CNC Machining Tolerances: Precision Without Unnecessary Cost
Understand CNC machining tolerances, achievable accuracy, GD&T, hole and shaft tolerances, and how tolerance requirements affect manufacturing cost and lead time.
What Are CNC Machining Tolerances?
CNC machining tolerance is the permissible variation allowed from the nominal dimension specified on an engineering drawing.
For example, if a shaft is specified as 20.00 ± 0.05 mm, the manufactured component can measure between 19.95 mm and 20.05 mm and still meet the dimensional requirement.
Tolerance is one of the most important factors in CNC manufacturing because it directly influences machining strategy, inspection requirements, tooling, cycle time, process capability and ultimately part cost.
The key is not simply achieving the tightest possible tolerance. The objective is to achieve the required functional tolerance reliably and economically.
Before specifying ±0.01 mm everywhere on a drawing, determine which dimensions actually affect function, fit, alignment or performance.
READ OUR DFM GUIDETolerance vs Accuracy vs Precision
These terms are often used interchangeably, but they describe different aspects of CNC manufacturing.
Tolerance
The acceptable dimensional variation specified by the designer or engineering drawing.
Accuracy
How closely the actual manufactured dimension matches the intended or nominal dimension.
Precision
The ability of a machining process to repeatedly produce similar results.
What Is a Standard CNC Machining Tolerance?
CNC machines can achieve different levels of dimensional control depending on the machine, material, geometry, tooling, inspection method and production volume.
| Requirement | Typical Approach | Best Application |
|---|---|---|
| General dimensions | Standard shop tolerance | Non-critical features |
| Tighter dimensional tolerance | Controlled CNC process | Functional components |
| Precision fits | Fine machining + inspection | Shafts, bores and mating parts |
| Very tight tolerance | Specialized machining and process control | Critical precision components |
There is no single CNC tolerance that applies to every component. The appropriate tolerance depends on the feature, material, machine capability and functional requirement.
For a broader explanation of manufacturing tolerances, see our guide: Manufacturing Tolerances Explained .
CNC Milling vs CNC Turning Tolerances
The achievable tolerance depends partly on the machining process and the geometry of the component.
CNC Milling
CNC milling is commonly used for prismatic components, housings, brackets, fixtures and complex 3D geometries.
- Flatness and parallelism
- Hole location accuracy
- Profile and contour control
- Perpendicularity
- Position and orientation
CNC Turning
CNC turning is particularly effective for shafts, pins, bushings, spacers and rotational components.
- Diameter control
- Concentricity
- Roundness
- Surface finish
- Thread dimensions