PART%20DESIGN%20SPECIFICATION

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PART DESIGN SPECIFICATION
Fall 2008 Dr. R. A. Wysk
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Agenda

• Go over engineering specifications
• Functional requirements
• Form, fit and function
• Dimensioning
• Tolerancing
• Engineering drawings
• datum

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Materials

• Read Chapter 2 and 3 from Computer Aided
  manufacturing (3rd Edition)
• Overview of engineering design
• Mechanical design representations
• Engineering drawing
• Geometric dimensioning and tolerancing
• AMSE Y14.5

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THE DESIGN PROCESSProduct Engineering
Design Process Off-road bicycle that ... 1.
Conceptualization 2. Synthesis 3. Analysis 4.
Evaluation 5. Representation

• Design Process
• How can this be accomplished?
• 1. Clarification of the task
• 2. Conceptual design
• 3. Embodiment design
• 4. Detailed design

Functional requirement -gt Design
Steps 1 2 Select material and properties,
begin geometric modeling (needs
creativity, sketch is sufficient) 3
mathematical, engineering analysis
4 simulation, cost, physical model
5 formal drawing or modeling
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DESIGN REPRESENTATION
Design
Engineering Representation
Manufac- turing

• Verbal
• Sketch
• Multi-view orthographic drawing (drafting)
• CAD drafting
• CAD 3D surface model
• Solid model
• Feature based design

Requirement of the representation method
precisely convey the design concept easy to use
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A FREE-HAND SKETCHOrthographic Projection
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A FORMAL 3-VIEW DRAWING
4 holes 1/4" dia
around 2" dia , first
hole at 45

2.000
0.001
A
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DESIGN DRAFTING
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Third angle projection
Drafting in the third angle
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INTERPRETING A DRAWING

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DESIGN DRAFTING
Partial view
A
-
A
Cut off view and auxiliary view Provide more
local details
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DIMENSIONING

• Requirements
• 1. Unambiguous
• 2. Completeness
• 3. No redundancy

Incomplete dimensioning
0.83 '
Redundant dimensioning
0.86 '
0.83 '
Adequate dimensioning
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TOLERANCE

• Dimensional tolerance - conventional
• Geometric tolerance - modern

nominal dimension

0.95 - 1.05
means a range
1.00 0.05
-
tolerance
0.10 - 0.00
0.00 - 0.10
0.95
1.05
unilateral bilateral

1.00 0.05
-
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TOLERANCE STACKING
1. Check that the tolerance dimension
specifications are reasonable - for
assembly. 2. Check there is no over or under
specification.
"TOLERANCE IS ALWAYS ADDITIVE" why?
0.80 '
0.01
?
What is the expected dimension and tolerances?
d 0.80 1.00 1.20 3.00 t (0.01
0.01 0.01) 0.03
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TOLERANCE STACKING (ii)
?
0.80 '
0.01
1.20 '
0.01
What is the expected dimension and tolerances?
d 3.00 - 0.80 - 1.20 1.00 t (0.01
0.01 0.01) 0.03
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TOLERANCE STACKING (iii)
x
?
0.80 '
0.01
Maximum x length 3.01 - 0.79 - 1.19
1.03 Minimum x length 2.99 - 0.81 - 1.21
0.97 Therefore x 1.00 0.03
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TOLERANCE GRAPH
d,t
d,t
d,t
A B C D
E
d,t

• G(N,d,t)
• N a set of reference lines, sequenced nodes
• d a set of dimensions, arcs
• t a set of tolerances, arcs

d dimension between references i j t
tolerance between references i j
ij
ij
Reference i is in front of reference j in the
sequence.
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EXAMPLE TOLERANCE GRAPH
d,t
d,t
d,t
A B C D
E
d,t
different properties between d t
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OVER SPECIFICATION

• If one or more cycles can be detected in the
  graph, we say that the dimension and tolerance
  are over specified.

d1
d2
A B C
d3
d1,t1
d2,t2
Redundant dimension
d3,t3
A






B






C
t1
t2
A B C
t3
Over constraining tolerance (impossible to
satisfy) why?
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UNDER SPECIFICATION
When one or more nodes are disconnected from the
graph, the dimension or tolerance is under
specified.
d2
d1
A B C D
E
d3
A






B






C








D



















E
C D
is disconnected from the rest of the graph. No
way to find
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PROPERLY TOLERANCED
A






B






C








D



















E
d,t
d,t
d,t
A B C D
E
d,t
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TOLERANCE ANALYSIS

• For two or three dimensional tolerance analysis
• i. Only dimensional tolerance
• Do one dimension at a time.
• Decompose into X,Y,Z, three one dimensional
  problems.
• ii. with geometric tolerance
• ? Don't have a good solution yet. Use
  simulation?

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A circular tolerance zone, the size is influenced
by the diameter of the hole. The shape of
the hole is also defined by a geometric tolerance.
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3-D GEOMETRIC TOLERANCEPROBLEMS
datum surface
datum surface
t
Reference frame
perpendicularity
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TOLERANCE ASSIGNMENT

• Tolerance is money
• Specify as large a tolerance as possible as
  long as functional and assembly requirements can
  be satisfied.
• (ref. Tuguchi, ElSayed, Hsiang, Quality
  Engineering in Production Systems, McGraw Hill,
  1989.)

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function
cost

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Tolerance value
Quality cost
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REASON OF HAVING TOLERANCE

• No manufacturing process is perfect.
• Nominal dimension (the "d" value) can not be
  achieved exactly.
• Without tolerance we lose the control and as a
  consequence cause functional or assembly failure.

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EFFECTS OF TOLERANCE (I)
1. Functional constraints e.g.
flow rate
d t
Diameter of the tube affects the flow. What is
the allowed flow rate variation (tolerance)?
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EFFECTS OF TOLERANCE (II)
2. Assembly constraints
e.g. peg-in-a-hole
dp
How to maintain the clearance?
dh
Compound fitting
The dimension of each segment affects others.
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RELATION BETWEENPRODUCT PROCESSTOLERANCES

• Machine uses the locators as the reference. The
  distances from the machine coordinate system to
  the locators are known.
• The machining tolerance is measured from the
  locators.
• In order to achieve the 0.01 tolerances, the
  process tolerance must be 0.005 or better.
• When multiple setups are used, the setup error
  need to be taken into consideration.

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Design specifications
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Process tolerance
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TOLERANCE CHARTING

• A method to allocate process tolerance and
  verify that the process sequence and machine
  selection can satisfy the design tolerance.

Not shown are process tolerance assignment
and balance
blue print
Operation sequence
produced tolerances
process tol of 10 process tol of 12 process
tol of 20 process tol 22 process tol of 22
setup tol
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SURFACE FINISH
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Usually simplified
waviness height
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waviness width
roughness height
0.002 - 2
63
roughness width cutoff default is 0.03" (ANSI
Y14.36-1978)
0.010 0.005
(m inch)
roughness width
(inch)
Lay
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PROBLEMS WITH DIMENSIONALTOLERANCE ALONE
As designed
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As manufactured
1
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1
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Will you accept the part at right? Problem is
the control of straightness. How to eliminate
the ambiguity?
1
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6
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GEOMETRIC TOLERANCES
ANSI Y14.5M-1977 GDT (ISO 1101, geometric
tolerancing ISO 5458 positional tolerancing
ISO 5459 datums and others), ASME Y14.5 - 1994

• FORM
• straightness
• flatness
• Circularity
• cylindricity

ORIENTATION perpendicularity angularity parallelis
m
Squareness
roundness
LOCATION concentricity true position symmetry
RUNOUT circular runout total runout PROFILE prof
ile profile of a line
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DATUM FEATURE CONTROL FRAME

• Datum a reference plane, point, line, axis where
  usually a plane where you can base your
  measurement.
• Symbol
• Even a hole pattern can be used as datum.
• Feature specific component portions of a part
  and may include one or more surfaces such as
  holes, faces, screw threads, profiles, or slots.
• Feature Control Frame

A
datum
// 0.005 M A
modifier
tolerance value
symbol
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MODIFIERS

• Maximum material condition MMC assembly
• Regardless of feature size RFS (implied unless
  specified)
• Least material condition LMC less frequently used
• Projected tolerance zone
• Diametrical tolerance zone
• T Tangent plane
• F Free state

maintain critical wall thickness or critical
location of features.
MMC, RFS, LMC MMC, RFS RFS
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SOME TERMS

• MMC Maximum Material Condition
• Smallest hole or largest peg (more material left
  on the part)
• LMC Least Material Condition
• Largest hole or smallest peg (less material left
  on the part)
• Virtual condition
• Collective effect of all tolerances specified on
  a feature.
• Datum target points
• Specify on the drawing exactly where the datum
  contact points should be located. Three for
  primary datum, two for secondary datum and one or
  tertiary datum.

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DATUM REFERENCE FRAME
.

• Three perfect planes used to locate the imperfect
  part.
• a. Three point contact on the primary plane
• b. two point contact on the secondary plane
• c. one point contact on the tertiary plane

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Secondary
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STRAIGHTNESS
Tolerance zone between two straightness lines.
Value must be smaller than the size tolerance.
0
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Design
Meaning
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(No Transcript)
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FLATNESS
Tolerance zone defined by two parallel planes.
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CIRCULARITY (ROUNDNESS)
a. Circle as a result of the intersection by any
plane perpendicular to a common axis. b. On
a sphere, any plane passes through a common
center. Tolerance zone bounded by two concentric
circles.
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0
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At any section along the cylinder
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CYLINDRICITY
Tolerance zone bounded by two concentric
cylinders within which the cylinder must lie.
0
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0
1
Rotate in a V
0
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Rotate between points
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PERPENDICULARITY
A surface, median plane, or axis at a right angle
to the datum plane or axis.
.
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A
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ANGULARITY
A surface or axis at a specified angle (orther
than 90) from a datum plane or axis. Can have
more than one datum.
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A
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A
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PARALLELISM
The condition of a surface equidistant at all
points from a datum plane, or an axis
equidistant along its length to a datum axis.
.
0
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PROFILE
A uniform boundary along the true profile within
whcih the elements of the surface must lie.
0
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B
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A
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RUNOUT
A composite tolerance used to control the
functional relationship of one or more features
of a part to a datum axis. Circular
runout controls the circular elements of a
surface. As the part rotates 360 about the
datum axis, the error must be within the
tolerance limit.
A
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TOTAL RUNOUT
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TRUE POSITION
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2
Dimensional tolerance
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Hole center tolerance zone
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True position tolerance
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HOLE TOLERANCE ZONE
Tolerance zone for dimensional toleranced hole is
not a circle. This causes some assembly problems.
For a hole using true position tolerance the
tolerance zone is a circular zone.
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TOLERANCE VALUE MODIFICATION
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B

• Produced True Pos tol
• hole size
• 0.97 out of diametric tolerance
• 0.98 0.01 0.05 0.01
• 0.99 0.02 0.04 0.01
• 1.00 0.03 0.03 0.01
• 1.01 0.04 0.02 0.01
• 1.02 0.05 0.01 0.01
• 1.03 out of diametric tolerance

1
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M L S
B
1
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2
0
MMC LMC
A
The default modifier for true position is MMC.
For M the allowable tolerance specified
tolerance (produced hole size -
MMC hole size)
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MMC HOLE
,

• Given the same peg (MMC peg), when the produced
  hole size is greater than the MMC hole, the hole
  axis true position tolerance zone can be enlarged
  by the amount of difference between the produced
  hole size and the MMC hole size.

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PROJECTED TOLERANCE ZONE
Applied for threaded holes or press fit holes to
ensure interchangeability between parts. The
height of the projected tolerance zone is the
thickness of the mating part.
.
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5

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1
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p
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SOME NUMBERS
Krulikowski, A., GDT Challenges the Fast Draw,
MFG ENG, Feb 1994.

• GDT drawings are more expansive to make,
  however, saves revision cost.
• Drawing revision costs 500 - 2000 on the paper
  work
• How much does it cost to put a part number onto
  a part? Estimates range from 1,000 -10,000.