Why Tolerance Matters in CNC Machining

In CNC machining, tolerance defines the permitted variation from a specified dimension. It directly influences assembly, performance, consistency, and manufacturing cost.

CNC Machining Tolerances

Our applies several common CNC tolerance levels according to the function and precision requirements of each component. Ultra-precision tolerances generally require dedicated fixtures, slower finishing operations, temperature control, and more advanced inspection methods.

Tolerance Level	Reference Tolerance	Typical Applications
General Tolerance	±0.10 mm	Cosmetic features, structural brackets, mounting clearance, and non-critical dimensions
Precision Tolerance	±0.05 mm	Mechanical interfaces, assembly surfaces, locating features, and standard hole-and-shaft features
High-Precision Tolerance	±0.01 mm	Bearing seats, precision guides, gear locations, and motion components
Ultra-Precision Tolerance	±0.005 mm	Critical features produced through grinding, precision boring, wire EDM, or specialized secondary operations

CNC Milling Tolerances

CNC milling is suitable for planes, pockets, holes, slots, contours, and complex three-dimensional surfaces. Achievable accuracy depends on part size, workholding rigidity, tool overhang, material stability, and the number of setups.

Milled Feature	Standard Capability	Precision Capability
General Linear Dimensions	±0.05 mm	±0.01 mm
Hole Position	±0.05 mm	±0.02 mm
Reamed Hole Diameter	±0.025 mm	±0.01 mm
Flatness	0.05 mm	0.01–0.02 mm
Parallelism	0.05 mm	0.01–0.02 mm
Profile Dimensions	±0.10 mm	±0.03–0.05 mm

Main Factors Affecting Milling Accuracy

• Thin walls, deep pockets, and long unsupported features may deform during machining.
• Multiple setups can introduce datum-transfer errors.
• Surface treatments may change hole diameters, external dimensions, and fit clearances.
• Large parts are more affected by machine travel, thermal variation, and residual material stress.

CNC Turning Tolerances

CNC turning is mainly used for cylindrical surfaces, end faces, tapers, threads, grooves, and internal or external diameters. Because the workpiece rotates around a central axis, turning can provide good roundness and concentricity.

Turned Feature	Standard Capability	Precision Capability
Outside and Inside Diameter	±0.05 mm	±0.01 mm
Length Dimensions	±0.05 mm	±0.02 mm
Concentricity	0.03–0.05 mm	0.01–0.02 mm
Circular Runout	0.03–0.05 mm	0.01 mm
Threads	Standard thread class	According to drawing or gauge requirements
Surface Roughness	Ra 1.6–3.2 μm	Ra 0.4–0.8 μm

5-Axis CNC Machining Tolerances

Five-axis CNC machining is suitable for complex surfaces, multi-angle hole patterns, and precision parts that benefit from fewer setups. Its main advantage is not that every feature automatically achieves a tighter tolerance, but that multiple surfaces can be machined while maintaining a consistent datum relationship.

5-Axis Machining Feature	Reference Capability
General Dimensions	±0.05 mm
Critical Dimensions	±0.01–0.02 mm
Multi-Surface Hole Position	0.02–0.05 mm
Curved Surface Profile	0.03–0.10 mm
Relative Position Between Machined Faces	0.02–0.05 mm

Swiss Turning Tolerances

Swiss turning is suitable for small-diameter parts, slender shafts, and precision components containing multiple turned, drilled, milled, and threaded features.

Swiss-Turned Feature	Standard Capability	Precision Capability
Outside Diameter	±0.02 mm	±0.005–0.01 mm
Length	±0.05 mm	±0.02 mm
Concentricity	0.02 mm	0.005–0.01 mm
Small Hole Diameter	±0.03 mm	±0.01 mm
Threads	Standard thread class	According to drawing or gauge requirements

CNC Machining Tolerances by Material

Material hardness, thermal conductivity, elasticity, and internal stress directly influence achievable machining accuracy.

Material	Reference Achievable Tolerance	Engineering Notes
Aluminum Alloys	±0.01 mm	Good machinability and low cutting forces; thin walls may still deform
Carbon and Alloy Steel	±0.01 mm	Good rigidity and predictable mechanical properties
Stainless Steel	±0.015 mm	Work hardening and tool wear require optimized cutting parameters
Brass	±0.01 mm	Excellent chip control and good suitability for precision turning
Copper	±0.02 mm	Soft and highly conductive; requires sharp tools and controlled clamping
Titanium Alloys	±0.02 mm	Low thermal conductivity concentrates heat near the cutting edge
Engineering Plastics	±0.05 mm	High elasticity and thermal expansion require specialized workholding and inspection

Sheet Metal Fabrication Tolerances

Sheet metal accuracy is affected by cutting, material thickness variation, bend springback, welding distortion, and coating thickness.

Sheet Metal Process	Standard Tolerance	Precision Tolerance
Laser-Cut Profile	±0.10 mm	±0.05 mm
Punched Holes and Slots	±0.10 mm	±0.05 mm
Bent Dimensions	±0.30 mm	±0.10–0.20 mm
Bend Angle	±1°	±0.5°
Welded Assemblies	±0.50–1.00 mm	±0.20–0.50 mm
Flatness	Depends on part size	May require straightening or dedicated fixtures

Injection Molding Tolerances

Injection molding accuracy is affected by material shrinkage, part size, wall thickness, gate location, mold temperature, holding pressure, and cooling conditions.

Standard Molding Tolerance Levels

Tolerance Level	Reference Range	Typical Applications
Commercial Grade	±0.10–0.30 mm	Housings, covers, consumer products, and general structural parts
Precision Grade	±0.05–0.10 mm	Interlocking features, assembly interfaces, and small mechanical components
Ultra-Precision Grade	±0.01–0.05 mm	Small parts with uniform walls and specialized mold and process control

Typical Material Shrinkage Rates

Material	Reference Shrinkage	Dimensional-Control Characteristics
ABS	0.4%–0.7%	Low shrinkage and good dimensional stability
PC	0.5%–0.7%	Good dimensional stability and impact resistance
PP	1.0%–2.5%	Higher shrinkage; gate and cooling design are important
PA6	0.8%–1.5%	Moisture absorption may cause post-molding dimensional change
PA66	0.8%–1.5%	Good heat resistance; fiber orientation can affect shrinkage
POM	1.8%–2.5%	High shrinkage but good wear resistance and stability after processing
PEEK	1.1%–1.5%	Requires high mold temperatures and stable process control
HDPE	1.5%–3.0%	High shrinkage with good flexibility and chemical resistance
PET	2.0%–3.0%	Shrinkage depends strongly on crystallinity and cooling
PBT	1.5%–2.5%	Good electrical properties; glass-fiber orientation affects dimensions
TPU	0.5%–2.0%	Shrinkage varies significantly with material hardness

Part Size and Injection Molding Tolerances

Part Size	Commercial Grade	Precision Grade
Below 10 mm	±0.05–0.10 mm	±0.02–0.05 mm
10–100 mm	±0.10–0.20 mm	±0.05–0.10 mm
100–500 mm	±0.20–0.40 mm	±0.10–0.20 mm
Above 500 mm	±0.30–0.50 mm	±0.15–0.30 mm

Die Casting Tolerances

Die casting is commonly used for aluminum, zinc, and other suitable metal alloys. Dimensional accuracy is affected by casting shrinkage, mold temperature, wall thickness, draft angle, parting lines, and ejection.

Die-Cast Feature	Reference Tolerance
General Dimensions on Small Castings	±0.10–0.20 mm
General Dimensions on Medium or Large Castings	±0.20–0.50 mm
Hole Position and Assembly Features	±0.10–0.30 mm
Features Near the Parting Line	Evaluated according to flash and die mismatch
Secondary CNC-Machined Features	±0.02–0.05 mm
Precision-Machined Holes	±0.01–0.02 mm

Design Considerations：

Vacuum Casting Tolerances

In rapid prototyping, vacuum casting generally refers to pouring polyurethane or silicone materials into flexible silicone molds to reproduce small quantities of plastic or rubber-like parts.

Vacuum-Cast Feature	Reference Tolerance
Small Rigid Parts	±0.20 mm
Medium and Large Rigid Parts	±0.30–0.50 mm
General Proportional Tolerance	±0.30%–0.50%
Flexible or Rubber-Like Parts	±0.50 mm or wider
Insert Position	±0.20–0.50 mm

Main Factors Affecting Vacuum Casting Accuracy

Vacuum casting is more appropriate for appearance validation and low-volume replication than for highly precise mechanical interfaces.

3D Printing Tolerances

3D printing accuracy depends on the printing process, layer thickness, build orientation, support structure, material shrinkage, and post-processing requirements.

3D Printing Process	Standard Reference Tolerance	Typical Applications
SLA	±0.10–0.20 mm	High-detail appearance models and transparent parts
SLS Nylon Printing	±0.20–0.30 mm	Functional parts and complex structures
FDM	±0.30–0.50 mm	Concept models and large structural prototypes
Metal Powder Printing	±0.10–0.30 mm	Complex metal structures; critical faces require machining
Flexible Material Printing	±0.30–0.60 mm	Flexible, cushioning, and sealing components

Design Considerations：

International Standards Reference

ISO 2768 — General Tolerances

ISO 2768 is commonly used for linear and angular dimensions that do not have individually specified tolerances. It provides four general tolerance classes:

Dimension Range (mm)	f Fine	m Medium	c Coarse	v Very Coarse
0.5–3	±0.05	±0.10	±0.20	—
3–6	±0.05	±0.10	±0.30	±0.50
6–30	±0.10	±0.20	±0.50	±1.00
30–120	±0.15	±0.30	±0.80	±1.50
120–400	±0.20	±0.50	±1.20	±2.50
400–1000	±0.30	±0.80	±2.00	±4.00
1000–2000	±0.50	±1.20	±3.00	±6.00
2000–4000	—	±2.00	±4.00	±8.00

ASME Y14.5 and ISO 1101 — GD&T

Geometric Dimensioning and Tolerancing provides a standardized engineering language for expressing functional requirements that cannot be communicated clearly through basic plus-or-minus dimensions.

Main GD&T Categories

Category	Common Controls
Form	Straightness, flatness, circularity, cylindricity
Orientation	Parallelism, perpendicularity, angularity
Location	Position, concentricity, symmetry
Runout	Circular runout, total runout
Profile	Profile of a line, profile of a surface

Feature-Control Frame Example

Position | Ø0.05 | M | A | B | C

This indicates a cylindrical true-position tolerance zone of 0.05 mm at maximum material condition, referenced to primary datum A, secondary datum B, and tertiary datum C.

Material Condition Modifiers

• RFS — Regardless of Feature Size: The geometric tolerance remains fixed regardless of the actual feature size.
• MMC — Maximum Material Condition: Additional bonus tolerance becomes available as the feature moves away from maximum material size.
• LMC — Least Material Condition: Used when minimum wall thickness or minimum material is functionally important.

How to Specify the Right Tolerance

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