Should parts that can be bought off the shelf be designed to do so?

• A. Always
• B. Often
• C. Never
• D. Sometimes

Answer: (C)

The main idea here is to avoid designing a component around the expectation that an off-the-shelf part will be used. You should define the function, interfaces, tolerances, and mounting in a way that is independent of any single supplier or specific off-the-shelf item. This keeps the design flexible and resilient to changes in availability, price, or lifecycle of parts.

If you tailor a design to fit a particular off-the-shelf part, you risk future compatibility problems: the part could be discontinued, its price could rise, or its tolerances and interfaces might change, forcing costly redesigns or supply-chain delays. By designing for the required function and standard interfaces rather than for a specific off-the-shelf component, you preserve option to substitute different parts without altering the core design. Off-the-shelf parts can still be used when they meet the specs, but the design itself shouldn’t be built around them.