ENGINEERING DRAWING (CHAPTER-3)



๐Ÿ“˜ ENGINEERING DRAWING

Chapter 3 – Lines, Lettering, and Dimensioning


3.1 Introduction

In Engineering Drawing, lines, lettering, and dimensioning are the foundation.

  • Lines → Represent shapes, edges, surfaces, and details.

  • Lettering → Provides notes, titles, and labels in a clear readable style.

  • 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
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)

๐Ÿ‘‰ 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:

  • Notes, titles, scales, dimensions, and instructions.

  • It must be legible, uniform, and neat.

(B) Types of Lettering (BIS recommends “Vertical, Gothic lettering”)

  1. Gothic Lettering – Simple, uniform thickness.

    • Vertical Gothic (upright letters).

    • Inclined Gothic (slanted at 75°).

  2. Roman Lettering – Thick and thin strokes (not recommended for engineering drawings).

  3. Freehand Lettering – Used for quick sketching only.

๐Ÿ‘‰ Standard engineering drawing uses Vertical Gothic lettering.

(C) General Rules for Lettering

  • Uniform height (as per drawing scale).

  • Spacing between letters = 1/6th height.

  • Minimum lettering height (mm):

Application Height (mm)
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

  • To specify the size, shape, and position of features in a drawing.

  • To give all necessary information for manufacturing.


(B) Types of Dimensioning

  1. Linear Dimensions

    • Shows length, width, height of objects.

    • Example: 100 mm × 50 mm rectangle.

  2. Angular Dimensions

    • Shows size of angles (in degrees).

    • Example: ∠ABC = 45°.

  3. Radial Dimensions

    • For arcs and circles.

    • Example: R25 means radius = 25 mm.

  4. Diameter Dimensions

    • Example: ⌀50 means diameter = 50 mm.

  5. Chain Dimensioning

    • Dimensions given one after another (end-to-end).

  6. Parallel Dimensioning

    • All dimensions taken from one reference line (common datum).


(C) Rules of Dimensioning (BIS Standards – IS:10714)

  1. All dimensions in mm (no need to write mm, unless other units are used).

  2. Dimension lines: thin continuous lines with arrows at ends.

  3. Extension lines: start 2 mm away from object.

  4. Arrows: 3 mm long, 15° angle.

  5. Dimensions placed above dimension line, parallel to it.

  6. Avoid duplication of dimensions.

  7. Give overall dimension as well as necessary details.

  8. Notes like "⌀", "R", "SQUARE" must be added clearly.

  9. 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.

  • Draw rectangle → Give length (100 mm) and width (60 mm).

  • Locate center of hole (50 mm, 30 mm from sides).

  • Give hole dimension as "⌀20".

  • Overall size must also be mentioned (100 × 60).


3.5 Summary of Chapter

  • Lines are of various types (continuous, dashed, chain, thick, thin) with specific meanings.

  • Lettering must be uniform and legible; Gothic vertical lettering is recommended.

  • Dimensioning conveys the size, shape, and position; must follow BIS rules.

  • Proper placement of dimensions avoids confusion and ensures accurate manufacturing.



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