Knowledge Base
CNC Machining Tolerances & Precision
ISO 2768 tolerance standards, GD&T capabilities, and tolerance specification recommendations
Tolerance System
Four Tolerance Dimensions
Gain a comprehensive understanding of CNC machining tolerance capabilities and appropriately specify precision requirements.
Linear Dimensional Tolerances
- — Fine class f: ±0.05mm (small parts)
- — Medium class m: ±0.1mm (standard)
- — Coarse class c: ±0.2mm (non-mating)
- — Very coarse class v: ±0.5mm (weldments)
Reference Standard
Default standard: ISO 2768-m
Cost Tip
Tighter tolerances = higher machining cost
Geometric Tolerances (GD&T)
- — Flatness up to 0.02mm per 100mm
- — Parallelism up to 0.01mm per 100mm
- — Concentricity up to 0.02mm
- — Position up to ±0.01mm
Reference Standard
CMM-verified via inspection report from our group's shared metrology center
Cost Tip
GD&T requires matching inspection capabilities
Surface Roughness
- — Rough milling: Ra 3.2-6.3μm
- — Finish milling: Ra 1.6-3.2μm
- — Grinding: Ra 0.4-1.6μm
- — Polishing: Ra 0.05-0.2μm
Reference Standard
Default delivery: Ra 3.2μm
Cost Tip
Higher roughness requirements = more operations
Tolerance Specification Guidelines
- — Only specify tight tolerances on mating surfaces
- — Non-functional surfaces follow default unspecified tolerances
- — Use GD&T for critical characteristics
- — Avoid full-dimension tight tolerance annotation
Reference Standard
Mating surfaces: ±0.02 to ±0.05mm is sufficient
Cost Tip
Rational tolerancing can reduce costs by 30%+
Tolerance Principles
Four Principles of Rational Tolerancing
Tolerance specification is not about being as tight as possible — it is about being as appropriate as possible.
Tolerance Accumulation
The longer the dimension chain, the greater the accumulated tolerance — consider this during design.
Datum First
Establish the machining datum first, then specify dimensional tolerances.
Economic Precision
Choose the loosest tolerance that meets functional requirements to reduce cost.
Inspection Compatibility
Specified tolerances must be measurable — otherwise they are meaningless.
Not Sure How to Specify Tolerances?
Send us your drawings — our engineers will provide reasonable tolerance recommendations based on functional requirements.