Basic Dimensioning with AutoCAD How to annotate simple drawings for use in constructing an object

Basic Dimensioning with AutoCAD

How to annotate simple drawings for use in constructing an object

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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,

Extension line, Reference dimension, and Leader

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

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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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Dimension Terminology:

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

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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 minimum of 2 text heights apart

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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 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.

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

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.

See Essentials of Engineering Design Graphics Chapter 4

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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 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.

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 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.

Dimensioning Rules


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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 (√) surface


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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 position with ordinate dimensions to hole centersSee Essentials of Engineering Design Graphics Chapter 4

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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 (1/4 ”or 6 mm) away from each other


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


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12. Do not duplicate dimensions and avoid using unnecessary or superfluous dimensions


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13. When all of the dimensions are expressed as inches. DO NOT use inch marks ('') or the abbreviation (.in)


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14. For drawings dimensioned in inches, values less than 1 inch should not be proceeded with a zero.


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15. For metric dimensions less than 1 mm DO place a zero in front of the decimal point.


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


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


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


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19. Reference dimensions should be placed in parenthesis or should include the abbreviation “REF”. Basic sizes (to be toleranced) should be placed inside of a rectangular box


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20. Extend leaders from the first or last word in a note. Point them toward (but not touch) the center of the circular features that they are specifying.


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


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Angular features:Dimension angular features with either:

X,Y coordinate location or Vertex location & degrees


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Dimension StylesUnidirectional dimensions are read from the lower left corner of the sheet

Aligned dimensions align with individual dimension lines.


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A short review on Dimension “elements”•What is…

•an Extension line ?•an Extension line gap ?•an Extension line extension ?•a Dimension line ?•a Dimension numeral ?•a leader ?

•When are…

•Decimals used ? Not used ?•Unit designators (in. or mm) listed on dimension lines ?

•What size…

•Is an Arrowhead ?

Dimensioning Dimensioning In AutoCADIn AutoCAD

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Dimensioning in AutoCAD

AutoCAD does semi-automatic dimensioning You select the features which should be

dimensioned and the location for each dimension AutoCAD measures the distance in question and

applies the appropriate dimension complete with arrowheads, extension lines, and dimension lines

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The Dimensioning Toolbar

The dimensioning commands can be easily accessed by: Right Click on an existing toolbar and

activate the dimensioning toolbar; Or, From the “pull down” menu

View

Toolbars

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Linear Dimensions

Linear dimensions are use to show basic dimensions of parts

They can be orientated either in a horizontal or vertical (or even aligned) fashion

AutoCAD will automatically measure the length specified and insert all the parts of the dimension.

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Leaders

Leaders are used to apply thread notes and other annotations

Leaders begin at the arrow and continue from their elbow to their annotation

The annotation can be any text string you want to type

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Radius and Diameter

By selecting a circular feature (arc or circle) with the Radius or Diameter option AutoCAD will measure the size and place the dimension at a user selected point

Using the Center option will place automatic centerlines on arcs or circles

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AutoCAD Dimension Variables:

See Essentials of AutoCAD by Whiteacre appendix 6

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AutoCAD Dimension Variables:

Variable Default Description

dimaso on All parts of the dimension are one entity.

dimscale 1.00 Changes sizes of text arrows.

dimasz .18 Sets arrowhead length.

dimexe .18 Extension beyond dimension line.

dimtad off Places text above the dimension line.

dimtxt .18 Sets height of text.

dimtix off Forces text inside of extension lines.

dimdec ?? Sets number of decimal places of dimension

dimcen ?? Sets center tick mark size

See Essentials of AutoCAD appendix 6

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Special Characters in AutoCAD: %%c will show as the Diameter symbol: Ø %%d will show as the Degree symbol: %%p will show as the Plus-minus symbol: %%u will start/stop text underlined %%o will start/stop text overline

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Spotfaced holes:No depth required. Cleans an uneven surface deep enough to allow full contact with bolt or washer.

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Counterbore holesRequire both a diameter and a depth dimension.

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Counterdrilled holes:Require both a diameter and a depth dimension. (measured to the shoulder).

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Countersunk holes:

Used with flat head screws and bolts. They become flush when tightened. Used extensively in the aerospace industry. The 82 angle is common for cutting tools and screw threads

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Chamfers:

Break away sharp edges and help guide the part into a mating assembly. May be specified with one distance and one angle, or with two distances.

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Necks:

Necks are used for “O”ring seals, parts turned on lathes, and designed failure points

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Knurls:

Knurling is a process of rolling patterns onto cylindrical objects to make them easier to grip.

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Dimensioning in small places

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Repetitive features

A combination of dimensioning & notes is used. A number in parenthesis indicates how many are required. The word TYPICAL (or TYP) indicates that all shown are the same.

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SI symbol

When dimensions on the drawing are all stated in millimeters, place an SI symbol near the title block.

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SI symbol: 1st vs 3rd angle projection.

Metric Units and 3rd angle projection

Metric Units and 1st angle projection

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Finish marks:

Rough surfaces that need to be machined to provide a smooth, finished surface are indicated with finish marks. The vertex of the √ needs to touch the edge view of the surface.

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Individual Assignment: AutoCAD

Plate 51………………..…….…all Plate 52……………………C & D Plate 53 …………….……..B & D Plate 54 ……………….…..A & D

Due: Beginning of class 12.2

Preview: Tolerances:………………. Plate 57-63

You will need your “paper copy” of the Vinson plate book for this assignment

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Dimensioning Errors: What is wrong with:

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Learning Objectives

Apply dimensions to objects using AutoCAD

Be able to write correct notes for machined holes

Documents

Basic Dimensioning with AutoCAD How to annotate simple drawings for use in constructing an object