Design for Manufacturing and Assembly

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.


DFM and DFA BenefitsIt reduces part count thereby reducing cost. If a

design is easier to produce and assemble, it can be 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 the same reason as why it increases the reliability.


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 an understanding of design goals. Make sure that the application of a guideline improves the design concept on those goal.


DFM and DFA Design Guidelines• Minimize part count by incorporating multiple functions into

single parts. Several parts could be fabricated by using different manufacturing processes (sheet metal forming, injection molding). Ask yourself if a part function can be performed by a neighboring part.


DFM and DFA Design Guidelines

• Modularize multiple parts into single sub-assemblies.



• Design to allow assembly in open spaces, not confined spaces. Do not bury important components.


DFM and DFA Design Guidelines• 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.



• Standardize parts to reduce variety.



• Design parts so they do not tangle or stick to each other.



• Distinguish different parts that are shaped similarly by non-geometric means, such as color coding.



• 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



• Design parts with orienting features to make alignment easier.



• Provide alignment features on the assembly so parts are easily oriented.


DFM and DFA Design Guidelines• Design the mating parts for easy insertion. Provide

allowance on each part to compensate for variation in part dimensions.


DFM and DFA Design Guidelines• Design the first part large and wide to be stable and

then assemble the smaller parts on top of it sequentially.

Insertion from the top is preferred.


DFM and DFA Design Guidelines• If you cannot assemble parts from the top down

exclusively, then minimize the number of insertion direction. Never require the assembly to be turned over.


DFM and DFA Design Guidelines• Joining parts can be done with fasteners (screws,

nuts and bolts, rivets), snap fits, welds or adhesives.




Minimizing 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?


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 not needed to be a separate entity.

KISS – Keep It Simple Stupid


DFM Design Guidelines

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 engineer knowledgeable about the process. The manufacturing processes are constantly refined.






















DFM Design GuidelinesInjection Molding

Injection Molding

Fabrication of Plastics



Minimize section thickness, cooling time is proportional to the square of the thickness, reduce cost by reducing the cooling time.

Provide adequate draft angle for easier mold removal.



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 and sinks.



Provide smooth transition, avoid changes in thickness when possible.

Keep section thickness uniform around bosses.



• 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.


DFM Design GuidelinesRotational 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


Rotational Molding Machines

Rock and roll machine

Vertical wheel machine

Shuttle machine

Turret machine


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


Rotational MoldingLimitations

• 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).


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 .75 inch.

Nominal wall thickness


Rotational Molding Examples


Rotational Molding Examples


DFM Design GuidelinesSheet-metal Forming






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 from any 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).


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.


Sand Casting Terminology


Casting DefectsHot spots – thick sections cool slower than other sections causing abnormal shrinkage. Defects such as voids, cracks and porosity are created.


Casting Defects and Design Consideration



Recommended minimum section thickness




DFM Design Guidelines – Machining

Design for Manufacturing and Assembly

Documents

dfa design guidelinesminimize

dfa design guidelinesparts

design concept

design team

manufacturing dfm

assembly dfa

understanding of design

dfa benefitsit