What are the rules for dimensioning an orthographic projection?

Short Answer:

In orthographic projection, dimensioning means adding accurate measurements to describe the size, position, and features of a part. To do this properly, certain rules must be followed so that the drawing is clear, correct, and easy to understand.

The main rules include placing dimensions outside the view, avoiding repetition, using standard symbols, keeping proper spacing, and showing all necessary lengths, angles, holes, and threads. Following these rules ensures that the drawing can be easily read by machinists, engineers, and technicians for manufacturing and inspection.

Detailed Explanation:

Rules for Dimensioning an Orthographic Projection

Orthographic projection is a method used to show multiple 2D views (like front, top, and side) of a 3D object. These views represent the true shape and size of each part of the object. To make these views useful for manufacturing or fabrication, we must add dimensions using proper rules and standards.

Dimensioning is not just about adding numbers. It is about presenting information in a neat, readable, and accurate way. The rules of dimensioning make sure that anyone using the drawing can understand the size, location, and features without confusion.

Important Rules of Dimensioning in Orthographic Projection

  1. Dimensions Must Be Placed Clearly
  • Always place dimensions outside the object view if possible.
  • Keep dimensions away from hidden lines or crowded areas.
  • Make sure the dimension lines are neat and do not cross each other unnecessarily.
  1. Avoid Repetition of Dimensions
  • Do not repeat the same dimension in more than one view.
  • Show each dimension only once where it is most clear.
  • Repeated dimensions make the drawing cluttered and confusing.
  1. Use Standard Dimension Lines and Symbols
  • Use thin solid lines with arrowheads for dimension lines.
  • Arrowheads should be neat and point to the extension lines.
  • Use standard symbols like:
    • Ø for diameter
    • R for radius
    • ± for tolerance
    • M for metric thread
  • Follow BIS/ISO/ANSI standards based on your country or company rules.
  1. Maintain Proper Spacing
  • Keep a uniform gap between object and dimension line (usually 10mm).
  • Maintain space between multiple dimension lines (at least 6mm apart).
  • This makes the drawing clean and easy to read.
  1. Dimension in Actual Size (True Length)
  • Always measure and write the real size, not the scaled-down drawing size.
  • All dimensions should reflect the actual part size, even if the drawing is reduced.
  1. Place Dimensions Between Views When Useful
  • If two views are next to each other, place the dimension between them to save space and improve clarity.
  • Choose the most appropriate view to show a particular feature.
  1. Show All Necessary Features
  • Include:
    • Length, width, and height
    • Hole sizes, positions, and types
    • Thread details
    • Fillet radii, chamfers, slots, and angles

Avoid missing any detail that is needed for manufacturing or checking.

  1. Use Aligned or Unidirectional Method
  • Aligned method: Dimensions are aligned with the feature and readable from the bottom or right side.
  • Unidirectional method: All dimensions are written horizontally and read from the bottom only.
  • Choose one method and use it consistently in the entire drawing.
  1. Avoid Dimensioning Hidden Lines
  • Always try to dimension visible features.
  • If it’s necessary to show hidden parts, use sectional views instead.
  • This improves clarity.
  1. Use Decimal or Fractional Units Clearly
  • Write dimensions with clear units (mm, cm, inches).
  • If using decimals, be consistent (e.g., 25.0, not just 25).
  • Avoid mixing metric and imperial units in the same drawing.
Conclusion

Following the rules of dimensioning in orthographic projection is very important in mechanical engineering. These rules ensure that the drawing is clear, accurate, standardized, and easy to understand by anyone using it. Correct dimensioning helps avoid mistakes in manufacturing, assembly, and inspection, and saves time and cost. A well-dimensioned drawing acts like a blueprint for production and supports smooth communication between design and workshop teams.