Specification
A specification defines what is being purchased in engineering terms: drawings, dimensions and tolerances, material callouts, performance requirements, and acceptance criteria. Suppliers quote and build to the spec, not to the designer's intent, so the document itself sets cost and quality. Over-specification buys precision nothing needs; under-specification invites quality escapes and disputes.
Examples
Tolerance audit: A machined shaft carries plus or minus 0.01 mm on a diameter that mates with a slip-fit bushing. Relaxing it to 0.04 mm removes a grinding operation and cuts price from $7.10 to $5.85. Across 80,000 annual units, one tolerance line was worth $100,000.
The unwritten requirement: A bracket spec omits surface treatment. The supplier ships bare steel, parts corrode in a coastal warehouse, and the claim fails because every stated requirement was met. The fix costs one drawing note; the lesson cost $62,000 in rework and freight.
Quoting to revision: Three suppliers quote a housing, two against revision D and one against older revision C with looser flatness. The low bid sits $0.60 under the others and is not comparable. Restating the RFQ at revision D for all three closes the spread to $0.08.
Definition
Every requirement on a drawing has a price. Tolerances tighter than function requires force slower machining, extra operations, and higher scrap; a plus or minus 0.01 mm callout where 0.05 mm would serve can add a grinding step that nearly doubles part cost. This is the core argument of design for manufacturability: suppliers price what the spec demands, and they rarely volunteer that the demand is excessive.
Under-specification is the mirror failure. Whatever the spec does not state, the supplier decides: surface finish, burr condition, packaging, test method. A part can be fully conformant and still unusable, and the buyer has no recourse because the requirement was never written. Ambiguity discovered at first article inspection is cheap; ambiguity discovered in the field is not.
Practical spec discipline: tolerance only what function needs, cite standard test methods rather than describing them, control revisions tightly, and state the revision on every RFQ so quotes are comparable. Periodic value engineering reviews exist largely to find requirements that cost money without adding function. LightSource ties supplier quotes to specific drawing revisions so pricing and spec stay in sync as designs change.
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