Transcript Chapter 8.1-8.5: Basic Dimensioning

Engineering
Drawing and Design
Chapter 10
Basic Dimensioning
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Dimensions are given on drawings by extension lines, dimension lines, leaders,
arrowheads, figures, notes and symbols
◦ Length
◦ Diameter
◦ Angle
◦ Location
◦ NOT duplicated
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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…
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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
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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
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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
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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 Dimensioning
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3 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.
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Tolerance
◦ The total amount that the feature on the actual part
can vary from the drawing or model.
Definitions continued…
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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
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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
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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 measurement
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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)
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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
◦ 27 inches

2’-0 (inch marks “ not shown)
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 mentioning
Dual Dimensioning specify both English/SI
units but not generally used anymore
 Angular Units decimal degree is preferred
over degrees, minutes, seconds
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◦ 60.5° instead of 60°30’
DUAL DIMENSIONING and
COMBINATION 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.
2.
Position Method
Bracket Method
DIMENSIONS IN () ARE MILLIMETERS
Unidirectional dimensioning: read
from the bottom of the drawing
Rules for Basic Dimensioning
 Symmetrical Outlines
 Reference dimensions placed in
parentheses
 Not to scale underlined with a straight
thick line
 Abbreviations: Table 4 Appendix A-4
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DIMENSION SYMBOLS
Dimensioning 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 VIEWS
What are the absolute minimum views
required to completely define an object?
One-View Drawing
Two-View Drawing
Three-View Drawing
BASIC RULES FOR DIMENSIONING
UNIDIRECTIONAL
USED ON ENGINEERING DRAWINGS
BASIC RULES FOR DIMENSIONING
ALIGNED
USED 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 Features
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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, Spotface
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Countersink is an angular-sided recess that
accommodates the head of flathead screws,
rivets, and similar items [ф.40ф.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 features
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)
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◦ Dimensioned normally by their angle and linear length
◦ Example: 45°X.10
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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)
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