Why tolerance is the biggest lever on price
At standard tolerances (around ±.005"), a part can be roughed and finished quickly and checked with basic gauges. As the band shrinks, each feature may need a separate finishing pass, a more rigid setup, tool wear compensation, and measurement on a CMM or optical system. Below ±.0005", even the shop's temperature starts to matter.
Typical tolerance bands and what they imply
| Tolerance | What it usually means | Relative cost |
|---|---|---|
| ±.005" | Standard machining, basic gauging | 1× |
| ±.001" | Controlled setups, in-process checks | 1.5–2× |
| ±.0005" | Finishing passes, CMM inspection | 2–3× |
| ±.0001" | Grinding/EDM or micro-finishing, climate control, 100% inspection of the feature | 3–5×+ |
These multipliers are illustrative; geometry, material, and quantity change the math. The pattern holds, though: cost climbs steeply as tolerance tightens.
How to spend tenths wisely
- Put tight tolerances on functional features only: bearing fits, sealing surfaces, mating datums.
- Use a general tolerance block for everything else instead of tight defaults.
- Tolerance to the datum that matters, so the shop isn't holding stacks that don't affect function.
- Ask for DFM feedback before you release the drawing. A quick review often finds features that can open up.
When ±.0001" is worth it
Implant and instrument interfaces, precision bores, valve seats, and optical mounts often do need tenths. In those cases, pick a supplier that measures to that level and documents it, because an unverified tenth is just a guess.
Frequently asked questions
What is a standard machining tolerance?
Around ±.005" for general features is common when no tolerance is specified, though many shops hold ±.001" routinely.
How can I reduce the cost of a precision part?
Hold tight tolerances only on functional features, simplify geometry where possible, choose a machinable material, and ask the shop for DFM feedback before finalizing the drawing.
Have a part like this? Send the print and an engineer will reply, typically within 48 hours.
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