ieng 248 d. h. jensen 10/2/2015engineering graphics & 3-d modeling1 lecture 06 dimensioning...
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IENG 248
D. H. Jensen04/19/23 Engineering Graphics & 3-D Modeling 1
Lecture 06
Dimensioning & Fastener Specification
IENG 248
D. H. Jensen04/19/23 Engineering Graphics & 3-D Modeling 2
Assignment: HW 06
Reading Skim CH 9, 11
Review pp. 380 – 381 (pp. 314 – 318 in old text) (Dimensioning Do’s & Don’ts)
Assignment: Ex. 9.2, p. 387 (Fig. 9.67, p.327 in old text):
Sketch dimensioned views for b and d, onlyScale drawing so that the parts are roughly double
size on your paper (smallest hole diameter is 2 squares in width)
Draw orthographic, multi-view with straight edge
(not CAD)Use English units – 1 square is .20 inches / side
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Dimensioning
Why dimension drawings? Historical practicality Production inaccuracies Clarity of:
SizePositionGeometric Form
How to dimension Know dimensioning techniques Select dimension placement Choose dimensions shown to convey:
Functionality of part (very similar to design intent)Manufacturing steps for part
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Dimensioning Terms
Nominal Size The approximate size used for naming purposes
Limits The extreme allowable sizes for a feature
Basic Size The theoretical exact size from which limits for
permissible variation are derived Basic Dimension
The theoretical exact location, to which tolerances are applied so as to limit permissible geometry variation
Datum A feature that is assumed to be exact for the purposes
of manufacture and inspection Tolerance
The total amount the feature is allowed to vary(upper limit - lower limit)
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D. H. Jensen04/19/23 Engineering Graphics & 3-D Modeling 5
Dimensioning Terms
Allowance The minimum space between mating parts
Purposes of Dimensions Specify the size of a feature Specify the location of a feature
Purpose of Tolerances Specify the allowable variation of a feature
Variation in sizeVariation in positionVariation in form (shape)
Dimensions & Tolerances specify relationships Functional relationships Manufacturing relationships Inspection relationships
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Dimensioning Lines
Dimensioning Line Types Dimension Line
Thin, dark, solid, with arrowheads at ends
Extension (Witness) LineThin, dark, solid, starting a small distance from the
feature and running to just past the dimension line
CenterlineThin, dark, long-short-long pattern; may be extended
for dimensioning
LeaderThin, dark, solid, with a single arrowhead/dot, at a
large angle with respect to feature being referenced
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Dimensioning Systems
Dimension Text Directions Aligned Unidirectional
Dual Dimensioning Metric on top / left, English on bottom / right Metric in square brackets, position consistent
Decimal Dimensioning Systems Complete decimal dimensioning Combination dimensioning
mixed decimal and nominal dimensions Number of decimals shown reflects tolerance
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Dimension Rounding
Three part rule:1. If the number following the rounding position
is less than 5, the positional digit is unchanged
2. If the number following the rounding position is exactly 5, round the position to the closest even digit
3. Otherwise, increment the positional digit
Exception: Diameters and other dimensions likely to be
divided by 2 should be rounded to an even digit, unless the precision is critical for strength, fit, etc.
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D. H. Jensen04/19/23 Engineering Graphics & 3-D Modeling 9
Dimensional Values
Clarity is most important Decimal point is bold Avoid crowding Always break lines crossing dimension values
Metric Has leading zeros on fractions less than one mm There are no right zeros in fractions unless it is
expressing the tolerance
English Has NO leading zeros on fractions less than 1 inch The number of right zeros in fractions are the same as
the feature tolerance
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D. H. Jensen04/19/23 Engineering Graphics & 3-D Modeling 10
Placement
Dimension lines may NOT cross other dimension
lines
Extension lines may cross other extension lines
and visible lines, but must be broken if crossing a
dimension line. There is a small gap between an
extension line and the object it references
Centerlines may be used as extension lines for
dimensioning purposes
Leaders may be used to specify radii and hole
diameters, but NOT protrusion diameters
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Rules for Dimensions
Prime Directive: Dimensions should not be shown on a view
unless they promote the clarity of the drawing Important Rules:
Show dimensions where shapes are shown in profile and contours are shown true size
Align dimension lines and group dimensions as much as possible for reading ease
Prefer placing dimensions between views Dimension lines are drawn at right angles to
extension / centerlines whenever possible, but always parallel to the measured distance in any case
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Standard Symbols
Counterbore / Spotface
Countersink (not a finish mark)
Depth (blind hole)
Diameter / Round shape
Radius
Places / times / by
Reference Square shape
R
( )
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Manufacturing Considerations
Dimension from datums (finished measuring surfaces) Finish Mark (examples)
Avoid tolerance stacks Chain vs. Datum dimensioning
Do not over-dimension Leave one dimension missing in a locational chain
Do not over-specify tolerances / conditions Over specification can result in excessive cost
Design for manufacturability / assembly / X See Figure 9.51, p. 314 (old text, only)
Append notes to clarify intent
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Assignment: HW 06
Reading Skim CH 9, 11
Review pp. 380 – 381 (pp. 314 – 318 in old text) (Dimensioning Do’s & Don’ts)
Assignment: Ex. 9.2, p. 387 (Fig. 9.67, p.327 in old text):
Sketch dimensioned views for b and d, onlyScale drawing so that the parts are roughly double
size on your paper (smallest hole diameter is 2 squares in width)
Draw orthographic, multi-view with straight edge
(not CAD)Use English units – 1 square is .20 inches / side
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D. H. Jensen04/19/23 Engineering Graphics & 3-D Modeling 15
Fastener Terminology
Screw Thread a helical ridge of uniform section on the
curved surface of a cylinder (internal or external)
External Thread a thread on the outside of a member (shaft)
Internal Thread a thread on the inside of a member (hole)
Major Diameter the largest diameter of a screw thread
Minor Diameter the smallest diameter of a screw thread
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Fastener Terminology
Angle of Thread the angle between the two sides of a thread,
measured on a plane through the axis of the screw
Crest the top surface/edge joining the thread sides
Root the bottom surface/edge joining the thread
sides
Side the surface connecting the crest with the root
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Fastener Terminology
Pitch the distance from one thread to the next,
measured parallel to the axis. The reciprocal of the number of threads per inch.
Pitch Diameter the diameter of a cylinder if it passed through
the point on the thread where the area of the threads and the space between threads is equal.
Lead the distance a thread advances axially in one
turn.
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Fastener Terminology
Axis of Screw the longitudinal centerline through the screw
Depth of Thread distance from crest to root measured normal
to the axis
Form of Thread cross section of thread cut by a plane through
the axis
Series of Thread standard number of threads per inch and
based on the screw diameter
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Thread Forms
Sharp V Whitworth Standard American National Unified Metric Standard Worm Square Acme Knuckle Buttress
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Thread Notes
Thread Series Coarse Fine Extra Fine 8-pitch 12-pitch 16-pitch
Thread Fits Class 1 Class 2 Class 3
Internal / External A - external B - internal
Thread Engagement S - short N - normal L - long
Multiple Threads DOUBLE TRIPLE QUADRUPLE
Left / Right Hand RH assumed LH must be noted Rule of Thumb:
point thumb into material, and fingers will curl in turning direction
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Coding a Thread Note
American National1. Major Diameter
2. Threads Per Inch
3. Form & Series
4. Fit Class
5. LH/RH
6. Thread Depth
Metric1. Metric Designation
2. Major Diameter
3. Pitch
4. Tolerance Grades
5. LH/RH
Unified National1. Major Diameter
2. Threads Per Inch
3. Form & Series
4. Fit Class
5. LH/RH
6. Thread Depth
Examples 3/4-10NC-2A LH-1.00
.75-8UNF-3B DOUBLE-.50 DEEP
M10X1.5-6H/5g6g LH
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Thread Representation
Detailed shows thread profile approximate helix by straight lines between crests
Simplified shows thread depth by hidden lines parallel to the axis length is determined by thread length
Schematic show alternating crest (thin) lines and root (thick) lines
perpendicular to the axis length is determined by thread length
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Standard Fasteners
Screws Nuts & Bolts Washers Keys Rivets (note spelling!)
Pins Comments:
Text gives typical shapes & dimensions for the heads
Appendices give standard dimensions for nominal sizes of fastener geometry
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Specifying a Fastener
Bolts & Nuts: Nominal body size Thread spec/note Length Finish Head style Name*
CAP SCRMACH SCRSET SCRNUTEtc.
Examples: M10 X 1.5 X 12 HEX HD SCR M10 X 1.5 X 12 HEX SOC HD .375 X 1.25 SQ HD SS 10-32 X 5/8 SLOT PAN HD MACH
SCR
Abbreviations: See Appendix 4,
pp. a21 - a 24
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