design fma(ok2)

8/10/2019 Design Fma(Ok2)

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Ken Youssefi UC Berkeley 1

Design for Manufacturing and Assembly

Design for manufacturing (DFM) is design based on

minimizing the cost of production and/or time to market

for a product, while maintaining an appropriate level of

quality. The strategy in DFM involves minimizing the

number of parts in a product and selecting the appropriate

manufacturing process.

Design For Assembly (DFA) involves making attachment

directions and methods simpler.


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DFM and DFA Benefits

It reduces part count thereby reducing cost. If a

design is easier to produce and assemble, it canbe done in less time, so it is less expensive.Design for manufacturing and assembly should

be used for that reason if no other.

It increases reliability, because if the production

process is simplified, then there is less

opportunity for errors.

It generally increases the quality of the product for thesame reason as why it increases the reliability.


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DFM and DFA

DFM and DFA starts with the formation of the

design team which tends to be multi-disciplinary,including engineers, manufacturing managers,

cost accountants, and marketing and sales

professionals.

The most basic approach to design for

manufacturing and assembly is to apply design

guidelines.

You should use design guidelines with anunderstanding of design goals. Make sure that the

application of a guideline improves the design

concept on those goal.


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DFM and DFA Design Guidel ines

Minimize part count by incorporating multiple functions into

single parts. Several parts could be fabricated by using different

manufacturing processes (sheet metal forming, injectionmolding). Ask yourself if a part function can be performed by a

neighboring part.


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Modularize multiple parts into single sub-assemblies.


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Design to allow assembly in open spaces, not

confined spaces. Do not bury importantcomponents.


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Parts should easily indicate orientation for insertion.

Parts should have self-locking features so that the

precise alignment during assembly is not required. Or,

provide marks (indentation) to make orientation

easier.


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Standardize parts to reduce variety.


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Design parts so they do not tangle or stick to each

other.


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Distinguish different parts that are shaped

similarly by non-geometric means, such as color

coding.


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Design parts to prevent nesting. Nesting is when

parts are stacked on top of one another clamp to

one another, for example, cups and coffee lids


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Design parts with orienting features to make

alignment easier.


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Provide alignment features on the assembly

so parts are easily oriented.


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Design the mating parts for easy insertion. Provide

allowance on each part to compensate forvariation in part dimensions.


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Design the first part large and wide to be stable and

then assemble the smaller parts on top of itsequentially.

Insertion from the top

is preferred.


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If you cannot assemble parts from the top down

exclusively, then minimize the number ofinsertion direction. Never require the assembly tobe turned over.


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Joining parts can be done with fasteners (screws,

nuts and bolts, rivets), snap fits, welds oradhesives.


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M inimizing the Number of PartsTo determine whether it is possible to combine neighboring

parts, ask yourself the following questions:

If the answer to all questions is NO,you should

find a way to combine the parts.

Must the parts move relative to each other?

Must the parts be electrically or thermally

insulated?

Must the parts be made of different material? Does combing the parts interfere with

assembly of other parts?

Will servicing be adversely affected?


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Minimizing the Number of Parts

The concept of the theoretical minimum number of parts

was originally proposed by Boothroyd (1982).During the assembly of the product, generally a part

is required only when;

1. A kinematic motion of the part is required.

2. A different material is required.

3. Assembly of other parts would otherwise be

prevented.

If non of these statements are true, then the part is notneeded to be a separate entity.

KI SSKeep I t Simple Stupid


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DFM Design Guidel ines

Another aspect of design for manufacturing is to make

each part easy to produce.

The up to date DFM guidelines for different processes

should be obtained from production engineerknowledgeable about the process. The manufacturing

processes are constantly refined.

DFM D i G id li


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DFM Design Guidelines

I njection Molding

Injection Molding

Fabr ication of Plastics


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I njection Molding

Minimize section thickness,cooling time is proportional to

the square of the thickness,

reduce cost by reducing the

cooling time.

Provide adequate draftangle for easier mold

removal.


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I njection Molding

Avoid sharp corners, they

produce high stress and

obstruct material flow.

Keep rib thickness less than

60% of the part thickness in

order to prevent voids andsinks.


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I njection Molding

Provide smooth transition,

avoid changes in thickness

when possible.

Keep section thickness uniformaround bosses.


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I njection Molding

Use standard general tolerances, do not tolerance;Dimension Tolerance Dimension Tolerance

0 d 25 0.5 mm 0 d 1.0 0.02 inch

25 d 125 0.8 mm 1 d 5.0 0.03 inch

125 d 300 1.0 mm 5 d 12.0 0.04 inch

300 1.5 mm 12.0 0.05 inch

Standard thickness

variation.

Minimum thickness recommended;

.025 inch or .65 mm, up to .125 for large

parts.

Round interior and exterior corners to

.01-.015 in radius (min.), prevents an

edge from chipping.


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Rotational Molding

A predetermined amount of plastic, powder or liquid form,

is deposited in one half of a mold.

The mold is closed.

The mold is rotated biaxially inside an oven.

The plastics melts and forms a coating over the inside

surface of the mold.

The mold is removed from the oven and cooled.

The part is removed from the mold.

Rotational molding process consists of six steps

R t ti l M ldi M hi


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Rotational M olding Machines

Rock and roll machine

Vertical wheel machine

Shuttle machine

Turret machine


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Rotational Molding

Advantages

Molds are relatively inexpensive.

Rotational molding machines are much less

expensive than other type of plastic processing

equipment.

Different parts can be molded at the same time.

Very large hollow parts can be made.

Parts are stress free.

Very little scrap is produced


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Rotational Molding

Limitations

Can not make parts with tight tolerance.

Large flat surfaces are difficult to achieve.

Molding cycles are long (10-20 min.)

Materials

Polyethylene (most common), Polycarbonate (high heat

resistance and good impact strength), Nylon (good wear

and abrasion resistance, good chemical resistance, good

toughness and stiffness).


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Rotational Molding

Polycarbonate wall thickness is typically between

.06 to .375 inches, .125 inch being an ideal

thickness. Polyethylene wall thickness is in the range of .125

to .25 inch, up to 1 inch thick wall is possible.

Nylon wall thickness is in the range of .06 to .75inch.

Nominal wall thickness

R t ti l M ldi E l


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Rotational M olding Examples



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Sheet-metal Forming



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Sheet-metal Forming

DFM Design G idelines


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Sheet-metal Forming

DFM Design Guidelines Casting


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DFM Design Guidelines - Casting

Casting, one of the oldest manufacturing processes, dates

back to 4000 B.C. when copper arrowheads were made.

Casting processes basically involve the introduction of a

molten metal into a mold cavity, where upon

solidification, the metal takes on the shape of the mold

cavity.

Simple and complicated shapes can be made fromany metal that can be melted.

Example of cast parts: frames, structural parts,

machine components, engine blocks, valves, pipes,

statues, ornamental artifacts..

Casting sizes range form few mm (teeth of a zipper)

to 10 m (propellers of ocean liners).


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Casting Processes

1. Preparing a mold cavity of the desired shape with

proper allowance for shrinkage.

2. Melting the metal with acceptable quality and temp.

3. Pouring the metal into the cavity and providing

means for the escape of air or gases.4. Solidification process, must be properly designed

and controlled to avoid defects.

5. Mold removal.

6. Finishing, cleaning and inspection operations.


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Casting Defects

Hot spotsthick sections cool slower than other sections

causing abnormal shrinkage. Defects such as voids, cracks

and porosity are created.


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Casting Defects and Design Consideration

DFM Design G idelines Casting


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Recommended minimum section thickness


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DFM Design GuidelinesMachining

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