Product design for manual assemblyDFA tool: To reduce manufacturing and assembly costs. Effectively analyses the ease of assembly, Quick results simple and easy to use.Free association of ideas, comparison of alternative designs,identification of assembly problem areas, evaluation of solutions logically,

Ideas, reasoning and decisions made during the design process become useful for future reference.

Database for assembly times and cost factors for various design situations and production conditions.

The ease of assembly depends on manual or general purpose automation or special purpose automation.

Manual assembly: Handling, insertion and fastening

Guide lines for part handling:

Design parts with end to end symmetry and rotational symmetry about the axis of insertionProvide features that will prevent jamming of parts stacked in bulkAvoid features that allow tangling of parts

Guide lines for part insertion and fastening:

Provide chamfers to guide insertion of two mating partsGenerous clearance should be providedWhere ever possible avoid holding down of partsUse pyramid assembly, progressive

Manual insertion and fastening consist of a finite variety of basic assembly tasks. Peg in hole, screw, weld, rivet, press fit.

Factors effecting handling times.Part symmetry, Part thickness and size, Part weightParts requiring two hands: Heavy, Very precise, large, flexible, part does not posses holding featuresCombination of factorsParts that severely nestle or tangle: Small, vision obscured, high temp.Chamfer on insertion operations: peg into a hole, part with a hole onto peg.Chamfer on peg is better, curved chamfers are better.Insertion time depends on length, diameter ,chamfer and clearance.Avoid jams and disc assembly problems: length and clearanceHolding down time: clearance, grip size and insertion length

Further Design guidelinesAvoid connections Design so that access for assembly operations is not restricted.Avoid adjustmentsUse kinematic design principles

Design parts to prevent nesting. Nesting is when parts that are tacked on top of one another clamp to one another, for example, cups and coffee lids. Design parts with orienting features to make alignment easier.

To determine whether it is possible to combine neighboring 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? If the answer to all questions is “NO”, you should find a way to combine the parts. 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 parts do not need to be separate entities and may be combined.

Design for Assembly Principles

• Minimize part count• Design parts with self-locating features• Design parts with self-fastening features• Minimize reorientation of parts during assembly• Design parts for retrieval, handling, & insertion• Emphasize ‘Top-Down’ assemblies• Standardize parts…minimum use of fasteners.• Encourage modular design• Design for a base part to locate other components• Design for component symmetry for insertion

DFA ProcessProduct Information: functional requirementsFunctional analysisIdentify parts that can be standardizedDetermine part count efficiencies

Step 2

Step 1

Analyze data for new design

Step 3

Identify handling (grasp & orientation) opportunitiesStep 4

Identify insertion (locate & secure) opportunitiesStep 5

Step 6 Identify opportunities to reduce secondary operations

Identify quality (mistake proofing) opportunities

Benchmark when possible

Determine your practical part count

Step 7

DFA Analysis Worksheet

Product Information: functional requirementsFunctional analysisIdentify parts that can be standardizedDetermine part count efficiencies

Step One

Considerations/Assumptions

• The first part is essential (base part)

• Non-essential parts:– Fasteners– Spacers, washers, O-rings– Connectors, leads

• Do not include liquids as parts (e.g.. glue, gasket sealant,)

Part Identification

• List parts in the order of assembly

• Assign/record part number

So take it apart!

Count Parts and Interfaces

• List number of parts (Np)

• List number of interfaces (Ni)

Determine if Parts Can be Standardized

• Can the current parts be standardized?:

• Should they be?• (Only put a “Y” if

both answers are yes…)

Theoretical Part Count EfficiencyTheoretical Part

Count Efficiency

Theoretical Min. No. Parts Total Number of Parts

Theoretical Part 1Count Efficiency 10

Theoretical Part Count Efficiency

=

= * 100

= 10%

* 100

GoalGoalRule of Thumb – Part Count Efficiency Goal >

60%

DFA Complexity Factor – Definition

• Assessing complexity of a product design• Two Factors

• Np – Number of parts• Ni – Number of part-to-part interfaces

– Multiply the two and take the square root of the total

– This is known as the DFA Complexity Factor

S Np x S Ni

DFA Complexity Factor – Target

• Smaller is better (Minimize Np and Ni)• Let Npt = Theoretical Minimum Number of parts

– from the Functional Analysis– Npt = 5

• Let Nit = Theoretical minimum number of part to part interfaces– Nit = 2(Npt-1)– Nit = 2(5-1) = 8

Part 2Part 3

Part 4

Part 5

Part 1

DCF = S Np x S Ni

DCFt = S Npt x S Nit

DCFt = 5 x 8 = 6.32

Determine Relative Part Cost Levels

• Subjective estimate only • Low/Medium/High

relative to other parts in the assembly and/or product line

Cost Breakdown

• Media paper 21.4%• Centertube 3.6%• Endplates (2) 3.0%• Plastisol 2.6%• Inner Seal 4.0%• Spring 0.9%• Shell 31.4%• Nutplate 21.0%• Retainer 4.8%• Loctite 0.3%• End Seal 7.0%

Step Two

Determine Practical Minimum Part Count

Determine Practical Minimum Part Count

• Team assessment of practical changes

• Tradeoffs between part cost and assembly cost

Implementation

RiskHighMediumLow

ShortTerm

MediumTerm

LongTerm

Idea Classification

Fastener Cost

• Select the

most

inexpensive

fastening

method

requiredplastic bending

riveting

screwing

snap fit

General Design Principles

Self-fastening features

General Design Principles

Asymmetric Part Symmetry of a partmakes assembly easier

Symmetry eliminates reorientation

Step Three

Identify quality (mistake proofing) opportunities

Mistake Proofing Issues

• Cannot assemble wrong part• Cannot omit part• Cannot assemble part wrong

way around.

symmetrical parts

asymmetrical parts

Step Four

Identify handling (grasp and orientation) opportunities

Quantitative criteria

• Handling Time: based on assembly process and complexity of parts– How many hands are required?– Is any grasping assistance needed?– What is the effect of part symmetry on assembly?– Is the part easy to align/position?

Handling Difficulty

• Size• Thickness• Weight• Fragility• Flexibility• Slipperiness• Stickiness• Necessity for using 1) two hands, 2) optical

magnification or 3) mechanical assistance

Handling Difficulty

size slipperiness

sharpness flexibility

Eliminate Tangling/Nesting

Step Five

Identify insertion (locate & secure) opportunities

Quantitative criteria

• Insertion time: based on difficulty required for each component insertion– Is the part secured immediately upon insertion?– Is it necessary to hold down part to maintain location?– What type of fastening process is used? (mechanical,

thermal, other?)– Is the part easy to align/position?

Insertion Issues

• Provide self-aligning and self locating parts

Insertion Issues• Ensure parts do not need to be held in position

Insertion Issues

• Parts are easy to insert. • Provide adequate access and visibility

Insertion Issues

• Provide adequate access and visibility

Step Six

Identify opportunities to reduce secondary operations

Eliminate Secondary Operations

• Re-orientation (assemble in Z axis)• Screwing, drilling, twisting, riveting, bending,

crimping.

Rivet

Eliminate Secondary Operations

• Welding, soldering, gluing.• Painting, lubricating, applying liquid or gas.• Testing, measuring, adjusting.

Error = Sum all Y’s in Error Columns Proofing Theoretical Min. No. Parts Handling = Sum all Y’s in Handling Columns Index Theoretical Min. No. PartsInsertion = Sum all Y’s in Insertion Columns Index Theoretical Min. No. Parts

2nd Op. = Sum all Y’s in 2nd Op. Columns Index Theoretical Min. No. Parts

Assembly Metrics

Analyze All Metrics

First consider:Reduce part count and type Part Count Efficiency

and DFA Complexity Factor

Then think about:Error Proofing Error Index

Then think about:Ease of handling Handling IndexEase of insertion Insertion IndexEliminate secondary ops. 2nd Op. Index

Set Target Values for These Measures

Step Seven

Analyze data for new design

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).

Modularize multiple parts into single sub-assemblies.

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

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, or hard to distinguish, by non-geometric means, such as color coding

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

Design the mating parts for easy insertion. Provide allowance on each part to compensate for variation in part dimensions.

Case 1

Case 2

Design the first part large and wide to be stable and then assemble the smaller parts on top of it sequentially.

Case1

Case 2

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.

Case 1

Case 2

Joining parts can be done with fasteners (screws, nuts and bolts, rivets), snap fits, welds or adhesives.