ENGINEERING DRAWING (CHAPTER-3)
๐ ENGINEERING DRAWING
Chapter 3 – Lines, Lettering, and Dimensioning
3.1 Introduction
In Engineering Drawing, lines, lettering, and dimensioning are the foundation.
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Lines → Represent shapes, edges, surfaces, and details.
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Lettering → Provides notes, titles, and labels in a clear readable style.
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Dimensioning → Specifies the size, location, and other details necessary for manufacturing.
3.2 Types of Lines (as per BIS: IS 10714 / SP-46)
Different line types are used in technical drawings. Each has a specific meaning.
Type of Line | Appearance | Thickness | Use |
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Continuous thick | ——————— | Thick | Visible outlines, edges |
Continuous thin | ——————— | Thin | Projection lines, dimension lines, leader lines, hatching |
Continuous thin freehand | ~~~~~~~ | Thin | Short break lines |
Continuous thin with zig-zag | _///_ | Thin | Long break lines |
Dashed thin | – – – – – – | Thin | Hidden edges |
Chain thin | –– ⃝ –– ⃝ –– | Thin | Center lines, axis, paths of motion |
Chain thick at ends | –– ⃝ –– ⃝ –– (thicker ends) | Thick | Cutting-plane lines |
Continuous thin with arrow | →————— | Thin | Leader lines (pointing to features) |
๐ Correct line thickness and type is essential, otherwise the drawing may be misinterpreted.
3.3 Lettering in Engineering Drawing
(A) Importance
Lettering is used for:
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Notes, titles, scales, dimensions, and instructions.
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It must be legible, uniform, and neat.
(B) Types of Lettering (BIS recommends “Vertical, Gothic lettering”)
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Gothic Lettering – Simple, uniform thickness.
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Vertical Gothic (upright letters).
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Inclined Gothic (slanted at 75°).
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Roman Lettering – Thick and thin strokes (not recommended for engineering drawings).
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Freehand Lettering – Used for quick sketching only.
๐ Standard engineering drawing uses Vertical Gothic lettering.
(C) General Rules for Lettering
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Uniform height (as per drawing scale).
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Spacing between letters = 1/6th height.
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Minimum lettering height (mm):
Application | Height (mm) |
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Title | 10, 12, 14 |
Main Headings | 6, 8, 10 |
Sub-headings | 3, 5, 7 |
Notes & dimensions | 2.5, 3, 5 |
3.4 Dimensioning
(A) Purpose of Dimensioning
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To specify the size, shape, and position of features in a drawing.
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To give all necessary information for manufacturing.
(B) Types of Dimensioning
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Linear Dimensions
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Shows length, width, height of objects.
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Example: 100 mm × 50 mm rectangle.
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Angular Dimensions
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Shows size of angles (in degrees).
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Example: ∠ABC = 45°.
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Radial Dimensions
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For arcs and circles.
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Example: R25 means radius = 25 mm.
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Diameter Dimensions
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Example: ⌀50 means diameter = 50 mm.
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Chain Dimensioning
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Dimensions given one after another (end-to-end).
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Parallel Dimensioning
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All dimensions taken from one reference line (common datum).
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(C) Rules of Dimensioning (BIS Standards – IS:10714)
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All dimensions in mm (no need to write mm, unless other units are used).
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Dimension lines: thin continuous lines with arrows at ends.
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Extension lines: start 2 mm away from object.
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Arrows: 3 mm long, 15° angle.
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Dimensions placed above dimension line, parallel to it.
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Avoid duplication of dimensions.
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Give overall dimension as well as necessary details.
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Notes like "⌀", "R", "SQUARE" must be added clearly.
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Place dimensions outside the view if possible.
(D) Example of Dimensioning Rules
Suppose we have a rectangle of 100 mm × 60 mm with a hole of ⌀20 mm at its center.
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Draw rectangle → Give length (100 mm) and width (60 mm).
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Locate center of hole (50 mm, 30 mm from sides).
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Give hole dimension as "⌀20".
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Overall size must also be mentioned (100 × 60).
3.5 Summary of Chapter
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Lines are of various types (continuous, dashed, chain, thick, thin) with specific meanings.
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Lettering must be uniform and legible; Gothic vertical lettering is recommended.
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Dimensioning conveys the size, shape, and position; must follow BIS rules.
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Proper placement of dimensions avoids confusion and ensures accurate manufacturing.
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