Engineering Drawing

1. Projections

Projections are methods of representing a 3D object on a 2D surface. Common types include:

  • Orthographic Projection: Front, top, and side views of an object.
  • Isometric Projection: A 3D representation where angles between axes are 120°.
  • Oblique Projection: A 3D view where the front face is shown in true shape, and depth is drawn at an angle.
  • Perspective Projection: Used in realistic drawings; lines converge at vanishing points.

2. Views in Engineering Drawing

  • Primary Views: Front view, top view, and side views.
  • Sectional Views: Internal features are shown by cutting the object.
  • Auxiliary Views: Used when an inclined surface needs to be shown in true shape.
  • Exploded Views: Illustrate how parts fit together.

3. Drawing Instruments

Common tools used in engineering drawing:

  • Drawing board – Provides a flat surface for sketching.
  • T-square – Ensures horizontal lines are straight.
  • Set squares – Used for drawing angles (30°, 45°, 60°).
  • Compass – Draws circles and arcs.
  • Divider – Measures distances.
  • Protractor – Measures angles.
  • French curves – Draws irregular curves.

4. Types of Lines

Lines communicate different information in drawings:

  • Continuous Thick Line – Represents visible edges.
  • Continuous Thin Line – Used for dimensioning and hatching.
  • Dashed Line – Represents hidden edges.
  • Chain Line – Indicates centerlines and axes.
  • Long-Dash Short-Dash Line – Represents cutting planes.

5. Geometric Figures

Basic geometric shapes used in engineering drawing:

  • Triangles, Squares, Rectangles, and Circles – Used in design layouts.
  • Ellipses and Parabolas – Common in mechanical and architectural drawings.
  • Polygons (Pentagons, Hexagons, etc.) – Frequently used in gear design and architecture.

6. Symbolic Representation

Engineering drawings use standardized symbols for clarity:

  • Electrical Symbols: Resistors, capacitors, transformers, etc.
  • Welding Symbols: Types of welds (fillet, groove, etc.).
  • Mechanical Symbols: Threads, gears, springs.
  • Architectural Symbols: Doors, windows, stairs, materials

What is Projection in Engineering Drawing?

A projection in engineering drawing is the method of representing a three-dimensional (3D) object on a two-dimensional (2D) surface using lines of sight. These projections help engineers and manufacturers visualize and understand the shape, size, and details of an object accurately.

Types of Projections in Engineering Drawing

Projections are broadly classified into two categories:

  1. Orthographic Projection (Parallel Projection)
  2. Perspective Projection

1. Orthographic Projection (Parallel Projection)

In orthographic projection, the projection lines are parallel to each other and perpendicular to the projection plane. It is mainly used for technical drawings as it provides accurate dimensions.

Types of Orthographic Projections

Orthographic projections are further divided into:

A. First-Angle Projection (European Method)

  • The object is placed in front of the projection plane.
  • The front view is drawn first, followed by other views.
  • The right side of the object appears on the left side of the drawing.
  • Used in Europe and India.

🔹 Symbol Representation:
⭘🡒■ (Circle in front of a square)

🔹 Views:

  • Front View
  • Top View
  • Side View (Left/Right)

B. Third-Angle Projection (American Method)

  • The object is placed behind the projection plane.
  • The right side of the object appears on the right side of the drawing.
  • Used in USA, Canada, and other countries.

🔹 Symbol Representation:
■🡒⭘ (Square in front of a circle)

🔹 Views:

  • Front View
  • Top View
  • Side View (Left/Right)

C. Isometric Projection

  • The object is rotated 30° from the horizontal.
  • Equal foreshortening along all three axes (X, Y, Z).
  • Used for pictorial representation of objects.

🔹 Advantages:

  • Gives a realistic 3D view of an object.
  • Used in assembly drawings and technical sketches.

D. Oblique Projection

  • The object is placed at an angle (usually 45°) to the projection plane.
  • One face is drawn in true shape, while the other faces are inclined.

🔹 Types of Oblique Projection:

  1. Cavalier Projection – Full depth (not foreshortened).
  2. Cabinet Projection – Half depth (foreshortened by 50%).

🔹 Uses:

  • Used in furniture design, industrial components, and simple illustrations.

2. Perspective Projection

  • Lines of sight converge at a single point.
  • Used for realistic representation of objects (e.g., buildings, vehicles).

🔹 Types of Perspective Projection:

  1. One-Point Perspective – One vanishing point (e.g., railway tracks).
  2. Two-Point Perspective – Two vanishing points (e.g., cube at an angle).
  3. Three-Point Perspective – Three vanishing points (e.g., skyscraper view).

🔹 Uses:

  • Architectural and artistic drawings.

Comparison Table: First Angle vs. Third Angle Projection

Feature First-Angle Projection Third-Angle Projection
Used in Europe, India USA, Canada
Object Placement In front of the projection plane Behind the projection plane
Right-Side View Drawn on the left Drawn on the right
Symbol ⭘🡒■ ■🡒⭘

Conclusion

  • Orthographic projections are best for technical accuracy.
  • Isometric and oblique projections help visualize objects.
  • Perspective projections create realistic 3D effects.

Engineering drawings use different types of lines to represent various features, dimensions, and details of an object. These lines help in conveying accurate information about the design, structure, and function of components. Here are the main types of lines used in engineering drawing along with their uses:

1. Object (Visible) Line

  • Description: Thick and continuous.
  • Use: Represents the visible edges and contours of an object.

2. Hidden Line

  • Description: Dashed line (short dashes).
  • Use: Represents edges or surfaces that are not visible from the current viewpoint.

3. Center Line

  • Description: Long and short alternating dashes.
  • Use: Indicates the center of circles, arcs, and symmetrical objects.

4. Construction Line

  • Description: Very light and thin continuous line.
  • Use: Used as a guide for constructing other lines and shapes.

5. Dimension Line

  • Description: Thin continuous line with arrowheads at both ends.
  • Use: Shows the measurement of an object.

6. Extension Line

  • Description: Thin continuous line extending from an object to indicate the start and end of a dimension.
  • Use: Helps in defining the points between which dimensions are measured.

7. Leader Line

  • Description: Thin line with an arrow or dot at one end.
  • Use: Used for annotations, notes, and symbols.

8. Cutting Plane Line

  • Description: Thick dashed line with long dashes and short dashes.
  • Use: Represents an imaginary plane where a sectional view is taken.

9. Section Line (Hatching Line)

  • Description: Thin diagonal lines.
  • Use: Represents the surfaces of an object that have been cut in a sectional view.

10. Break Line

  • Description: Zig-zag or wavy continuous line.
  • Use: Indicates that a part of an object is removed for clarity.

11. Phantom Line

  • Description: Alternating long and double short dashes.
  • Use: Represents alternative positions of moving parts or repeated details.

In engineering drawing, different types of lines are categorized into Type A, Type B, Type C, etc., as per BIS (Bureau of Indian Standards) IS 10714 or ISO 128. These classifications define the thickness, style, and purpose of lines in technical drawings.

10 Types of Lines in Engineering Drawing

  • Type A – Thick continuous line
    • A1 – Visible outline
    • A2 – Visible edges
    • Used for visible outlines and edges
  • Type B – Thin continuous line
    • B1 – Dimension Line
    • B2 – Leader line
    • B3 – Extension line
    • B4– imaginary line for intersection Part
    • B5– Hatching
    • B6– outline of revolved section
    • B7– short center lines
    • B8 – Diagonal line
    • B9- THREAD LINE
    • Used for dimension lines, projection lines, hatching, and short center lines
  • Type C – Thick dashed line
    • Used for hidden edges and outlines
  • Type D – Thin dashed line
    • Used for hidden edges when clarity is needed
  • Type E – Thin continuous line with freehand zigzag
    • Used for break lines in long objects
  • Type F – Thin continuous line with curve
    • Used for limits of partial views and sections
  • Type G– Chain line (long dash and dot)
    • Used for center lines and symmetry axes
  • Type H – Chain line (long dash, two short dashes)
    • Used for cutting plane lines
  • Type J – 
  • Type K – Thick continuous with freehand
    • Used for irregular boundaries or breaks
  • Thick Lines are used for visible objects, cutting planes, and break lines.
  • Thin Lines are used for dimensioning, projection, and hatching.
  • Dashed Lines represent hidden features.
  • Chain Lines are used for center lines, cutting planes, and special limits.
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