13M305 MACHINE DRAWING

Dimensioning

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By

G.Prabu ram

Assistant Professor/Mechanical

Learning Objectives� Be able to understand the basic rules of

dimensioning

� Apply dimensions to objects in accordance with engineering standards

Define the following items: Dimension line,

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� Define the following items: Dimension line, Extension line, Reference dimension, and Leader

� Be able to recognize the following machined holes: Spotfaced, Counterbored, Countersunk, & Counterdrilled.

Basic Dimensioning

� Dimensions allow a part to be manufactured

� Dimensions are ALWAYS in real world units

� Dimensions should be applied in a concise,

coherent manner

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coherent manner

Dimension Terminology:

EXTENSION LINE

1.80

1.00

.90

DIMENSION LINE

EXTENSION LINE:

EXTENSION

GAP

.80

ARROWHEAD3.00

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1.201.00

LEADER

GAP

2.00

Extension lines Extension lines should be offset from the part by one half the text height and should extend one text height beyond the

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height beyond the dimension line

Dimension Spacing The first row of dimensions must be a minimum of 3 text heights away from the part.Any dimensions beyond it must be a

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beyond it must be a minimum of 2 text heights apart

Arrowheads: Should be 3 times longer than they are wide.

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Dimensioning Rules � Avoid placing any dimensions on

the part (inside the view) unless there is no other option.

� Avoid dimensioning to hidden features

� Always place the dimension where the characteristic shape is shown in the most descriptive view. (This means don’t place a dimension on object lines making a “T joint”.)

� Always dimension holes in their

� Dimension cylindrical objects as diameters in their rectangular view.

� Always place the first row of dimensions a minimum distance of 3 text heights away from the edge of the part. Additional stacks can be a minimum of 2 text heights away from each other.

� Keep dimensions between the views whenever possible

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� Always dimension holes in their circular view by stating the diameter of drilled holes. Specify the hole depth of special features such as countersinking with a note following the dimension.

� Dimension rounded corners and arc features as radii where they appear in their rounded views.

� If the same value is repeated many times, then use a general note for the features.

views whenever possible

� Extension lines may cross each other and over other lines on the part, but dimension lines should never be crossed.

� The overall dimension should always be given. It should be placed outside of smaller dimensions and be the furtherest dimension from the part.

� Do not duplicate dimensions and avoid using unnecessary or superfluous dimensions

� When all of the dimensions are expressed in inches, do not use inch mark (“) or the abbreviation for inches (in.)

� For drawings dimensioned in inches, values less that one inch should not be preceded with a

� Conserve space and time by using abbreviations and standardized symbols whenever possible.

� Reference dimensions should be placed in parentheses or should include the abbreviation “REF”.

� Basic sizes (to be toleranced) should be placed inside a rectangular box

� Extend leaders from the first of

Dimensioning Rules

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should not be preceded with a zero.

� For metric drawings, omit the use of the millimeter (mm) notation following the numeral, as millimeters are the default units.

� The origin for baseline or ordinate dimensions used as a datum should be extended from a finished edge of the part.

� Extend leaders from the first of last word in a note. Point them toward the center of circular features that they are specifying.

� Place dimensions among the various views to avoid crowding. Stagger horizontal dimensions to avoid contact or crowding of values.

1. Avoid placing dimensions on the part (inside of the view).

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2. Avoid dimensioning to hidden features

There is one exception:……When the hidden line is a finished

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line is a finished (√) surface

3. Always place dimensions where the characteristic shape is shown in the most descriptive view

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4. Always dimension holes in their circular view with the Ø. Specify special features (hole types) with a note.

Also:Also:

Remember to locate hole

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locate hole position with ordinate dimensions to hole centers

5. Dimension rounded corners and arc features as radii where they appear in their rounded view.

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6. If the same value is repeated many times, then use a general note for the feature. (ALL FILLETS AND ROUNDS ARE .125R)

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7. Dimension cylinders in their rectangular view with a diameter symbol….Ø.

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8. Place the first row of dimensions 3 text heights (3/8 ”or 10 mm) away from the edge of the part.

Additional stacks of dimensions can be a minimum of two text heights

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two text heights (1/4 ”or 6 mm) away from each other

9. Keep dimensions between views whenever possible .

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10. Extension lines may cross each other and over other lines on the part, but dimension lines should never be crossed. (Hint no “arrow-headed” lines can cross “arrow-headed” lines)

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Dimensioning Errors:� What is wrong with:

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Dimensioning Errors:� What is wrong with:

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Dimensioning Errors:� What is wrong with:

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Dimensioning Errors:� What is wrong with:

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