dimensioning -...

Dimensioningwith Tolerances

Learning Objectives

After completing this chapter, you will be able to do thefollowing:/ Define and use dimensioning and tolerancing terminology./ Identify different types of tolerance dimensions./ Create dimension styles with specified tolerance settings.

Chapter 16 introduced the creation of dimension styles and explained how to setthe specifications for dimension geometry, fit format, primary units, alternate units,and text.This chapter describes the basics of tolerancingand explains how to preparedimensions with tolerances for mechanical manufacturing drawings.

c Dimensioning Conventions 3Dimension styles help standardize drawings within a company or industry.

Dimensioning for mechanical drafting usually uses the following AutoCAD settings,depending on company practices.

Lines and Arrows

Thefollowing conventions apply to most mechanical drawings.Thesesettings areusually incorporated into appropriate dimensions styles that are, in turn, included indrawing templates.

• The minimum dimension line spacing for baseline dimensioning is .375(10 mm). A value of .5 (12 mm) or .75 (19 mm) is usually more appropriate.

• The extension line extension is .125 (3 mm), and the extension line offset is .063(1.5 mm).

• Arrowheads are usually closed filled, but closed blank, closed, or open arrowheads are sometimes used.

• A small dot is used on a leader pointing to a surface.• The centerline option is used for center marks for circles and located arcs. Fillets

and rounds generally have no center marks.

633

i

fit format: The

arrangement ofdimension text and

arrowheads on a

drawing.

Fit Format

As you may recall, fit format refers to the arrangement of dimension text andarrowheads on a drawing. AutoCAD provides several options for fit format. Keep thefollowing points in mind when you set up the fit format for a drawing or template:

• The manually defined format is convenient for flexible text placement.• The best-fit option for text and arrows is common, but other format options

work better for some applications.• Horizontal and vertical justifications are usually in centered format.• Text placement is normally inside and outside horizontal for unidirectional

dimensioning.

Primary Units, Text, and TolerancesAlthough exact settings are often dictated by company policy or the intended use

of a drawing, the following guidelines apply to most mechanical drawings:• Objects are dimensioned in inches or millimeters.• The primary units are typically decimal, with the number of decimal places

controlled by the feature tolerance.• Using alternate units for dual dimensioning isnot a recommended ASME practice.• The text is usually placed using the romans.shx font, a height of .12 (3 mm), and

a gap of .063 (1.5 mm).• The tolerance method depends on the application.

c Tolerancing Fundamentals )tolerance: Total A tolerance is the total amount a specificdimension is permitted to vary.TolerancesdimeTsiOTis60'''0 are not given tovalues identified asreference, maximum, minimum, or stock sizes. Apermitted to vary. tolerance may be applied directly to the dimension, indicated by a general note, or

identified in the drawing title block. See Figure 19-1.

634

Figure 19-1.Tolerances can be specifiedon the dimension, in a general note,or in the drawing title block.

12.50+0.25

Specified Tolerance(Tolerance Placed

on the Dimension)

— 12.50

Unspecified Tolerance(Tolerance Placed

in Note or Title Block)

UNLESS OTHERWISE SPECIFIEDTOLERANCES FOR:

INCHES mm

1 PLACE DIMS.; ± .1 ± 2.5

2 PLACE DIMS.; ± .01 ± 0.25

3 PLACE DIMS.; ± .005 ± 0.100

ANGULAR; ± 30"FRACTIONAL; ± 1

32

FINISH; 125 fl in. 3.2 /J, M

2-PLACE DECIMALS ARE ±0.25.

Tolerance in a General Note

Tolerances in a Title Block

AutoCAD and Its Applications—Basics

The limits ofa dimension are the largest and smallestnumerical values the featurecan be. In Figure 19-2, the dimension stated as 12.50±0.25 is in a style known asplus-minus dimensioning. The tolerance of this dimension is the difference betweenthe maximum and minimum limits. This tolerance style can be used when the variance is the same in the positive and negative directions. In this case, the upper limitis 12.75 (12.50 + 0.25), and the lower limit is 12.25 (12.50 - 0.25). To find the tolerance,subtract the lower limit from the upper limit. The tolerance is .50.

The specified dimension is the part of the dimension from which the limits arecalculated. In Figure 19-2, the specified dimension of the feature shown is 12.50. Atolerance on a drawing may be calculated and shown using limits dimensioning. Inthis style, the specified dimension is not shown. Many schools and companies preferthis method because the limits are givenand calculations are not required.

A bilateral tolerance permits different variance in the positive and negativedirections from the specified dimension. An equal bilateral tolerance has the samevariance in both directions. In an unequal bilateral tolerance, the variance from thespecified dimension is not the same in both directions. See Figure 19-3. A unilateraltolerance permits a variance in only one direction from the specified dimension. SeeFigure 19-4.

Figure 19-2.Examples of plus-minus dimensioning and limits dimensioning.

—r12.50±0.25

Plus-Minus Dimensioning

Figure 19-3.Examples of bilateral tolerances.

O/1+0.08Z4-0.20

112.7512.25

1

.750

Limits Dimensioning

+ .002-.003

Metric Inch

Unequal Bilateral Tolerance

24+0.1 .750 ±.005Metric Inch

Equal Bilateral Tolerance

Figure 19-4.Thevarianceof a unilateral tolerance is in only one direction from the specifieddimension.

24

24

0-0.2

+ 0.20

Metric

Chapter 19 Dimensioning with Tolerances

.625

.625

+ .000-.004

+ .004-.000

Inch

limits: The largestand smallest

numerical values

the feature can be.

plus-minusdimensioning: Atolerance style inwhich the positiveand negativevariance is equaland is preceded bya ± symbol.

limits

dimensioning:Method in which the

limits are given andcalculations are not

required.

bilateral tolerance:

A tolerance stylethat permitsvariance in both

the positive andnegative directionsfrom the specifieddimension.

equal bilateraltolerance: A

tolerance that has

the same variance

in both directions.

unequal bilateraltolerance: A

tolerance in which

variance from the

specified dimensionis not the same in

both directions.

unilateral

tolerance: Permits

an increase or a

decrease in onlyone direction

from the specifieddimension.

635

Q

Q

DDIM

Dimensions

>Dimension Style

X

Dimension Style.

BEE flirt Mil

Format

> Dimension

Style

636

c Assigning Decimal Places 3The ASME Y14.5M Dimensioning and Tolerancing standard has separate recommen

dations for the display of decimal places in inch and metric dimensions. Examples ofdecimal dimension values in inches and metric units are shown in Figure 19-3 andFigure 19-4.

Inch DimensioningGeneral rules for displaying dimensions in drawings dimensioned in decimal

inches include.

• A specified inch dimension is expressed to the same number of decimal placesas its tolerance. Zeros are added to the right of the decimal point if needed. Forexample, the inch dimension .250±.005 has an additional zero added to the .25to match the three-decimal tolerance. Similarly, the dimensions 2.000±.005 and2.500±.005 both have zeros added to match the tolerance.

• Both values in a plus-minus tolerance for an inch dimension have the samenumber of decimal places. Zeros are added to fill in where needed. Forexample:+.005 , +.005-.010 not -.01

Metric DimensioningGeneral rules for displaying dimensions in drawings dimensioned using metric

units include.

• The decimal point and zeros are omitted from the dimension when the metricdimension is a whole number. For example, the metric dimension 12 has nodecimal point followed by a zero. This rule is true unless tolerance values aredisplayed.

• When a metric dimension includes a decimal portion, the last digit to the rightof the decimal point is not followed by a zero. For example, the metric dimension 12.5 has no zero to the right of the 5. This rule is true unless tolerancevalues are displayed.

• Both values in a bilateral tolerance for a metric dimension have the same

number of decimal places. Zeros are added to fill in where needed.• Zeros are not added after the specified dimension to match the tolerance. For

example, both 24±0.25 and 24.5+0.25 are correct. Somecompaniesprefer to addzeros after the specified dimension to match the tolerance, however, in whichcase 24.00+0.25 and 24.50+0.25 are both correct.

c Setting Primary Units 3As described in Chapter 16, a dimensionstylecan be created with specific format

ting, justification, and text settings. The Dimension Style Manager dialog box is usedboth to create and to modify dimension styles.SeeFigure 19-5.Tomodify a dimensionstyle, highlight the style you want to modify and pick the Modify... button to accessthe Modify Dimension Style dialog box.The Primary Units tab is used to set the type ofunits and precision of the dimension. See Figure 19-6.


Figure 19-5.The Dimension Style Manager dialog box.

Pick to accessthe Modify Dimension Style

dialog box|

r51 Dimension Style Manager C3®Currentdimensionstyle:Standard

Styles: 'review of: Standard

A Annotative

HBBBEI

_y\

1 SetCyrrent 1

1.01591— | dew... |. 1 >

11.1955

!_

1 [ Modify... ]

(7)\ 2.0207 [ Qverride... ]

\V^/ | Compare... ]R0.8045-

List:

|AH styles v-• Qon't li:.t styles in

Close | Help |

Figure 19-6.Settings for the unit format and precision of linear dimensions are located in the PrimaryUnits tab.

Set the precisionfor specified vdimensions

Set the zero

suppression

Modify Dimension Style: Standard

Lines Symbolsand Arrows Text Fit Primary Units [Alternate Units Tolerances!

Linear dimensions

Unit format:

Precision

Fraction format:

Decimal separator:

Round off:

Prefix:

Suffix:

Measurement scale

Scale factor

• Apply lolayout dimensions only

Zero suppression

0 Leading

• Trailing

XDecimal

0.0000

Horizontal

'.' (Period)

0.0000

I LV7J

-• Ofeei

0 0 inches

Jr

L.0159

h1.1955

3 60*

R.8045 J

Angulardimensions

Unitsformat: DecimalDegrees

Precision: 0

Zero suppression

0 Leading

• Trading

OK Cancel

2.0207

Help

In the Linear dimensions area of the Primary Units tab, the Precision drop-downlist allows you to specify the number of zeros displayed after the decimal point ofthe specified dimension. The ASME standard recommends that the precision for thedimension and the tolerance be the samefor inchdimensions, but it may be differentfor metric values, as previously described.

The Zero suppression settings were explained in Chapter 16. For metric dimensions, the Leading optionsshould be off, and theTrailing optionsshould be on. Forinchdimensions, the Leading optionsshouldbe on,and theTrailing optionsshould be off.

Chapter 19 Dimensioning with Tolerances 637

symmetricaltolerance:

AutoCAD's term for

an equal bilateraltolerance.

638

C Setting Tolerance Methods )The Tolerances tab of the New (or Modify) Dimension Style dialog box is used to

apply a tolerance method to your drawing. See Figure 19-7. The default option in theMethod: drop-down list is None. This means no tolerance method is used with yourdimensions. As a result, most of the options in this area are disabled. If you pick atolerance method from the drop-down list, the resulting image in the tab reflects themethod selected. The drop-down list options are shown in Figure 19-8.

Symmetrical Tolerance MethodAutoCAD refers to an equal bilateral tolerance as a symmetrical tolerance. The

Symmetrical tolerance dimensioning option is used to draw dimension text thatdisplays an equal bilateral tolerance in the plus-minus format. When the Symmetricaloption is selected, the Upper value: text box, Scaling for height: text box, and Verticalposition: drop-down list are activated. The preview image displays an equal bilateraltolerance. See Figure 19-9. You can enter a tolerance value in the Upper value: text box.Although it is inactive, you can see that the value in the Lower value: text box matchesthe value in the Upper value: text box.

Figure 19-7.The Tolerances tab contains formatting settings for tolerance dimensions.

Select a

tolerance-

method

Set the

precision fortolerance

dimensions

Settingsshould match

the Zero

suppressionlinear

dimension

settings in thePrimary

Units tab

f'oj Modify Dimension Style: Standard

Lines ||Symbols and Arrows Text If Fit II Piimaiy Units Alternate Units TolerancesTolerance lormal

Method:

Vertical position:

Tolerance alignment

.Align decimal separators

ASgn opetatioi i rymbota

Zero suppression

soVig

[ | Trailing

Middle

00 feet

3

1.1955

R.8045

Alternate unit tolerance

Precision:

Zero suppression

• Leading 00 reel

Ti.= 'r.;1 0Oinch«

Cancel Help

Figure 19-8.A tolerance dimensioning method can be selected from the options in the Method: dropdown list, located in the Tolerance format area of the Tolerances tab.

Select a

tolerance

method

|Modify Dimension Style: Standard

Lines Symbolsand Arrows Text Fit Primary I.

Tolerance format

Method-

Precision

Upper value:

SymmetricalDeviationLimitsBasic


Figure 19-9.Setting the Symmetrical tolerance method option current, with an equal bilateral tolerancevalue of .005.

c

Specifiedtolerance

method

Equalbilateral

tolerance

value

Modify Dimension Style: Standard

Exercise 19-1

2.750±.005

Drawing

Complete the exercise on the Student CD.

Deviation Tolerance Method

AutoCAD refers to an unequal bilateral tolerance as a deviation tolerance. Thismeans the tolerance deviates (departs) from the specified dimension with two differentvalues. The deviation tolerance method can be set by selecting Deviation in the Method:drop-down list of the Tolerances tab. After selecting this option, the Uppervalue: andLower value: text boxes are activated so you can enter the desired upper and lowertolerance values. See Figure 19-10. The preview image in the tab changes to match arepresentation of an unequal bilateral tolerance.

The deviation option can alsobe used to draw a unilateral tolerance by entering 0for either the Upper value: or Lower value: setting. Ifyou areusinginchunits,AutoCADincludes the plus or minus sign before the zero tolerance. When metric units are used,the sign is omitted for the zero tolerance. See Figure 19-11.

Exercise 19-2Complete the exercise on the Student CD.

Figure 19-10.Settingthe Deviation tolerance methodoptioncurrent, with unequalbilateral tolerance values.

2 Modify Dimen ion Style: Standard n| Lines || Symbols and Arrows || Text || F* Primary

Method

Precision

Upper value:

Lpwervalue*

Scaling (orheight

Vertical position-

Specifiedtolerance —

method

_-:-• Deviationv

n omn V

^^,\ 0.0020v

Enter the

desired -e-tolerance

values

___^_| 0.0050 *

11.0000 A

iMidde V

Setting


3.625+ .002-.005

Drawing

deviation

tolerance:

AutoCAD's term foran unequal bilateraltolerance.

639

basic dimension: A

theoretically perfectdimension.

640

Figure 19-11.When a unilateral

tolerance is specified,AutoCADautomaticallyplaces the plus or minussymbol in front of thezero tolerance, if Englishunits are used. The

symbol isomitted withmetric units.

2.000+ .000

-.005

English Units

-0.2"

Metric Units

Limits Tolerance Method

In limits dimensioning, the tolerance limits are given, and no calculations fromthe specified dimension are required (unlike plus-minus dimensioning). The limitstolerance method can be set by picking Limits in the Method: drop-down list in theTolerances tab. When this option is set, the Upper value: and Lower value: text boxesare activated. Youcan then enter the desired upper and lower tolerance values that areadded and subtracted from the specified dimension. The values you enter can be thesame or different. See Figure 19-12.


Basic Tolerance Method

The basic tolerance method is used to draw basic dimensions. A basic dimension is atheoretically perfectdimensionand isused in geometric dimensioningand tolerancing,which is covered in Chapter 20. The basictolerancemethod can be set by picking Basicin the Method: drop-down list in theTolerances tab. With this setting, the Upper value:and Lower value: options in the Tolerance format area are disabled because a basicdimension has no tolerance. A basicdimension is distinguished from other dimensions bya rectangle placed around the dimension number, as shown in Figure 19-13.

DPicking the Draw frame around text check box in the Text tab of theNew (or Modify) Dimension Style dialog box also activates the basictolerance method.

Figure 19-12.Selecting the limitstolerance method

and setting limitvalues. Specified

tolerance

method

Enter the

desired

tolerance

values

3 Modify Dimensior Style: Standard

Lines Symbols and Arrows Text Fit Primary, . ,

Method -^

Precision

Upper value:

_^Lew€rvaKjt

Scaling lor height:

Vertical position:

-| Limitsv

10.0000 -

,_|0.0050 \y

____^- [0.0050 :

10000 *

| Middle V

Setting

2.755

2.745

Drawing

ALitoCAD and Its Applications—Basics

Figure 19-13.The basic tolerance method is used for basic dimensioning. The dimension text for a basicdimension is placed inside a rectangle.

I ilalModify Dimension Style:Standard

Lines | Symbols andArrows || Text || Fit Primarv

T 1 11

Method. _5

Precision

Upper value:

J^wgTvalue.

Specifiedtolerance —

j- Basic V

method

:pr|a0050

Tolerance ^40.0050values are-

Scaling lor height:

Verticalposition:

disabled h.oooo

iMUdtev

Setting

- 2.000

Drawing

Tolerance Precision and Zero SuppressionAfter a tolerance method is specified in the Method: drop-down list of the

Tolerances tab, you can set the precision of the tolerance. By default, when you setthe primary unit precision on the Primary Units tab, AutoCAD automatically makesthe tolerance precision in the Tolerances tab the same unit precision. If the setting doesnot reflect the level of precision you want, change it using the Precision drop-down listin the Tolerance format area.

As is the case with the precision settings, a tolerance method must be selectedbefore the Zero suppression tolerance format options can be specified. The suppression settings for linear dimensions in the Tolerances tab should be the same as theZero suppression tolerance format settings in the Primary Units tab. AutoCAD doesnot automatically match the tolerance setting to the primary units setting.

Tolerance JustificationUsing the options in the Vertical position: drop-down list in the Tolerance format

area of the Tolerances tab, you can control the alignment, or justification, of deviation tolerancedimensions. The Middle option centers the tolerance with the specifieddimension and is the default. This is also the recommended ASME practice. The otherjustification options are Top and Bottom. Deviation tolerance dimensions displayingeach of the justification options are shown in Figure 19-14.

The options in the Tolerance alignment area become available for selection whenyou use a deviation or limits tolerance method. The selected option controls the leftand right tolerance justification. When using a deviation tolerance method, pick theAligndecimal separators radio button to vertically align the upper and lower tolerancevalue decimal points. Select the Align operational symbols radio button to verticallyalign the upper and lowertolerance plus and minus symbols. See Figure 19-15. When

Figure 19-14.Examples of the tolerance justification options for deviation tolerance dimensions.

2.625+ .002

-.005"

Middle


2.625 + .002-.005

Top

+ .0022.625-.005

Bottom

641

642

Figure 19-15.Changing tolerancealignment whenusing a deviationtolerance method.

2.0"-1.0-.5

Aligned on Decimal Separator

2.01.0

-.5

Aligned on Operational Symbols

using the limits tolerance method, pick the Align decimal separators radio button tovertically align the upper and lower limit decimal points. Select the Align operationalsymbols radio button to left-justify the upper and lower limits. See Figure 19-16.

Tolerance HeightYoucan set the text height of the tolerance dimension in relation to the text height

of the specified dimension using the Scaling for height: text box in the Toleranceformat area of the Tolerances tab. The default of 1.0000 makes the tolerance dimension

text the same height as the specified dimension text. This is the recommended ASMEstandard. If you want the tolerance dimension height to be three-quarters as highas the specified dimension height, type .75 in the Scaling for height: text box. Somecompanies prefer this practice to keep the tolerance part of the dimension from takingup additional space. Examples of tolerance dimensions with different text heights areshown in Figure 19-17.

Figure 19-16.Changing tolerancealignment whenusing a limitstolerance method.

Aligned on Decimal Separator

Aligned on Operational Symbols


Figure 19-17.Using different scale settings for the text height of tolerance dimensions.

3.250+ .005

-.002

Tolerance Scale Setting = 1

3.2553.248

Tolerance Scale Setting = 1

•3.250+ .005

-.002

Tolerance Scale Setting = .75

3.255

3.248

Tolerance Scale Setting = .75

Tolerance Method Review

/

/

/

/

/

Each tolerance method option you pick is represented by an image preview inthe Modify Dimension Style dialog box.Choose a tolerance method based on the characteristics of the tolerance. If the

upper and lower variance is equal, you can choose the Symmetrical option tocreate an equal bilateral tolerance. If the upper and lower variance differs, usethe Deviation option. Use the Limits option to show only the minimum andmaximum allowed values.

Specify the decimal precision for the primary units and tolerances accordingto the ASME guidelines for inch-based or metric drawings.When drawing inch tolerance dimensions, you should activate the Leadingcheck box in the Zero suppression area of the Tolerances tab. The same optionshould be activated for linear dimensions in the Primary Units tab. You canthen properly draw inch tolerance dimensions without placing the zero beforethe decimal point, as recommended by ASMEstandards. These settings allowyou to draw a tolerance dimension such as .625±.005.When drawing metric tolerance dimensions, deactivate the Leading check boxin the Zero suppression area of the Tolerances tab. Deactivate the same optionfor linear dimensions in the Primary Units tab. This allows you to place a metrictolerance dimension with the zero before the decimal point, as recommendedby ASME standards, for example, a dimension such as 12±0.2.



Chapter TestAnswer the following questions. Write your answers on a separate sheet ofpaperorcomplete the electronic chapter teston the Student CD.

1. Define the term tolerance.

2. What are the limits of the tolerance dimension 3.625±.005?

3. Give an example of an equal bilateral tolerance in inches and in metric units.4. Give an example of an unequal bilateral tolerance in inches and in metric units.5. Give an example of a unilateral tolerance in inches and in metric units.6. Which dialog box is used to create dimension styles? How is it accessed?7. How do you open the Tolerances tab?8. How do you set the number of zeros displayed after the decimal point for a toler

ance dimension?

9. Which Zero suppression settings should be specified for linear and tolerancedimensions when using metric units?

10. Which Zero suppression settings should be specified for linear and tolerancedimensions when using inch units?

11. What happens to the preview image in the Tolerances tab when a tolerancemethod option is picked from the Method: drop-down list?

12. What is the purpose of the Symmetrical tolerance method option?13. What is the purpose of the Deviation tolerance method option?14. What is the purpose of the Limits tolerance method option?15. Name the tolerance dimension justification option recommended by the ASME

standards.

16. Explain the results of setting the Scaling for height: option to 1 in the Tolerancestab.

17. What setting would you use for the Scaling for height: option if you wantedthe tolerance dimension height to be three-quarters of the specified dimensionheight?

644 AutoCAD and Its Applications—Basics

Drawing ProblemsSet the limits, units, dimension style options, and other parameters as neededfor thefollowingproblems. Use the guidelines given below.

A. Draw and dimension the necessary views for the following drawings to exactsize.Theseproblems are presented in 3D. Draw the proper 2Dviews for each.

B. Apply dimensions accurately using ASME standards. Create dimensionstyles that suit the specific needs of each drawing. For example, save different dimension styles for metric and inch dimensions.

C. Create separate layers for the views and dimensions.D. Plot the drawings with 0.6 mm object lines and 0.3mm thin lines.E. Place the following general notes in the lower-left corner of each drawing.

3. UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS.

(or INCHES, as applicable)2. REMOVE ALL BURRS AND SHARP EDGES.

1. INTERPRET PER ASME Y14.5M-1994.

NOTES:

F. Save the drawings as P19-1 through P19-7.1.

2.

S0.562

Title: Washer

Material: SAE 1020

Inch

m 1-4990 1.497

>.375 FLAT

0.249

^7.400

+ .000

-.001

Title: Handle

Material: Bronze

Inch


I r\

646

ALL OTHER THREE5

Title,Material«dild Stei'l

Irfflff W

0.750

Title: Screw

Material: SAE 4320

Inch

0.250+ .001

DECIMALS ±.010


5. This object is shown as a section for clarity. Do not draw a section.

6.

%0a "*Title: Thrust Washer

Material: SAE 5150

Inch

%

Title: SpacerMaterial: Cold Rolled Steel

Metric


Ik

648

7.

Title: Locking CollarMaterial: SAE1080

Metric

8. Draw the vise clamp shown below. Save the drawing as P19-8.

PITCH 64 RAISEDDIAMOND KNURL

0.625

R.250-/ R-063

.250-

0.4688

IX-.205

+ .005["•zuo-.ooo

17 in,J103-

- .438 /-5-44UNF-3

SR0.3125


dimensioning -...

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