Technical Drawing Guide: Understanding Projections, Sections, and Dimensioning

Posted on May 24, 2024 in Visual arts

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FORMATS

  • Drawing sheets used for technical drawings.
  • Special characteristics:
    • Margins
    • Dimensions
    • Labeling table
    • Orientation signs
    • Centering marks
    • Folding marks
  • Standardized by UNE noma: 1026-2 83 PART 2.

SYSTEM

  • Most common system: A-series (A0, A1, A2, A3, A4, A5, etc.)
  • A0 dimensions: 1189 mm x 841 mm
  • Subsequent formats obtained by halving the longest side of the previous format.

Dimension:
A0: 841 x 1189
A1: 594 x 841
A2: 420 x 594
A3: 297 x 420
A4: 210 x 297
A5: 148 x 210
A6: 105 x 148
A7: 74 x 105
A8: 52 x 74
A9: 37 x 52
A10: 26 x 37

MARGINS

  • Minimum margins:
    • 20 mm for A0 and A1 formats
    • 10 mm for A2, A3, and A4 formats
  • Side folding margin: 20 mm

TITLE BLOCK

  • Designated area for placement of:
    • Legends
    • Addresses
    • Titles
    • Relevant information

FOLDING

  • Standardized for storage and transportation.

REPRESENTATION SYSTEMS

  • Purpose: Represent three-dimensional objects on a two-dimensional surface.
  • Based on the projection of objects onto a flat plane.

CYLINDRICAL ORTHOGONAL PROJECTION

  • Cylindrical Orthogonal:
    • Projecting rays are parallel and perpendicular to the projection plane.
    • All dimensions are represented in true magnitude.
    • Used for designing parts and machinery.
  • Cylindrical Oblique:
    • Projecting rays are parallel but at an angle other than 90° to the projection plane.
    • Dimensions of certain edges may be scaled.
  • Multiple Views:
    • Uses minimum number of views to represent the object.
    • Main views:
      • FRONT: Elevation
      • TOP: Plan
      • SIDE: Right Profile, Left Profile
      • BOTTOM: Bottom
      • BACK: Rear

REPRESENTATION SYSTEMS

  • European System:
    • Front view is shown behind the object.
  • American System:
    • Front view is shown in front of the object.

OBLIQUE CYLINDRICAL PROJECTIONS

  • Projecting rays form a 45° angle with the projection plane.
  • No scaling or reduction coefficients are applied.
  • Used to represent edges in their true length.
  • Does not accurately represent the foreshortening effect.

AXONOMETRIC PROJECTION (OBLIQUE)

  • Projection plane is not parallel to any of the object’s three axes.
  • Types:
    • Isometric Projection:
      • Three axonometric axes form equal angles.
      • All three isometric axes use the same scale.
      • Allows for direct measurement of dimensions from the views.
    • Dimetric Projection:
      • Two of the three axonometric axes form equal angles.
      • Two axes use the same scale, the third axis uses a different scale.
      • Provides a somewhat realistic view but makes it difficult to obtain true dimensions.
    • Trimetric Projection:
      • All three axonometric axes form different angles.
      • Each axis has its own scale.
      • Provides a dynamic and realistic view but makes it difficult to obtain edge dimensions.

CONIC PROJECTION

  • Provides a view similar to observing the object from a relatively close point.
  • Used for architectural design.
  • Dimensions of most edges are scaled.
  • Provides a more realistic representation.
  • Vanishing point: Point where lateral edges converge to create depth.
  • Types:
    • One-Point Perspective:
      • Projection plane is parallel to one side of the object.
      • Front view is in true magnitude.
    • Two-Point Perspective:
      • Projection plane is parallel to two of the object’s axes.
      • Only one edge is in true scale.
    • Three-Point Perspective:
      • Projection plane is not parallel to any of the object’s axes.
      • None of the edges are in true scale.

STANDARD SELECTION

  • Refer to NORMA UNE: 1-032-82.
  • Choose the most characteristic view of the object as the primary view.
  • Select views that minimize the number of hidden edges.
  • Prioritize views that provide the most simplified representation.

CUTTING AND SECTIONS

CUT

  • Representation technique that removes a portion of the object to reveal internal features.
  • Cut surface is shown with hatching lines.
  • Types of cuts:
    • Full Section
    • Half Section
    • Offset Section
    • Broken-Out Section
    • Revolved Section
    • Removed Section

SECTION

  • View showing the intersection of a cutting plane with the object.
  • Does not show the portion of the object behind the cutting plane.
  • Provides a clearer and more direct representation.

BREAK LINES

  • Used to shorten the representation of long and uniform objects.
  • Advantages:
    • Saves drawing space.
    • Eliminates repetitive details.
    • Simplifies the representation.
    • Defines the extent of the object.

DIMENSIONS

GENERAL PRINCIPLES

  • Dimensions should be minimal, sufficient, and appropriate for manufacturing.

GENERAL DIMENSIONING GUIDELINES

  1. Indicate each dimension only once, unless repetition is necessary.
  2. Do not omit any dimensions.
  3. Place dimensions on views that clearly show the elements being dimensioned.
  4. Use consistent units for all dimensions, unless clarity requires otherwise.
  5. Do not dimension features that are the result of manufacturing processes.
  6. Place dimensions outside the object, unless clarity is improved by placing them inside.
  7. Avoid dimensioning to hidden edges, unless additional views are necessary.
  8. Distribute dimensions logically, clearly, and aesthetically.
  9. Indicate hole depths and diameters in the same view.
  10. Avoid chain dimensioning, as it can lead to errors.

DIMENSION ELEMENTS

  • Dimension line
  • Dimension value
  • Dimension arrowheads

VIEWS

  • Orthogonal projections of an object from different viewpoints.
  • Types:
    • Oblique: Perspective view where projecting rays form a 45° angle.
    • Isometric: View where all three dimensions are drawn to the same scale.
    • Perspective: Representation of an object as it appears to the eye from a specific viewpoint.