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DFM / DFA Review

Design for manufacture and assembly: fewer parts, simpler joins, standard processes — cost falls with complexity.

  • Time1 h
  • FormatSmall group
  • StageGenerate

DFM / DFA Review: what it is and why it works

A DFM/DFA review examines a design against two sets of rules: design for manufacture, which makes each part easier and cheaper to produce, and design for assembly, which makes the product easier to put together. The review starts with the parts carrying the highest process cost. Manufacturing rules cover tolerances, radii, wall thicknesses, tool access and standard processes; assembly rules cover part count, fastener count, insertion directions and self-locating features. The saving from each proposed change is estimated before deciding, and recurring findings are added to the design rules checklist.

Much of a product's manufacturing cost is locked in by design decisions such as tolerances, geometry, materials and part count, and those decisions are expensive to change once tooling is built. Reviewing them against DFM and DFA rules early catches cost that production would otherwise have to live with. The approach developed by Geoffrey Boothroyd and Peter Dewhurst in the early 1980s asks, for each part, whether it moves relative to other parts, must be of a different material, or must be separate for assembly or service; if not, it is a candidate for combination. DFM/DFA turns value engineering ideas into concrete design changes and, over time, feeds design rules that prevent the same mistakes on the next product.

What you need

  • Current drawings or 3D models and the bill of materials
  • Process cost per part, from the cost breakdown or should-cost models
  • Assembly sequence and times, observed or estimated
  • Manufacturing capabilities and standard processes available in-house and at suppliers
  • The existing design rules checklist, if any

What you get

  • A list of proposed design changes with estimated savings per change
  • Reduced part count, fastener count and assembly time, where changes are adopted
  • Relaxed tolerances where no functional reason justifies them
  • Updated design rules capturing recurring findings

When to use it

When prototypes assemble beautifully and production bills keep growing.

How to do it, step by step

  1. Pick the parts with the highest process cost share.
  2. Review each against design-for-manufacture rules: tolerances, radii, tool access.
  3. Review assemblies against design-for-assembly: part count, fastener count, insertion direction.
  4. Estimate the saving per design change before deciding.
  5. Feed the recurring mistakes back into the design rules checklist.

Worked example: Stainless steel guard assembly for food conveyors

Illustrative scenario — figures are realistic but not from a real company.

A conveyor manufacturer for food processing plants used a stainless steel side guard assembly on most of its lines. Each guard had 23 parts, including 12 fasteners, took about 14 minutes to assemble and cost around $212. Production volumes were rising, and the assembly had become a bottleneck.

  1. The team chose the guard because its process cost share was among the highest of all fabricated assemblies.
  2. The DFM review found tolerances of plus or minus 0.004 inch on laser-cut edges that mated with nothing, and four different bend radii requiring four press brake tool changes.
  3. The DFA review applied the minimum part count questions: most brackets and spacers did not move, did not need a different material and did not need to be separate for service. They could be combined with the main panel through bent tabs.
  4. Fasteners were reduced to four captive screws. Because the guard is a fixed guard, the team kept tool-operated fasteners so it cannot be removed by hand, in line with machinery safety requirements.
  5. Savings were estimated for each change before approval, and the findings on tolerances and bend radii were added to the sheet-metal design rules.

Result. The redesigned guard had 9 parts, assembled in about 5 minutes, and cost around $148, a saving of about 30 percent. Fewer crevices also made cleaning easier for customers. The team learned that the tolerance and bend radius issues appeared across many other sheet-metal parts, so the design rule update was worth more than this single redesign.

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Common pitfalls and how to avoid them

  • Reviewing parts at random rather than where process cost is highest.Start with the parts and assemblies with the largest process cost share.
  • Combining parts in ways that make service or safety harder.Check each part combination against maintenance access, safety and regulatory requirements.
  • Adopting changes without estimating savings and tooling costs.Estimate the net saving of each change, including any new tooling or validation effort.
  • Fixing the same issues project after project.Feed recurring findings into the design rules checklist and train designers on them.

Frequently asked questions

What is the difference between DFM and DFA?

Design for manufacture focuses on making each individual part easier and cheaper to produce, for example through suitable tolerances, radii, wall thicknesses and standard processes. Design for assembly focuses on the product as a whole, reducing the number of parts and fasteners and making parts easy to handle and insert. The two are usually applied together, because changes in one affect the other.

How do you decide whether a part can be eliminated?

A common approach asks three questions for each part: does it move relative to the parts already assembled, must it be made of a different material, and must it be separate to allow assembly or service? If the answer to all three is no, the part is a candidate for combination with another part. Engineering judgment then confirms whether the combination is practical.

When should DFM/DFA reviews be done?

As early as possible, ideally during concept and detailed design before tooling is ordered, because that is when changes are cheapest. Reviews on existing products are still worthwhile when volumes are significant or when a redesign is planned. A final check before design release helps catch issues that slipped through.

Origin

Design for Manufacture and Assembly — Geoffrey Boothroyd & Peter Dewhurst, early 1980s.

Used in these playbooks

Cost redesign sprint 1 week

One week to redesign the expensive parts: analyze functions, run the workshop, apply DFM rules and cost the prototype honestly.

  1. Functional Analysis
  2. Value Engineering Workshop
  3. DFM / DFA Review
  4. Prototype Costing

Related methods

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