assembly-oriented design

AOD for LGO 1© Daniel E Whitney04/21/23

Assembly-Oriented Design

Dan Whitney

April 5, 2002


Poll

1. We design assemblies explicitly as part of our product development process

2. Our suppliers design our assemblies3. We design things and our manufacturing

engineers try to get us to change them4. 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


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


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


Sony Does DFA During Concept Design


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


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


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


“Chain of Delivery” of Quality

No single part “delivers” the KC.

Inner Fenders(Budd-Philadelphia) Body Frame

(Ford LAP)

Radiator Sup port(DECO)

C owl Top(Hawthorne)

Assembly Too ling (TESCO)

Fen der Skin(Bud d-Shelbyville)

Reinforcements(Wise, Metalform)

Fix ture Vendor C)

Parts Vendor BFixture Vendor F Part Vendor A

Part Vendor C

Structural Check Fixture:Fixture Vendor E

D-pillar Assembly Station:Fixture Vendor B

Fen der(Part:Bu dd -Shelbyville

C hecking fixture: M&M)Check Fixture Vendor D)

Hood(Ford-Chicago)

Assembly Fixture for Fender:Fixtu re Vendor A

CustomerFeature:Hood Fit toFender

Assembly Fixture for Hood: Fixture Vendor A

Closure Panel Check Fixture: Fixture Vendor G

OrganizationalBoundary

PART COUNT: 9PART SOURCES: 7TOOL COUNT: 5TOOL SOURCES: 4CHECK FIXTURE COUNT: 2CHECK FIXTURE SOURCES: 2DISPERSAL INDEX: 81%

LIF RIF

RS

LOF ROF

H

KC KC

BF

G

Liaison Diagram:


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


Office Stapler.

BASE

HANDLE

CARRIER

ANVIL

ANVIL

CARRIER

PUSHER

STAPLES

RIVET

RIVET

PIN

AX

IS "A

"

AX

IS "B

"

"X" DIRECTION

"Y" D

IRE

CT

ION

SIDE VIEW

TOP VIEW

HAMMER

"Z" D

IRE

CT

ION

BASE

ANVILRIVET

CARRIER

HANDLE

PIN

STAPLES

PUSHER

Liaison Diagram


Datum Flow Chains in the Stapler

BASE

ANVILRIVET

CARRIER

HANDLE

PIN

STAPLES

PUSHER

BASE

ANVILRIVET

CARRIER

HANDLE

PIN

STAPLES

PUSHER

BASE

ANVILRIVET

CARRIER

HANDLE

PIN

STAPLES

PUSHER

BASE

ANVILRIVET

CARRIER

HANDLE

PIN

STAPLES

PUSHER

The datum flow chain is a chain of constraining mates from one end of the KC to the other.


Mates, Contacts, and KC Delivery

BASE

ANVILRIVET

CARRIER

HANDLE

PIN

STAPLES

PUSHER

MateContact

Mates give location.Contacts reinforce location.Variation travels from part to part along the chain of mates.


Coordinate Frames

.

BASE

HANDLEHAMMER

CARRIER

ANVIL

ANVIL

CARRIER

RIVET

RIVET

PIN

AX

IS "A

"AX

IS "B

"

"Y" D

IRE

CT

ION

SIDE VIEW

TOP VIEW

"Z" D

IRE

CT

ION

HANDLE

HAMMER

"X" DIRECTION

PIN

.

HANDLEHAMMER

BASE

CARRIER

ANVIL

ANVIL

CARRIER

STAPLES

RIVET

RIVET

AX

IS "A

"AX

IS "B

"

"X" DIRECTION

"Y" D

IRE

CT

ION

SIDE VIEW

TOP VIEW

"Z" D

IRE

CT

ION

HANDLE

HAMMER

PIN

CRIMPER

CRIMPER

STAPLE

PIN


Chains of F

rames =

A

ssembly

.

BASE

HANDLEHAMMER

CARRIER

ANVIL

ANVIL

CARRIER

RIVET

RIVET

PINA

XIS

"A"A

XIS

"B"

"Y" D

IRE

CT

ION

SIDE VIEW

TOP VIEW

"Z" D

IRE

CT

ION

HANDLE

HAMMER

"X" DIRECTION

STAPLES

KC

KC

PIN

STAPLESKC

KC


Dash

L. Fender

R. Fender

L. Body Side R. Body side

L. DoorR. DoorUnderbody

R. ApronL. Apron

L. I. Shot.R. I. Shot.L. O. Shot.

R. O. Shot.

L. O. RailR. O. Rail

Hood

L. Hinge R. Hinge

Bolster

Hood Latch

L. Lamp R. Lamp

Fascia

L. I. Rail R.I. Rail

y, z

F1

L. BracketL. Bracket

x, x,y, z

y,z

x, z, x,

y, z

y

x

yy

z, y

x, x

66

x, z, x, y, z

x, x,y, z

66

Hood fixture

F F

F F6

6

66

FF Fx, z, x, y x, z x, y

6

F

F

F

F

6

z, x y

z, x y

Fx, x

y, z, y, z y, z, y, z

z, y

y

x, y, z

x, y, z

xy, z

x, y, z x, y, z

F

F

Datum Flow Chain for Car Front End

Drawn by Gennadiy Goldenshteyn, MIT Student


DFC for Aircraft Circumference

KEEL ANDLOWER LOBESUPPLIER'S

FIXTURE

FRAMESUPPLIER'S

FIXTURE

SIDE PANELSUPPLIER'S

FIXTURE

CROWN PANELSUPPLIER'S

FIXTUREFRAMESUPPLIER'S

FIXTURE

FINAL ASSEMBLER'STHIRD FIXTURE

FINAL ASSEMBLER'S FEATURE

SUPPLIER'S FEATURE

SAME FEATURE USED BYSUPPLIER AND FINAL ASSEMBLER

DATUMSHIFT

FINAL ASSEMBLER'SSECOND FIXTURE

FINAL ASSEMBLER'SFIRST FIXTURE

HANDTOOL

OVER-CONSTRAINT


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


Connective Assembly Model

Parts A and B are joined by two features

The nominal location of part B can be calculated from the nominal location of part A using 4x4 transform math

TT

T

AF

FB

AB

A

B

TAF

AT

FB

B


Varied Part Location Due to Variation

TT

T

AFFB'

AB'

AB'

TBB'

The varied location of Part B can be calculated from the nominal location of Part A. This processcan be chained to Part C, etc., including errors onPart B. It uses the same math as the nominal model.


Stapler Variations.

CARRIER

STAPLE

ANVILRIVETCRIMPER

X ERROR

ANVIL

RIVETCRIMPER

CARRIER

STAPLES

Y ERROR


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


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


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


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


Assembly Design Process

Nominal Design:

Identify each KCDesign a DFC for it

Choose features to build constrained DFCCheck for proper constraint

Check for KC conflictFind a suitable assembly sequence

Variational Design:

Check for robustness of DFC against variationsCheck achievement of each KC using tolerance analysis

See if a different assembly sequence gives better variation


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