1

MME445: Lecture 02

The design process

A. K. M. B. RashidProfessor, Department of MME

BUET, Dhaka

Learning Objectives

Knowledge &

Understanding

Understanding of the functional structure of a design process and

the types of design tools and materials data required for design

Skills & AbilitiesAbility to determine the elements of a design flow chart and

identify the design tools and materials data required for design

Values &

Attitudes

Appreciation of the design process and the need for design tools

and materials data in design

Resources

• M F Ashby, Materials Selection in Mechanical Design, 4th Ed., Ch. 02

2

Outline of this lecture….

Introduction

The design process

Types of design

Design tools and materials data

Function, material, shape, and process

Case study

Introduction

We are primarily concerned here with mechanical design: the physical

principles, the proper functioning, and the production of mechanical system.

• this does not ignore industrial design – pattern, colour, texture, and (above all)

consumer appeal

The optimum starting point in product development is good mechanical design,

and the ways in which the selection of materials and processes contribute to it.

Our aim is to develop a methodology for selecting materials and process that

is design-led.

• that means it uses, as inputs, the functional requirements of the design.

3

The design process

The starting point of a design is a market need or a new idea;

the end point is the full specification of a product that fills the

need or embodies the idea.

A need must be identified before it can be met.

It is essential to define the need precisely—that is, to formulate a need statement,

often in this form:

“A device is required to perform task X”

expressed as a set of design requirements.

solution is neutralto avoid narrow thinking constrained by preconceptions

Concept

Embodiment

Detail

Product Specification

Market Need:design requirement

iterate

4

Products are technical systems composed of sub-assemblies

and components

It is like describing a cat

(the system) as made up

of one head, one body,

one tail, four legs, and so

on (the sub-assemblies),

each composed of

components: femurs,

quadriceps, claws, fur.

• useful way to analyze

an existing design

• not of much help in

the design process

itself, that is, in

devising new designs

The type of arrangement is called the function structure or function decomposition

of the system, which gives a systematic assessment of design options.

It is like describing a cat as an appropriate linkage of a respiratory system, a cardio-

vascular system, a nervous system, a digestive system, and so on.

This design converts

inputs into outputs.

e.g., an electric motor

converts electrical energy

into mechanical energy

The system is broken

down into interconnected

sub-systems, each of

which performs a specific

function.

5

Concept

Embodiment

Detail

Product Specification

Market Need:design requirement

Determine function structure Seek working principles Evaluate and select concepts

Develop layout, scale, formModel and analyse assemblies Evaluate and select layout

Analyse components in detail Optimise performance and cost Final choice of material and process

iterate

FIGURE 1 The design flow chart

Example of good design

cell phones iPod

These design employ a high degree of functionality

(market need), proper materials selection, aesthetics

and price.

All that sounds well and good.

If only it were so simple.

The linear process suggested in Fig. 1

obscures the strong coupling between

the three stages.

Thus a key part of design, and of

selecting materials for it, is flexibility,

the ability to explore alternatives

quickly.

FIGURE 2 The convoluted path of design. Here the

C-blobs represent concepts; the E-blobs, embodiments of

the Cs; and the D-blobs, detailed realizations of the Es.

The consequences of choices made at

the concept or the embodiment stages

may not become apparent until the detail

is examined.

Iteration, looping back to explore

alternatives, is an essential part of the

design process.

The process is complete when a

compatible path from “need” to

“specification” can be identified.

6

Types of design

New idea or working principle

• e.g., the ballpoint pen or the CD

New materials can offer new, unique combinations of properties

that enable original design.

• high-purity silicon enabled the transistor

• high-purity glass for the optical fiber

• high coercive-force magnets for the miniature earphone

• solid-state lasers for the compact disc

Original design

The new material often suggests the new product.

Sometimes, instead, the new product demands the development

of a new material.

• nuclear technology drove the development of a series of new zirconium alloys

and low-carbon stainless steels

• space technology stimulated the development of light weight composites

• gas turbine technology today drives development of high-temperature alloys

and ceramic coatings.

Original design sounds exciting, and it is.

But most design is not like that.

7

Almost all designs are adaptive or developmental

• e.g., beverage cans, automobiles,…

Adaptive or developmental design

Takes existing concept (product or product range) and, through

a refinement in working principle, seeks to

• enhance performance

• reduce cost

• adapt it to changing market conditions ….

This type of design, too, is often made possible by developments

in materials.

• polymers replacing metals in household appliances

• carbon fiber replacing wood in sports equipment

Change of scale or dimension or detailing

without change of function or method of achieving it

• desktop to laptop computer

• Scaling up of boilers or turbines

Question:

Microsoft Windows 10 – What type of design is this?

Variant design

Change of scale or circumstances of use may require

change of material.

• small boats are made of fiberglass, large ships are made of steel

• small boilers are made of copper, large ones of steel

• subsonic planes are made of one alloy, supersonic of another

8

So far we learned and be able to indentify ....

The design flow chart

Functional structure of a technical system

Types of design

Design tools and materials data

9

Function, material, shape and process

FIGURE 3 The central problem of

materials selection in mechanical design:

the interaction between function, material,

process, and shape.

To make a shape, the material is subjected to the manufacturing processes.

• Primary forming processes (e.g., casting and forging)

• Material removal processes (machining, drilling)

• Joining processes (e.g., welding), and

• Finishing processes (e.g., painting or electroplating)

The interaction between function, material,

shape, and process lies at the heart of the

material selection process.

• Function influences material choice.

• Material choice influences processes.

• Process determines shape, size, precision,

and, of course, cost.

The more sophisticated the design, the tighter

the specifications and the greater the interactions.

Case study

We need to express the above functional requirement

of the design in a solution neutral fashion in a

reasonably detailed need statement.

“A device is required to pull corks

from wine bottles”

A simple design problem

We need a design solution for a device that will allow us

to gain access to the wine contained in a corked bottle.

“A device is required to allow access to wine in a corked bottle”

with convenience, at modest cost, and without contaminating the wine

Is there anything wrong about this need statement?

10

A few ideas / concepts …..

pulverized

cork

Solutions (a) through (c) are

feasible and are implemented in

devices available on the market.

Solutions (d) and (e) are feasible,

but can be eliminated on the

ground that they have the potential

of (dangerously) contaminating the

wine.

How can we define the

requirements for solutions

(a) through (c) into a device?

shear traction high internal

pressure

break bottle neck

(bypass coke)

axial traction

or pulling

Axial traction – a screw or similar

device is threaded into the cork and

axial traction is applied.

Shear traction – thin blades of an

elastic material slide between the cork

and bottle and the cork removed by

traction when twisted and pulled

High internal pressure – the cork is

pierced and gas (air) pumped into the

bottle such that the cork is pushed out

All solutions seek to apply a force and transmit this force to the cork

such that it is removed from the bottle

axial traction

or pullingshear

traction

high internal

pressure

Working principles for implementing

the first three concepts

11

Embodimentof the 1st concept

Each of the solutions embodies the use

of a screw with various means of applying

the axial traction

(a) direct,

(b) via a lever,

(c) via gears attached to beams, and

(d) spring-assisted traction

The embodiments identify the functional requirements of each

component of the device, which might be expressed in statements

such as

• A cheap screw to transmit a prescribed load to the cork

• A light lever (that is, a beam) to carry a prescribed bending moment

• A slender elastic blade that will not buckle when driven between the

cork and the bottle neck (for concept 2)

• A thin, hollow needle, stiff and strong enough to penetrate a cork

(for concept 3)

The functional requirements of each one of the components are

inputs to the materials selection through their respective design

equations or requirements.

12

The function structure

Detailed Design

Detailed design of the lever

of embodiment with material

choice.

120.0

13

Summary and conclusions

Design is an iterative process.

The starting point is a market need captured in a set of design requirements.

Concepts for a product that meet the need are devised.

If initial estimates and exploration of alternatives suggest that the concept is

viable, the design proceeds to the embodiment stage:

• working principles are selected,

• size and layout are decided, and

• initial estimates of performance and cost are made.

If the outcome is successful, designer proceeds to the detailed design stage:

• optimization of performance,

• full analysis of critical components,

• preparation of detailed production drawings (usually as CAD files), and

• specifications of tolerance, precision, assembly, and finishing methods.

Materials selection enters at each stage, but at different levels of breadth

and precision.

Data exist that meet the needs of all these levels.

Each level requires its own data management scheme.

The management system must be design-led, yet must recognize the

richness of choice and embrace the complex interaction between the

material, its shape, the process by which it is given that shape, and the

function it is required to perform.

And it must allow rapid iteration — back-looping when a particular path

proves to be unprofitable. Tools now exist to help with all of this.

14

But given this complexity, why not opt for the safe bet :

Stick to what you used before?

Many have chosen that option.

Off them, few are still in business.

MME445: Lecture 03

Engineering materials and their properties

Next Class