FUNDAMENTAL RULES

The fundamental rules control the clarity and engineering intent of the dimensions on an engineering drawing. The fourteen fundamental rules are summarized below.

  1. Each dimension has a tolerance. Exceptions include; basic, reference maximum, minimum, or stock dimensions.
  2. Dimensioning and tolerancing shall be complete. Assumptions or measuring distances from the drawing to obtain missing dimensions is not allowed.
  3. No duplicate dimensions. The use of reference dimensions should be minimized.
  4. Dimensions should be selected and arranged based on the function and mating relationship of the part.
  5. Specifying manufacturing methods on a drawing should be avoided. However, where manufacturing, processing, quality assurance, or environmental information is essential to the definition of engineering requirements, it shall be specified on the drawing or in a document referenced on the drawing.
  6. It is permissible to identify as nonmandatory certain processing dimensions that provide for finish allowance, shrink allowance, and other requirements, provided the final dimensions are given on the drawing.
  7. Dimensions should be arranged to enhance readability.
  8. The decimal diameter or thickness should be stated for wires, cables, sheets, rods, and other materials manufactured to gage or code numbers. The gage or code number may be placed in parenthesis next to the decimal dimension.
  9. A 90o angle is assumed where center lines and lines depicting features are shown on a drawing at right angles and no angle is specified.
  10. A 90o basic angle is assumed where center lines of features in a pattern or surfaces shown at right angles on the drawing are located or defined by basic dimensions and no angle is specified.
  11. Unless otherwise specified, all dimensions are applicable at 20oC.
  12. All dimensions and tolerances apply in a free state (no applied forces) condition.
  13. Unless otherwise specified, all geometric tolerances apply for full depth, length, and width of the features.
  14. Dimensions and tolerances apply only at the drawing level where they are specified.