Technical Drawing Not³es

Technical Drawing

Elements of Projection

  • Object
  • Plane of projection
  • Point of sight
  • Rays of sight

Orthographic Projection

  • Point of sight is at infinity.
  • Rays of sight are parallel and intersect the plane at right angles.

Planes of Projection

  • Six directions: top, front, right side, left side, rear, and bottom.

Orthographic vs. Pictorial Drawings

  • Pictorial drawings: show three views (height, width, depth) simultaneously.
  • Orthographic drawings: show a two-dimensional view at a time; require at least two views to show an object's exact shape.
  • Orthographic views: perpendicular projections from the object to the plane of projection.

Scaling

  • Enlarged Scale: Drawing is proportionally bigger than the actual size (e.g., 2:1, 1.5:1, 5:1).
  • Scaled drawings must specify original dimensions, not scaled dimensions.
  • Scales should be indicated in or near the title block; use "scales as shown" if multiple scales are used.

Engineering Lettering

  • Use single-stroke Gothic lettering.
  • Lettering should be plain and legible.
  • Poor lettering detracts from the drawing's appearance.
Letter Proportions
  • C=L=NC = L = N
  • s=23Cs = \frac{2}{3} C
  • Where:
    • CC = capital letters
    • LL = long stem letters
    • NN = numerals
    • ss = short stem letters
  • Example:
    • If C=6C = 6 mm, then s=23×6=4s = \frac{2}{3} \times 6 = 4 mm

Guidelines for Lettering

  • Use light horizontal lines (capital line, waist line, base line, drop line) for uniform letter height.
  • Capital line and base line for capitals only; add waist and drop lines for lowercase letters.

Fractions

  • Horizontal bar separates numerator and denominator.
  • Clearance between numerator/denominator and bar.
  • Numerator and denominator are equal in size, 2/3 the height of the integer.
  • The bar is proportional to the width of the numerator and the denominator.

Letter Composition

  • Uniform style, height, width, slope (75° inclination), and spacing.
  • Consistent spacing between letters and words.

Lines in Technical Drawings

  • Lines are part of a standardized graphic language.
  • Each line type has a precise symbolic meaning.

Order of Priority for Coinciding Lines

  1. Visible outlines (Continuous thick lines, type A).
  2. Hidden outlines (Dashed line, type E).
  3. Cutting planes (Chain thin, thick at ends, type H).
  4. Centre lines and lines of symmetry (Chain thin line, type G).
  5. Centroidal lines (Chain thin double dashed line, type K).
  6. Projection lines (Continuous thin line, type B).

Drawing Sheet - ISO A-Series

  • Based on A0 sheet (1 m2m^2, width-to-length ratio of 1:2\sqrt{2}).
  • Smaller sheets obtained by halving the basic sheet.

Title Blocks

  • General information source: drawing title, student's name, instructor, course, school, scale, date.
  • Located in the bottom right-hand corner.

Drawing Objectives

  • Legibility, Neatness, Accuracy, and Speed.