An Aerospace Manufacturing Perspective Introduction to Assembly

Course Overview Introduction Assembly Concepts – Constraint – Fixtures – Assembly features – Tolerance stacks copyright J. Anderson, 2008

Assembly – The Necessary Evil Assembly is inherently integrative – brings parts together – brings people, departments, companies together – can be the glue for concurrent engineering Assembly is where the product comes to life – there aren’t many one-part products Assembly is where quality is “delivered” – quality is delivered by “chains” of parts, not by any single most important part copyright J. Anderson, 2008

Assembly The term assembly covers a wide field – From a lowly pencil sharpener with less than 20 parts to an advanced fighter aircraft like the F-35 Joint Strike Fighter with hundreds of thousands parts copyright J. Anderson, 2008

The Study of Assembly copyright J. Anderson, 2008 Traditional unit processes studied for 150+ years Assembly studied perhaps 40 years Most assembly process design and actual assembly is manual Surge in interest in robot assembly in the 70s Interest in “appropriate technology” today

Manual vs. Automated Assembly copyright J. Anderson, 2008 People “just do it” Machines can’t “just do it” It was hoped that robots could “just do it” Early robot research focused on imitating what people do o behave flexibly o use their senses o fix mistakes

What happened…… copyright J. Anderson, 2008  Too slow and too costly  No one knew how to do an economic analysis and most didn’t care at first  People do what they do because of their strengths and weaknesses - same with robots  Today there is a place for robots, people, and fixedautomation in assembly  The issue is to decide which is best and how to prepare the “environment”

Robotics as a Driver for Assembly Automation copyright J. Anderson, 2008 Robotics raises a number of generic issues: flexibility vs efficiency generality vs specificity responsiveness or adaptation vs preplanning absorption of uncertainty vs elimination of uncertainty lack of structure vs structure

Assembly = Constraint copyright J. Anderson, Assembly = removal of dof = application of constraint 2.As constraint is applied, degrees of freedom are taken away so that a part gets to where it is supposed to be. 3.When parts are where they are supposed to be, the key characteristics of the assembly can be delivered, assuming no variation 4.This is called the nominal design

Constraint is Accomplished by Surfaces in Contact copyright J. Anderson, 2008

Degrees of Freedom copyright J. Anderson, 2008 An object's location in space is completely specified when three translations (X, Y, Z) and three rotations (X,Y, Z ) are specified How many DOFs are constrained for a cube on table (x-y plane)? -rotation about x & y and translation along z; therefore 3 degrees of freedom are constrained

Assembly Constraint copyright J. Anderson, Proper constraint provides a single value for each of a body’s 6 degrees of freedom (dof) 2.This is done by establishing surface contacts with surfaces on another part or parts 3.If less than 6 dof have definite values, the body is under-constrained 4.If an attempt is made to provide 2 or more values for a dof, then the body is over-constrained because rigid bodies have only 6 dof 5.Any extra needed dof must be obtained by deforming the object

Example of Proper and Over Constraint copyright J. Anderson, 2008 Proper constraint permits an assembly to have unambiguous chains of delivery of KCs

"Good" Over-constrained Assemblies copyright J. Anderson, 2008 Preloaded angular contact bearing systems Preload increases contact stress, creating a stiff bearing system (see next page) Planetary gears - redundant locators, no stress Shrink fit Heated wheel slips on over shaft, shrinks upon cooling to make a super-tight joint Beam built in at both ends It's stiffer for the same cross section than a simply ‑ supported beam because the ends can support a moment A good design permits longitudinal motion at the ends In each case there is an underlying properly constrained system!

Why Does Over-Constraint Occur? copyright J. Anderson, 2008 Forces or torques are deliberately inserted, e.g. Shrinking Tightening a lock nut The design attempts to fix more than 6 degrees of freedom of a part, e.g. The x position is determined by the part's left end The part's x position is determined by the part's right end There is a fight whose outcome is compression in the x direction and no easy way to calculate the x position

Tipoffs for Over-constraint copyright J. Anderson, It takes skill to put the parts together and get them just right 2.The assembly task is operator- dependent 3.Fasteners have to be tightened in a particular sequence 4.It is hard to get welded parts out of the fixture 5.Some parts will assemble easily but other "identical" ones will not 6.You can never get everything to line up the way you want it to 7.Results are inconsistent

Location and Stability copyright J. Anderson, 2008

Force Closures and Form Closures copyright J. Anderson, 2008 Force closures are one-sided They support force in one direction at a definite location They can provide proper constraint Form closures are two-sided They can support unlimited force They will generate over-constraint unless some clearance is provided If clearance is provided, then the location is no longer definite

One-Side and Two-Side Constraints copyright J. Anderson, 2008 One-side (AKA force closure) Needs an effector Gives perfect knowledge of location but can't support an arbitrary force in all directions Two- or multi-side constraint (AKA form closure) Needs no effector and can support arbitrary force Contains its own stabilizer Actually contains over-constraint If we relax this over-constraint with a little clearance then we lose perfect knowledge of location

When Parts are Joined, Degrees of Freedom are Fixed copyright J. Anderson, 2008 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

F35 Horizontal Stabilizer Fixture copyright J. Anderson, 2008 Stabilizer structure Fixture

How Airplanes are Built copyright J. Anderson, 2008 Boeing: Ensure that there is open space at max material condition Fill the gap with shims, reducing gap to XXX Report remaining gap to Engineering Lately: use better process control to predict gaps and prepare standard shims in as many cases as possible Airbus: Make parts from 3D CAD/NC Join them directly No shims Both attempt to limit locked-in stress

F/A 18 Horizontal Stabilizer copyright J. Anderson, 2008 Install Torque Clecos Cure Liquid Shim Position Skin Uses Hard Tool Suspended by a Crane Typical Tool on Storage Rack Suction Cups for Holding Skin Remove Skin Inspect Liquid Shim and Repair Install Skin Current Cure Time is 8 Hours Using Hard Tool Opportunity for Automation

F/A 18 Horizontal Stabilizer, contd copyright J. Anderson, 2008 Move Structure into Workstand Move Structure into Automated Drill Machine Drill & Countersink Holes Full Size Drill & Countersink Tack Rivets to Full Size Install Fasteners Inspect Holes Using Renishaw Probe Sample Skin and Frame

Examples of Engineering Features copyright J. Anderson, 2008

Statistical and Worst Case Compared copyright J. Anderson, 2008