Identify the functional dimensions first
For a travel bottle, the interface between the neck and closure may deserve attention. For a palm-style facial brush, the hand opening and contact area may be more relevant. Select the locations that affect assembly, handling or packaging instead of assigning the same priority to every decorative curve. A drawing or dimension sheet should show the measurement location, direction and part condition: assembled or unassembled, compressed or free, and when measured after sampling.
A generic tolerance is not an automatic specification
“Use the usual industry tolerance” does not select the critical dimensions for a customer. A bottle-neck fit, a palm opening and an outer package shape perform different functions; the acceptable variation can differ too. The drawing, mating part, handling action and sample results should guide the agreement. If a project adopts a specific standard or tolerance class, verify its scope, edition and measurement method in the project documents rather than applying an unconfirmed table by default.
Record measurements that can be repeated
At sampling, record the part revision, measurement location, condition, instrument, reviewer, sample count and result. If results vary, do not present only the best part as proof of production capability. Review whether the variation affects fit or use, then agree how future samples will be selected and judged. Where relevant, retain an assembly observation alongside the dimensional readings: similar measurements do not always create the same handling experience.
If the buyer already has a rigid mating part, provide the physical item or reliable dimensional information with the brief. Measuring the silicone part alone may miss variation in that component and compression during assembly. Agree whose mating part and which revision will be used for sample approval. Recheck the critical interface if the mating part later changes.
For a Zhongding OEM/ODM project, bring the drawing, reference sample and mating component. Tell Zhongding which interface matters, what the part must do and what variation would be unacceptable. This is more useful than demanding “the tightest tolerance” without a function. Move from the OEM page to contact once the project brief, sample path and acceptance method are clear.