PICTORIAL DRAWING

ENGINEERING DRAWING: LECTURE 7 - PICTORIAL DRAWING

1. Introduction to Pictorial Drawings

• Definition: Pictorial means picture. It refers to any realistic form of drawing.
• Purpose: Facilitates the communication of technical information during all phases of a design process.
• Characteristic: Produces a three-dimensional view of an object, showing height, width, and depth simultaneously.

2. Major Types of Pictorial Drawings

• Oblique Drawing:
  • Simple but least realistic.
• Perspective Drawing:
  • Most realistic and most complex.
• Isometric Drawing:
  • Falls in between perspective and oblique drawings in terms of realism and complexity.

3. Oblique Drawing

• Description: Shows objects with one or more parallel faces having true shape and size.
• Front View: One face, typically the front, appears exactly as it would in a multi-view drawing.
• Circles: Are true circles and can be drawn with a compass or circle template.
• Rectangles: Parallel to the front plane are true size and have square corners. Projectors are parallel but oblique to the plane.
• Axes: Uses three axes: two at right angles (X and Y axes) and the third at any convenient angle (usually 30° or 45°).
• Scaling:
  • A scale must be selected for the orthographic or front faces.
  • An angle must be selected for the receding axis or depth.

4. Perspective Drawing

• Realism: The most realistic form of pictorial drawing.
• Vanishing Points: Lines appear to converge at an imaginary point (vanishing point), similar to how walls and floor lines converge when looking down a long hall.
• Types: Can be one-point, two-point, or multiple points.

5. Isometric Drawing

• Meaning: "Isometric" means equal measure.
• Axes: Uses three axes at 120° to each other to show three faces of an object simultaneously.
• Scale: The three axis lines are measured using the same scale.
• Usage: The most common type of pictorial drawing used in industry.
• Viewpoint: Provides a single view, making the entire object appear as if it is tilted towards the observer.
• Isometric Angles: Characterized by angles of 30° x 90° x 30°.

Rules for Isometric Lines:

• Parallelism: All lines parallel in the orthogonal view must remain parallel in the isometric view.
• Horizontal Lines: Lines that appear horizontal in the orthographic view are drawn at a 30° angle in isometric views.
• Vertical Lines: Lines that appear vertical in orthographic views remain vertical in isometric views.
• Isometric Lines: Lines that are parallel to the axis and are drawn vertically or at a 30° angle. These lines can be measured.
• Non-isometric Lines: Lines that are not parallel to the axes. They cannot be measured.
• Hidden Lines: Never shown in isometric views.

Drawing a Square Box in Isometric (Example Steps):

1. Draw a horizontal line.
2. Erect a vertical line at any convenient point O on the horizontal line.
3. Draw a line inclined at 30° at point O to the left and right of the vertical line.
4. Transfer the length of the square to the three lines you now have (using an outline).
5. Generate vertical lines on both sides of point O through the ends of the lines.
6. Complete the box (Isometric box).
7. Outline the complete box.

6. Orthographic to Isometric Drawings Conversion

• It is crucial to be able to convert orthographic drawings into pictorial drawings, especially isometric drawings.
• The same level of knowledge is required for converting isometric drawings back to orthographic.
• Various examples demonstrate this conversion process for different objects.

7. Isometric Curves and Circles

• Appearance: Circles and curves on isometric planes appear as an ellipse.
• Methods for Drawing Isometric Curves:
  • Ordinate Method: Involves selecting points on the curved profile in the orthographic view and transferring their ordinates to the isometric view. (Used for small fractions of a circle where approximate methods may deviate significantly).
  • Approximate Parallelogram Method (Four-Center Method): A more convenient and preferred method for drawing isometric curves and circles.
  • Circumscribing Square Method: Can also be used to draw isometric curves.
• Steps for Drawing an Isometric Circle (Four-Center Method):
  1. Draw a square (isometric box) to contain the circle on the desired isometric plane.
  2. Draw center line PQ parallel to two opposite sides of the isometric square (e.g., AB and DC).
  3. Draw center line XY parallel to the other two opposite sides (e.g., AD and BC) to locate the center.
  4. Join the longest diagonal (BD) through the center.
  5. From corner A (the obtuse angle of the parallelogram), draw two lines to points Q and X (midpoints of opposite sides) to cut the diagonal BD at R and S respectively.
  6. With A as the center and radius AX, draw an arc to join X and Q.
  7. With C as the center and radius CY, draw an arc to join Y and P.
  8. With R as the center and radius RP, draw an arc to join P and X.
  9. With S as the center and radius SQ, draw an arc to join Q and Y to obtain the full isometric circle (ellipse).

8. Isometric Problems and Examples

• The lecture includes various examples demonstrating the application of isometric drawing principles for different mechanical components and shapes, often involving the conversion from orthographic views.