A drawing arrives with one bore called out at ±0.05 mm and a blanket ±0.13 mm everywhere else. Before that part goes out for quote, someone has to decide whether the tight feature is a standard milling tolerance, a grinding tolerance, or a sign the part needs a secondary operation the drawing does not mention. Get that wrong and the quote either cannot actually hold the number, or the part costs three times what the design needed.

What the Number Commits a Shop To

CNC milling and turning is listed to hold ±0.005 mm as a standard capability. A callout tighter than that is not just "harder machining" — it usually means the feature needs to move to a secondary process. Wire, sinker, or small-hole EDM is listed to ±0.003 mm, and ID, OD, surface, or centerless grinding to ±0.002 mm. Reading a tolerance callout correctly means reading it as a routing decision, not just a difficulty rating. On rotational features — pins, screws, shafts — that routing decision usually starts with CNC turning rather than milling.

Reserve the Tight Numbers for the Features That Earn Them

A part with every dimension pulled to the tightest achievable number does not fit better than one where tolerance is concentrated on the features that actually mate, bear load, or align with another component. It just adds inspection time and cost across geometry that never needed it. This distinction matters most on surgical instrument and orthopedic implant components, where a handful of bearing surfaces or mating interfaces carry the real requirement and the rest of the part can run at a standard machining tolerance. When those mating features sit on different planes or compound angles, the tolerance callout is often the clue that the part needs single-setup 5-axis machining rather than repositioning across multiple setups.

Put the Tolerance in Context With Material and Inspection

The same numeric callout means a different process and a different lead time depending on the material behind it — titanium, stainless, and cobalt-chrome do not machine, grind, or hold a finish the same way. A tolerance also is not a complete requirement on its own. It needs the inspection expectation attached to it: whether the feature gets measured on a CMM, reported with the shipment, or simply held to print without documentation. The quality page lays out what to define before the part is made — drawing revision, critical features, material condition, and inspection deliverables — for exactly this reason.

What to Send With the RFQ

Mark which features are critical instead of leaving one blanket tolerance in the title block. State the material and required condition, note whether a dimensional report should ship with the part, and flag if the drawing already anticipates EDM or grinding rather than assuming milling will cover it. That is the difference between a quote built on a guess and one built on the actual requirement.

Engineering Takeaway

Read a tolerance callout as a routing decision, not a difficulty rating: standard milling and turning hold ±0.005 mm, EDM holds ±0.003 mm, and precision grinding holds ±0.002 mm, so a number tighter than ±0.005 mm usually means the feature needs a secondary process. Reserve the tight numbers for features that actually mate, bear load or align, name the material and inspection requirement alongside the tolerance, and flag on the RFQ if a feature looks too fine or too internal for cutting tools at all.

If the tight features are internal, lattice, or otherwise impractical to reach with cutting tools at all, the better routing question is not which secondary operation to add but whether the part should be metal-printed instead — see the DMLS vs SLM guide for how that decision gets made. Otherwise, send the marked-up drawing to the team with the material and inspection requirement attached, and the quote will reflect the tolerance that is actually being asked for.