The document discusses the principles of Design for Assembly (DFA), emphasizing the importance of reducing the number of parts in a product to enhance quality and cost-effectiveness. It outlines methodologies for identifying essential versus non-essential parts and offers examples highlighting the impact of design improvements on assembly efficiency. Key strategies include simplifying product structure, optimizing part accessibility, and applying systematic evaluation techniques to streamline the assembly process.

1. Design for Assembly
Dr Richard FarrDr Richard Farr

2. Content
Introduction to assembly
Which parts are ‘essential’?
Conducting a Product Autopsy
General principles
Assessing ease of
assembly
Let’s make a Stealth
Bomber
Concluding remarks

3. Learning outcomes
Understand why good products actually have
fewer parts
Learn to identify surplus parts, and eliminate
them
See how money can be saved while quality is
improved
Master some simple design principles that will
speed assembly operations, and save you
money

4. Assemblies are the product of a complex
design process
Designers can have a major influence on the
cost and quality of an assembly
The assembly task also involves…
Introduction to Assembly

5. This forms a significant part of Design for
Manufacture (DFM) – sometimes considered
together as DFMA
DFA addresses product structure
simplification since the total number of parts in
a product is a key indicator of design quality
Useful in the optimisation of manual assembly
tasks, and also automated assembly (although
with somewhat different rules for each)
Design for Assembly (DFA)

6. Complexity costs money
Intricacy
Tolerances
Surface finish
Symmetry
Uniformity
Accessibility
Orientation
Handling
Time

7. Both the Boothroyd & Dewhurst, and Lucas
methodologies use the idea of fundamental or
essential parts.
All non-essential parts should be evaluated in
case they can be designed out.
The Lucas methodology labels parts as ‘A’
(essential) and ‘B’ (target for designing out).
Boothroyd & Dewhurst use ‘1’ and ‘0’, but the
result is similar.
Which Parts are Essential?

8. Design Efficiency is the number of essential
parts divided by the total number of parts,
expressed as a percentage.
Maybe use Design Efficiency in a decision gate,
by specifying a target efficiency percentage.
Design Efficiency
A
A + B
x 100%

9. Does this part move
relative to all parts
which have already
been analysed?
Is this part of a different
material to all parts
already analysed, with
which there was no
relative movement?
Is the movement
essential for the
product to function?
Is a different
material essential
for the product
to function?
Is the part separate
to allow for its
in-service adjustment
or replacement?
Is the adjustment
or replacement
essential?
Must the part be
separate to provide
the required
movement?
Must the part be
separate to satisfy the
different material
requirement?
Must the part be
separate to enable
the adjustment or
replacement?
It’s a “B” PART – non-essential
It’san“A”PART–essential

10. Original design for a
thermal gunsight reticle in a
US tank, made by Texas
Instruments, Inc.
There are a large number
of fasteners.
Example
ource: Boothroyd, Dewhurst and Knight (1994)

11. Redesigned thermal
gunsight reticle:
simpler to assemble,
and less to go wrong!
Example
ource: Boothroyd, Dewhurst and Knight (1994)

12. Original Redesign Improvement
Assembly time (h) 2.15 0.33 84.7%
Number ofdifferent
parts
24 8 66.7%
Total number of parts 47 12 74.5%
Total number of
operations
58 13 77.6%
Metal fabricationtime
(h)
12.63 3.65 71.1%
Weight (lb) 0.48 0.26 45.8%
Measuring Improvement

13. Redesign
Another example
Source: Boothroyd, Dewhurst
and Knight (1994)
Original

14. Some simple rules can save you some time when trying to identify fundamental parts…
The first part analysed will always be ‘essential’ because there is nothing to compare it to (but try to choose
something sensible...)
Fasteners (nuts, bolts, screws, pins, rivets…) are never ‘A’ parts, and should always be considered as a target
for designing out.
Basic Principles

15. Any product that you no longer have a use for might be dismantled to see if it’s as simple as it could possibly
be.
Learn by doing!
Use the matrix of
nine questions
to see which parts
are essential.
Don’t dismantle
anything you might
want to use again!
Product Autopsy

16. Fasteners…
Five screws
hold two
halves of the
telephone
base
together

17. Fasteners…
Better designs
use integral
fasteners
(printer by
Hewlett-Packard)

18. More waste inside…
“Ballast”

19. Discussion point #1
What is the “ballast” for?

20. What is the “ballast” for?
Possible answers…
Makes the phone base more likely to stay where it’s put, perhaps?
Makes it feel like a durable, ‘quality’ product because it’s heavy?
Other reasons?

21. Discussion point #2
Supposing the product needs to have a heavy
base… is this the best way to achieve that design
requirement?
What’s wrong with the approach taken?

22. What’s wrong with the design?
Possible answers…
Why are three pieces of sheet steel required?
Why must they be held in place by two screws, and two washers?
Is the plastic shim really necessary?
Couldn’t the phone base just have a single piece of steel, a big lump of putty, or a sandbag, or something?

23. DFA for electronics too…
Is all that interconnection necessary? Point-to-point
wiring and a ribbon connector means more work.

24. Final verdict
Too many components overall
Too many fasteners (should be integral clips)
Too much variety (different part types)
Multiple parts to do a simple job (“ballast”)
Too many assembly operations required
Assembly operations that would be difficult to
automate (including point-to-point wiring)
Product was needlessly expensive to make
Reliability was compromised by excessive
interconnection via manual processes

25. A study at Ford in the
1990s found that
threaded fasteners
featured in 75% of
assembly line defects.
Threaded fasteners

26. General principles
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 of parts from bulk
6. Minimise the need for reorientations during assembly
7. Design parts that cannot be installed incorrectly
8. Maximise part symmetry if possible or make parts
obviously asymmetric
Boothroyd & Dewhurst (1989)

27. General principles (continued)
Ulrich & Eppinger (1995)
9. Eliminate processing steps
10. Choose the appropriate economic scale for the
process
11. Standardise components and processes
12. Adhere to “Black Box” component procurement
(give a description of what the component has to
do, not how to achieve it)
13. Minimise system complexity

28. 1. Reduce part types
Remember that each part needs to be...
Designed
Prototyped and tested
Manufactured (or procured)
Inspected
Controlled
Documented
Work smarter, not harder! A good design has fewer parts.

29. 3. Self aligning, self locating
Get rid of “redundant
constraints” (features
that demand accurate
manufacturing)
These two
components will still fit
together even if they
are manufactured
really crudely… which
means money is saved

30. 5. Ease of handling
Specify components
that can’t get tangled
together when they’re
mixed together in a
box or a bin

31. 5. Ease of handling (continued)
Specify components
that can’t nest tightly
together

32. 7. Mistake-proofing
Use physical
obstructions to stop
components being
fitted in the wrong
place, or the wrong
way round
The name for this is
“poka-yoke” ポカヨ
ケ

33. 8. Maximise part symmetry
Make components fit
either way round
whenever you can
Ideally, parts will
have rotational and
end-to-end
symmetry.

34. 8. Make asymmetry obvious
These irregularly-sized
and spaced holes force
the worker to figure out
which way it fits
The addition of a flat
side or similar feature
helps to achieve correct
orientation during
manual assembly (but
symmetry would
probably be better)

35. Formal evaluation method...
‘TeamSET’

36. TeamSET software
A software-based version of the ‘Lucas Methodology’
(Lucas, 1989)
Supports designers in many tasks, including Design for
Assembly (DFA), Design for Manufacture (DFM),
Quality Function Deployment (QFD), Design to
Target Cost (DTC) and Failure Mode & Effect
Analysis (FMEA)
TeamSET is no longer available to buy: more recent
versions are only used in consultancy
The paper-based version of the Lucas Methodology is
still relevant, for both manual and automated
assembly

37. Lucas Methodology
Assigns ‘penalty points’ based on the characteristics
of each part. For example…
Need to use tweezers? – score 1.5
Sticky component? – score 0.5
Must be held in place until another operation is
completed? – score 2.0
Restricted vision? – add 1.5
Evaluate all parts in an assembly, then add up the
penalties. The best design is the one that has the
lowest total score.
Note: penalties differ for automated assembly

38. Estimating assembly time
To work out the cost of assembly, you need to know how much time it will take. Performing a mime of the
assembly process is a good start. When the design is firming up, additional possibilities such as prototypes can
be experimented upon.
Keep in mind that an assembly operation typically involves
three stages:
Grasping
Moving
Inserting (or attaching)

39. Improving assembly time
Think about how a part is presented: does it have
to come from a box where many are jumbled
together?
Remember that the best assembly processes
involve moving parts in a straight line, from above
Any operation that scores a penalty under the
Lucas Methodology will be troublesome, and will
take longer

40. B2 stealth bomber
Lots of parts, obviously…

42. Low-budget version...

43. Upper body moulding:
Five screws (of two different
sizes) hold the upper and lower
body together
Upper body moulding:
Five screws (of two different
sizes) hold the upper and lower
body together

44. Lower body subassemblyLower body subassembly

45. Motor/fan subassemblyMotor/fan subassembly

46. Battery compartment coverBattery compartment cover

47. Captive screw on battery
compartment cover:
A screw fixing is required
on battery compartments
for toys, for safety
Captive screw on battery
compartment cover:
A screw fixing is required
on battery compartments
for toys, for safety

48. Other bits and pieces include
a “ceiling mount kit”
Mount plate, swivel, cable,
hook, and three more screws
Other bits and pieces include
a “ceiling mount kit”
Mount plate, swivel, cable,
hook, and three more screws

49. Procedure
The toy was dismantled until each individual
component could be seen
Components were listed in an indented Bill of
Materials format
Components were identified as either ‘A’ parts or
‘B’ parts, using the questions given earlier
The design efficiency of the toy was calculated
‘B’ parts could then be focused upon during a
redesign exercise

50. Bill of Materials (partial)
The indented Bill
of Materials
(BOM) lists all
the components,
and assemblies
required
Indents show the
“parent-children
relationships”, or
what belongs
where

51. Bill of Materials analysed
Only parts need to
be identified as
essential or non-
essential – not
subassemblies

52. Discussion points
Your own analysis may differ a little bit, but…
We decided that the battery compartment cover was a ‘B’ part because theoretically the lower body moulding could
serve as a battery compartment cover
The screw fastening for securing batteries on children’s toys is a required safety feature in some markets… but we
decided it was still a ‘B’ part for the purposes of calculating design efficiency, since this functionality could be achieved
by other means

53. Results of Analysis
Eleven parts were identified as essential:
The box the toy came in
The ceiling mount plate, swivel and cable
The upper and lower body mouldings
The motor, and attached fan (this had been described as bought in, complete)
The switch
The spring steel battery contacts (three parts)

54. Results of Analysis
Eleven essential parts
Thirty parts or bought-in elements in total
A
A + B
x 100%
Formula:
11
30
x 100%
… design efficiency is 36.7%

55. Making use of design efficiency
Some companies use design efficiency as a decision
gate (for example, proceed with the design if
efficiency is over 45%)
Use design efficiency to compare two or more
alternative design concepts, and go with the best one
Examine each ‘B’ part in turn, and state how it might
be designed out (if reasons such as manufacturing
complexity prevent its elimination, record the
reasons)
…but consider ease of assembly as well

56. Concluding Remarks
Always try to simplify products
Counting the number of parts in a product is a
good indication of how well-designed it is
Good designs for reasonably-priced, reliable
products tend to have fewer parts
Fasteners (nuts, bolts, screws, rivets etc.) can
usually be designed out – but leave them in
where access or disassembly are required

57. References
Boothroyd, G., Dewhurst, P. & Knight, W.A.
(2011) Product Design for Manufacture and
Assembly, Boca Raton: CRC Press
Lucas (1991) Mini-Guide: The Lucas
Manufacturing Systems Handbook, Solihull: Lucas
Engineering & Systems Ltd
• Pahl, G., Beitz, W., Feldhusen, J. & Grote, K.H.
(2007) Engineering Design: A Systematic
Approach, London: Springer-Verlag

58. References (continued)
• Redford, A.H. & Chal, J. (1994) Design for
Assembly: Principles and Practice, New York:
McGraw Hill
• Ulrich, K.T. & Epping, S.D. (1995) Product Design
and Development, Singapore: McGraw-Hill

59. Want more?
Further material from Richard Farr on
Capacify, the Sustainable Supply Chain blog
http://capacify.wordpress.com
On Twitter: @Capacified

60. Tutor notes
The first part you analyse is always an essential part, even if something daft is chosen.
Consider, for example, a manufacturer’s badge on an engine block. If you were wise you would
start with the engine block (clearly essential) but let’s suppose you start with the badge, and
move on with the analysis. The next part analysed should be something it’s connected to, so
now you look at the engine block.
Does it need to move relative to all other parts? No. Is it of a different material? That’s likely
true… but is a different material essential? No. Is it separate to allow for replacement or
adjustment? No.
The result is that you end up with a “badge” that
can acquire all the functionality of the engine block it
is connected to; the badge becomes the engine block,
complete with bores for cylinders and so on.
In reality, what happens is that the separate badge
is found to be non-essential, and the manufacturer’s
name ends up being included in the casting for the
engine block.

61. Tutor notes – product autopsy
You may find it useful to choose a product of your own for the “autopsy” so you can
show the students exactly what you’re finding. Photos can only convey so much! We
never yet found a product that didn’t exhibit some assembly problems, or at least
trade-offs made for reasons of manufacturability, so it should be simple to find a
substitute.
If time permits, it can be a good idea to have the students perform the autopsy
themselves. We once did an exercise where each pair of students was given a ‘dead’
computer mouse and a set of screwdrivers, and they worked to identify what was good
and bad about the design.
Regarding the telephone that was used in the autopsy shown in these slides, and the
discussion points…
The “ballast” is an awful solution because it requires a total of eight parts, including
what appears to be a plastic shim to stop it all rattling. The pieces that have been
punched from steel are triplicate because you couldn’t punch through a thicker sheet
easily… but why are they zinc plated? And why not just clip in a slug of cast iron, or
something? Using two screws with small heads and then adding washers to fit the
large holes in the steel weights is simply unforgivable.
The only possible reason for persevering with this bad design and making needlessly
expensive telephones must have been because the moulds for the telephone’s plastic
parts already existed, and altering the design would have been a large one-off
expense.

62. • The assembly game works well as an interactive part of the lesson, but you will need to make some simple props. You’ll need a number of
paired pieces of wood with matching holes drilled in them, to take nuts and bolts.
• All parts are kitted in advance of the lesson, in a disassembled state and boxed such that they cannot be seen in advance.
• One student is called upon to use a stopwatch, to time the “employees” that perform each assembly task. The first task is the baseline
against which all others will be considered, and it can be worth running it a few times with different “employees” to get an average time.
• Unknown to the participants, subsequent challenges make the job harder, including:
• A set with bolts much longer than they need to be
• A set with four different sizes of holes, nuts and bolts, requiring additional thought
• “Dirty” parts (the student is required to wear disposable gloves)
• “Dangerous” parts (the student is required to wear protective gauntlets)
• A miniature set, with tiny nuts and bolts

63. Tutor notes – assembly game
Nothing brings home the importance of Design for
Assembly as much as actually trying an assembly task
for yourself.
Tutors may also wish to introduce “quality
problems” as a theme, by forcing the students to find
good parts from a set that includes damaged nuts and
bolts.

64. Assembly game

65. Discussion points
• Were the tests ‘fair’?
• Which ones were the hardest to assemble, and
why?
• What other experiments would you like to try?
What else might have been very difficult to
work with?
• How could we improve the design?