The CNC Tolerance Guide: ISO 2768, Standard vs Precision Tolerance

Every dimension on a drawing needs a tolerance, whether it’s written down or not — if it’s not specified, a default standard fills the gap, and that default might be looser than the part actually needs. This CNC tolerance guide covers how tolerances work, what ISO 2768 actually specifies for unmarked dimensions, and how to tell when a feature genuinely needs tight tolerance machining instead of the shop’s standard default.

What Is a CNC Machining Tolerance?

A tolerance is the acceptable range of variation a dimension can have and still let the part fit and function correctly. Machined parts are never produced at the exact nominal dimension — cutting forces, tool wear, and material behavior all introduce small variations, so every feature on a drawing needs either an explicit tolerance or a general default to fall back on.

ISO 2768: The Default Standard for General Tolerances

ISO 2768-1 is the standard most shops reference for dimensions that don’t carry an individual tolerance on the drawing. It defines four classes — fine (f), medium (m), coarse (c), and very coarse (v) — with permissible deviation increasing as the class gets coarser and as the nominal dimension gets larger.

Nominal Size RangeFine (f)Medium (m)Coarse (c)Very Coarse (v)
6–30 mm±0.1 mm±0.2 mm±0.5 mm±1.0 mm
30–120 mm±0.15 mm±0.3 mm±0.8 mm±1.5 mm
120–400 mm±0.2 mm±0.5 mm±1.2 mm±2.5 mm

A drawing marked “ISO 2768-m” applies the medium class to every unmarked dimension — the most common default in mechanical design. Only dimensions that need something tighter should carry an explicit tolerance override; applying a tight value to the whole print instead adds cost without improving the part.

Standard vs Precision Tolerance: What Actually Changes

FactorStandard Tolerance (ISO 2768-m/c)Precision / Tight Tolerance
Typical range±0.1–0.5 mm±0.005–0.013 mm
Inspection methodManual gauging, spot-checksCMM verification, documented report
Cycle time impactStandard single-pass machiningSlower — multiple finishing passes
Best fitBrackets, housings, non-critical featuresBearing bores, mating faces, seal surfaces

Practical Examples by Tolerance Class

  • General bracket dimensions: ISO 2768-m (±0.2–0.3 mm) applied across the whole part, since fit isn’t critical outside mounting holes.
  • Bearing bore in a gearbox housing: flagged individually at ±0.01 mm while the rest of the housing stays at standard tolerance.
  • Sealing face on a enclosure: flatness called out separately at ±0.013 mm to ensure a gasket seats correctly.
  • Connector pin diameter: ±0.005 mm on a Swiss-turned feature critical to repeatable mating.

Tight Tolerance Machining: When ISO 2768 Isn't Enough

Tight tolerance machining applies when a feature’s fit or function depends on precision beyond what a general standard provides — a press-fit bore, a sealing face, or a feature that mates with another part across thousands of duty cycles. These features should be flagged individually with GD&T rather than tightening the entire drawing, since only the functional dimension needs the extra process control and inspection.

What Affects How Tight a Tolerance Can Actually Be Held

Material matters as much as machine capability — harder, more dimensionally stable materials with a lower coefficient of thermal expansion hold tighter tolerance more consistently than soft or thermally reactive ones. Raw stock condition, machine rigidity, and tooling precision all stack together, which is why two shops quoting the same tolerance on paper can produce very different real-world consistency across a batch.

How to Specify Tolerances Correctly on a Drawing

Reference a general standard like ISO 2768-m for the whole print, then call out only the features that actually need tighter control, using GD&T rather than a blanket linear tolerance. Confirm the shop provides quality certifications and CMM inspection reporting for flagged features, and check process capability across CNC milling, turning, and 5-axis machining before committing a critical-tolerance drawing.

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FAQ: CNC Tolerance Guide

It applies the medium tolerance class to every dimension that doesn’t carry its own explicit tolerance, balancing manufacturability and cost for most mechanical features.
Generally anything tighter than ±0.05 mm; precision work commonly falls in the ±0.005–0.013 mm range for critical features.
No. ISO 2768 covers unmarked general dimensions, while GD&T defines form, orientation, and location requirements for features that need explicit functional control.
They require slower cutting passes, more frequent tool changes, and CMM inspection, so cost should reflect only the features that genuinely need that level of control.

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CNC Tolerance Guide: Get Your Quote

Kintec machines to ISO 2768 standard tolerance by default and holds critical features to ±0.005 mm using 5-axis and Swiss capability with CMM-verified inspection.

  • ✅ Standard ISO 2768-m tolerance with tight-feature options to ±0.005 mm
  • ✅ CMM inspection reports available on request
  • ✅ No minimum order — prototype from 1 piece
  • ✅ ISO 9001:2015 certified facility
  • ✅ 24-hour quote turnaround

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