Dimensioning is a method of accurately communicating size information about objects and structures so they can be produced. After completing this chapter you will be able to apply the standard dimensioning practices for mechanical drawings, differentiate current ANSI standards and past practices for dimensioning.
Dimensioning Definition A dimension is a numerical value that defines the: Size and shape Position Surface texture or Geometric characteristic Of a feature Units The default dimensional units in most CAD systems are: Millimeters (mm) in the metric system Inches in the English system NOTE: • Dimensions in millimeters typically expressed as whole numbers • Dimensions in inches typically expressed to two decimal place accuracy Terminology NOTE: Dimension text heights are typically 3 mm, and are uniform throughout the drawing Leader – Line (w/ arrow) drawn from a feature to a note Reference dimension – Numerical value provided for information only and not used in fabrication of the part Dimensioning Guidelines for Prisms1 Do not repeat dimensions Dimension most descriptive view of feature being dimensioned Dimension between views Omit one intermediate dimension Smaller dimensions inside of larger dimensions Extension lines may cross object lines and other extension lines Dimensioning Guidelines for Prisms2 Avoid dimensioning inside an object Dimension to visible object lines, not to hidden lines Dimensioning Rules/Guidelines for Cylinders and Arcs Dimension diameter of cylindrical parts in their rectangular views Dimension diameter of cylindrical holes in their circular views Dimension radius of fillets and rounds (180 degrees or less) Locate center of holes C’est tout! ... object lines and other extension lines Dimensioning Guidelines for Prisms2 Avoid dimensioning inside an object Dimension to visible object lines, not to hidden lines Dimensioning Rules/Guidelines for ... Reference dimension – Numerical value provided for information only and not used in fabrication of the part Dimensioning Guidelines for Prisms1 Do not repeat dimensions Dimension most descriptive view of feature being