Chapter Eight Pictorial drawing: Isometric, 3D Solids Modelling and Oblique Parallel Projection

1. Chapter Eight Pictorial drawing: Isometric, 3D Solids Modelling and Oblique Parallel Projection

2. Purpose This chapter provides an overview of how to: • produce isometric, 3D solids modelling and oblique parallel pictorial drawings from orthogonal views • select the best viewing direction when making a pictorial drawing • understand the relationship between two- and three-dimensional drawing.

3. Introduction • Pictorial views are not normally dimensioned. • Three general classifications of pictorial drawings: • axonometric projection • oblique projection • perspective projection (used mainly by architects)

4. Axonometric projection • Axonometric projection – turning the object so that any three principal faces can be seen from the one viewing position. • Any number of viewing angles; however, certain positions are classified as isometric, dimetric and trimetric. • The most common used is isometric.

5. Isometric projection • Isometric – means equal measure. • Isometric projection – it is necessary to view an object so that its principal edges are equally inclined to the viewer and hence are foreshortened equally; see Figure 8.2, p.214. When an isometric view is drawn using an isometric scale, it is termed an isometric projection. • Isometric scale – for correct isometric projection, a scale is used which allows for the foreshortening of isometric lines.

6. Isometric projection • Isometric drawing – prepared without shortening measurements, about 22.5 per cent larger than the isometric projection and is used for most purposes. • The main purpose of an isometric view is to provide a pictorial view which reveals as much detail as possible, and this fact should be remembered when selecting the principal edges as the isometric axes; see Figure 8.4 (a)–(h), p.215.

7. Isometric projection • Circles may be drawn whole or in part in isometric view by the use of ordinates constructed on an orthogonal view and transferred to the isometric view; see Figure 8.5, p.216. • Circles may also be constructed using the four-centre method; see Figure 8.6, p.216.

8. Isometric projection • Isometric curves – points on these curves are plotted by the method of ordinates taken from an orthogonal view, as shown in Figure 8.7., p.217. • Isometric angles and non-isometric lines – these have to be plotted by the use of horizontal and vertical measurements as shown in Figure 8.8.

9. Making an isometric drawing Making an isometric drawing (refer to Figure 8.9, p.218): • Orthogonal views for making the isometric drawing • Draw in light construction lines (circles and curves full thickness) • Remove excess lines (simplified if construction lines lightly drawn) • Line in 30° right lines • Line in 30° left lines • Line in vertical lines to complete view

10. Representation of details common to pictorial drawing • Filleted corners and rounded edges may be represented by either straight or curved lines as shown in Figure 8.10, p.219. • Pictorial drawings should be sectioned along centre lines, the cutting plane cutting parallel to one of the principal viewing planes of the object (Fig. 8.12(a)). • If dimensioning required either unidirectional (read from the bottom of drawing) or principal plane dimensioning (where dimensions lie in one or more of the three pictorial planes (Fig. 8.13(b)) are used.

11. Basic 3D solid modelling concept • A solid model is a 3D representation that has the properties of mass, volume, centre of gravity and moments of inertia. This information can be used in other computer programs, e.g. numerical control machining or stress analysis. • A solid part (3D model) generally consists of a group of features, added one at a time, until the model is complete. Engineering solid models are generally built from a base sketch and then either ‘extruded’ or ‘revolved’ to create a 3D model.

12. Basic 3D solid modelling concept • There are many 3D modelling software packages available; however, this information is directly related to the Autodesk® Inventor Suite. • Inventor has the ability to produce the following: • Part – (.ipt) • Drawing – (.idw) • Assembly – (.iam) • Presentation – (.ipn) • Sheet Metal – (.ipt) • Project – (.ipj) • iFeature – (.ide) • Design Variables – (.idv)

13. Basic 3D solid modelling concept Each model is created using the same basic steps: • Sketch (to produce basic shape) • Add constraints (either dimensions or physical constraints) • Extrude/revolve (both can be used to add or remove material) • Add features (chamfers, fillets, holes etc.) • This process is repeated to complete the model

14. Oblique parallel projection • Front face is parallel to the picture plane, depth lines are drawn at an angle (45°, 30° or 60°); however, any suitable angle may be used. • Cavalier projection – depth lines drawn to full length (Figure 8.39(a)) appearance is unnatural. • Cabinet projection – depth lines drawn to half length (Figure 8.39(b)) appearance more natural, used in most drawings.

15. Oblique parallel projection Rules worth remembering when making oblique drawings: • Rule 1 Place the object so that the view with the most detail is parallel to the picture plane, especially if the view consists of arcs and circles. This is illustrated in Figure 8.40. • Rule 2 Place the object so that the longest dimension runs horizontally across the sheet, as shown in Figure 8.41.

16. Oblique parallel projection • Rule 3 In some cases the above two rules conflict, and when this is so, Rule 1 has preference as the advantage gained by having the irregular face without distortion is greater than that gained by observing Rule 2. This rule is illustrated in Figure 8.42. • Rule 4 Decisions about viewing an object in oblique projection should aim to show the object so that its shape is most clearly presented and is conducive to showing its dimensions.

17. Oblique parallel projection • Circles can be plotted using a similar method as isometric circles except that measurements along the 45° axis are taken from the half size quadrant (refer to Figure 8.43, p.229) . • Alternatively, oblique circles may be plotted using true shape semicircles located on the edges of the oblique face and projecting points on the oblique circles as shown in Figure 8.43.

18. Oblique parallel projection • Angles on oblique drawings are constructed as per Figure 8.44, p.229. • A number of views which can be obtained by varying the angle of the receding axis are shown in Figures 8.45(a)–(d); each view is chosen because it reveals the maximum amount of detail for that particular orientation of the object.

19. Summary Pictorial drawings present information in an easily identified manner as a 3D model is more simply identified than a 2D drawing. They are constructed from the dimensions recorded on 2D orthogonal drawings either manually or on a CAD system. Isometric projection is commonly used, but oblique projections are a viable alternative.