Assembly-Oriented Design
Dan Whitney
April 5, 2002
AOD for LGO
3/19/2016
© Daniel E Whitney
1
Poll
1. We design assemblies explicitly as part of our
product development process
2. Our suppliers design our assemblies
3. We design things and our manufacturing
engineers try to get us to change them
4. We design parts using the best CAD system in
the world and then we wonder why we have
trouble assembling them
5. We don’t have any assembly problems
AOD for LGO
3/19/2016
© Daniel E Whitney
2
Scope of “Assembly”
• Assembly spans the entire range from point
processes to business strategy
• Assemblies are things that do something
– Attributes
– Architecture
– Families, platforms…
• Assembly is a process of putting things together
–
–
–
–
On the factory floor
Operations
Equipment
Ergonomics
AOD for LGO
3/19/2016
© Daniel E Whitney
3
Scope of Assembly - 2
• Design for assembly
– Part handling and mating
– Part consolidation
• Integral architecture favors performance
• Modular architecture favors business issues
• Design of assemblies - technical and business issues
–
–
–
–
–
Design intent
CAD representation
Key Characteristics
Math models, constraint, tolerances
Architectures, families, delayed commitment, flexibility
AOD for LGO
3/19/2016
© Daniel E Whitney
4
Sony Does DFA During Concept Design
AOD for LGO
3/19/2016
© Daniel E Whitney
6
Things an Assembly Theory Must Do
• Represent top-level goals for the assembly
• Link these goals to requirements on the assembly
and the parts
• Represent nominal and varied location of parts in
space
• Provide for declaration of mutual constraint
between parts
• Merge design of assembly and of assembly
processes, including adjustments and fixtures
• Support a design process for assemblies that can
be added to CAD
AOD for LGO
3/19/2016
© Daniel E Whitney
7
This Theory of Assembly...
• Focuses on “Kinematic Assemblies”
• Emphasizes Delivery of Key Characteristics (KCs)
• Documents KC Delivery and Constraint with the
Datum Flow Chain (DFC)
• Achieves Constraint with Assembly Features
• Achieves DFC Robustness via Tolerance Analysis
• Exploits Underconstrained Assemblies to Achieve
Adjustments
AOD for LGO
3/19/2016
© Daniel E Whitney
8
What Happens During Assembly
• People think assembly is fastening
• Assembly is really the chaining together of
coordinate frames
• These chains of frames “deliver” certain parts or
features on parts to desired places in space relative
to other parts or features on them within tolerances
• Complete chains describe Key Characteristics of
the assembly
• This theory of assembly generates a design process
for assemblies based on creating these chains
AOD for LGO
3/19/2016
© Daniel E Whitney
9
“Chain of Delivery” of Quality
Closure Panel Check Fixture: Fixture Vendor G
PART COUNT: 9
PART SOURCES: 7
TOOL COUNT: 5
TOOL SOURCES: 4
CHECK FIXTURE COUNT: 2
CHECK FIXTURE SOURCES: 2
DISPERSAL INDEX: 81%
Assembly Fixture for Hood: Fixture Vendor A
Hood
(Ford-Chicago)
Customer
Feature:
Hood Fit to
Fender
Assembly Fixture for Fender:
Fixtu re Vendor A
Organizational
Boundary
Li ai son
Di agr am:
H
KC
Fen der
(Part:Bu dd -Shelbyville
C
hecking
fixture:
Check Fixture
VenM&M)
dor D)
KC
L OF
Structural Check Fixture:
Fixture Vendor E
Assembly Too ling (TESCO)
Fix ture Vendor C )
Fen der Skin
(Bud d-Shelbyville)
D-pillar Assembly Station:
Fixture Vendor B
ROF
BF
LIF
RI F
RS
G
Reinforcements
(Wise, Metalform)
Parts Vendo r B
Fixture Vendo r F
Radiator Sup port
(DECO)
Inner Fenders
(Budd-Philadelphia)
Part Vendor C
Body Frame
(Ford LAP)
C owl Top
(Hawthorne)
Part
Vendor A
No single part “delivers” the KC.
AOD for LGO
3/19/2016
© Daniel E Whitney
10
Maintaining Oversight on KCs
• To design the chains that deliver the KCs, we have
developed the Datum Flow Chain (DFC)
• A DFC is an assembly-level statement of design
intent that– documents the chain that delivers the KC
– identifies the parts that make up the chain
– provides a skeleton for the strategy by which the parts
will be located in space as links in the chain
• Each step in the assembly process adds links to the
chain and each subassembly is kinematically
constrained
AOD for LGO
3/19/2016
© Daniel E Whitney
11
Office Stapler
AXIS "A"
.
AXIS "B"
PIN
PUSHER
"Z" DIRECTION
TOP VIEW
STAPLES
CARRIER
RIVET
ANVIL
HANDLE
CARRIER
PIN
HAMMER
RIVET
STAPLES
ANVIL
SIDE VIEW
HANDLE
CARRIER
RIVET
BASE
Liaison Diagram
AOD for LGO
3/19/2016
© Daniel E Whitney
ANVIL
"Y" DIRECTION
PUSHER
BASE
"X" DIRECTION
12
Datum Flow Chains in the Stapler
HANDLE
HANDLE
CARRIER
CARRIER
PUSHER
PUSHER
PIN
PIN
RIVET
RIVET
STAPLES
STAPLES
ANVIL
ANVIL
BASE
BASE
HANDLE
HANDLE
CARRIER
CARRIER
PUSHER
PIN
RIVET
STAPLES
ANVIL
PUSHER
PIN
RIVET
STAPLES
ANVIL
BASE
BASE
The datum flow chain is a chain of constraining mates from one end of the KC to the other.
AOD for LGO
3/19/2016
© Daniel E Whitney
13
Mates, Contacts, and KC Delivery
HANDLE
CARRIER
PUSHER
Contact
PIN
Mate
RIVET
STAPLES
ANVIL
BASE
AOD for LGO
3/19/2016
© Daniel E Whitney
Mates give location.
Contacts reinforce location.
Variation travels from part
to part along the chain
of mates.
14
AXIS "A"
AXIS "A"
AXIS "B"
AXIS "B"
Coordinate Frames
HAMMER
HAMMER
HANDLE
HANDLE
TOP VIEW
PIN
CRIMPER
CARRIER
TOP VIEW
PIN
RIVET
RIVET
ANVIL
ANVIL
HAMMER
HAMMER
"Z" DIRECTION
CARRIER
"Z" DIRECTION
STAPLES
HANDLE
HANDLE
PIN
CARRIER
SIDE VIEW
CRIMPER
CARRIER
SIDE VIEW
RIVET
RIVET
"Y" DIRECTION
STAPLE
"Y" DIRECTION
PIN
ANVIL
ANVIL
BASE
BASE
"X" DIRECTION
"X" DIRECTION
AOD for LGO
3/19/2016
© Daniel E Whitney
15
Chains of Frames =
Assembly
AOD for LGO
3/19/2016
© Daniel E Whitney
16
Datum Flow Chain for Car Front End
F1
L. Body Side
R. Body side
6
Underbody
L. Door
6
R. Door
6
F
F
6
L. O. Rail
y,z
F
x, y, z
x, x,y, z
L. Apron
y
L. O. Shot.
z, x y
F
x, y, z
z, y
F
x, x,y, z
F
y, z, y, z
R.I. Rail
F
F
x, z, x, y
x, x
F
F
6
Hood
x, z
x, y
F
Hood fixture
x, x
L. I. Shot.
x
y, z
y
R. Apron
x, y, z
y
z, x y
R. O. Shot.
F
x, y, z
z, y
6
R. Hinge
L. Bracket
F
y, z, y, z
R. I. Shot.
L. Hinge
L. Fender
6
R. O. Rail
6
L. I. Rail
F
6
Dash
6
R. Fender
L. Bracket
Hood Latch
x
y, z
y
Bolster
R. Lamp
L. Lamp
x, z, x, y, z
x, z, x,
y, z
AOD for LGO
Fascia
3/19/2016
© Daniel E Whitney
Drawn by Gennadiy Goldenshteyn, MIT Student
17
DFC for Aircraft Circumference
AOD for LGO
3/19/2016
© Daniel E Whitney
18
DFC Carries Design Intent
• Designer declares how KCs will be delivered
• Intent is expressible in CAD terms
• Intent expressed this way is independent of CAD
vendor
• DFCs can be bookshelved for future use
AOD for LGO
3/19/2016
© Daniel E Whitney
19
Connective Assembly Model
T AF
T
A
FB
B
Parts A and B are joined by two features
T AF
T
A
T AB
FB
B
The nominal location of part B can be calculated from
the nominal location of part A using 4x4 transform math
AOD for LGO
3/19/2016
© Daniel E Whitney
20
Varied Part Location Due to Variation
T
AF
TFB'
A
T AB'
B'
TBB'
The varied location of Part B can be calculated
from the nominal location of Part A. This process
can be chained to Part C, etc., including errors on
Part B. It uses the same math as the nominal model.
AOD for LGO
3/19/2016
© Daniel E Whitney
21
Stapler Variations
AOD for LGO
3/19/2016
© Daniel E Whitney
22
When Parts are Joined, Degrees of
Freedom are Fixed
• Parts join at places called assembly features
• Different features constrain different numbers and
kinds of degrees of freedom of the respective parts
(symmetrically)
• Parts may join by
– one pair of features
– multiple features
– several parts working together, each with its own
features
• When parts mate to fixtures, dofs are constrained
AOD for LGO
3/19/2016
© Daniel E Whitney
23
Overconstrained and “Properly”
Constrained Assemblies
• Assemblies that function by geometric
compatibility and force/moment equilibrium are
called
– statically determinate
– “properly” constrained
– “kinematic” or “semi-kinematic”
• Assemblies that require the other principle of
statics (stress-strain relations) are called
– statically indeterminate
– “over-constrained”
• Constraint is a property of the nominal design
AOD for LGO
3/19/2016
© Daniel E Whitney
24
Summary of Assembly Theory Nominal Design
• An assembly is a set of parts that deliver their quality,
defined by the KCs, as a result of the geometric
relationships between the parts (and fixtures)
• Designing an assembly means designing these
relationships in terms of one DFC per KC
– The DFC documents the nominal relationships in terms of
constraint
– The DFC passes from part to part via mates
• The nominal design is a constraint structure
• Assembly features create the constraint relationships
at each mate
AOD for LGO
3/19/2016
© Daniel E Whitney
32
Summary of Assembly Theory Variation Design
• Tolerances should assure the robustness of the DFC
• KC delivery is verified by a tolerance analysis of
each DFC
• Tolerances on parts derive from tolerances on the
KCs
• Part tolerances are sublinks of the DFC
• Type-1 assembly-level tolerances come from part
tolerances
• Type-2 assembly-level tolerances can be altered by
adjustments to the assembly process
AOD for LGO
3/19/2016
© Daniel E Whitney
33
Assembly Design Process
Nominal Design:
Identify each KC
Design a DFC for it
Choose features to build constrained DFC
Check for proper constraint
Check for KC conflict
Find a suitable assembly sequence
Variational Design:
Check for robustness of DFC against variations
Check achievement of each KC using tolerance analysis
See if a different assembly sequence gives better variation
AOD for LGO
3/19/2016
© Daniel E Whitney
35
Assembly Course Topics
• Assembly in the small:
– Physics of part mating
• Assembly in the large:
–
–
–
–
–
Key characteristics
Constraint
Tolerances
DFA
Product architecture, customization
• A class project on these topics lasts all term
AOD for LGO
3/19/2016
© Daniel E Whitney
36