9

Concurrent Engineering

Eng. K.C Wickramasinghe

BSc(Hon’s), AMIESL

Dept. of Mechanical & Manufacturing

Engineering,

Faculty of Engineering,

University of Ruhuna.

Lecture Session 05

01

Rule base method :- Design for Assembly

Definition: A process for improving product design for easy and

low-cost assembly, focusing on functionality and on assembly

concurrently.

Aim: To simplify the product so that the cost of assembly is

reduced.

Initially developed by Geoffrey Boothroyd in mid 1970s.

Introduction

02

Assembly Methods

03

Manual Assembly

-Workers manually

assemble the product or

components of a product

-Hand tools are used

-Flexible

-Problem when

production increases

-Labor costs

Automatic Assembly

-Special purpose assembly

machines

-Assemble one specific

product

Robotic Assembly

-Programmable

assembly machines

-Use robotic assembly

systems

-Controlled by PLC or

computer

Assembly Methods

04

Relative costs of different assembly methods by type and production

volume.

05

Evaluation methods for DFA

Boothroyd-dewhurst method

Based on two principles

1. The application of criteria to each part to determine if

it should be separate from all other parts.

2. Estimation of the handling and assembly costs for each

part using the appropriate assembly process.

Lucas method (early 1980s by Lucas Corp, UK)

Boothroyd-dewhurst method

06

Relies on an existing design which is iteratively evaluated and

improved.

Steps:

1. Select an assembly method for each part

2. Analyze the parts for the given assembly methods

3. Refine the design in response to shortcomings identified by

the analysis

4. Loop to step 2 until the analysis yields a sufficient design

Worksheet for analysis

Sample Boothroyd-Dewhurst DFA worksheet

a b c d e f g h i*

Nam

e

of

asse

mbly

Part

ID#

# of

Consecutive

identical

operations

2-digit

handling

code

Manual

handling

time/

part

2-digit

insertion

code

Manual

insertion

time/

part

Operati

on time

(bd+f)

Operation

cost

Essential

part?

Totals go here -> Tm= Cm= Nm=

Boothroyd-dewhurst method

Parts are evaluated as to whether it is really necessary (in the

assembly) by asking three questions:

Does the part move relative to another part?

Are the material properties of the part necessary?

Does the part need to be a separate entity for the sake of

assembly?

All parts are then evaluated to obtain the minimum number of

theoretically needed parts, denoted by Nm.

Boothroyd-dewhurst method

Design efficiency = (Assembly time for a part x Nm) / Tm

Lucas method

Boothroyd-Dewhurst method is based on timing each of the

handling and insertion motions

Lucas method is based on a "point scale" which gives a relative

measure of assembly difficulty

Lucas method is based on three separate and sequential analyses

Functional analysis

Feeding analysis

Fitting analysis

Functional Analysis

The components of the product are reviewed only for their function

and divided into two groups A and B

Group A - those that are essential to the product's function

Group B - those that are not essential to the product's function

Typically, a design efficiency of 60% is targeted for initial designs.

Design efficiency = A / (A+B)x100%

Feeding/Handling Analysis

Both the part handling and insertion times are examined

(similar to Boothroyd-Dewhurst analysis)

Component is awarded a score on its ease of handling

Feeding Ratio = (Total feeding/handing index)

Number of essential components

Ideal feeding ratio 2.5

Lucas DFA method - Manual Handling Analysis

Handling Index = A+B+C+D B. Handling difficulties

All that apply

Delicate 0.4

Flexible 0.6

Sticky 0.5

Tangible 0.8

Severely nest 0.7

Sharp/Abrasive 0.3

Untouchable 0.5

Gripping problem / slippery 0.2

No handling difficulties 0

A. Size & Weight of Part

One of the following

Very small - requires tools 1.5

Convenient - hands only 1

Large and/or heavy

requires more than 1 hand

1.5

Large and/or heavy

requires hoist or 2 people

3

C. Orientation of Part

One of the following:

Symmetrical, no orientation req'd 0

End to end, easy to see 0.1

End to end, not visible 0.5

D. Rotational Orientation of Part

One of the following

Rotational Symmetry 0

Rotational Orientation, easy to see 0.2

Rotational Orientation, hard to see 0.4

Fitting Analysis

Need to know the proposed assembly sequence

Relative difficulty in assembling the complete product is

determined.

– Whether the component needs to holding in a fixture

– The assembly direction

– Alignment problems

– Restricted vision

– Required insertion force

Fitting ratio =Total fitting index/No. of essential components

Lucas DFA method - Manual Fitting Analysis

Fitting Index = A+B+C+D+E+F

A. Part Placing and Fastening

One of the following

Self-holding orientation 1.0

Requires holding Plus 1 of the following

2.0

Self-securing (i.e. snaps) 1.3

Screwing 4.0

Riveting 4.0

Bending 4.0

D. Access and/or Vision

One of the following

Direct 0

Restricted 1.5

B. Process Direction One of the following

Straight line from above 0

Straight line not from above 0.1

Not a straight line 1.6

E. Alignment One of the following

Easy to align 0

Difficult to align 0.7

C. Insertion One of the following

Single 0

Multiple insertions 0.7

Simultaneous multiple insertions 1.2

F. Insertion Force One of the following

No resistance to insertion 0

Resistance to insertion 0.6

Manufacturing Analysis

Find the cost of manufacturing each component

Not a true cost but guide the designer by giving a relative

measure of manufacturing cost