Drawing Fundamentals and Scale

Engineering Drawing Fundamentals and Scale

Engineering drawing is often referred to as the universal language of engineers. It is a highly formalized system of graphical communication used to precisely define the requirements for engineered items.

Drawing as Technical Communication

Unlike artistic drawing, which is subjective and intended to evoke emotion or interpretation, an engineering drawing is objective. Its primary purpose is to capture all the geometric features of a product or component so a manufacturer or builder can produce it exactly as intended, without ambiguity.

A complete engineering drawing provides authoritative information regarding:

  • Geometry: The exact shape and features of the object.
  • Dimensions: The precise physical size of the object.
  • Tolerances: The permissible variation in the physical dimensions.
  • Material & Finish: What the object is made of and the required surface quality.

To ensure this communication is universally understood, engineering drawings adhere to strict international or national standards (such as ASME or ISO). These standards dictate how lines, views, and symbols are applied, ensuring that a drawing created in one country can be accurately interpreted and manufactured in another.

Fundamental Concept of Drawing Scale

Because engineered objects range in size from microscopic electronic components to massive bridges and skyscrapers, it is rarely practical to draw an object at its actual physical size. Therefore, engineering drawings are scaled.

Scale is the ratio of the linear dimension of an element of an object as represented in the original drawing to the real linear dimension of the same element of the object itself.

Common Scale Types

  1. Full Scale (1:1):

    • The drawing is exactly the same size as the real object.
    • Example: A small bracket may easily fit on a standard sheet of paper at a 1:1 scale.

  2. Enlarged Scale (e.g., 2:1, 10:1):

    • The drawing is larger than the real object.
    • Used for very small components where detail would be difficult to see or dimension at actual size. A 10:1 scale means the drawing is ten times larger than the real part.

  3. Reduced Scale (e.g., 1:2, 1:100):

    • The drawing is smaller than the real object.
    • Used for large objects like machinery, buildings, or geographical layouts. A 1:100 scale means 1 unit of measurement on the drawing represents 100 units on the actual object.

Rules of Scale in Dimensioning

A critical rule in engineering drawing is that the dimensions written on the drawing always state the actual physical size of the object, regardless of the drawing's scale.

  • If a drawing is scaled 1:2 (half size), a feature that is physically 100 mm long will be drawn 50 mm long on the paper.
  • However, the dimension text placed next to that feature must explicitly state 100 mm.
  • A manufacturer should never attempt to measure the printed drawing with a ruler to determine a size; they must always read and rely on the written dimensions.

The overall scale of the drawing is always prominently noted in the drawing's Title Block. If a specific view or detail on the sheet uses a different scale than the main drawing, that specific scale must be clearly noted immediately below the view's title.