A drawing that calls out ±0.005 mm on every dimension is asking for something very different — in cost and lead time — than one that reserves that tolerance for the two or three features that actually need it. For orthopedic prototypes, where a part often has to mate with an implant, instrument or bone model, knowing which tolerance band a feature really needs is as important as hitting the number once it's specified.

What tighter tolerance actually costs

Every tolerance band tighter than standard shop practice adds setup time, inspection time, or a secondary process. A ±0.005 mm callout on a bearing surface is routine for 3, 4 and 5-axis CNC milling and turning. The same callout on a large flat face or a non-critical mounting hole usually isn't necessary — and holding it there anyway adds cost and lead time without adding anything the part needs to function.

Where each process lands

  • Milling & turning: ±0.005 mm — the standard band for most machined prototype features.
  • EDM (sinker, wire, small-hole): ±0.003 mm — used for internal geometry and hardened materials that are difficult or impossible to mill.
  • ID / OD / surface / centerless grinding: ±0.002 mm — reserved for bearing surfaces, shafts and other features where fit is the whole point of the tolerance.

These aren't arbitrary numbers — they reflect what each process is mechanically capable of holding repeatably, not just on a best-case part. Full material and process detail lives on the Materials & Finishing page.

Matching tolerance to the feature, not the whole part

The most useful thing an engineer can do on a prototype drawing is separate “needs to fit” from “needs to exist.” A trial implant's articulating surface or an instrument's mating feature earns a tight tolerance and, often, a grinding or EDM pass. A boss, a chamfer or a non-mating wall doesn't — and calling it out at ±0.002 mm anyway is a common way prototype quotes come back higher and slower than expected. This kind of review is part of the design-for-manufacturing check built into every Lab Ready quote.

If you're not sure which features on your next orthopedic implant or instrument prototype need tight tolerance and which don't, send the drawing — that review is part of every quote, not an extra step.