8.1-8.3 Basic Dimension Ing

8/3/2019 8.1-8.3 Basic Dimension Ing

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Chapter 8Chapter 8

EngineeringDrawing and Design

EngineeringDrawing and Design

Seventh EditionSeventh Edition


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Chapter 8.1: Basic DimensioningChapter 8.1: Basic Dimensioningy Dimensions are given on drawings by extension lines, dimension lines, leaders,

arrowheads, figures, notes and symbols

Length

Diameter

Angle

Location NOT duplicated

y Dimension lines are used to determine the extent and direction of dimensions

Terminated by uniform arrows

Arrows: 3:1 length/width recommended

Center lines never used for dimension lines

Avoid dimensioning to hidden lines

Avoid crossing dimension lines

Place dimension lines outside extension lines when space is tight (fig.8.3)


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Definitions continuedDefinitions continued

y Extension lines are used to indicate the point or line on thedrawing to which the dimension applies (fig.8-5)

Gap between extension line and visible object line

Perpendicular to dimension lines

Center lines may be used as extension lines

y Leaders are used to direct notes, dimensions, symbols, itemnumbers, or part numbers to features on a drawing (fig.8-6)

Arrowheads terminate on a line; dots within the outline of the object

Leaders do not cross eachother

Leaders are pointed to the center of circles or arcs

y

Notes are used to simplify or complement dimensioning by givinginformation on a drawing in condensed and systematic manner

General notes placed in a central portion below the view which they apply

Local notes are connected by a leader to the point to which they apply


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BASIC DIMENSIONING

DIMENSION ANDEXTENSION LINES


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BASIC DIMENSIONING


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PLACEMENT OF DIMENSIONS

INCORRECT


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INCORRECT


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INCORRECT


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CORRECT


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Units of measurementUnits of measurementy Decimal-Inch parts are designed in basic decimal increments,

preferably .02 in, and expressed with a minimum of two figures tothe right of the decimal point (fig.8-7)

Whole dimensions: 24.00 NOT 24

Decimal dimensions: .44 NOT 0.44 (no zero before the decimal point)

y

Fractional-Inch is not ANSI recommended. 1/64 inch. not usedy Foot-and-InchAll dimensions 12 inches and greater are specified in

feet and inches

24 inches 2-0 (inch marks not shown)

27 inches 2-3

y SI Metric Units mm or m (micrometer)

Whole numbers 2 NOT 02 or 2.0 (numbers 1-9 shown without zero to left ofdecimal)

Decimal 0.2 NOT .2 or .20 (numbers


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DIMENSIONING UNITS

DECIMAL INCH


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DIMENSIONING UNITS

FEET AND INCHES


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DIMENSIONING UNITS

MILLIMETERS


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Worth mentioningWorth mentioning

y Dual Dimensioning specify both English/SI

units but not generally used anymore

y Angular Units decimal degree is preferredover degrees, minutes, seconds

60.5 instead of 6030


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Unidirectional dimensioning: readUnidirectional dimensioning: read

from the bottom of the drawingfrom the bottom of the drawingy Rules for Basic Dimensioning (p.183)y Symmetrical Outlines (fig.8.13)

y Reference dimensions (fig.8.14) placed in

parentheses

y Not to scale (fig.8.15) underlined with a

straight thick line

y Abbreviations: Table 4 Appendix A-4


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BASIC RULES FOR DIMENSIONING

UNIDIRECTIONALUSED ON ENGINEERINGDRAWINGS


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ALIGNEDUSED ON ARCHITECTURALAND STRUCTURAL DRAWINGS


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PLACE DIMENSIONS BETWEEN VIEWS


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PLACE SMALLEST DIMENSIONS NEAREST

THE VIEW BEING DIMENSIONED


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88--2 Dimensioning Circular Features2 Dimensioning Circular Features

y Diameters

One view drawing-longitudinal view(fig.8-16)

End view drawing (fig.8-16c)

y Radii-circular arc

Passes through radius center and terminates with arrow touchingarc (fig.8-18)

R precedes the numerical value

Cross at center of radius (fig.8-18b)

Simple fillets and rounds dimensioned with a general note

y

Rounded ends (fig.8-20)y Cylindrical Holes-leader usually used (fig.8-22,8-23)

y Multiple Holes example 4 X 8.4 (fig.8-25)

y Slotted Holes-(fig.8-26)


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Countersink,Countersink, CounterboreCounterbore,, SpotfaceSpotface

(fig. 8(fig. 8--26 & 826 & 8--27)27)y Countersinkis an angular-sided recess that

accommodates the head of flathead screws,rivets, and similar items [.40.80x82]

y

Counterbore is a flat-bottomed, cylindricalrecess that permits the head of a fasteningdevice, such as a bolt, to lie recessed into thepart[.38v .75x.25]

y

Spotface is an area in which the surface ismachined just enough to provide smooth,level seating for a bolt head, nut, or washer[.38 v .75]


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88--3 Dimensioning common features3 Dimensioning common features

y Repetitive features and dimensions use an X in conjunctionwith the numeral to indicate the number of times

y Chamfering is the process of cutting away the inside oroutside piece (fig.8-29) Dimensioned normally by their angle and linear length

Example: 45X.10y Slopes and tapers

Slope is the slant of a line

A taper is the ratio of the difference in the diameters of twosections (fig.8-31)

y Knurls is specified in terms of type, pitch, diameter before

and after knurling (fig. 8-32) Types include: Straight, Diagonal,Spiral, Convex, Raised Diamond

y Undercutting or necking is the operation of cutting a recessin a diameter that is done to permit two parts to cometogether (fig.8-34a)

8.1-8.3 Basic Dimension Ing

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