles and notes
Two rules to remember:
Each dimension necessary to describe a component should be given, and it should not be necessary to deduce a functional dimension from other dimensions on the drawing.
There should be no more dimensions than are necessary to define a component.

13. Dimensioning and insertion of subtitles and notes
Functional dimensions – govern the working of a component.
Non-functional dimensions – all other dimensions.

14. Dimensioning and insertion of subtitles and notes
Three correct methods of dimensioning lengths are given in Figures 6.11(a), (b) and (c), p.152.
Note the following:
Where two dimensions together give the length of an object as in Figure 6.11(a), the overall dimension is omitted.
When an overall length is shown, as in Figure 6.11(b) and (c), a non-functional intermediate dimension is omitted.

15. Dimensioning and insertion of subtitles and notes
An exception to Rule 2 previously are ‘Auxiliary dimensions’, indicated by placing brackets around the dimension (Figure 6.11(d)), which are used to show overall dimensions, even though all intermediate dimensions are supplied; however, they are in no way binding as far as machining operations are concerned.

16. Production of a mechanical drawing
A drafter must fully understand the working of components to be able to indicate functional dimensions, therefore ensuring features are correctly located on the finished product.

17. Production of a mechanical drawing
Referring to Figure 6.14, p.154, the following features are considered functional:
The axis of the bored hole is vertical, centrally located and is a toleranced distance from the back surface, which is machined.
The top surface of the boss must be correctly located in relation to the three 16mm diameter fixing holes.
The bore of the boss is a toleranced size.

18. Production of a mechanical drawing
It is often necessary when dimensioning a drawing to decide on one or more base or datum lines from which functional dimensions are taken.
Figure 6.14, p.154, follows the previous rules as well as those given in Chapter 2.

19. Drawing of title block, parts list and revisions table
A general description is provided on pages 11–15, Chapter 1.
Figure 6.12 shows an isometric view of a cast steel wall bracket.
We are to draw:
a front view in direction A
a side view in direction B
a top view.

20. Drawing of title block, parts list and revisions table
Figure 6.13 shows the rough sketch for the calculation of the positions of the three views.
The completed orthogonal projection is shown in Figure 6.14, p.154.

21. Summary
Generally, we draw in third-angle orthogonal projection as it gives a clear indication of the relationship of all projected views to the front view.
When producing a mechanical drawing it is important to identify the available drawing space then calculate the position of the views. The views are first lightly constructed to ensure they are correct, then lined in. Finally, all other details are included, such as dimensioning, subtitles, notes, title block, parts list and revision table.