CNC Machining Tolerances

ISO 2768, Precision Standards & Cost

Learn about standard CNC machining tolerances, ISO 2768 classifications, GD&T fundamentals, achievable precision ranges, and how tolerances impact machining costs.

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What Are CNC Machining Tolerances?

CNC machining tolerances define the allowable variation in a part’s dimensions. They ensure proper fit, function, and interchangeability in assembly. Tighter tolerances increase machining time, inspection requirements, and production cost.

  • Engineering Tip: Apply tight tolerances only to critical features that affect part function and assembly. This helps reduce machining time and lower manufacturing costs.

Standard CNC Machining

Tolerances

Feature

Feature

Feature

Linear Dimensions

± 0.1 mm

± 0.004 in

Angular dimensions

± 0.1 ° – ± 1 °

± 0.1 ° – ± 1 °

Radius/Chamfer

± 0.2 mm

± 0.008 in

Surface Finish

Ra 3.2 – 6.3 μm

Ra 125 – 250 μin

  • Actual achievable machining tolerances depend on factors such as material properties, part geometry, tool wear, surface finish requirements, and fixturing methods.

ISO 2768 Tolerance Standards

ISO 2768 defines general tolerances for linear and angular dimensions when specific tolerances are not explicitly stated on drawings.

Class

Description

Application

Example

ISO 2768-f (Fine)

Fine tolerance

 Aerospace, medical, precision components

10 ± 0.05 mm
.3937 ± 0.002 in

ISO 2768-m (Medium)

Medium tolerance

General engineering, machined parts

10 ± 0.1 mm
.3937±0.0039 in

ISO 2768-c (Coarse)

Coarse tolerance

Structural parts, non-critical components

10 ± 0.2 mm
.394 ± 0.008 in

  • Tip: When in doubt, use ISO 2768-m as the default standard for most CNC machined parts.

Tight Tolerance CNC Machining

Achieving tight tolerances requires not only advanced CNC equipment, but also mature machining expertise, well-designed manufacturing processes, stable machining conditions, and precise inspection methods.

At 6C Proto, parts requiring tolerances tighter than ±0.01 mm typically involve:

  • Your Content Goes Here
  • Precision Fixturing
  • Optimized Cutting Parameters

  • Controlled Machining Environment

  • Full CMM Inspection

  • Note: Thin walls, deep cavities, and unstable materials may reduce achievable machining accuracy.

Factors Affecting CNC Machining Tolerances

CNC machining tolerances can be affected by many factors, including machine accuracy, tooling, thermal deformation, material stability, and fixturing methods. Understanding these key factors helps improve machining stability, optimize production efficiency, and achieve more precise dimensional control.

Machine Calibration

Regular calibration and maintenance help ensure machine positioning accuracy and repeatable machining stability.

Tooling

Long and slender cutting tools may bend under cutting forces during machining, which can affect dimensional accuracy.

Thermal Expansion

Heat generated between the cutting tool and the workpiece during machining can cause thermal expansion of both the tool and the part, resulting in dimensional variation.

Material Properties

Different materials have different characteristics, such as hardness, stability, and machinability, all of which may affect the final machining result.

Workholding

Poor fixture design or improper clamping methods may cause part movement or deformation, reducing machining accuracy.

Surface Finish

Achieving a higher surface finish often requires optimized machining parameters and may also involve additional finishing processes.

Tolerance vs Machining Cost

Tighter machining tolerances increase machining time and inspection requirements.

Tolerance Range Relative Cost Production Time
± 0.1 mm Low Normal
± 0.05 mm Medium +20% ~ 50%
± 0.01 mm High +50% ~ 100%
± 0.005 mm Very High +100% or more

Engineering Tip: Unnecessarily tight tolerances can significantly increase machining costs and extend production time, while part performance may not necessarily improve.

GD&T in CNC Machining

In CNC machining, there are not only linear dimensional tolerances, but also GD&T tolerances used to control the geometric characteristics of a part, such as form and position.

Symbol GD&T Definition Application Cost Impact

▱

 Flatness Controls flat surface Sealing surfaces Medium

⌖

 Position Controls location Hole patterns High

◎

 Concentricity Controls concentricity Shafts, holes High

↗

 Runout Controls rotation Rotating components High

⊥

 Perpendicularity Controls 90° angles Assembly features Medium

Note: Geometric tolerances such as position, runout, and concentricity usually require CMM inspection and stricter process control.

Real Manufacturing Examples

Automotive
Material: AL6063-T5
Tolerance: ±0.01 mm
Quantity: 100 pcs
Part: Motor Housing

Machinery Manufacturing
Material: Stainless Steel 304
Tolerance: ±0.01 mm
Quantity: 50 PCS
Part: Mechanical Component

Medical
Material: PEEK
Tolerance: ±0.02 mm
Quantity: 2 pcs
Part: Mounting Component

Inspection & Quality Control

We ensure every part meets your tolerance requirements.

CMM
Inspection

Height
Gauge

Coating Thickness
Inspection

Surface Roughness
Tester

We perform full dimensional inspection on all products. All critical dimensions are inspected according to customer drawings and tolerance specifications. Bubble inspection reports are available upon request.

FAQS

CNC machining tolerance generally refers to the acceptable dimensional variation that can be achieved under standard machining conditions. For most CNC machined parts, the standard tolerance typically follows ISO 2768-mK, with a minimum general tolerance of ±0.1 mm. Actual achievable tolerances may also be affected by factors such as material properties, part geometry, and machining processes.

In general, tolerances smaller than ±0.05 mm are considered tight machining tolerances. Achieving tight tolerances requires not only high-precision CNC equipment, but also stable machining conditions, well-designed manufacturing processes, and proper process control.

Yes. High-precision CNC machining can achieve tolerances of ±0.01 mm or even tighter. However, whether this level of accuracy can be achieved depends on factors such as material properties, part geometry, dimensions, and machining methods. Thin walls, deep cavities, and materials prone to deformation can significantly increase machining difficulty.

Tighter machining tolerances usually require longer machining time, more complex process control, and higher inspection requirements, which increase manufacturing costs. In addition, tight tolerances may reduce machining efficiency and extend lead times. It is recommended to apply tight tolerances only to critical functional dimensions in order to balance performance and cost.

ISO 2768 is an international general tolerance standard used to define default tolerances for linear and angular dimensions when specific tolerances are not individually specified on engineering drawings. Common tolerance classes include:

  • ISO 2768-f (Fine)
    ISO 2768-m (Medium)
    ISO 2768-c (Coarse)

Among them, ISO 2768-m is the most commonly used general tolerance standard in CNC machining.

For most CNC prototypes and functional verification parts, ISO 2768-m medium tolerance is generally recommended. This standard helps balance part functionality, machining cost, and production lead time. If the part involves assembly fits, sealing surfaces, or motion mechanisms, tighter tolerances can be specified for critical dimensions.

Dimensional tolerances are mainly used to control allowable variations in linear dimensions such as length, diameter, and angle. GD&T (Geometric Dimensioning and Tolerancing), on the other hand, is used to control geometric characteristics such as form, orientation, location, and runout. Compared with traditional dimensional tolerances, GD&T defines functional requirements more accurately and is widely used in high-precision assemblies and complex engineering components.

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