Question 6 of 7: Design for manufacturing and assembly, the environment, and universal design

Question 6: Design for manufacturing and assembly, the environment, and universal design (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part A — Two aspects addressed by each technique

Part B — How each technique impacts the design process

All three change the process in the same structural way — they pull knowledge that used to arrive downstream into the concept and embodiment phases — but they do it with different mechanisms and different examples.

DFMA inserts an explicit, quantified review at the end of the embodiment phase and brings manufacturing and assembly engineers into the design team from the start. The Boothroyd–Dewhurst procedure asks of every part three questions — does it move relative to its neighbours, must it be of a different material, must it be separate for assembly or service — and any part that answers no to all three is a candidate for elimination. A worked example is given in part C. The process consequence is that a design review now has a number attached to it, so "this could be simpler" becomes an argument that can be won or lost on evidence.

Design for the Environment adds a screening life-cycle assessment at the concept stage and an end-of-life strategy as a specification line rather than an afterthought. The green bin of Question 1 is a direct example: designing out the steel axle and the elastomer gasket turned a four-material assembly into a single polymer that can be granulated whole, and that decision had to be taken at concept, because by detail design the pinned hinge was in the tool. The process consequence is a new deliverable (a materials and end-of-life declaration) and a new gate criterion.

Universal Design changes who is in the room and what the specification says. Anthropometric and strength data enter the requirements as numbers — the 25 N lid force of Question 1 was set from the fifth-percentile one-hand pull, not from what felt reasonable — and usability trials with a genuinely diverse participant group become a scheduled activity rather than an optional one. A second example: a lever handle instead of a knob costs nothing extra to make and can be operated with a closed fist, an elbow or a full shopping bag, which is why it is the standard illustration of the principle that inclusive design usually improves the product for everyone.

The common process risk is worth naming: three techniques applied simultaneously generate conflicting recommendations — DFMA wants parts consolidated and permanently joined, design for the environment wants them separable for material recovery, and universal design wants the generous grips and clearances that add material. Resolving those conflicts is a design-management task, and the honest way to do it is to bring them into the same weighted decision matrix used in Question 1 rather than to let whichever technique has the loudest advocate win.

Part C — Short-term and long-term costs

Given. The lid module of the green bin, redesigned under DFMA. The existing module has 11 parts and takes 68 s to assemble; the theoretical minimum part count is 4. The redesigned module has 5 parts and takes 27 s. Production is 250 000 units per year, assembly labour and overhead is 48 dollars per hour, and the redesign plus new tooling costs 310 000 dollars.

Find. The design-for-assembly index before and after, the annual saving, and the payback period — that is, the short-term cost and the long-term return.

Approach. Compute the Boothroyd–Dewhurst index for each design, convert the assembly-time reduction to an annual cash saving at the stated labour rate, and divide the investment by it.

1. Compute the design-for-assembly index for the existing module. With an ideal handling-and-insertion time $t_{\text{ideal}} = 3$ s per part, the index is $E_{\text{ma}} = N_{\min} t_{\text{ideal}} / t_{\text{total}} = (4)(3)/68 = 0.176$, or 17.6 per cent. Anything below about 0.2 is conventionally treated as poor and worth attacking.
2. Compute it for the redesign. Consolidating the latch, the gasket and the pinned hinge into moulded features removes six parts and the reorientation they forced: $E_{\text{ma}} = (4)(3)/27 = 0.444$. The index has improved by a factor of 2.52, and note that the numerator never moved — the theoretical minimum is a property of the function, not of the design, which is exactly what makes it a fair yardstick.
3. Convert the time saved into money. The saving is $68 - 27 = 41$ s per unit, so at 48 dollars per hour $$\text{saving per unit} = \frac{41}{3600} \times 48 = 0.547 \ \text{dollars}.$$ Over the annual volume, $0.547 \times 250\,000 = 136\,667$ dollars per year.
4. Set that against the short-term cost and conclude. The investment is 310 000 dollars of engineering and tooling, spent up front, so $$\boxed{\ \text{payback} = \frac{310\,000}{136\,667} = 2.27\ \text{years}.\ }$$ That is comfortably inside the tool life and inside the twelve-year product life, so the technique pays for itself with a wide margin — and the recurring saving continues for the remaining nine years at no further cost.

Generalising from that calculation, the costs divide cleanly.

Short-term costs are all incurred before launch and are real: additional engineering hours in concept and embodiment, when the schedule is already tight; training, software and data — DFMA databases, life-cycle inventory databases, anthropometric data sets; specialist involvement from manufacturing, environmental and accessibility experts; extra prototyping and usability testing; a longer front-end schedule, which is the cost programme managers resist most; sometimes higher unit material cost, since recycled resin, a mono-material substitute or a larger grip may cost more per part; and tooling investment such as the 310 000 dollars above.

Long-term returns accrue over the life of the product and are almost always larger: lower assembly labour and fewer parts to purchase, store, inspect and document, which is where the 136 667 dollars a year came from; higher quality and lower warranty cost, because a part that is not there cannot be fitted wrongly; lower material and disposal cost, and access to extended-producer-responsibility credits; a larger addressable market, since an inclusively designed product is usable by older and less able customers without a separate variant; reduced regulatory and reputational risk; and lower cost of change, because a design that was made simple early is cheaper to modify later.

The strategic point that a full-mark answer should end on is the one the figure makes: by the end of detail design roughly 80 per cent of whole-life cost is committed while only about 14 per cent has actually been spent. All three techniques are therefore cheap when applied early and nearly worthless when applied late — and a firm that applies them late experiences them as pure cost, which is the usual reason they get abandoned.