CNC Machining Tolerance Guidelines

Author: Date Published: Oct 19,2023


CNC machining is a precise manufacturing process that involves the use of computer-controlled

machines to create intricate parts and components. One crucial aspect of CNC machining is tolerance,

which refers to the allowable variation in dimensions and specifications for a given part. 

This article provides a comprehensive guide on CNC Machining Tolerance guidelines, 

covering common tolerances, the importance of tolerance restrictions,

and key considerations when selecting tolerances.


I. Common Tolerances in CNC Machining

In CNC machining, various types of tolerances are commonly used to define the

acceptable limits for dimensional variations. Some common tolerances include:

1. Geometric Dimensioning and Tolerancing (GD&T): GD&T is a symbolic language used to

specify complex geometric features and their allowable variations. It encompasses symbols, 

feature control frames, and datums to communicate precise requirements for form,

orientation, location, profile, and other geometric aspects.

2. Linear Dimensions: Linear dimensions specify the permissible range of variations

along straight lines or distances between specific points on a part.

3. Angular Dimensions: Angular dimensions define acceptable angular 

variations or orientations between two or more features.

4. Surface Finish: Surface finish tolerance specifies the acceptable 

roughness or smoothness level on machined surfaces.

5. Positional Tolerance: Positional tolerance determines the permissible deviation from 

true position or location of features relative to specified datums.

II. Why Tolerance Restrictions Are Needed

Tolerance restrictions play a vital role in ensuring functional compatibility and quality

in CNC machined parts. Here are some reasons why tolerance restrictions are necessary:

1. Interchangeability: Tight tolerances ensure that parts can be easily 

interchanged without compromising fitment or functionality within an assembly.

2. Functionality: Parts with tight tolerances help maintain precise functionality

by minimizing unwanted clearances or interference between mating components.

3. Performance Optimization: Strict tolerance requirements enable engineers to optimize

performance by achieving desired characteristics such as improved sealing capabilities,

reduced frictional losses, or enhanced structural integrity.

4.Quality Control: Well-defined tolerances facilitate effective quality control processes by

providing measurable criteria against which parts can be inspected for conformance during production.

III.What to Remember When Selecting Tolerances

When selecting tolerances for CNC machining projects,it's important to consider several factors:

1.Design Intent:The intended function,purpose,and operating conditions of the part should guide

tolerance selection.For example,a high-precision aerospace component may require tighter

tolerances compared to a non-critical consumer product component.

2.Manufacturing Capabilities:The capabilities of the CNC machine shop should be considered.

Some machines may have limitations that affect achievable precision levels.

Choosing realistic tolerances ensures feasibility within manufacturing constraints.

3.Cost Considerations:Tighter tolerancessometimes involve additional time,labor,

and resources,resulting in higher production costs.Striking an optimal 

balance between functional requirements and cost-efficiency is crucial.

4.Material Properties:Different materials exhibit varying levels of dimensional stability.

Consideration should be given to material properties,such as thermal expansion coefficients,

to select appropriate allowances for dimensional changes due to temperature fluctuations.

5.Application Requirements:Certain applications,such as press-fit assemblies,may require

specific clearance fits that dictate tight positional or size-related tolerancing.

Consulting relevant industry standards,guidelines,and customer specifications

helps ensure compliance with application-specific requirements.


CNC machining relies heavily on well-defined tolerance guidelines for producing accurate and reliable parts.

With knowledge about common tolerancing methods,the significance of restriction necessity,and considerations

when selecting appropriate allowances,CNC machinists can achieve optimal precision while meeting 

functional requirements,cost constraints,and industry standards.

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