Chapter 10Chapter 10

Basic Dimensioning

EngineeringDrawing and DesignEngineeringDrawing and Design

Basic DimensioningBasic Dimensioning Dimensions are given on drawings by extension lines, dimension

lines, leaders, arrowheads, figures, notes and symbols◦ Length

◦ Diameter

◦ Angle

◦ Location

◦ NOT duplicated

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)

Definitions continued…Definitions continued… Extension lines are used to indicate the point or line on the

drawing 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

Leaders are used to direct notes, dimensions, symbols, item numbers, 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

Notes are used to simplify or complement dimensioning by giving information 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

Basic DimensioningBasic Dimensioning Overview:

◦ Dimensions and notes define the size, finish, and other requirements to fully define what you want manufactured.

◦ Drawings for products must be dimensioned so that production personnel all over the world can make mating parts that will fit properly when assembled or when used to replace parts.

Good DimensioningGood Dimensioning3 Aspects of Good Dimensioning

◦ Technique: Good appearance of lines, spacing and arrows to allow

others to read the drawing.

◦ Placement: Logical placement according to standard practices, so

they are legible and east to find.

◦ Choice: Dimension for function and best manufacturing

practices.

Tolerance◦ The total amount that the feature on the actual

part can vary from the drawing or model.

Definitions continued…Definitions continued… Extension lines are used to indicate the point or line on the

drawing to which the dimension applies Gap between extension line and visible object line

◦ Perpendicular to dimension lines

◦ Center lines may be used as extension lines

Centerlines are used to locate holes or other symmetrical features. Thin, dark lines alternating long and short dashes

◦ Can cross other lines with out requiring gaps

Leaders are used to direct notes, dimensions, symbols, item numbers, or part numbers to features on a drawing Arrowheads terminate on a line; dots within the outline of the object

◦ Leaders do not cross each other

◦ Leaders are pointed to the center of circles or arcs

Notes are used to simplify or complement dimensioning by giving information 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

BASIC DIMENSIONING

DIMENSION AND EXTENSION LINES

BASIC DIMENSIONING

PLACEMENT OF DIMENSIONS

INCORRECT

PLACEMENT OF DIMENSIONS

INCORRECT

PLACEMENT OF DIMENSIONS

INCORRECT

PLACEMENT OF DIMENSIONS

CORRECT

Units of measurementUnits of measurement Decimal-Inch parts are designed in basic decimal

increments, preferably .02 in, and expressed with a minimum of two figures to the 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)

Fractional-Inch is not ANSI recommended. 1/64 inch. “ not used

Foot-and-Inch All dimensions 12 inches and greater are specified in feet and inches◦ 24 inches 2’-0 (inch marks “ not shown)◦ 27 inches 2’-3

SI Metric Units mm or m (micrometer)◦ Whole numbers 2 NOT 02 or 2.0 (numbers 1-9 shown without zero to

left of decimal)◦ Decimal 0.2 NOT .2 or .20 (numbers <1 shown with a zero)◦ Large numbers 32545 (no comma or spaces to separate digits)◦ General note like: unless otherwise specified dimensions are in

millimeters

DIMENSIONING UNITS

DECIMAL INCH

DIMENSIONING UNITS

FEET AND INCHES

DIMENSIONING UNITS

MILLIMETERS

Worth mentioningWorth mentioningDual Dimensioning specify both

English/SI units but not generally used anymore

Angular Units decimal degree is preferred over degrees, minutes, seconds◦60.5° instead of 60°30’

DUAL DIMENSIONING and DUAL DIMENSIONING and COMBINATION UNITSCOMBINATION UNITS

Dual dimensioning is used to show metric and decimal-inch dimensions on the same drawing. Two methods of displaying the dual dimensions are:

1. Position Method2. Bracket Method DIMENSIONS IN () ARE MILLIMETERS

Unidirectional dimensioning: Unidirectional dimensioning: read from the bottom of the read from the bottom of the drawingdrawingRules for Basic Dimensioning Symmetrical Outlines Reference dimensions placed in

parenthesesNot to scale underlined with a

straight thick lineAbbreviations: Table 4 Appendix

A-4

DIMENSION SYMBOLSDIMENSION SYMBOLSDimensioning symbols are used to replace traditional terms or abbreviations.

Form and Proportion of Dimensioning Symbols. (Reprinted from ASME Y14.5M-1994 (R2004),by permission of The American Society of Mechanical Engineers. All rights

reserved.)

NECESSARY VIEWSNECESSARY VIEWS

One-View Drawing

Two-View Drawing

Three-View Drawing

What are the absolute minimum viewsrequired to completely define an object?

BASIC RULES FOR DIMENSIONING

UNIDIRECTIONALUSED ON ENGINEERING DRAWINGS

BASIC RULES FOR DIMENSIONING

ALIGNEDUSED ON ARCHITECTURAL AND STRUCTURAL DRAWINGS

BASIC RULES FOR DIMENSIONING

PLACE DIMENSIONS BETWEEN VIEWS

BASIC RULES FOR DIMENSIONING

PLACE SMALLEST DIMENSIONS NEAREST THE VIEW BEING DIMENSIONED

8-2 Dimensioning Circular 8-2 Dimensioning Circular FeaturesFeatures Diameters

◦ One view drawing-longitudinal view◦ End view drawing

◦ Radii-circular arc◦ Passes through radius center and terminates with

arrow touching arc ◦ R precedes the numerical value◦ Cross at center of radius ◦ Simple fillets and rounds dimensioned with a general

note Rounded ends Cylindrical Holes-leader usually used Multiple Holes – example 4 X ф8.4 Slotted Holes-

Countersink, Counterbore, Countersink, Counterbore, SpotfaceSpotfaceCountersink is an angular-sided recess

that accommodates the head of flathead screws, rivets, and similar items [ф.40ф.80x82°]

Counterbore is a flat-bottomed, cylindrical recess that permits the head of a fastening device, such as a bolt, to lie recessed into the part[ф.38v ф .75x.25]

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

8-3 Dimensioning common 8-3 Dimensioning common featuresfeatures Repetitive features and dimensions use an X in

conjunction with the numeral to indicate the “number of times”

Chamfering is the process of cutting away the inside or outside piece (fig.8-29)◦ Dimensioned normally by their angle and linear length◦ Example: 45°X.10

Slopes and tapers ◦ Slope is the slant of a line◦ A taper is the ratio of the difference in the diameters

of two sections (fig.8-31) Knurls is specified in terms of type, pitch, diameter

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

Undercutting or necking is the operation of cutting a recess in a diameter that is done to permit two parts to come together (fig.8-34a)