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.
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Dimensioning
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Definition
Size and shape
Position
Surface texture or
Geometric characteristic
Of a feature
A dimension is a numerical value that
defines the:
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Units
Millimeters (mm) in the metric system
Inches in the English system
The default dimensional units in most
CAD systems are:
NOTE:
• Dimensions in millimeters typically expressed
as whole numbers
• Dimensions in inches typically expressed to two decimal place accuracy
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Terminology
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
NOTE: Dimension text heights are typically 3
mm, and are uniform throughout the drawing
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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
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Dimensioning Guidelines for Prisms2
Avoid dimensioning
inside an object
Dimension to visible
object lines, not to
hidden lines
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Dimensioning Rules/Guidelines for
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
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C’est tout!