Unit 1: Introduction to Engineering Drawing

1. Conceptual Framework of Drawing Instruments

Engineering drawing is the universal language of engineers. Unlike artistic drawing, which expresses aesthetics, engineering drawing is a graphical language used to convey precise information required for the construction or analysis of machines and structures.

1.1 Classification of Instruments

The quality of a drawing depends on the quality, adjustment, and manipulation of the instruments.

• Mini-Drafter (Drafting Machine): Combines the functions of a T-square, set-square, scales, and protractor. It consists of a parallelogram linkage that keeps the scales parallel to a set datum.
  • Function: Used to draw horizontal, vertical, and inclined parallel lines and to measure angles.
• Drawing Board: Made of well-seasoned soft wood (pine or fir) with a smooth, flat surface. It holds the drawing paper.
• Set Squares: Usually made of transparent plastic. Two types exist: and . They are used to draw specific angles and vertical/inclined lines relative to the T-square or drafter.
• Compass:
  • Large Compass: Used for drawing circles with a radius greater than 25mm.
  • Bow Compass: Used for small circles (radius < 25mm) to ensure stability and precision.
• Dividers: Used to transfer dimensions from the scale to the drawing or to divide lines into equal parts. They do not mark the paper but prick points.
• Pencils (Grading): Pencils are graded by hardness.
  • H-Grades (Hard): Used for construction lines and guidelines. Light lines. (4H, 3H, 2H).
  • B-Grades (Black/Soft): Used for artistic work or rough sketches. (HB, B, 2B).
  • Standard Usage:
    • 2H: Construction lines, center lines, section lines (Thin).
    • H: Lettering, dimensions, arrowheads.
    • HB: Visible outlines, boundary lines (Thick/Dark).

2. Line Types (The Alphabet of Lines)

In engineering drawing, different lines represent different features. The variation is based on thickness (weight) and style (dashed, continuous, etc.).

3. Dimensioning

Dimensioning is the process of specifying the part's size and location on the drawing.

3.1 Elements of Dimensioning

• Extension Line: Extends from the object to indicate the points between which the dimension applies. Must have a 1-2mm gap from the object outline.
• Dimension Line: Drawn between extension lines with arrowheads at ends.
• Leader Line: An inclined line with a horizontal tail and an arrowhead, used to label circles or features.
• Arrowheads: Triangular shape, filled. Standard ratio of length to width is 3:1.

3.2 Systems of Dimensioning

1. Aligned System:
   • Dimensions are placed perpendicular to the dimension line.
   • Readable from the bottom or the right-hand side of the sheet.
   • Used in general engineering drawings.
2. Unidirectional System:
   • Dimensions are broken in the middle of the dimension line or placed above it.
   • All text is upright and readable from the bottom of the sheet only.
   • Common in large automotive/aerospace drawings.

3.3 General Rules

• Never cross dimension lines with other dimension lines.
• Place dimensions outside the view whenever possible.
• Small dimensions should be placed nearer to the view than large dimensions.
• Do not repeat dimensions.

4. Single Stroke Vertical Gothic Lettering

Lettering refers to writing titles, dimensions, and notes on a drawing. It must be legible, uniform, and capable of rapid execution.

• Single Stroke: Means the width of the line is the same as the thickness of the pencil point (no varying thick/thin strokes like calligraphy).
• Vertical Gothic: Refers to sans-serif (no "feet" on letters) and upright orientation ( to the horizontal).

4.1 Rules and Proportions

• Height (h): Standard heights are 2.5, 3.5, 5, 7, 10, 14, and 20 mm.
• Aspect Ratio:
  • Most letters are approx. 7:5 or 6:5 (Height : Width).
  • Exceptions:
    • W and M are the widest (approx. 7:8).
    • I is the narrowest.
• Spacing: The space between letters should appear visually equal (area balancing), not mechanically equal.
• Guidelines: Always draw very light (4H) horizontal guidelines to maintain uniform height.

5. Scales

Scales are used to represent large objects on a small sheet (Reduction) or small objects on a large sheet (Enlargement).

5.1 Representative Fraction (R.F.)

The ratio of the length of the object on the drawing to the actual length of the object.

• Full Scale (1:1): R.F. = 1
• Reducing Scale (1:10, 1:50): R.F. < 1
• Enlarging Scale (10:1, 2:1): R.F. > 1

Note: Both numerator and denominator must be in the same units before calculating R.F.

5.2 Plain Scale

• Used to measure up to two units (e.g., Decimeters and Centimeters) or a unit and its first subdivision.
• Consists of a line divided into equal main divisions (representing the primary unit) and the first division subdivided (representing the secondary unit).
• Reading: The zero is placed at the end of the first main division. Main units are read to the right of zero; subdivisions are read to the left.

5.3 Diagonal Scale

• Used to measure up to three units (e.g., Meters, Decimeters, and Centimeters) or up to two decimal places.
• Principle of Diagonal Scale (Similar Triangles):
  • To divide a short vertical line into small parts (e.g., 10 parts), a diagonal is drawn.
  • Moving horizontally across the diagonal changes the vertical height proportionally.
• Structure:
  • Horizontal movement: Main units.
  • Sub-division movement (left of zero): Secondary units.
  • Vertical movement: Tertiary units (diagonal division).

6. Introduction to AutoCAD Interface

AutoCAD (Computer-Aided Design) replaces manual drafting tools with digital precision.

6.1 Interface Components

• Application Button (Top Left): File management (New, Open, Save, Print).
• Quick Access Toolbar: Frequently used commands next to the icon.
• Ribbon: The main palette containing tabs (Home, Insert, Annotate) and panels (Draw, Modify, Layers).
• Drawing Area (Workspace): The infinite space where drawing occurs.
• Command Line: Located at the bottom. Used to type commands directly (e.g., LINE, CIRCLE). This is the most efficient way to use AutoCAD.
• UCS Icon: Shows X and Y orientation.
• Status Bar: Bottom right toggle buttons for drafting aids (Ortho, Osnap, etc.).

6.2 Setting Up a Drawing

Before drawing, the environment must be defined.

Units

Sets the unit of measurement and precision.

• Command: UNITS or UN
• Length Type: Decimal (Metric), Engineering/Architectural (Feet/Inches).
• Precision: Number of decimal places (e.g., 0.00).
• Insertion Scale: Controls scaling when blocks are inserted (e.g., Millimeters).

Limits

Sets the imaginary boundaries of the drawing area (essentially the paper size).

• Command: LIMITS
• Step 1: Specify lower-left corner (usually 0,0).
• Step 2: Specify upper-right corner (e.g., 210,297 for A4).
• Toggle: LIMITS -> ON (prevents drawing outside these bounds).

6.3 Navigation

How to move around the drawing area.

• Zoom: Enlarges or reduces the view (not the actual object size).
  • Command: Z (Enter).
  • Options: A (All), E (Extents - fits everything drawn on screen), W (Window).
  • Mouse Wheel: Scroll forward to zoom in, backward to zoom out.
• Pan: Shifts the view horizontally or vertically without changing magnification.
  • Command: P (Enter).
  • Mouse Wheel: Press and hold the scroll wheel to pan (Hand icon appears).

6.4 Coordinate Systems

• WCS (World Coordinate System): The fixed, default system where X is horizontal, Y is vertical, and Z is perpendicular to the screen. Origin is (0,0,0).
• UCS (User Coordinate System): A movable coordinate system defined by the user. Used for drawing on inclined planes or specific orientations.
  • Command: UCS.

6.5 Drafting Aids (Status Bar & Toggles)

These tools ensure precision.

• ORTHO Mode (F8):
  • Restricts cursor movement to perfectly horizontal (0°, 180°) or vertical (90°, 270°) directions.
  • Essential for drawing straight lines.
• OSNAP (Object Snap) (F3):
  • Magnetically snaps the cursor to precise geometric points on an existing object.
  • Types: Endpoint (square), Midpoint (triangle), Center (circle), Intersection (X), Perpendicular, Tangent.
  • Config: Type OS to select which points to snap to.

6.6 Function Keys (F-Keys Cheat Sheet)