import json
import os

os.makedirs(os.path.join('content', 'Sem1', 'MEC136', 'unit1'), exist_ok=True)

notes_md = """---
unitNumber: 1
unitTitle: "Introduction to Engineering Drawing"
readTime: "40 min read"

Unit 1: Introduction to Engineering Drawing

1. Why It Matters: The Universal Graphic Language of Engineering

Engineering drawing is the formal graphic language employed by engineers, architects, and industrial designers across the globe to communicate technical intent, spatial geometry, and manufacturing tolerances. While spoken languages are subject to regional ambiguity and translation errors, standardized technical drawings communicate unambiguous physical specifications that can be interpreted and manufactured with micron-level precision anywhere on Earth.

CODECopy Code
+-------------------------------------------------------------------------+
|                  THE TECHNICAL COMMUNICATION PIPELINE                   |
+-------------------------------------------------------------------------+
| Design Conceptualization -> Standardized Graphic Modeling -> Production |
|   (Ideation & CAD)       |   (ISO/BIS Standards, Tolerances)   |  (CAM/CNC, Fab) |
+-------------------------------------------------------------------------+

Mastering the foundational tools, line types, dimensioning systems, scales, and Computer-Aided Design (CAD) environments is essential for transforming mental concepts into physical reality.

2. Drawing Instruments and Sheet Standards

Precision technical drawings require standardized tools and designated drawing sheets to maintain geometric accuracy.

2.1 Essential Drawing Instruments

• Drawing Board: Manufactured from seasoned soft woods (such as pine or fir) to prevent warping and allow paper fixing pins or drafting tape. Standard sizes range from $D_0$ ($1500 \\times 1000\\text{ mm}$) to $D_3$ ($500 \\times 350\\text{ mm}$).
• Mini-Drafter: A modern articulated instrument combining the functions of a T-square, set-square, protractor, and scales into a single clamped drafting arm. Its two graduated blades are fixed at exactly 90^\\circ.
• Set-Squares (Triangles): Two transparent celluloid triangles—the 45^\\circ triangle (45^\\circ-45^\\circ-90^\\circ) and the 30^\\circ-60^\\circ triangle (30^\\circ-60^\\circ-90^\\circ)—used independently or in combination to project angles in 15^\\circ increments.
• Compass and Dividers: Precision instruments with steel points used for drawing circles, arcs, and transferring dimensional measurements without scaling distortion.

2.2 Standard Drawing Sheet Sizes (ISO 216 / BIS SP 46)

Standard drawing sheets follow the geometric relationship where the surface area of the base size ($A_0$) is exactly $1\\text{ m}^2$, and the ratio of width to length is strictly $1 : \\sqrt{2}$:

Relationship: Halving an $A_n$ sheet parallel to its shorter edge yields the next smaller sheet size $A_{n+1}$.

2.3 Sheet Layout and Title Block

• Margins: A border line of $20\\text{ mm}$ width on the left-hand filing edge (to accommodate binding) and $10\\text{ mm}$ on the remaining three edges for $A_2, A_3, A_4$ sheets ($20\\text{ mm}$ on all edges for $A_0, A_1$).
• Title Block: Located strictly at the bottom right-hand corner of every drawing sheet. Standard dimensions specified by Bureau of Indian Standards (BIS) are $185\\text{ mm} \\times 65\\text{ mm}$, containing legal metadata: Course/Company Name, Title of Drawing, Sheet Number, Scale, Date, Projection Symbol, and Approvals.

3. Pencil Hardness Grades and Line Conventions

3.1 Pencil Hardness Spectrum

Drafting leads consist of a kiln-fired mixture of graphite and kaolin clay. Increasing the clay proportion increases hardness and lightness; increasing graphite increases softness and darkness:

$\\underbrace{9\\text{H} \\quad 8\\text{H} \\quad 7\\text{H} \\quad 6\\text{H} \\quad 5\\text{H} \\quad 4\\text{H}}_{\\text{Extremely Hard / Faint (Layouts)}} \\quad \\longleftrightarrow \\quad \\underbrace{3\\text{H} \\quad 2\\text{H} \\quad \\text{H}}_{\\text{Medium Hard (Construction)}} \\quad \\longleftrightarrow \\quad \\underbrace{\\text{HB} \\quad \\text{F}}_{\\text{Standard (Outlines, Text)}} \\quad \\longleftrightarrow \\quad \\underbrace{\\text{B} \\quad 2\\text{B} \\dots 7\\text{B}}_{\\text{Soft / Very Dark (Artistic)}}$

• $2\\text{H}$ / $3\\text{H}$: Used for light construction lines, projection rays, dimension extension lines, and center line paths.
• $\\text{H}$: Used for visible outlines, finished geometric features, section hatching, and dimension lines.
• $\\text{HB}$: Used for lettering, dimension numerals, title block entries, and prominent arrowheads.

3.2 Standard Line Types (ISO 128 / BIS SP 46)

Precedence of Coinciding Lines: Visible Outlines $>$ Hidden Lines $>$ Cutting Plane Lines $>$ Center Lines $>$ Projection Lines.

4. Engineering Lettering and Dimensioning Systems

4.1 Engineering Lettering Standards

• Style: Single-stroke sans-serif Gothic lettering executed vertically (90^\\circ) or inclined at 75^\\circ to the horizontal.
• Legibility Rules: All characters must be uniform in height, stroke width, and spacing. Lowercase letters are avoided on general production drawings except for standard metric abbreviations (e.g., $\\text{mm}$).
• Nominal Heights ($h$): Standardized as $2.5, 3.5, 5, 7, 10, 14, 20\\text{ mm}$. Main titles typically use $7\\text{ mm}$ or $10\\text{ mm}$; subheadings use $5\\text{ mm}$; dimension numerals and notes use $3.5\\text{ mm}$.

4.2 Anatomy of a Dimension

1. Dimension Line: Continuous thin line indicating the extent and direction of the measured distance, terminated by arrowheads.
2. Extension Line (Projection Line): Continuous thin line projected perpendicular to the measured feature, extending roughly $2-3\\text{ mm}$ beyond the arrowhead.
3. Leader Line: Continuous thin line directing a note, label, or callout to a specific feature, terminated with an arrowhead (pointing to an edge) or a dot (placed on a face).
4. Arrowhead: Standardized closed and filled triangular tip with an aspect ratio of $3 : 1$ (length to maximum base width).

CODECopy Code
   Extension Line                                Extension Line
        │                                             │
        │<────────────── 50 mm ──────────────────────>│
        │▲                                           ▲│
        ││              Dimension Line               ││
   ┌────┴┴───────────────────────────────────────────┴┴────┐
   │                                                       │
   │                    Engineering Feature                │

4.3 Dimensioning Systems: Aligned vs. Unidirectional

5. Engineering Scales and Representative Fraction (RF)

A scale is the ratio between the linear dimensions of an element as represented on a drawing and the actual physical dimensions of the corresponding object.

5.1 Representative Fraction (RF)

The Representative Fraction is a dimensionless ratio defined as:

$\\text{RF} = \\frac{\\text{Dimension of object on drawing}}{\\text{Actual physical dimension of object}} = \\frac{\\text{Length on Drawing}}{\\text{Actual Length}}$

Rule: Both numerator and denominator must be converted to identical units before calculating RF.

5.2 Classification of Scales

1. Full Size Scale ($\\text{RF} = 1:1$): Object is drawn to its exact physical size.
2. Reducing Scale ($\\text{RF} < 1$): Used for large objects (buildings, aircraft, engine blocks). Standard ratios: $1:2, 1:5, 1:10, 1:50, 1:100, 1:500$.
3. Enlarging Scale ($\\text{RF} > 1$): Used for micro-components (watch gears, semiconductor dies, precision surgical needles). Standard ratios: $2:1, 5:1, 10:1, 50:1$.

5.3 Types of Scales in Technical Drafting

• Plain Scale: Reads two consecutive metric units (e.g., meters and decimeters, or centimeters and millimeters).
• Diagonal Scale: Utilizes the principle of similar triangles to read three consecutive units (e.g., meters, decimeters, and centimeters) or measurements up to two decimal places.
• Vernier Scale: Employs a primary scale and a sliding fractional vernier scale to achieve micro-fractional readings.

6. Introduction to Computer-Aided Design (AutoCAD)

Computer-Aided Design (CAD) replaces physical drawing boards with digital vector geometry engines, enabling parametric modeling, layer separation, and automated precision.

CODECopy Code
+-------------------------------------------------------------------------+
|                  AUTOCAD DIGITAL DRAFTING ENVIRONMENT                   |
+-------------------------------------------------------------------------+
| Graphics Screen | User Coordinate System (UCS) Icon | Command Line Prompt|
| Ribbon Toolbars | Object Snaps (OSNAP)              | Layer Properties   |
+-------------------------------------------------------------------------+

6.1 AutoCAD Coordinate Systems

All geometric entities are positioned relative to the Cartesian coordinate plane $(X, Y, Z)$:

1. Absolute Cartesian Coordinates ($X, Y$): Points are referenced directly from the global origin $(0,0)$. Example: 100,75.
2. Relative Cartesian Coordinates (@ΔX,ΔY): Points are positioned relative to the previous point entered. Example: @50,30 moves $50$ units right and $30$ units up.
3. Relative Polar Coordinates (@Distance<Angle): Points are defined by a linear distance and an angular inclination measured counter-clockwise from the positive X-axis. Example: @100<45.

6.2 Essential AutoCAD Command Taxonomy

7. Common Pitfalls & Exam Tips

• Incorrect Dimensioning Spacing: Dimension lines must never be drawn too close to object outlines. Maintain at least $8-10\\text{ mm}$ clearance from outlines and $6-8\\text{ mm}$ between parallel dimension lines.
• Crossing Extension and Dimension Lines: Extension lines may cross each other, but dimension lines should never cross other lines. Place smaller dimensions closer to the object and larger overall dimensions further out.
• RF Unit Conversion Errors: Always convert kilometers, meters, or centimeters to millimeters before evaluating Representative Fractions ($1\\text{ m} = 1000\\text{ mm}$; $1\\text{ km} = 10^6\\text{ mm}$).
• Precedence Confusion: When a hidden line coincides with a visible outline, the visible outline always takes visual precedence.

8. Quick Revision Summary

• Standard Sheet Sizes: $A_0$ is $1\\text{ m}^2$ ($841 \\times 1189\\text{ mm}$) with a $1 : \\sqrt{2}$ aspect ratio; halving yields $A_1, A_2, A_3, A_4$.
• Title Block: Standard dimensions are $185 \\times 65\\text{ mm}$, placed at the bottom-right corner.
• Pencil Grades: $2\\text{H}/3\\text{H}$ for construction, $\\text{H}$ for outlines/dimensioning, $\\text{HB}$ for text and arrows.
• Line Standards: Continuous thick for visible edges; dashed thin for hidden details; chain thin for centerlines.
• Dimensioning: Arrowheads follow a $3:1$ ratio; Aligned system places text along dimension lines, Unidirectional places text horizontally.
• Scales: $\\text{RF} = \\text{Drawing Length} / \\text{Actual Length}$; plain scales read 2 units, diagonal scales read 3 units.
• AutoCAD: Uses Absolute ($X,Y$), Relative (@ΔX,ΔY), and Polar (@Dist<Angle) coordinate systems controlled via commands and OSNAP precision modes.
  """

with open(os.path.join('content', 'Sem1', 'MEC136', 'unit1', 'notes.md'), 'w', encoding='utf-8') as f:
f.write(notes_md.strip() + '\n')

print("Wrote notes.md for MEC136 Unit 1")