Dimensioning & Tolerancing

Dr. Fairuz I. RomliSemester 2 2009/2010

Introduction: Dimensioning Working drawings must show dimensions and contain notes conveying

sizes, specifications and other information Techniques of dimensioning presented are based primarily on the

standards of the American National Standards Institute (ANSI) Two commonly used units of measurement

Metric units – in millimeter – round the millimeters to the nearest whole number

English units – in inches – always show up to two decimal places, even for whole numbers

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dual Dimensioning On some drawings, both metric and English units might be required

Place the millimeter equivalent either under or over the inch units Place the converted dimension in brackets to the right of the

original dimension Be consistent in the arrangement throughout the whole drawing

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Metric Designation In metric system (SI)

First-angle projection positions the front view over the top view and the right-side view to the left of the front view

American system – use third-angle projection – places top view over the front view

Label metric drawings with one of the symbols

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Numeric & Symbolic Dimensioning Vertical dimensions – aligned or uni-directional

Uni-directional – all dimensions appear in the standard horizontal position

Aligned – numerals are parallel with vertical and angular dimension lines and read from the right-hand side of the drawing, never from the left-hand side

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Numeric & Symbolic Dimensioning Placement

Dimensions should be placed on the most descriptive views of the part being dimensioned

First row of dimensions should be at least 3 times the letter height (3H) from the object

Successive rows of dimensions should be spaced equally at least 2 times the letter height (2H)

Regardless of space limitations – numerals have to be the same size throughout the entire drawing

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Numeric & Symbolic Dimensioning

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Numeric & Symbolic Dimensioning Symbols

To save some drawing times – use these symbols instead of words Size usually based on the letter height – often 1/8 inch

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules To place dimensions and notes on drawings most effectively

However, from time to time, rules of dimensioning must be violated due to the complexity of the part or shortage of space

Rules for prisms

Place the first row of dimensions at least 3H from the object Successive rows at least 2H from each other

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Place the dimensions between the views sharing these dimensions

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Place the dimensions on the most descriptive views of an object

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Dimension visible features, not hidden features

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Leave the last dimension blank in a chain of dimensions when you also give an overall dimension

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Place dimensions in well-organized lines for uncluttered drawings

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Do not duplicate dimensions on a drawing to avoid errors or confusion

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Dimension lines should cross any other lines unless absolutely necessary

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Extension lines may cross other extension lines or object if necessary

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Leave a small gap from the edges of an object to extension lines that extend from them. Do not leave gaps where extension lines cross object lines or other extension lines

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for prisms

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for angles

Place angular dimensions outside angular notches by using extension lines

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for angles

Dimension a bent surface rounded corner by locating its theoretical point of intersection with extension lines

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for cylindrical parts and Holes

Dimension the diameter (not the radius) of a cylinder in the rectangular view

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for cylindrical parts and Holes

Dimensions on concentric cylinders are easier to read if they are staggered within their dimension lines

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for cylindrical parts and Holes

Dimension holes in their circular view with leaders whenever possible, but dimension them in their rectangular views if necessary

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rules for cylindrical parts and Holes

Draw leaders pointing toward the centers of holes

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rule for leaders

Extend leaders from the first or the last word of a note with a horizontal elbow

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rule for arcs and radii

When space permits, place dimensions and arrows between the center and the arc

When space is not available for the numbers, place the arrow between the center and the arc number outside

If there is no space for the arrow inside, place both the dimension and arrow outside the arc with a leader

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Dimensioning Rules Rule for location dimensions

Locate cylindrical holes in their circular views by coordinates to their centers

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Introduction: Tolerancing Technique of dimensioning parts within a required range of variation

Each dimension is allowed a certain degree of variation within a specified zone

A tolerance should be as large as possible without interfering with the function of the part to minimize production costs Manufacturing costs increase as tolerances become smaller

Positioning of tolerances

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Rules of Tolerancing Order of numbers

Place upper limits either above or to the right of lower limits In plus minus tolerancing, place the plus limits above the minus

limits

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

Rules of Tolerancing Positioning of numbers

Spacing and ratios of numerals used to specify tolerances on dimensions

Earle, J. H. “Graphics for Engineers”, Addison-Wesley Publishing, 1996

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