The Chain Method for Thinking About Complex Systems. Daniel E Whitney September 22, 2016

Transcription

1 The Chain Method for Thinking About Complex Systems Daniel E Whitney September 22, /23

2 The Chain Method for Thinking About Assemblies and Other Complex Systems Topics Introduce the chain method as a way of designing, diagnosing, and reverse engineering complex systems See how to think about a complex system as a set of cooperating parts linked into one or more closed chains Applying this to mechanical assemblies: Identify important dimensions (called Key Characteristics) at the assembly level that enable the system to do its job Identify a chain of elements that drive the KC Use this knowledge to understand how the system works and to solve problems Examples from Dan s experience and work by his students 2/23

3 Some History My career includes lots of reverse engineering of other people s products or systems in order to solve some problem These items are addressed by describing them as systems and diagramming the relationships between the elements My research also includes interest in network models of complex systems Here, again, a diagram of the relationships helps to understand what is happening Examples include mechanical assemblies, manufacturing processes, and supply chains 3/23

4 Network Model of Bicycle The colors represent communities or modules identified by an algorithm due to Newman and Girvan following the tradition of Social Network Theory. 4/23

5 Network Model of V8 Engine 5/23

6 V8 Engine with Communities and Important Functional Chains 6/23

7 Car Hood Lexus TV ad 1992 All sheet metal parts supposedly made by a master supplier (called a Full Service Supplier) All fixtures and tools made by other suppliers During ramp-up of a new model, problems arise aligning the hood to the front fenders This is a big fit and finish issue No one can figure out what is wrong or how to structure a search for the solution Dan asks two students to draw a diagram and identify all the suppliers 7/23

8 Chain of Delivery of Quality Shows clearly who delivers what and how long the chains of delivery are Key Characteristic Hood Ford-Chicago Closure Panel Check Fixture: Fixture Vendor G Outer Fender Part:part Vendor H Check Fixture Vendor D Fender Skin Part Vendor H Assembly Fixture For Hood: Fixture Vendor A Customer Feature: Hood Fit To Fender Reinforcements Parts Vendor B Fixture Vendor F Assembly Fixture Vendor C Assembly Fixture For Outer Fender: Fixture Vendor A Organizational Boundary Structural Check Fixture: Fixture Vendor E Radiator Support Part Vendor C Inner Fenders Part Vendor G D-pillar Assembly Fixture Vendor B Body Frame Ford PART COUNT: 9 PART SOURCES: 7 TOOL COUNT: 5 TOOL SOURCES: 4 CHECK FIXTURE COUNT: 2 CHECK FIXTURE SOURCES: 2 DISPERSAL INDEX: 81% Cowl Top Part Vendor A Oh, we buy the radiator support Minho Chang Narendra Soman 8/23

9 Jet Engine Accessory Door The door is part of the engine housing assembly on an aircraft Several suppliers make parts of this assembly When the door doesn t fit at final assembly, the maker of another subassembly, the inlet, is blamed Analysis consists of diagramming the whole assembly process, including the fixtures The problem is traced to uncoordinated datums in the fixture for assembling the inlet, where a surface unrelated to the final assembly process is used as the base for building it 9/23

10 Engine Assembly with Suppliers saving-the-galaxy-the-c-5-amprerpprogram-03938/ Tim Cunningham 10/23

11 Chain of Locating Surfaces (Including Incorrect Fixturing on the Inlet Lip) 11/23

12 Laser Car Body Measuring System The laser system works at every factory except one Same story as with the hood and accessory door: a problem no one can diagnose or figure out how to solve Same approach: draw the diagram, visualize a chain, identify the owners of the links, find errors Dan visits the factory, asks for the specs for the system, can t find a top level spec Turning over rocks reveals one worm after another including problems coordinating trades, reuse of old equipment, lack of system awareness, multiple reporting paths, multiple owners of chain segments Michael Rutz 12/23

13 Laser Measuring System and Owners Laser camera OCMM = optical coordinate measuring machine 13/23

14 Chain of Coordinate Systems with Many Owners 14/23

15 Internal System Owners When the System is Working Correctly Repair org is a different group that is available only on the night shift 15/23

16 The Broader Context Problem-solving has to encompass managerial, organizational, and technical elements Managerial: roles and responsibilities, incentives Organizational: lines of command and communication, assignment of tasks, mustering of expertise Technical: what engineers think about Social network theory looks for clusters or communities The chain method looks for functional loops that link the communities 16/23

17 Chain Method Process Steps Identify the problem in terms of a missing capability at a high level Door that won t close, apparatus that breaks and can t be fixed, assemblies that can t pass final inspection even though all the parts supposedly pass Identify the elements that support correct behavior Parts, people, organizations Don t assume that there is one faulty item Identify links between these elements Contracts, specifications, CAD models Formalize these into a chain of links and nodes 17/23

18 Chain Method Steps - 2 Identify the owner(s) of the links and nodes in the chain and their relationships Identify the key relationships that must exist in order for the chain to function correctly and find out what s wrong at the inter-relationship level Turn over the rocks Keep doing this until no new worms are found Try to fix the situation at the lowest practical level and elevate if this doesn t work 18/23

19 Chain Method Process Applied to Mechanical Assemblies Develop a concept, sketch, assign some dimensions, identify the key assembly-level dimensions (KCs) that deliver the function If the delivered function is a force or torque, then identify sources: fluid pressure, spring deformation, actuator, etc Draw the chain of parts and fixtures that delivers these dimensions List the coordinate directions (x, y, z, etc) in the KC that need to be delivered 19/23

20 Process Steps - 2 Consider and verify how each part in the chain is located relative to its neighbors in the chain Specify the surfaces that convey location Try to avoid over-constraint Define a common datum structure for the whole assembly Identify the dimensions inside each part that carry the chain through it from entrance surface to exit surface Make sure that each coordinate direction (x, y, z, etc) of the KC can be traced to the chain segment and surfaces inside each part Critique the datum structure to see that it supports creation of the chain segment 20/23

21 Process Steps - 3 Make a quick check of tolerance capability Calculate 0.1*sqrt(number of chain links) [mm] If this is a lot bigger than the range that you think will permit delivery of the KC then you are probably in trouble because the design is too sensitive to variations in the members of the chain For example, this check on the force to extract a circuit card from its connector yields 8.8 N for each 0.1 mm of total uncertainty compared to about 40 N max that is tolerable and 2 N min to maintain electrical contact in a chain of 6 elements 6 *8.8 = 21.55N! 21/23

22 Elements of Assembly Design Intent The KC What it is, why it matters, what performance it supports The coordinate directions that must be controlled An estimate of how well they must be controlled The definition and members of the chain that delivers the KC How they locate to each other Datums, key surfaces, supporting dimensions inside parts, including inspection and QC notes Sources of sensitivity and estimate of its size 22/23

23 Summary Functions at many levels arise from proper operation of closed chains of things, people, and organizations Proper operation depends on defining and implementing these chains systematically Failures or breakdowns can be diagnosed by analyzing (if necessary discovering) the chains The chains span managerial, organizational, and technical domains 23/23