Design to Cost · Toolkit

Design-for-Cost Rules

Write ten costing rules your designers can check in seconds: standard tolerances, preferred materials, no one-off parts.

Time30 min
FormatSolo
StageToolkit

Design-for-Cost Rules: what it is and why it works

Design-for-Cost Rules condense a company's costliest recurring design mistakes into a short list of positive rules that designers can check in seconds. Each rule states what to do rather than what to avoid: use the preferred tolerance class unless a function requires tighter, choose from the preferred material list, use standard fasteners and pipe sizes, design sheet metal to standard bend radii. The list is kept to about ten rules, integrated into the CAD checklist and the design gate criteria, and refreshed after each project postmortem so it reflects current processes and suppliers.

The method works because most cost is designed in through small, repeated choices rather than big decisions, and designers under schedule pressure default to habit. A long design manual is rarely read; ten well-chosen rules embedded in tools and checklists are applied every day, including when no cost engineer is available. The rules capture organizational learning that would otherwise stay in the heads of a few experienced people. The approach draws on the checklist tradition of DFM/DFA associated with Boothroyd and Dewhurst. It complements full DFM/DFA analysis on critical parts, supports Standardization by pointing to preferred items, and uses Cost of Poor Quality data to find the design choices that cause recurring failures.

What you need

Records of costly design issues from past projects: changes, quotes, scrap, field failures
Preferred lists already in use: materials, fasteners, components, processes
Process capability data from the shop and key suppliers
The CAD checklist and design gate criteria where rules will be embedded

What you get

A list of about ten positive, checkable rules, each with its reason and cost effect
The rules embedded in the CAD checklist and gate reviews
A waiver process for justified exceptions
A refresh schedule tied to project postmortems

When to use it

When the same expensive mistakes are re-invented in every new design.

How to do it, step by step

Collect the costliest design mistakes from past projects.
Turn each into a positive rule: preferred tolerances, materials, processes.
Keep the list to ten rules a designer can check in seconds.
Integrate the rules into the CAD checklist and design gates.
Review and refresh the rules after each project postmortem.

Worked example: Ten rules for a pump skid fabricator

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

A company building pump and dosing skids for water treatment and chemical plants found that its design reviews repeatedly flagged the same issues: nonstandard pipe schedules, tight tolerances on welded frames, one-off brackets and unusual flange ratings. Each issue added cost and lead time, and new designers kept repeating them.

The engineering manager and a cost engineer reviewed change orders and quotes from the last 20 projects and listed 27 recurring cost issues.
They grouped and rewrote them as positive rules, such as: use Schedule 10S for stainless process piping up to 4 in unless the calculation requires more; use the default frame tolerance of +/-1/8 in; select brackets from the standard bracket library; use ASME B16.5 Class 150 flanges unless process conditions require a higher class.
The list was cut to ten rules, each with a one-line reason and an order-of-magnitude cost effect, and added to the CAD release checklist.
Exceptions required a short written justification approved by the lead engineer.

Result. Over the next year, design review comments on these topics dropped sharply and the average skid frame used fewer unique parts. Two rules were revised after a postmortem showed that a customer standard required heavier piping on some sites; the rule now points designers to check the customer specification first.

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Common pitfalls and how to avoid them

Writing a long manual that nobody reads.Keep to about ten rules that can each be checked in seconds, and move detail to supporting references.
Writing rules as prohibitions without alternatives.Phrase each rule positively, pointing to the preferred choice.
Rules that conflict with codes, customer specifications or safety requirements.State that applicable codes, customer specifications and safety requirements always take precedence, and provide a simple waiver process.
Never updating the rules.Review the list after each project postmortem and retire rules that no longer reflect current processes or suppliers.

Frequently asked questions

What are design for cost guidelines?

They are practical rules that help designers choose lower-cost options without detailed cost analysis for every decision: preferred materials, standard components, tolerance classes, process-friendly features and part-count reduction. Effective guidelines are short, specific to the company's processes and suppliers, and embedded in the tools designers use.

What is the difference between DFM and design for cost?

Design for manufacturing focuses on making parts easy and reliable to produce with the chosen processes. Design for cost is broader: it considers materials, sourcing, logistics, assembly, testing and life-cycle costs, and sets targets. DFM rules are a major source of design-for-cost rules, but not the only one.

How many design rules should a checklist have?

Few enough that designers apply them routinely, often around ten for a product family. More rules can exist in references, but the checklist should contain the ones with the highest cost impact and the most frequent violations. Review usage and results periodically and replace rules that no longer matter.

Origin

Design-for-cost guidelines — DFM/DFA rule traditions (Boothroyd-Dewhurst checklists).

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