Design for Assembly (DFA)

Design for AssemblyDr Richard FarrDr Richard Farr

Content

Introduction to assemblyWhich parts are ‘essential’?Conducting a Product AutopsyGeneral principlesAssessing ease ofassemblyLet’s make a StealthBomberConcluding remarks

Learning outcomes

Understand why good products actually have fewer partsLearn to identify surplus parts, and eliminate themSee how money can be saved while quality is improvedMaster some simple design principles that will speed assembly operations, and save you money

Assemblies are the product of a complex design processDesigners can have a major influence on the cost and quality of an assemblyThe assembly task also involves…

Introduction to Assembly

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

Design for Assembly (DFA)

Complexity costs money

IntricacyTolerancesSurface finishSymmetryUniformityAccessibilityOrientationHandlingTime

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?

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 + Bx 100%

Does this part moverelative to all partswhich have already

been analysed?

Is this part of a differentmaterial to all parts

already analysed, withwhich there was norelative movement?

Is the movementessential for the

product to function?

Is a differentmaterial essential

for the productto function?

Is the part separateto allow for its

in-service adjustmentor replacement?

Is the adjustmentor replacement

essential?

Must the part beseparate to provide

the requiredmovement?

Must the part beseparate to satisfy the

different materialrequirement?

Must the part beseparate to enablethe adjustment or

replacement?

It’s a “B” PART – non-essential

It’s an “A” P

AR

T – e ssen tial

Original design for a thermal gunsight reticle in a US tank, made by Texas Instruments, Inc.

There are a large number of fasteners.

Example

Source: Boothroyd, Dewhurst and Knight (1994)

Redesigned thermal gunsight reticle: simpler to assemble, and less to go wrong!

Example


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

Redesign

Another example


Original

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

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 ofnine questionsto see which partsare essential.

Don’t dismantleanything you mightwant to use again!

Product Autopsy

Fasteners…

Five screws hold two halves of the telephone base together

Fasteners…

Better designs use integral

fasteners

(printer by Hewlett-Packard)

More waste inside…

“Ballast”

Discussion point #1

What is the “ballast” for?

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?

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?

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?

DFA for electronics too…

Is all that interconnection necessary? Point-to-point wiring and a ribbon connector means more work.

Final verdict

Too many components overallToo many fasteners (should be integral clips)Too much variety (different part types)Multiple parts to do a simple job (“ballast”)Too many assembly operations requiredAssembly operations that would be difficult to automate (including point-to-point wiring)Product was needlessly expensive to makeReliability was compromised by excessive interconnection via manual processes

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

Threaded fasteners

General principles

1. Reduce part count and part types2. Strive to eliminate adjustments3. Design parts to be self-aligning and self-locating4. Ensure adequate access and unrestricted vision5. Ensure the ease of handling of parts from bulk6. Minimise the need for reorientations during assembly7. Design parts that cannot be installed incorrectly8. Maximise part symmetry if possible or make parts

obviously asymmetric

Boothroyd & Dewhurst (1989)

General principles (continued)

Ulrich & Eppinger (1995)

9. Eliminate processing steps10. Choose the appropriate economic scale for the

process11. Standardise components and processes12. 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

1. Reduce part types

Remember that each part needs to be...DesignedPrototyped and testedManufactured (or procured)InspectedControlledDocumented

Work smarter, not harder! A good design has fewer parts.

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

5. Ease of handling

Specify components that can’t get tangled together when they’re mixed together in a box or a bin

5. Ease of handling (continued)

Specify components that can’t nest tightly together

7. Mistake-proofing

Use physical obstructions to stop components being fitted in the wrong place, or the wrong way roundThe name for this is “poka-yoke” ポカヨケ

8. Maximise part symmetry

Make components fit either way round whenever you canIdeally, parts will have rotational and end-to-end symmetry.

8. Make asymmetry obvious

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

Formal evaluation method...

‘Tea

mSE

T’

TeamSET softwareA 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 consultancyThe paper-based version of the Lucas Methodology is still relevant, for both manual and automated assembly

Lucas MethodologyAssigns ‘penalty points’ based on the characteristics of each part. For example…Need to use tweezers? – score 1.5Sticky component? – score 0.5Must be held in place until another operation is completed? – score 2.0Restricted vision? – add 1.5Evaluate 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

Estimating assembly timeTo 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:GraspingMovingInserting (or attaching)

Improving assembly timeThink 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 aboveAny operation that scores a penalty under the Lucas Methodology will be troublesome, and will take longer

B2 stealth bomber

Lots of parts, obviously…

Low-budget version...

“It really flies!”(Sort of.)

Upper body moulding:Five screws (of two differentsizes) hold the upper and lower body together

Upper body moulding:Five screws (of two differentsizes) hold the upper and lower body together

Lower body subassemblyLower body subassembly

Motor/fan subassemblyFour more screws (two sizes)

Motor/fan subassemblyMotor/fan subassembly

Battery compartment coverBattery compartment cover

Captive screw on batterycompartment cover:

A screw fixing is requiredon battery compartmentsfor toys, for safety

Captive screw on batterycompartment cover:

A screw fixing is requiredon battery compartmentsfor toys, for safety

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

ProcedureThe toy was dismantled until each individual component could be seenComponents were listed in an indented Bill of Materials formatComponents were identified as either ‘A’ parts or ‘B’ parts, using the questions given earlierThe design efficiency of the toy was calculated‘B’ parts could then be focused upon during a redesign exercise

Bill of Materials (partial)

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

Bill of Materials analysedOnly parts need to be identified as essential or non-essential – not subassemblies

Discussion pointsYour 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

Results of AnalysisEleven 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)

Results of AnalysisEleven essential partsThirty parts or bought-in elements in total

A

A + Bx 100%

Formula:

11

30x 100%

… design efficiency is 36.7%

Making use of design efficiencySome 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 oneExamine 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

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

References

Boothroyd, G., Dewhurst, P. & Knight, W.A. (2011) Product Design for Manufacture and Assembly, Boca Raton: CRC PressLucas (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

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

Want more?Further material from Richard Farr on Capacify, the Sustainable Supply Chain bloghttp://capacify.wordpress.com

On Twitter: @Capacified

http://capacify.wordpress.com/

Tutor notesThe 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” thatcan acquire all the functionality of the engine block itis connected to; the badge becomes the engine block,complete with bores for cylinders and so on.

In reality, what happens is that the separate badgeis found to be non-essential, and the manufacturer’sname ends up being included in the casting for theengine block.

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.

• 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

Tutor notes – assembly gameNothing 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.

Assembly game

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?