Dimensions explain:

• size of the object
• location of features (e.g. hole)
• characteristics of features (e.g. diameter and depth)
• tolerance (or accuracy)

Terminology

• Dimension Text – numerical value on drawing that defines size
• Dimension Line – thin, solid line; shows extent and direction of dimension
• Arrowheads – placed at ends of dimension lines; shows limits of dimension
• Extension Line – thin, solid line perpendicular to dimension line; indicates which feature is associated with dimension
• Leader Line – thin, solid line; used to indicate features associated with note, dimension or symbol
• Tolerance – amount a particular dimension is allowed to vary; all dimensions have them

 

Standards

• American National Standards Institute, ANSI
  • ANSI Y14.5 is widely used in the United States.
• British Standards Institution, BSI
• Deutsche Industries-Normen, DIN

 

Guidelines

The main goal for dimensioning is to provide clarity.

1. Do not over-define or under-define the object.
2. If you want to include unnecessary dimensions for reference, place them in parentheses (XX).
3. Keep dimensions on a separate layer.
4. The size and style of leader lines, text, and arrows should be consistent throughout the drawing.
5. Use OSNAP to select the features to be dimensioned.
6. AutoCAD uses the DEFPOINTS layer for the small points at the end of an extension line.  This layer will not print.

Do

1. place dimensions in the view that best describes the feature
   • use linear dimensions for cylinders, cones and frustums
   • use diameter dimensions for holes
   • use f for circles
   • use R for arcs
2. position dimensions between views
3. place as many dimensions in one view as possible
4. evenly space dimensions -- away from the object's edge and from other dimensions
5. display only the number of decimal places required for manufacturing precision
6. assume square corners and symmetry, unless otherwise noted

Do not

1. put dimensions inside the object
2. overlap dimensions
3. dimension to hidden lines

 

References

• feh.eng.ohio-state.edu/Lectures/191au05/18 H191 W05 D03 1-1 V1.2 - Introduction to Dimensioning.ppt
• courses.washington.edu/mengr123/current/Dimension%20Rules.pdf#search=%22dimensioning%20rules%22
• courses.washington.edu/mengr123/current/Dimensions.pdf#search=%22dimensioning%20guidelines%22
• www.we-r-here.com/cad/tutorials/level_4/4-11.htm

Shorthand

• 2X Ø12 -- Two holes with diameter 12 and depth of 30
• R20 TYP– Radius 20 –Typical
• 36 – square dimension of 36
• Ø8 Ø12 x 160º - Countersunk hole with diam.8 thru
• .25 – 20 UNC-2B – Thread note.