chapter 10 basic dimensioning engineering drawing and design engineering drawing and design
TRANSCRIPT
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Chapter 10Chapter 10
Basic Dimensioning
EngineeringDrawing and DesignEngineeringDrawing and Design
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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)
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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
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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.
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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.
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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
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BASIC DIMENSIONING
DIMENSION AND EXTENSION LINES
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BASIC DIMENSIONING
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PLACEMENT OF DIMENSIONS
INCORRECT
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PLACEMENT OF DIMENSIONS
INCORRECT
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PLACEMENT OF DIMENSIONS
INCORRECT
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PLACEMENT OF DIMENSIONS
CORRECT
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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
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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 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’
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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
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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
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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.)
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NECESSARY VIEWSNECESSARY VIEWS
One-View Drawing
Two-View Drawing
Three-View Drawing
What are the absolute minimum viewsrequired to completely define an object?
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BASIC RULES FOR DIMENSIONING
UNIDIRECTIONALUSED ON ENGINEERING DRAWINGS
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BASIC RULES FOR DIMENSIONING
ALIGNEDUSED ON ARCHITECTURAL AND STRUCTURAL DRAWINGS
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BASIC RULES FOR DIMENSIONING
PLACE DIMENSIONS BETWEEN VIEWS
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BASIC RULES FOR DIMENSIONING
PLACE SMALLEST DIMENSIONS NEAREST THE VIEW BEING DIMENSIONED
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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-
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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]
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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)