Drawing Conventions: Lines, Lettering, Dimensioning Systems & Symbols

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1. Standard Types of Lines and Their Applications (BIS SP 46: 2003 / ISO 128)

Lines in engineering drawing are characterized by their thickness ratio (standard thick-to-thin ratio is $2 : 1$, typically $0.7\text{ mm}$ thick and $0.35\text{ mm}$ thin).

Type A: ────────────────────────────────────── (Continuous Thick: 0.7 mm)
Type B: ────────────────────────────────────── (Continuous Thin: 0.35 mm)
Type C: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (Continuous Thin Freehand)
Type D: ──────────\/\/\──────────\/\/\──────── (Continuous Thin Zig-Zag)
Type E: ─── ─── ─── ─── ─── ─── ─── ─── ───    (Dashed Thick: 0.7 mm)
Type F: --- --- --- --- --- --- --- --- ---    (Dashed Thin: 0.35 mm)
Type G: ─────── · ─────── · ─────── · ───────  (Long Dash-Dotted Thin)
Type H: █────── · ─────── · ─────── · ──────█  (Dash-Dotted Thin, Thick Ends)
Type K: ─────── · · ─────── · · ─────── · · ── (Long Dash Double-Dotted Thin)

Line Type	Appearance	Description & Thickness	Specific Applications
A	Continuous Thick	Thick ($0.5\text{ to }0.7\text{ mm}$)	Visible outlines, visible edges, crests of screw threads.
B	Continuous Thin	Thin ($0.25\text{ to }0.35\text{ mm}$)	Dimension lines, extension lines, leader lines, section/hatching lines, imaginary intersection lines.
C	Continuous Thin Freehand	Thin, irregular freehand	Limits of partial or interrupted views and sections (short breaks).
D	Continuous Thin with Zig-Zags	Thin with straight zig-zags	Long-break lines for shafts, pipes, and structural beams.
E / F	Dashed (Medium/Thin)	Dashes $\approx 3\text{ mm}$, gaps $\approx 1\text{ mm}$	Hidden outlines and hidden edges.
G	Chain Thin (Long Dash-Dot)	Long dash $10\text{–}12\text{ mm}$, dot, gap $1\text{ mm}$	Center lines, lines of symmetry, pitch circle diameters of gears/holes, path of motion.
H	Chain Thin, Thick at Ends	Thin chain with thick ends & change of direction	Cutting planes (indicates section view direction).
J	Chain Thick	Thick chain line	Indication of surfaces receiving special treatment (heat treatment, plating).
K	Chain Thin Double-Dotted	Long dash, two dots, long dash	Outlines of adjacent parts, extreme positions of movable links, centroidal lines.

Precedence / Priority of Coinciding Lines:

When two or more lines of different types coincide in a drawing, the following order of priority applies:

1. Visible outlines (Type A) — Highest priority
2. Hidden outlines (Type E/F)
3. Cutting planes (Type H)
4. Center lines and lines of symmetry (Type G)
5. Centroidal lines (Type K)
6. Dimension and extension lines (Type B) — Lowest priority

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2. Engineering Lettering (BIS IS 9609 / ISO 3098)

Lettering conveys information regarding title, dimensions, tolerances, and manufacturing notes. It must be uniform, legible, and suitable for microfilming/scanning.

General Conventions:

• Style: Single-stroke commercial gothic letters (vertical or inclined 75° to horizontal).
• Proportion:
  • Height ($h$): Standard nominal sizes: $1.8, 2.5, 3.5, 5, 7, 10, 14, 20\text{ mm}$.
  • Main Title: $7\text{ or }10\text{ mm}$.
  • Sub-titles & headings: $5\text{ or }7\text{ mm}$.
  • Dimensions, notes, and general text: $2.5\text{ or }3.5\text{ mm}$.
• Spacing Guidelines (for Lettering Type B, thickness $d = h/10$):
  • Space between characters = $2d = 0.2h$
  • Space between words = $6d = 0.6h$
  • Space between base lines = $14d \text{ to } 20d = 1.4h \text{ to } 2.0h$

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3. Systems of Dimensioning

There are two standard methods of placing dimension numbers on drawings:

A. Aligned System (IS 11669)

• Dimension values are placed parallel to and above the dimension lines (not touching the line).
• Values must be readable from the bottom edge or the right-hand edge of the drawing sheet.
• Widely used in civil and structural engineering drawings.

       Aligned System
       
            |<-  30  ->|
            +----------+
            |          | ^
            |          | | 50 (reads from right)
            |          | v
            +----------+
            |<-  30  ->|

B. Unidirectional System

• All dimension values are oriented horizontally, regardless of the inclination of the dimension line.
• Dimension lines are interrupted (broken) in the middle to insert the value.
• Values are readable only from the bottom of the sheet.
• Preferred in modern mechanical and production drafting.

       Unidirectional System
       
            |-- 30 ----|
            +----------+
            |          | ^
            |          | |-- 50 --| (horizontal text)
            |          | v
            +----------+

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4. Standard Engineering Symbols & Abbreviations

Symbol / Abbreviation	Meaning / Definition	Standard Example
$\boldsymbol{\varnothing}$	Diameter of a cylindrical feature	$\varnothing 25$
$\boldsymbol{R}$	Radius of an arc or fillet	$R12$
$\boldsymbol{S\varnothing}$	Spherical diameter	$S\varnothing 40$
$\boldsymbol{SR}$	Spherical radius	$SR20$
$\boldsymbol{\square}$	Square cross-section	$\square 30$
$\boldsymbol{\times}$	Repetition or chamfer	$4 \times \varnothing 10$ or $2 \times 45^\circ$
PCD	Pitch Circle Diameter (holes on a circular path)	$\text{PCD } 100$
CSK	Countersunk hole	$\text{CSK } \varnothing 16 \times 90^\circ$
CBORE	Counterbored hole	$\text{CBORE } \varnothing 18 \downarrow 6$
CL or $\boldsymbol{\mathbb{E}}$	Centerline	Placed at axis ends
TYP	Typical (applies to all identical instances)	$R3 \text{ TYP}$

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5. Summary of Key Exam Points

1. Cutting Plane Line: Chain thin line with thick ends (Type H).
2. Hidden Edges: Dashed line (Type E/F), dash length $\approx 3\text{ mm}$, spacing $\approx 1\text{ mm}$.
3. Inclined Lettering Angle: Standard is 75° forward tilt with the horizontal.
4. Aligned vs Unidirectional: Aligned reads from bottom and right; Unidirectional reads strictly from bottom with broken dimension line.
5. Arrowhead Geometry: Solid black, $3:1$ length-to-depth proportion.