Ch 8 English

8/13/2019 Ch 8 English

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Design for Manufacturing and

Assembly


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Design for Assembly

Is a technique which can be used in many stages of product

design and development such as product teardown, reverse

engineering, improvement of product concept.

Benefits of DFM

SIMPLIFY the design of product

Reduce the number of parts, and the cost of parts

Reduce the time of manufacturing and assembly

Increase quality and reliability of product


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DFM Process


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Accuracy of manufacturing cost estimation

Level 1 estimation relies on experience of expert

engineer

Takes less than 10 minutes for a system with 50 parts,

and is within 20 % accuracy

Level 2 estimation list the costs explicitly by relying

on experience from similar product, expert experience,

and vendor information

Takes approx. 1 day for a system with 50 parts, and is

within 5 % accuracy


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Level 3 estimation Cost accounting

cost calculation of every part

Use database of material cost estimation and motion/ time

study

Take approx. 1 week for product with 50 parts and is

within 1 % accuracy


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How to reduce the cost of parts

Understand manufacturing process capability

Redesign the parts to eliminate manufacturing steps

Choose production size suitable with manufacturing

process


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How to reduce the cost of assembly

Even though assembly cost contributes only a small part

of total manufacturing cost, assembly cost reduction is

beneficial because of several indirect effects:

Number of parts is reduced

Process complexity is reduced

Cost of manufacturing support is reduced

A technique for reducing cost of assembly is design for

assembly (DFA)


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Design for Assembly

System design

Design for ease of handling

Design for ease of insertion

Design for fastening

Design for manufacturing processes


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System design

MODULARITY - a module is a self-contained component that is

equipped with standard interfaces that allow it to be integrated

into a larger system

Modules form building blocks that can be used interchangeably

in different products.

Design for modularity has several benefits:

Easy to reassemble

Easy to detect quality problems

() () ()


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Steel support bracket

Left: one-part product (simple, easy to manufacture)

Right: several part product (joined by spot welding)

Another way to achieve system design for assembly

simple design, and eliminate unnecessary parts


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(a) Original design (b) Redesigned housing unit

The redesigned part facilitates product assembly, as well

as the servicing of the units.


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A common electric outlet box.


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13 parts, including screws, and must be assembled

by tedious hand methods.


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A one-piece plastic electric box is injection molded

with the nails in place, and requires no assembly.


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Criteria for analyzing unnecessary parts

Boothroyd and Dewhurst (1994) suggests that unnecessary parts

are those that answer No to the following questions:

Does the part move relative to other parts in normal operatingcondition of product?

Is it necessary that the part is made of different materials or

isolated from other parts such as electrical insulation, heat

insulation, or vibration reduction?

Does the part have to be isolated from other parts otherwise it isimpossible to assemble the products?

If the answer is no, the part is unnecessary and can be

integrated with other parts.


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Another approach for system design

reduce variability of parts


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Handling Guidelines

Maximize part symmetry

Provide orienting features

on non-symmetries

Prevent nesting of parts

Eliminate tangly parts

Avoid sharp ends

Provide orienting features

on non-symmetries

()

()

()

()

()

()


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Parts made symmetrical for easier orientation

From Stoll (1999)


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Adding external features (such as chamfers, slots, and flats) to

facilitate orientation From Stoll ( 1999)


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Provide orienting features on non-symmetries

From Priest (2001)


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Provide orienting features for label

From Priest (2001)


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Shingling or overlapping can be avoided by providing thicker contact

edges, or vertical, or highly angled surfaces.

(Stoll, 1999)


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Insertion Guidelines

Minimize resistance

Provide chamfer

Design parts that locks into

place

Insert new parts into assembly

from above (z axis)

()

()

()

()


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(a) top-down Z axis assembly (b) avoid multi-motion insertion

(c) design assemblies as layered stacks with components positively located

(d) provide alignment features for guiding components (Stoll, 1999)


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Process in the open

(Boothroyd, Dewhurst, and Knight, 1994)


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Design features that facilitate inserting and mounting of components

(Stoll, 1999)


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Top-down assembly

Note - Bottom of computer case is used as a conveyor pallet assembly fixture and

support for parts. (From Priest, 2001)


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Fastening guidelines

Minimize the number of fasteners


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Minimize the number of fasteners


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Assembly Efficiency

Assembly Efficiency (Ema

) , DFA index

Ema

= Nmin

ta

/ tma

Nmin

= the theoretical minimum number of parts

ta

= 3 sec (average time used to assemble one part which is not

difficult to handle, insert, or fasten together)

tma

= approximate time to assemble the total number of parts

into a product


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Conclusion

DFM is a technique which is aimed at reducingmanufacturing cost by decreasing the number of parts

in the design

To do DFM, it is necessary to estimate manufacturing

cost.

DFA is part of DFM, invented by Boothroyd, Dewhurst

and Knight. It is aimed at facilitating part assembly.


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Eco-efficiency

Reduction of raw material

Reduction of energy usage

Reduction of emission

Increase of recyclability

Increase of sustainable use of renewable resources

Increase product durability

Increase the useful functions of product and service


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Regional and local environmental problems

Acid rain

Air pollution (smog)

Water pollution


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GLOBAL environmental problems

1. Biodiversity loss

2. Ozone depletion

3. Climate change


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Design for Environment (DFE)

is an umbrella term describing techniques used to incorporate an

environmental component into products and services before they

enter the production phase.

DFE seeks to discover product innovations that will meet cost and

performance objectives while reducing pollution and waste

throughout the life-cycle.


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DFE Techniques

1. Techniques that are used to identify the environmental

impact of a product throughout its life cycle such as life-

cycle assessment.

2. Techniques that help designers improve the environmental

performance of their products.

Design for recycling

Design for disassembly

Design for remanufacture

Hazardous material minimization

Design for finishing and labeling

Design for energy efficiency

Design for disposability


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DFE in System Design

Design multifunctional products

Aim for minimum number of parts

Avoid the use of spring, cable, pulley

Use modular design


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Design for recycling

Reduce material variability

Reduce the use of high impact materials

Use recycled materials

Design for easy access of highest value materials

Identify all materials in the product


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Design for disassembly

Design parts so that they are

secure during disassembly.

Avoid the use of metal inserts in

plastic parts

Other guidelines are similar to

DFMA.


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Fastening guidelines

Minimize the number of

fasteners

Minimize the use of

fastener-removing tools.

Provide easy access to

fasteners.


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Fastening (

)

Use fasteners which are made of materials

compatible with the parts.

Avoid the use of adhesives unless compatible

with the parts.

Minimize the use of cables.


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Design for remanufacturing/reuse

Identify the parts which can be

remanufactured.

Identify the packages which

can be remanufactured.

Other guidelines are similar to

design for disassembly


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Hazardous material minimization

Avoid the use of materials in the

controlled lists

Identify materials on all parts


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Energy efficiency guidelines

Specify best-in-class energy efficiency component

Have subsystems power down when not in use

Permit users to turn off systems in part or whole

Make parts whose movement is powered as light as

possible

Insulate heated systems

Avoid nonrechargable battery

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Ch 8 English