The document provides guidelines for designing products for manual assembly. It discusses minimizing part counts, using symmetry and self-locating features to guide insertion, avoiding nested parts, and designing for top-down assembly. The guidelines aim to reduce assembly time and costs by making products easy to handle, insert parts, and avoid secondary operations like holding parts in place.

1. Product design for manual assembly
DFA 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.

2. 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 insertion
Provide features that will prevent jamming of parts stacked in
bulk
Avoid features that allow tangling of parts
Guide lines for part insertion and fastening:
Provide chamfers to guide insertion of two mating parts
Generous clearance should be provided
Where ever possible avoid holding down of parts
Use pyramid assembly, progressive

3. 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 weight
Parts requiring two hands: Heavy, Very precise, large, flexible, part
does not posses holding features
Combination of factors
Parts 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 clearance
Holding down time: clearance, grip size and insertion length

4. Further Design guidelines
Avoid connections
Design so that access for assembly operations is not
restricted.
Avoid adjustments
Use 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.

5. 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.

6. 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

7. DFA Process
Product Information: functional requirements
Functional analysis
Identify parts that can be standardized
Determine 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

8. DFA Analysis Worksheet

9. Product Information: functional requirements
Functional analysis
Identify parts that can be standardized
Determine part count efficiencies
Step One

10. 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,)

11. Part Identification
• List parts in the order
of assembly
• Assign/record part
number

12. So take it apart!

13. Count Parts and Interfaces
• List number of parts
(Np)
• List number of
interfaces (Ni)

14. Determine if Parts Can be Standardized
• Can the current parts
be standardized?:
• Should they be?
• (Only put a “Y” if
both answers are
yes…)

15. Theoretical Part Count Efficiency
Theoretical Part
Count Efficiency
Theoretical Min. No. Parts
Total Number of Parts
Theoretical Part 1
Count Efficiency 10
Theoretical Part
Count Efficiency
=
= * 100
= 10%
* 100
GoalRule of Thumb – Part
Count Efficiency Goal >
60%

16. 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

17. 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 2
Part 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

18. Determine Relative Part Cost Levels
• Subjective estimate only
• Low/Medium/High
relative to other parts
in the assembly and/or
product line

19. 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%

20. Step Two
Determine Practical Minimum Part Count

21. Determine Practical Minimum Part Count
• Team assessment of
practical changes
• Tradeoffs between part
cost and assembly cost

22. Implementation
Risk
HighMediumLow
Short
Term
Medium
Term
Long
Term
Idea Classification

23. Fastener Cost
• Select the
most
inexpensive
fastening
method
required plastic bending
riveting
screwing
snap fit

24. General Design Principles
Self-fastening features

25. General Design Principles
Asymmetric Part Symmetry of a part
makes assembly easier
Symmetry eliminates reorientation

26. Step
Three
Identify quality (mistake proofing) opportunities

27. Mistake Proofing Issues
• Cannot assemble wrong
part
• Cannot omit part
• Cannot assemble part
wrong way around.
symmetrical parts
asymmetrical parts

28. Step Four
Identify handling (grasp and orientation) opportunities

29. 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?

30. Handling Difficulty
• Size
• Thickness
• Weight
• Fragility
• Flexibility
• Slipperiness
• Stickiness
• Necessity for using 1) two hands, 2) optical
magnification or 3) mechanical assistance

31. Handling Difficulty
size slipperiness
sharpness flexibility

32. Eliminate Tangling/Nesting

33. Step Five
Identify insertion (locate & secure) opportunities

34. 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?

35. Insertion Issues
• Provide self-aligning and self locating parts

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

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

38. Insertion Issues
• Provide adequate access and visibility

39. Step Six
Identify opportunities to reduce secondary operations

40. Eliminate Secondary Operations
• Re-orientation (assemble in Z axis)
• Screwing, drilling, twisting, riveting, bending,
crimping.
Rivet

41. Eliminate Secondary Operations
• Welding, soldering, gluing.
• Painting, lubricating, applying liquid or gas.
• Testing, measuring, adjusting.

42. 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. Parts
Insertion = 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

43. 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 Index
Ease of insertion Insertion Index
Eliminate secondary ops. 2nd Op. Index
Set Target Values for These Measures

44. Step
Seven
Analyze data for new design

45. 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).

46. Modularize multiple parts into single sub-assemblies.

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

48. 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.

49. Standardize parts to reduce variety.

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

51. Distinguish different parts that are shaped similarly, or hard to
distinguish, by non-geometric means, such as color coding

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

53. Design the mating parts for easy insertion.
Provide allowance on each part to compensate for variation in
part dimensions.
Case 1
Case 2

54. Design the first part large and wide to be stable and then
assemble the smaller parts on top of it sequentially.
Case1
Case 2

55. 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

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