How to specify dimensions and tolerances for an RFQ

A nominal size does not tell a manufacturer how much variation your assembly can accept. Yet arbitrary tight tolerances can also create an unnecessarily difficult quote. Start with the fit or function that the dimension controls, then decide what needs a measurable limit.

Define the requirement

Attach a drawing with units and revision, then identify the critical dimensions and how they will be measured. Use a tolerance only where the permitted variation is understood. For less critical features, ask the supplier to propose a normal production range and show why it works for the intended use.

Questions to send with the RFQ

Ask the supplier to flag dimensions that its proposed process cannot hold consistently. Request the measurement method and whether inspection occurs on a sample, first article or production lot. A number without a measurement method can still be interpreted differently.

Review the answer

Review the quote and sample against the same drawing. If one supplier changes a tolerance or measuring datum, treat that as a deviation, not a clerical detail. A lower price may simply reflect a wider acceptance range than another supplier assumed.

An example to adapt

Imagine a lid that must mate with an existing jar. The sealing diameter is more important than a decorative ridge. Label the interface dimension clearly and test the assembled fit; avoid spending the entire tolerance budget on a feature that does not affect function.

Decision before sending

A buyer should challenge every unusually tight tolerance with a practical question: what failure does this limit prevent? If the answer is unclear, the limit may be a legacy number rather than a functional need. Discuss the critical interface with the design owner and record the measuring reference. Then ask suppliers whether a proposed process can hold it consistently. This turns a drawing note into a testable procurement decision rather than an unexplained source of tooling cost.

Source context

NIST's STEP file explanation describes dimensions and tolerances as manufacturing information exchanged through CAD and downstream inspection. Give the supplier the controlled drawing revision and ask how the critical dimensions will be measured.