1. Boothroyd & Dewhurst’s DFA Analysis
B&D offer eight rules or guidelines which are important during design for manual assembly (listed in decreasing order of importance):
1. Reduce part count and part types
2. Strive to eliminate adjustments
3. Design parts to be self-aligning and self-locating
4. Ensure adequate access and unrestricted vision
5. Ensure the ease of handling parts from bulk
6. Minimize the need for re-orientations during assembly
7. Design parts that cannot be installed incorrectly
8. Maximize part symmetry if possible or make parts obviously asymmetrical

2. Boothroyd & Dewhurst’s DFA Analysis (cont.)
Step 1: Obtain information about the product or assembly from drawings, prototypes, or an existing product
Step 2: Take the product or assembly apart and assign an identification number to each item as it is removed
Step 3: Begin to reassemble the product beginning with the highest identification number and add the remaining parts one-by-one
Complete one row of the DFA worksheet for each part
Never assume that parts are grasped one in each hand and then assembled together before placing them in a partially-completed assembly
Step 4: Complete DFA worksheet, computing total manual assembly time, cost, and design efficiency

3. DFA Worksheet Obtained from B&D Manual Handling Worksheet
Obtained from B&D Manual Insertion Worksheet
Design Efficiency
EM = (3 x NM)/TM

4. Estimating Theoretical Minimum Number of Parts
First Part;
Theoretical #
of parts = 1
Add a part
Is there a need for significant
movement between parts?
Keep the two
parts separate
Yes
Is there a need for interfacing
parts to be isolated? No
Yes
Would assembly or disassembly of the
product be prevented by combining two parts? No
Yes
Combine the two parts No
Add zero to the
minimum theoretical
number of parts
Add one to the
minimum theoretical
number of parts
Add next part

5. Assumptions for BDI DFMA
Parts are provided in bulk and randomly oriented in a bin.
Parts are handled and inserted one at a time.
Products weigh a few grams to a few tens of kilograms and are electro-mechanical in nature.
Assembly sequence is optimized for one operator. 5

6. Assembly Time Estimate by Boothroyd/Dewhurst DFMA
Estimate assembly time:
Time to assemble a part:
Time to handle a part
Time to insert a part
Time to assemble all parts, one at a time 5

7. BDI DFMA: Assembly Time
Time to handle a part:
Pick a part, orient & get ready to insert it
1.13 ~10 seconds depending on:
Symmetry; Primary(-) & secondary(-)
Size: Thickness, dimension, weight, …
Handling ease: Tool, flexible, tangling, ... 5

9. BDI DFMA: Part Symmetry
Primary (a-) symmetry: Rotational symmetry perpendicular to the axis of insertion
a = 180o
a = 360o 5

10. BDI DFMA: Part Symmetry
Secondary (b-) symmetry: Rotational symmetry about the axis of insertion
b = 0o
b = 360o 5

11. ? DFA Worksheet Part Orientation 3600 1800 
asymmetry: rotational symmetry
about an axis perpendicular to the
axis of insertion 
3600
1800
? = =

12. DFA Worksheet Part Orientation  = ?  = 00  = 1800
 symmetry: rotational
symmetry about its axis of insertion
 = ?
 = 00
 = 1800

13. BDI DFMA: Part Symmetry5

15. BDI DFMA: Part Size 5

16. BDI DFMA: Assembly Time
Time to insert a part:
Time to insert and secure a part
1.5 ~ 12 seconds depending on:
Ease of access: Visual & hand access
Ease of positioning: Hold-down, alignment, insertion force, ..
Securing requirement: Screw, snap, bending, soldering, ….
Time to assemble all parts, one at a time 5

17. BDI DFMA: Assembly Time
Time to insert a part
Two-digit insertion codes for piston:
_ _ _ _ 5

18. BDI DFMA: Assembly Time
Time to insert a part
Two-digit insertion codes for washer:
_ _ _ _ 5

19. BDI DFMA: Assembly Time
Handling Time 5

20. BDI DFMA: Assembly Time
Insertion Time 5

21. BDI DFMA: Assembly Time
Total Assembly Time 5

22. Pneumatic Pump Exercise with Boothroyd/Dewhurst DFMA2

23. BDI DFMA: Pneumatic Pump
Assembly Sequence:
6. Two screws
5. One cover
4. One spring
3. One piston stop
2. One piston
1. One main body
Tool:
A screw driver 5

24. BDI DFMA: Pneumatic Pump
Assembly Time: 5

25. BDI DFMA: Pneumatic Pump
Assembly Time: 5

26. BDI DFMA: Pneumatic Pump
Assembly Time: 5

27. BDI DFMA: Pneumatic Pump
Assembly Time: 5

28. BDI DFMA: Pneumatic Pump
Assembly Time: 5

29. BDI DFMA: Pneumatic Pump
Assembly Time: 5

30. BDI DFMA: Pneumatic Pump
Assembly Time: 5

31. Part Reduction Method by Boothroyd/Dewhurst DFMA2

32. BDI DFMA: Part Reduction
Estimate assembly time:
Identify critical parts & candidates for elimination/combination. Let’s assume a product consists of a number of parts:
Are they all critical part?
What if a part is missing or broken?
What if two parts are replaced by a larger or different part
Determine design efficiency 5

33. BDI DFMA: Part Reduction
When ready to assemble a part, should this part:
Move relative to all already assembled parts?
Be made of different material from all already assembled parts?
Be a separate part from all already assembled parts for assembly and disassembly?
If any answer is a “yes”, this part is a critical part and may not be eliminated or combined! 5

34. BDI DFMA: Why Reduce Parts?
A non-existing part never:
Causes a quality problem
Adds the cost
Creates an inventory problem
Requires design changes
Creates a shortage problem
Has a lead time 5

35. BDI DFMA: Pneumatic Pump
Assembly Sequence:
6. Two screws
5. One cover
4. One spring
3. One piston stop
2. One piston
1. One main body 5

36. BDI DFMA: Pneumatic Pump
Part Reduction: 5

37. BDI DFMA: Pneumatic Pump
Part Reduction: 5

38. BDI DFMA: Pneumatic Pump
Part Reduction: 5

39. BDI DFMA: Pneumatic Pump
Part Reduction: 5

40. BDI DFMA: Pneumatic Pump
Part Reduction:
Yes 5

41. BDI DFMA: Pneumatic Pump
Part Reduction:
Yes 5

42. DFA of Pneumatic Pump
According to the piston stop, the cover is a candidate for elimination or combination:
Can the cover be eliminated?
Can the cover be combined? If so with what?
Fasteners are automatic candidates of elimination:
How can the fasteners be eliminated? 5

43. Part Reduction Is the Key for Design Improvement
Eliminating one part reduces at least 3 seconds.
In addition a non-existing part is the best since it never:
Requires purchasing and warehouse
Needs inspection
Causes a quality problem
Needs design changes and drawing
Be in shortage 5

44. DFA of Pneumatic Pump
According to the piston stop, the cover is a candidate for elimination or combination:
Can the cover be eliminated? No!
Can the cover be combined? Yes! Combine with piston!
Fasteners are automatic candidates of elimination:
How can the fasteners be eliminated? Yes! Press-fit the cover! 5

45. New Design of Pneumatic Pump

46. BDI DFMA: Design Efficiency
The pneumatic pump:
Has 7 parts & needs seconds to assemble
Has 4 critical parts & 3 non-critical parts
If the product is designed perfectly:
Pump would consists of only four parts
Each part would take 3 seconds to assemble
Total assembly time would be 12 seconds
Design efficiency from assembly view points is:
Design Efficiency = 12/40.75 = 29% 5

47. BDI DFMA: Pneumatic Pump
Assembly Time:
What should we do to reduce handling time of the cover and insertion times of the cover and screws? 5

48. BDI DFMA Assembly Time 1.13 ~10 seconds to handle a part depending on:
Symmetry; Primary(-) & secondary(-)
Size: Thickness, dimension, weight, …
Handling ease: Tool, flexible, tangling, …
To reduce part handling time:
More part symmetry: a- & b-symmetry
Right part size : Thickness, dimension, …
No flexible, tangling parts 5

49. To Reduce Handling Time
Increase symmetry by introducing redundancy: a - symmetry from 360o to 180o
Closed-end springs do not tangle 5

50. BDI DFMA Assembly Time 1.5 ~ 12 seconds to insert a part depending on:
Visual & hand access
Hold-down, alignment, insertion force,
Securing: Screw, snap, bending, soldering, ..
To reduce part insertion time:
Easy visual & hand access
No hold-down, easy alignment, low/no insertion force, ..
Easy/No securing requirement: Snaps instead of screw, bending, soldering, …. 5

51. To Reduce Insertion Time
The long shaft makes piston alignment easy
The washer seat eliminates needs of holding 5

52. New Design of Pneumatic Pump
Piston & piston stop are combined
Cover is press-fitted 5

53. New Design of Pneumatic Pump
BDI DFMA Assembly Time Analysis: 5

54. New Design of Pneumatic Pump
BDI DFMA Assembly Time Analysis:
Design comparison: 5

55. Design Guidelines for Assembly
How to design a product for ease of assembly?
Reduce the number of part
Design each part for ease of handling
Design each part for ease of insertion
Examine trade-offs, other cost and other X’s 5

56. Design Guidelines for Assembly
Reduce the number of part:
Eliminate unnecessary parts
Combine parts
Reduce the different kinds of parts
Reduce/eliminate fasteners 5

57. Design Guidelines for Assembly
Design each part for ease of handling:
Increase part symmetry but avoid near symmetric parts
Avoid tangling, nesting and sticky parts
Reduce/eliminate fasteners
Avoid custom-designed parts & use standard parts
Reduce part variability. If not possible, make difference pronounced 5

58. Design Guidelines for Assembly
Design each part for ease of insertion:
Design for top-down layered assembly
Enhance self-locating & alignment capability
Provide in-process verification process
Design the base part for transportation & orientation & the last part to lock the assembly
Avoid turnover & processing during assembly
Design for one-hand assembly
Design fixtures together with products 5

59. Design Guidelines for Assembly
Examine trade-offs, other cost and other X’s:
To hold a wash or provide a washer seat?
Several standard parts or a single custom part?
Different material?
Different joining method?
What about:
Service & repair
Packing and shipping
Recycle ...
Assembly is only one of many cost factors! 5