An Aerospace Manufacturing Perspective Introduction to Assembly

An Aerospace An Aerospace Manufacturing Manufacturing

PerspectivePerspective

Introduction to AssemblyIntroduction to Assembly

Course Overview Course Overview

Introduction Assembly Concepts

– Constraint– Fixtures– Assembly features– Tolerance stacks

copyright J. Anderson, 2008

Assembly – The Necessary EvilAssembly – 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


Assembly 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


The Study of Assembly


•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


•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

obehave flexiblyouse their sensesofix mistakes

What happened……


Too slow and too costlyNo one knew how to do an economic analysis and most didn’t care at firstPeople do what they do because of their strengths and weaknesses - same with robotsToday there is a place for robots, people, and fixed automation in assemblyThe issue is to decide which is best and how to prepare the “environment”

Robotics as a Driver for Assembly Automation


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


1. 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


Degrees of Freedom


An object's location in space is completelyspecified 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


1. 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


Proper constraint permits an assembly to have unambiguous chains of delivery of KCs

"Good" Over-constrained Assemblies


Preloaded angular contact bearing systemsPreload increases contact stress, creating a stiff bearing system (see next page)

Planetary gears - redundant locators, no stressShrink 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 momentA good design permits longitudinal motion at the ends

In each case there is an underlying properly constrained system!

Why Does Over-Constraint Occur?


Forces or torques are deliberately inserted, e.g.ShrinkingTightening 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 endThe part's x position is determined by the part's right endThere is a fight whose outcome is compression in the x direction and no easy way to calculate the x position

Tipoffs for Over-constraint


1. 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


Force Closures and Form Closures


Force closures are one-sidedThey support force in one direction at a definite locationThey can provide proper constraint

Form closures are two-sidedThey can support unlimited forceThey will generate over-constraint unless some clearance is providedIf clearance is provided, then the location is no longer definite

One-Side and Two-Side Constraints


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 ofFreedom 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 featuresmultiple featuresseveral parts working together,

each with its own features

When parts mate to fixtures, dofs are constrained

F35 Horizontal Stabilizer Fixture


Stabilizer structure

Fixture

How Airplanes are Built


Boeing:Ensure that there is open space at max material conditionFill the gap with shims, reducing gap to XXXReport remaining gap to EngineeringLately: use better process control to predict gaps and prepare standard shims in as many cases as possible

Airbus:Make parts from 3D CAD/NCJoin them directlyNo shims

Both attempt to limit locked-in stress

F/A 18 Horizontal Stabilizer


Install Torque ClecosInstall Torque Clecos

Cure Liquid ShimCure Liquid Shim

Position Skin

Uses Hard ToolSuspended by a Crane

Typical Tool on Storage Rack

Suction Cups for Holding Skin

Remove Skin

Inspect Liquid Shim and RepairInspect Liquid Shim and Repair

Install Skin

Current Cure Time is 8 Hours

Using Hard Tool

Using Hard Tool

Opportunity for Automation

F/A 18 Horizontal Stabilizer, contd


Move Structure into WorkstandMove Structure into Workstand

Move Structure into Automated Drill Machine

Move Structure into Automated Drill Machine

Drill & Countersink Holes Full SizeDrill & Countersink Holes Full Size

Drill & Countersink Tack Rivets to Full SizeDrill & Countersink Tack Rivets to Full Size

Install FastenersInstall Fasteners

Inspect HolesInspect Holes

Using Renishaw Probe

Sample Skin and Frame

Examples of Engineering Features


Statistical and Worst Case Compared


Documents

An Aerospace Manufacturing Perspective Introduction to Assembly