Intro to Assembly R2010

An Aerospace An Aerospace

Introduction to AssemblyIntroduction to Assembly

An Aerospace An Aerospace

Manufacturing Manufacturing

PerspectivePerspective

Course Overview Course Overview

� Introduction

� Assembly Concepts� Assembly Concepts

– Constraint

– Fixtures

– Assembly features

– Tolerance stacks

copyright J. Anderson, 2008

Assembly Assembly –– The Necessary EvilThe Necessary Evil

� Assembly is inherently integrative

– brings parts together

– brings people, departments, companies 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 partswith hundreds of thousands parts


The Study of Assembly

•Traditional unit processes

studied for 150+ years

•Assembly studied perhaps


•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


•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 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


their strengths and weaknesses - same with robots�Today there is a place for robots, people, and fixed automation in assembly�The 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


•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


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


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


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 stress


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.

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

1. It takes skill to put the parts together and get them just right

2. The assembly task is operator-dependent


dependent3. Fasteners have to be tightened

in a particular sequence4. It is hard to get welded parts out

of the fixture5. Some parts will assemble easily

but other "identical" ones will not6. You can never get everything to

line up the way you want it to7. Results are inconsistent

Location and Stability


Force Closures and Form Closures

Force closures are one-sidedThey support force in one direction at a definite

location


location

They can provide proper constraint

Form closures are two-sidedThey 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

One-side (AKA force closure)•Needs an effector•Gives perfect knowledge of location but can't support an arbitrary force in all


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


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

Fixture


Stabilizer structure

How Airplanes are Built

Boeing:Ensure that there is open space at max material conditionFill the gap with shims, reducing gap to XXX


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

Position Skin

Uses Hard Tool

Suspended by a Crane


Install Torque

Clecos

Install Torque

Clecos

Cure Liquid

Shim

Cure Liquid

Shim

Typical Tool on

Storage Rack

Suction Cups for

Holding Skin

Remove Skin

Inspect Liquid

Shim and Repair

Inspect Liquid

Shim and Repair

Install Skin

Current Cure

Time is 8 Hours

Using Hard

ToolUsing Hard

Tool

Opportunity for Automation

F/A 18 Horizontal Stabilizer, contd

Move Structure

into Automated

Drill Machine

Move Structure

into Automated

Drill Machine

Drill & Countersink

Holes Full Size

Drill & Countersink

Holes Full Size

Drill & Countersink Tack

Rivets to Full Size

Drill & Countersink Tack

Rivets to Full Size Inspect HolesInspect Holes

Using Renishaw

Probe


Move Structure

into Workstand

Move Structure

into Workstand Install FastenersInstall Fasteners

Sample Skin and

Frame

Examples of Engineering Features


Statistical and Worst Case Compared


Intro to Assembly R2010

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