Inst 12-2a

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ENGR 111 12.2aBasic 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 ofdimensioning

Apply dimensions to objects in accordance

with engineering standards Define the following items: Dimension line,

Extension line, Reference dimension, andLeader

Be able to recognize the following machinedholes: 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:

1.201.00

LEADER

EXTENSION LINE

1.80

1.00

.90

DIMENSION LINE

EXTENSION LINE:

EXTENSION

GAP

2.00

.80

ARROWHEAD3.00

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Extension linesExtension lines

should be offset

from the part byone halfthe text

height and should

extend one textheight beyond the

dimension line

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Dimension Spacing The first row ofdimensions must be

a minimum of 3 textheights away from

the part.

Any dimensions

beyond it must be aminimum 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) unlessthere is no other option.

Avoid dimensioning to hiddenfeatures

Always place the dimension wherethe characteristic shape is shownin the most descriptive view. (Thismeans dont place a dimension on

object lines making a T joint.) Always dimension holes in theircircular view by stating thediameter of drilled holes. Specifythe hole depth of special featuressuch as countersinking with a notefollowing the dimension.

Dimension rounded corners andarc features as radii where theyappear in their rounded views.

If the same value is repeatedmany times, then use a generalnote for the features.

Dimension cylindrical objects asdiameters in their rectangularview.

Always place the first row ofdimensions a minimum distance of3 text heights away from the edgeof the part. Additional stacks canbe a minimum of 2 text heightsaway from each other.

Keep dimensions between theviews whenever possible

Extension lines may cross eachother and over other lines on thepart, but dimension lines shouldnever be crossed.

The overall dimension should

always be given. It should beplaced outside of smallerdimensions and be the furtherestdimension from the part.

SeeEssentials of Engineering Design Graphics Chapter 4

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Do not duplicate dimensions and

avoid using unnecessary orsuperfluous dimensions

When all of the dimensions areexpressed in inches, do not useinch mark () or the abbreviationfor inches (in.)

For drawings dimensioned in

inches, values less that one inchshould not be preceded with azero.

For metric drawings, omit the useof the millimeter (mm) notationfollowing the numeral, asmillimeters are the default units.

The origin for baseline or ordinatedimensions used as a datumshould be extended from afinished edge of the part.

Conserve space and time by using

abbreviations and standardizedsymbols whenever possible.

Reference dimensions should beplaced in parentheses or shouldinclude 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 themtoward the center of circularfeatures that they are specifying.

Place dimensions among thevarious views to avoid crowding.

Stagger horizontal dimensions toavoid contact or crowding ofvalues.

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 hiddenline 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 . Specify special features (hole types) with a

note.

Also:

Remember to

locate hole

position with

ordinate

dimensions tohole centers


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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..


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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 cancross 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 azero 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 insideof 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 Styles

Unidirectional dimensionsare 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 elementsWhat is

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

When areDecimals used ? Not used ?Unit designators (in. or mm) listed on dimensionlines ?

What sizeIs an Arrowhead ?

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DimensioningIn AutoCAD

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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 ofthe 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:

dimtxt

dimtix

dimdec

dimgap

dimasz

.80

2.00

dimexe

.901.00

1.80

3.00

1.001.20

SeeEssentials of AutoCAD by Whiteacre appendix 6

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AutoCAD Dimension Variables:

Variable Default Descriptiondimaso 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

SeeEssentials of AutoCAD appendix 6

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Special Characters in AutoCAD:

%%c will show as the Diameter symbol:

%%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. Usedextensively 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 areused for Oring seals,parts turnedon lathes,and designedfailure points

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Knurls:

Knurling is aprocess of rollingpatterns ontocylindrical

objects to makethem easier togrip.

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Dimensioning in small places

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Repetitive features

A combination ofdimensioning & notesis used. A number in

parenthesis indicateshow many arerequired. The wordTYPICAL (or TYP)

indicates that allshown are the same.

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SI symbol

When dimensions onthe 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 finishmarks. The vertex of

the needs to touchthe edge view of the

surface.

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Individual Assignment: AutoCAD

Plate 51...all Plate 52C & 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

Di i i E

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Dimensioning Errors:

What is wrong with:

Di i i

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Dimensioning Errors: What is wrong with:

Di i i E

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Di i i E

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L i Obj i

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Learning Objectives

Apply dimensions to objects using

AutoCAD

Be able to write correct notes for

machined holes

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