CNC Machining Tolerances: What You Can Actually Hold
“Tight tolerance” is the phrase that quietly inflates more machining quotes than any other. Every extra decimal place you demand costs real money — and half the time the part doesn’t need it. This guide covers what CNC machining can actually hold, what the standards mean, and how to spec tolerances that give you a working part without paying for precision you’ll never use.
Here’s the core truth: tolerance is a cost lever, not a quality badge. A well-toleranced drawing calls out tight limits only where the part function demands them, and lets everything else ride on a sensible default. The standard baseline for general CNC work is about ±0.005″ (0.13 mm), equivalent to ISO 2768 medium class (MS Machining, 2025).
Key Takeaways
- Standard CNC tolerance is about ±0.005″ (0.13 mm), matching ISO 2768 medium — good enough for most features (MS Machining, 2025).
- Process sets the limit: milling holds ±0.05 mm, turning ±0.025 mm, boring/reaming ±0.01 mm, grinding ±0.005 mm or better.
- Cost climbs sharply below ±0.025 mm — tight tolerances need extra operations, fixturing, and inspection.
- Spec tight limits only on functional features (fits, bearings, seals); let the rest ride on a default block tolerance.
What does “tolerance” actually mean in CNC machining?
A tolerance is the allowable deviation from a nominal dimension — the range a feature can fall within and still be acceptable. No machine makes a part perfectly to size, so every dimension carries a tolerance, stated or implied. A hole called out as 10.00 mm ±0.05 is in spec anywhere from 9.95 to 10.05 mm.
Tolerances come in two flavors. Explicit tolerances are called out on specific dimensions that matter. General tolerances — usually referenced as an ISO 2768 class in the title block — catch everything not individually specified. Together they tell the machinist exactly how precise each feature must be, and that precision maps directly to cost.
A tolerance is the allowable deviation from a nominal dimension. Explicit tolerances are called out on critical features; general tolerances, referenced as an ISO 2768 class in the title block, govern everything else. Since no machine is perfect, every dimension carries a tolerance — and the tightness you demand maps directly to machining cost (Fictiv, 2025).
What tolerance can CNC actually hold?
The achievable tolerance depends on the process, and knowing the realistic limits keeps your drawing honest. Milling comfortably holds about ±0.05 mm; turning tightens to ±0.025 mm on diameters because the workpiece rotates against a fixed tool; boring and reaming reach ±0.01 mm; and grinding achieves ±0.005 mm or finer (MS Machining, 2025).
| Process | Achievable tolerance | Best for |
|---|---|---|
| Milling (3/4/5-axis) | ±0.05 mm (±0.002″) | General prismatic features |
| Turning | ±0.025 mm on diameters | Cylindrical parts, shafts |
| Boring / reaming | ±0.01 mm | Precise holes and bores |
| Grinding | ±0.005 mm or better | Bearing surfaces, hardened parts |
Notice the pattern: the tighter you go, the more specialized (and slower) the operation. Anyone can mill to ±0.1 mm on a decent machine. Holding ±0.01 mm demands a secondary boring or grinding pass, a rigid setup, and often a climate-controlled shop. That’s not a supplier being difficult — it’s physics and process, and it’s why the last few microns cost the most.
Achievable CNC tolerance is set by the process: milling holds ~±0.05 mm, turning ~±0.025 mm on diameters, boring and reaming ~±0.01 mm, and grinding ~±0.005 mm or better. Tighter tolerances require additional specialized operations, rigid fixturing, and temperature control — which is why the last few microns drive the steepest cost (MS Machining, 2025).
ISO 2768 explained: fine, medium, and coarse
ISO 2768 is the shorthand that keeps drawings clean. Instead of tolerancing every dimension, you name a general tolerance class in the title block, and it applies to all un-toleranced features. Part 1 covers linear and angular dimensions in four classes — fine (f), medium (m), coarse (c), and very coarse (v) — where the allowable deviation grows with feature size.
| Class | 0.5–3 mm | 6–30 mm | 30–120 mm | 120–400 mm |
|---|---|---|---|---|
| Fine (f) | ±0.05 | ±0.1 | ±0.15 | ±0.2 |
| Medium (m) | ±0.1 | ±0.2 | ±0.3 | ±0.5 |
| Coarse (c) | ±0.2 | ±0.5 | ±0.8 | ±1.2 |
For most machined parts, medium (m) is the sensible default — it’s what shops assume unless told otherwise, and it balances cost against precision. Specify fine (f) only when the whole part genuinely needs it, because it tightens every dimension and lifts cost across the board. Part 2 of the standard adds geometric tolerances (flatness, position) in grades H, K, and L.
ISO 2768 lets a drawing reference one general tolerance class — fine, medium, coarse, or very coarse — that governs all un-toleranced dimensions, with allowances that widen as feature size grows. Medium (m) is the standard default for machined parts; specifying fine (f) tightens every dimension and raises cost across the board, so reserve it for parts that truly need it (JLC, 2025).
Why do tighter tolerances cost more?
Every step tighter on tolerance adds cost in three ways: more operations, more scrap risk, and more inspection. A feature that could be milled in one pass at ±0.1 mm might need a roughing pass plus a finishing bore at ±0.01 mm — doubling cycle time. Tighter limits also raise the scrap rate, since more parts fall outside the band, and they demand slower, more careful cutting.
| Standard ±0.1 mm |
| ||
| Tight ±0.025 mm |
| ||
| Precision ±0.01 mm |
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| Ground ±0.005 mm |
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Inspection compounds it. A loose tolerance can be checked with calipers in seconds; a tight one needs a CMM, gauge pins, or an air gauge, plus documentation. None of this means tight tolerances are wrong — when a bearing fit or seal demands ±0.01 mm, you spec it. The waste is applying that same limit to a clearance hole that would work fine at ±0.2 mm.
In should-cost reviews, we routinely see 20–40% of a machining quote driven by tolerances the part doesn’t functionally need. One client’s bracket dropped 28% in price when we relaxed six non-critical dimensions from ±0.02 to the ISO 2768-medium default — with zero effect on fit or function. Tolerance is the cheapest cost lever you’re probably not pulling.
How do you spec tolerances the smart way?
The goal is simple: tight where it matters, loose everywhere else. Start by identifying the few features that actually control function — mating surfaces, bearing bores, sealing faces, locating datums — and tolerance those precisely. Then let a sensible general class (usually ISO 2768 medium) handle the rest. This single discipline captures most of the available savings.
| Feature | Tight tolerance? |
|---|---|
| Bearing / shaft fit | ✔ Yes — tight |
| Sealing surface | ✔ Yes — tight |
| Locating datum | ✔ Yes — tight |
| Clearance / bolt hole | ✘ No — standard |
| Cosmetic / outer face | ✘ No — standard |
| Non-mating edge | ✘ No — standard |
Smart tolerancing checklist
- Default first — put ISO 2768-m in the title block, then only tighten specific features.
- Tolerance the function — fits, bearings, seals, and datums get explicit limits; nothing else does.
- Avoid tolerance stacking — chained dimensions accumulate error; datum from a single reference.
- Match tolerance to process — don’t ask a mill for a ground finish; specify the operation if needed.
- Ask your supplier — a good machinist will flag over-tight callouts before quoting.
What else affects the tolerance you can hold?
Process is the headline, but three other factors move the achievable tolerance. Material matters: stable metals like steel, aluminum, and brass hold tight limits well, while plastics and some exotic alloys move with temperature and cutting stress. Geometry matters: thin walls, deep pockets, and long unsupported features flex under cutting force, widening real-world tolerance.
And setup matters most of all. Rigid fixturing, minimal tool overhang, a stable shop temperature, and good workholding are what separate a quoted tolerance from a delivered one. This is why a capable, well-equipped shop can hold tighter limits reliably — and why the cheapest quote sometimes can’t actually make the part. See our CNC machining capability for equipment and process detail.
A quoted tolerance means nothing without process capability behind it. We’ve seen shops accept ±0.01 mm callouts they had no realistic way to hold, then ship parts that failed incoming inspection. Ask a prospective supplier not just “can you hold this?” but “show me the Cpk data” — capability, not optimism, is what protects your line.
Get machined parts to print — from Rajkot to the world
Holding tolerance reliably takes the right equipment, the right process, and honest capability data. Magnus Sourcing connects global buyers to Rajkot’s multi-axis CNC shops — milling, turning, boring, grinding, and EDM — with dimensional inspection and full documentation under one accountable partner. Our supplier network holds ISO 9001:2015 and IATF 16949 standards, with CMM inspection and material traceability.
Have a drawing with tight callouts? Send it over with your volume, and our engineering team will confirm what’s achievable — and flag any tolerances that are costing you money for no functional gain.
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Frequently Asked Questions
What is the standard CNC machining tolerance?
The general standard is about ±0.005″ (0.13 mm), matching ISO 2768 medium class. Most shops assume this unless a drawing specifies otherwise. It’s sufficient for the majority of features; tighter limits are added only on functional dimensions like fits, bearings, and seals (MS Machining, 2025).
What is the tightest tolerance CNC can hold?
With grinding, CNC can hold ±0.005 mm (about ±0.0002″) or better on stable materials. Boring and reaming reach ±0.01 mm, turning ±0.025 mm, and milling ±0.05 mm. Achieving the tightest limits requires rigid fixturing, temperature control, and secondary operations, which raise cost significantly.
What is ISO 2768?
ISO 2768 is a general tolerance standard. Part 1 defines linear and angular tolerance classes — fine, medium, coarse, very coarse — applied to all un-toleranced dimensions via the title block. Part 2 covers geometric tolerances in grades H, K, and L. Medium is the common default for machined parts (JLC, 2025).
Why do tighter tolerances increase cost?
Tighter tolerances require additional machining operations, slower cutting, higher scrap rates, and more sophisticated inspection like CMM or gauge pins. Each step tighter compounds these. Relaxing non-critical dimensions to a standard default often cuts 20–40% from a machining quote with no effect on part function.
How should I decide which tolerances to tighten?
Tighten only the features that control function — mating surfaces, bearing bores, sealing faces, and locating datums. Let everything else ride on a general ISO 2768 medium tolerance. This “tight where it matters, loose everywhere else” approach captures most machining savings without compromising the working part.
The bottom line
CNC machining can hold remarkably tight tolerances — down to a few microns with grinding — but every micron has a price. The skill isn’t demanding the tightest number; it’s knowing which features truly need it. Spec function-critical dimensions precisely, default the rest to ISO 2768 medium, and you get a part that works without paying for precision you’ll never use.
Sourcing machined parts and want them right and right-priced? Magnus Sourcing’s Rajkot CNC partners confirm what’s achievable and flag costly over-tight callouts before you commit. Start with a free machining review →
