CNC Machining Tolerances: What to Specify and What to Expect
Typical CNC machining tolerances for milled and turned parts, how to specify them on drawings, and when tighter tolerances are worth the cost.
The Short Version
Unless your drawing says otherwise, IsoCNC machines general features to ±0.005 in (±0.13 mm). Where the part needs it — bearing seats, precision bores, mating features — we hold ±0.001 in (±0.025 mm)and tighter, quoted and inspected against the specific callout. The drawing governs; this page explains how to write one that gets you the part you need without paying for precision you don’t.
CNC machining tolerances are a purchasing decision as much as an engineering one. Every tightened callout adds machine time, measurement time, or both — so the goal is not the tightest drawing you can write, but the loosest drawing that still makes the assembly work.
Typical CNC Machining Tolerances by Feature
| Capability | Specification |
|---|---|
| Linear dimensions | Standard ±0.005 in (±0.13 mm) · Achievable (quoted) ±0.001 in (±0.025 mm) |
| Bores and reamed holes | Standard ±0.002 in (±0.05 mm) · Achievable (quoted) ±0.0005 in (±0.012 mm) |
| Turned diameters | Standard ±0.005 in (±0.13 mm) · Achievable (quoted) ±0.001 in (±0.025 mm) |
| True position | Standard 0.010 in (0.25 mm) · Achievable (quoted) 0.002 in (0.05 mm) |
| Flatness | Standard 0.005 in (0.13 mm) · Achievable (quoted) 0.001 in (0.025 mm) |
| Surface finish (as-machined) | Standard Ra 3.2 µm (125 µin) · Achievable (quoted) Ra 0.8 µm (32 µin) |
Tighter than “achievable” is possible with grinding, honing, or jig boring — tell us the requirement and we’ll quote the process honestly rather than promise a machining tolerance we can’t hold.
What Tight Tolerances Actually Cost
Tolerance is not linear in price. Moving a feature from ±0.005 in to ±0.002 in roughly adds slower feeds, more in-process measurement, and tighter environmental control. Moving to ±0.0005 in can move the feature to a different machine or a grinding operation entirely. A drawing that blankets ±0.001 in across every dimension can double or triple part cost — while a drawing that tightens only the three dimensions that matter adds very little.
How to Write a Tolerance-Friendly Drawing
- Default loose in the title block — ±0.005 in or ±0.13 mm general, then tighten individually.
- Tighten only functional features — bores, bearing seats, datums, sealing surfaces, mating interfaces.
- Define datums clearly — GD&T without datums is uninspectable; we measure what the datum structure tells us to measure.
- Watch stack-ups across parts — if two parts mate, the tolerance budget belongs on both drawings.
- Call out threads with class and depth — "M6-6H, 12 mm deep" is quotable; "M6" alone invites questions.
- Say what the part does — one sentence of application context lets our engineers flag a tolerance that's fighting the geometry.
More on designing parts that machine well — fillets, wall thickness, hole depths, thread choices — is in our DFM design guide for CNC parts.
Standard or Precision: How to Decide
Ask three questions of every feature. Does it locate another part? Does it seal, spin, or slide? Will a stack-up across the assembly land here? If all three answers are no, the feature belongs at standard tolerance — full stop. Typical tolerances exist because they are the natural accuracy of a well-run machining process: they cost nothing extra and they repeat batch after batch. Reserve precision callouts for the minority of features that genuinely need them, and your drawings get cheaper, faster to quote, and easier to inspect. And if a legacy drawing carries tight blanket tolerances you suspect are historical accident rather than intent, flag it at RFQ — relaxing an inherited callout has cut cost on plenty of parts with zero functional change.
Tolerances by Material
Material moves what’s practical. Aluminum and brass hold fine tolerances readily; stainless and titanium deflect more under the tool and need adjusted strategy; plastics creep with temperature and stress — hold critical plastic dimensions at room-temperature measurement conditions and expect wider practical limits than metal. Material-specific guidance is on our materials page.
One more note: parts are measured at controlled shop temperature. If your incoming inspection measures under different conditions, say so at quote stage — thermal expansion on a large aluminum part is measurable, not theoretical, and agreeing on measurement conditions up front avoids arguments later.
How We Verify
Every order passes dimensional inspection before shipment, with CMM verification for tight and geometric callouts. First article reports are available on new parts. For critical features, the inspection method is agreed at quote stage — if a callout needs a CMM program or a dedicated gauge, that scope is settled before production, not after. The full process is on our quality inspection page.
Have a Drawing with Tight Callouts?
Send it. DFM review is part of every quote — if a tolerance is driving cost or fighting the geometry, we'll tell you before you order.
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