CNC Machining Tolerances: What Standards Really Mean
A tolerance on a drawing isn't just a number — it drives cost, quality and lead time. This guide explains GB/T and ISO standards, fits, and what different tolerance grades actually cost you.
What Is a Tolerance?
A tolerance is the allowable variation in a dimension. A part specified at 25.00 mm with a ±0.05 tolerance can measure anywhere from 24.95 to 25.05 mm and still be accepted. Tighter tolerance = tighter control = more cost and often longer machining time.
The trade-off is real: the tighter the tolerance, the more it costs. Good design specifies the loosest tolerance that still works.
Common Tolerance Standards
| Standard | What it covers | Common use |
|---|---|---|
| GB/T 1800 (China) | Linear dimension tolerance grades (IT grades) | China machining shops; basis for linear fits |
| ISO 2768 | General tolerances for linear/angular dimensions not individually specified | Default tolerances on many engineering drawings |
| ISO 286 / ISO fits | Limit and fits (hole-basis & shaft-basis) | Clearance, transition and interference fits |
| ASME Y14.5 | Geometric dimensioning & tolerancing (GD&T) | International drawings with position, flatness, runout |
Tolerance Grades and What They Cost
Tolerance grades are numbered (IT5, IT6, IT7...). Lower number = tighter = more expensive.
| Grade | Typical ± for ~25mm | Feeling | Relative cost |
|---|---|---|---|
| IT5 | ±0.006 mm | Instrument/measurement grade; needs grinding or fine finishing | High |
| IT6 | ±0.009 mm | Tight general machining; matched fits | Medium-high |
| IT7 | ±0.013 mm | Common precision fit grade (e.g. H7 holes) | Medium |
| IT8 | ±0.019 mm | General engineering tolerance | Low-medium |
| IT9+ | ±0.03 mm and looser | Non-critical, clearance parts | Low |
Practical guideline
As a baseline, many Chinese CNC shops reliably hold a linear tolerance of GB/T 1800.1 grade 5–7. Beyond that (grade 3–4 or tighter), expect grinding, more inspection and higher cost.
Fits: Clearance, Transition, Interference
Fits describe how two mating parts relate. On drawings you'll see hole-basis fits like H7/g6 (clearance), H7/k6 (transition) or H7/p6 (interference):
- Clearance fit (H7/g6): parts slide freely — shafts, slides.
- Transition fit (H7/k6): slight interference possible — locating pins.
- Interference fit (H7/p6): press fit — hubs, bearings.
The uppercase letter (H) is the hole, the lowercase (g, k, p) is the shaft; the numbers are grade. Ensuring your drawing states the intended fit prevents costly surprises.
Geometric Tolerances (GD&T)
Type tolerances control shape/position, not size: flatness, straightness, position, concentricity, runout, angularity. These matter for assemblies where two parts must align. They add inspection cost, so apply them only where functionally needed.
How Tolerances Affect Cost & Lead Time
- Machine time: tighter tolerances may need slower feeds, more passes, or a finishing operation (grinding, honing).
- Inspection: tighter tolerances require CMM or gauge checks on more parts, adding QC time.
- Rejects: tighter tolerances increase scrap risk.
- Design intent: a loose tolerance where a tight one isn't needed saves money without losing function.
Need help specifying tolerances?
Send your drawing. We'll review the tolerances against the intended function and suggest the most cost-effective grades — then quote transparently.
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