Prototypes are easy; recurring production is where suppliers are made or broken. Use this framework to qualify a CNC partner you can rely on month after month.
ISO 9001:2015 is the baseline. Ask how they control variation: first-article inspection, in-process checks, SPC, and how non-conformances are documented and corrected.
Send a real part, not a toy. Evaluate dimensional accuracy, surface finish, burr control, packaging and — importantly — consistency across the whole batch, not just the first five pieces.
For special alloys and plastics, confirm mill certs and traceability. A supplier managing 100+ vetted sub-suppliers can absorb capacity shocks that sink a single-shop vendor.
Communication, engineering responsiveness and on-time delivery during the pilot predict the production reality. Slow answers now become missed deadlines later.
It is the baseline for a controlled process. For automotive-grade needs you may also want IATF 16949, but many equipment makers are well served by a strong ISO 9001 supplier with SPC discipline.
At least a full production batch with SPC data; review Cpk on the critical characteristics, not just pass/fail.