What is tolerance in CNC machining?
Tolerance is the acceptable variation in a part dimension. In CNC machining, tolerance defines how much a dimension may deviate from the nominal value.
Typical tolerances in CNC machining range from ±0.01mm for standard milling to ±0.002mm for Swiss turning.
What tolerance can CNC machining achieve?
| Process | Standard Tolerance | Precision Capability |
|---|---|---|
| CNC Milling | ±0.01mm | ±0.005mm |
| CNC Turning | ±0.01mm | ±0.005mm |
| Swiss Turning | ±0.005mm | ±0.002mm |
| Wire EDM | ±0.005mm | ±0.002mm |
Many buyers specify unnecessarily tight tolerances. Unless functionally required, ±0.01mm is sufficient for most industrial components.
How tolerance affects cost
Tighter tolerances increase cost in three ways:
- Slower machining — precision cutting requires reduced feed rates
- Additional inspection — tighter tolerances require more frequent measurement
- Specialized equipment — some tolerances require Swiss turning or grinding
Only specify tolerances tighter than ±0.01mm when the fit, function, or assembly requires it. For clearance holes or non-critical surfaces, ±0.1mm is often sufficient.
When tight tolerance actually matters
- Bearing fits — ±0.005mm or tighter
- Sealing surfaces — ±0.005mm or tighter
- Precision alignment — depends on assembly requirements
- Threads — standard tolerances are usually sufficient
Applying the same tight tolerance to every dimension. This adds unnecessary cost. Instead, specify tighter tolerances only where functionally required.
How to specify tolerance on drawings
- General tolerance note — specify ISO 2768-m or equivalent
- Dimension-specific callouts — add tolerances only to critical dimensions
- GD&T where applicable — for position, flatness, and runout control
A standard general tolerance of ±0.1mm on non-critical dimensions reduces cost significantly.
GD&T basics
GD&T (Geometric Dimensioning and Tolerancing) uses symbols to define allowable variation in form, orientation, and position.
- Position tolerance — controls hole locations
- Flatness — controls surface planarity
- Runout — controls concentricity
- Perpendicularity — controls squareness
GD&T is useful for parts that assemble with other components. For standalone parts, it may be unnecessary.
A common practice is to use general tolerance ISO 2768-m for most dimensions and add specific tolerances only to features that require precision control.
Frequently Asked Questions
What is the standard tolerance for CNC machining?
Standard tolerance is typically ±0.01mm (ISO 2768-m) for most CNC machining processes.
Can CNC machining achieve ±0.001mm?
Yes, but under specific conditions — typically with specialized equipment and significantly higher cost.
How do I know if my tolerance is too tight?
Ask: Does this feature require precision fit? If not, loosen the tolerance.
What is the difference between ±0.01mm and ±0.005mm?
±0.005mm is twice as tight and typically adds 30-50% to machining cost.