AOD for LGO 1 © Daniel E Whitney 9/11/2015 Assembly-Oriented Design Dan Whitney April 5, 2002

AOD for LGO 2 © Daniel E Whitney 9/11/2015 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 © Daniel E Whitney 9/11/2015 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 4 © Daniel E Whitney 9/11/2015 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 5 © Daniel E Whitney 9/11/2015 Conventional DFA Addresses local issues Is applied too late in the process Addresses some of the things that factory MEs worry about Favors integrated designs Any attempt to broaden its reach requires a great deal of additional knowledge –Business tradeoffs –Supply chain management during design and production –Ergonomics

AOD for LGO 6 © Daniel E Whitney 9/11/2015 Sony Does DFA During Concept Design

AOD for LGO 7 © Daniel E Whitney 9/11/2015 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 8 © Daniel E Whitney 9/11/2015 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 9 © Daniel E Whitney 9/11/2015 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 10 © Daniel E Whitney 9/11/2015 “Chain of Delivery” of Quality No single part “delivers” the KC.

AOD for LGO 11 © Daniel E Whitney 9/11/2015 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 12 © Daniel E Whitney 9/11/2015 Office Stapler Liaison Diagram

AOD for LGO 13 © Daniel E Whitney 9/11/2015 Datum Flow Chains in the Stapler The datum flow chain is a chain of constraining mates from one end of the KC to the other.

AOD for LGO 14 © Daniel E Whitney 9/11/2015 Mates, Contacts, and KC Delivery MateContact Mates give location. Contacts reinforce location. Variation travels from part to part along the chain of mates.

AOD for LGO 15 © Daniel E Whitney 9/11/2015 Coordinate Frames

AOD for LGO 16 © Daniel E Whitney 9/11/2015 Chains of Frames = Assembly

AOD for LGO 17 © Daniel E Whitney 9/11/2015 Dash L. Fender R. Fender L. Body Side R. Body side L. Door R. Door Underbody R. Apron L. Apron L. I. Shot. R. I. Shot. L. O. Shot. R. O. Shot. L. O. Rail R. O. Rail Hood L. HingeR. Hinge Bolster Hood Latch L. Lamp R. Lamp Fascia L. I. Rail R.I. Rail y, z F1 L. Bracket x,  x,  y,  z y,  z x, z,  x,  y,  z y xx y y z,  y x,  x 6 6 x, z,  x,  y,  z x,  x,  y,  z 66 Hood fixture FF F F F F F x, z,  x,  y x, z  x,  y 6 F F F F 6 z,  x  y F x,  x y, z,  y,  z z,  y y x, y,  z x  y,  z x, y,  z F F Datum Flow Chain for Car Front End Drawn by Gennadiy Goldenshteyn, MIT Student

AOD for LGO 18 © Daniel E Whitney 9/11/2015 DFC for Aircraft Circumference

AOD for LGO 19 © Daniel E Whitney 9/11/2015 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 20 © Daniel E Whitney 9/11/2015 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 T T T AF FB AB A B T AF A T FB B

AOD for LGO 21 © Daniel E Whitney 9/11/2015 Varied Part Location Due to Variation 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 22 © Daniel E Whitney 9/11/2015 Stapler Variations

AOD for LGO 23 © Daniel E Whitney 9/11/2015 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 24 © Daniel E Whitney 9/11/2015 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 25 © Daniel E Whitney 9/11/2015 Two Pegs + Two Holes (Line Fit) Example: Car engine cylinder head mate to block Is this situation fully-, over-, or under- constrained? Note: I-DEAS and Solidworks software say it is “fully constrained”

AOD for LGO 26 © Daniel E Whitney 9/11/2015 Car Seat Mounting Original Design: 25mm ± 0.5 mm 4 Holes Final Design 25mm ± 0.5mm 25mm x 40mm 40mm

AOD for LGO 27 © Daniel E Whitney 9/11/2015 Use of Screw Theory to Check Constraint Create library of elementary features Each such feature has a twist matrix representation –Don’t need geometry! Make constraining joints between parts by using one or more elementary features in combination Check degree of mobility or constraint using twist intersection algorithm by Konkar

AOD for LGO 28 © Daniel E Whitney 9/11/2015 Example Library Feature: Plate and Slotted Pin Joint twist = [  x  y  z vx vy vz] = [ ] The twist has two rows because the feature allows two different motions Z rotation Y translation

AOD for LGO 29 © Daniel E Whitney 9/11/2015 Analysis of a Combined Feature Made from Engineering Features Analysis results : Motion about Z is possible The rotation center is about f1 The amount of rotation can be calculated if peg and slot dimensions are known and slight clearance is assumed Overconstraint exists along Y and about Z Overconstraint exists about X and Y Overconstraint exists along Z

AOD for LGO 30 © Daniel E Whitney 9/11/2015 Second Example Analysis results : No motion is possible Overconstraint exists about X and Y Overconstraint exists along Z

AOD for LGO 31 © Daniel E Whitney 9/11/2015 DFCs, Variation, and Constraint Constraint OK at nominal dimensions Constraint not OK at nominal Overconstrained: there is no DFC as we define it (or else you need a stress analysis to find it) Constraint plan is not robust: mates and contacts do not maintain their identity - there is no unique and permanent DFC Constraint plan is robust to variation: there is a unique and permanent DFC

AOD for LGO 32 © Daniel E Whitney 9/11/2015 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 33 © Daniel E Whitney 9/11/2015 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 34 © Daniel E Whitney 9/11/2015

AOD for LGO 35 © Daniel E Whitney 9/11/2015 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 36 © Daniel E Whitney 9/11/2015 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 37 © Daniel E Whitney 9/11/2015 Software Tools Assembly Designer - front end user interface for designing the DFC along with selecting the features Constraint Checker - checks for constraints given the DFC and the features using motion and constraint analysis SPAS - derives the geometric precedence relations through a series of yes/no questions DFCPR - derives the precedence relations that result from the DFC PRED - translates the precedence relations into C code LSG - determines all feasible assembly sequences given the precedence relations EDIT - allows for interactive editing of sequences Tolerance Analysis - determines tolerances for a sequence

AOD for LGO 38 © Daniel E Whitney 9/11/2015 Example Assembly Throttle Body

AOD for LGO 39 © Daniel E Whitney 9/11/2015 Example Assembly

AOD for LGO 40 © Daniel E Whitney 9/11/2015 Assembly Designer

AOD for LGO 41 © Daniel E Whitney 9/11/2015 DFC Design

AOD for LGO 42 © Daniel E Whitney 9/11/2015 Feature Selection

AOD for LGO 43 © Daniel E Whitney 9/11/2015 Constraint Data Input

AOD for LGO 44 © Daniel E Whitney 9/11/2015 Assembly Sequence Constraint Editor

AOD for LGO 45 © Daniel E Whitney 9/11/2015 Precedence Relations

AOD for LGO 46 © Daniel E Whitney 9/11/2015 Assembly Sequence Editor

AOD for LGO 47 © Daniel E Whitney 9/11/2015 Assembly Sequence Selection