Design for Manufacturing and

Assembly

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

DFM Process

Manufacturing cost estimation

• In estimating manufacturing cost per unit of

product, there are 3 cost categories

–Cost of parts

–Cost of assembly

–Overhead cost

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

• 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

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

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)

Design for Assembly

• System design

• Design for ease of handling

• Design for ease of insertion

• Design for fastening

• Design for manufacturing processes

System design

• MODULARITY - a module is a self-contained component that is equipped with standard interfaces that allow it to be integratedinto 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

(a) Original design (b) Redesigned housing unit

The redesigned part facilitates product assembly, as well

as the servicing of the units.

• A common electric outlet box.

• 13 parts, including screws, and must be assembled

by tedious hand methods.

• A one-piece plastic electric box is injection molded

with the nails in place, and requires no assembly.

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 operating

condition 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 is

impossible to assemble the products?

• If the answer is “no”, the part is unnecessary and can be

integrated with other parts.

• Another approach for system design

– reduce variability of parts

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)

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

facilitate orientation From Stoll ( 1999)

Provide orienting features on non-symmetriesFrom Priest (2001)

Provide orienting features for labelFrom Priest (2001)

• Shingling or overlapping can be avoided by providing thicker contact

edges, or vertical, or highly angled surfaces.

(Stoll, 1999)

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)

Process in the open

(Boothroyd, Dewhurst, and Knight, 1994)

• Design features that facilitate inserting and mounting of components

(Stoll, 1999)

• Top-down assembly

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

support for parts. (From Priest, 2001)

Fastening guidelines

• Minimize the number of fasteners

• Minimize the number of fasteners

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

Conclusion

• DFM is a technique which is aimed at reducing

manufacturing 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.

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

Regional and local environmental problems

• Acid rain

• Air pollution (smog)

• Water pollution

GLOBAL environmental problems

1. Biodiversity loss

2. Ozone depletion

3. Climate change

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.

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

DFE in System Design

• Design multifunctional products

• Aim for minimum number of parts

• Avoid the use of spring, cable, pulley

• Use modular design

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

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.

Fastening guidelines

• Minimize the number of fasteners

• Minimize the use of fastener-removing tools.

• Provide easy access to fasteners.

Fastening (ตอ)

• 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.

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

Hazardous material minimization

• Avoid the use of materials in the

controlled lists

• Identify materials on all parts

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