Comprehensive Guide to CNC Machining Tolerance Specifications

CNC machining capabilities facilitate the attainment of exceedingly precise outcomes, with select machines achieving accuracies as fine as +/-0.0025mm. Nevertheless, executing milling operations at the utmost precision level of CNC machinery entails significant expenses and time investments. This underscores the criticality of CNC machining tolerances. Given the varying precision requirements across different components, it is prudent for designers to specify the precise dimensions and tolerances essential for each part.

What Do Tolerances Signify?

Tolerances serve as benchmarks denoting the requisite level of precision for the manufacturing of a given part. Specifically within CNC machining, tolerances delineate the permissible degree of deviation in the final dimensions or measured values of custom machined parts.

CNC machine shops gauge components using numerical parameters, typically prefaced with a ± symbol. For instance, a tolerance of ±0.001″ assigned to a 2.550-inch part signifies that the manufactured piece may have a length ranging between 2.549” and 2.551”. Similarly, a part measuring 1.5 inches in height, with a tolerance requirement of ±0.005”, mandates final dimensions falling within the range of 1.495” and 1.505” to meet quality standards.

Stipulating standard or non-standard tolerances in engineering drawings provides CNC machining services and machinists with guidance on the requisite precision levels for part production. Tighter tolerances—characterized by smaller numerical values—demand greater precision, while looser tolerances—indicated by larger numerical values—entail comparatively less stringent precision requirements.

Common Types of Tolerances in CNC Machining

Standard Tolerances

Machinists often adhere to standard CNC tolerances for commonly manufactured parts. This includes specific accuracy for elements like threads, pins, and pipes. Many milling services provide international standards of around +/-0.1mm, especially when customers do not specify their own tolerance levels. For a comprehensive view, refer to the detailed CNC machining chart. The scope of these standards is typically defined by various international organizations, including the International Organization for Standardization (ISO), the American Society of Mechanical Engineers (ASME), and the American National Standards Institute (ANSI).

Bilateral Tolerances

When employing bilateral tolerance, the deviation from the given dimension can be either negative or positive, allowing the part to be slightly larger or smaller than stated. An example is +/- 0.06mm, indicating that the machined part can be 0.06mm shorter or longer than the specified measurement. Such specifications are typically applied to exterior dimensions.

Geometric Dimensioning and Tolerancing (GD&T)

GD&T tolerances are much more thorough than the other systems of machining tolerances. It highlights the measurements and allowable deviations. It also outlines specific geometric characteristics for the machined part such as how flat it should be, its concentricity, and its true position. GD&T tolerances are often used for parts that have extremely precise dimensions.

Unilateral Tolerances

Unilateral tolerances permit deviations exclusively in one direction, either only positive or only negative. For example, a tolerance specified as +0.00/-0.06mm indicates that the finished part can be up to 0.06 mm smaller, but not larger, than the stated measurement. Such tolerances are typically employed in designs where a part must fit into another component. Ensuring the part does not exceed the specified dimensions is crucial, as a larger size would prevent it from fitting into its intended position.

Limit Tolerances

A limit tolerance is expressed as a range of values, where the part is fine as long as the measurement falls between that range. 13 – 13.5mm for example is a limit tolerance for CNC machining indicating that the part must have a measurement that falls between the upper and the lower limit (13mm is the upper limit and 13.5mm is the lower limit).

Considerations for Selecting Tolerances in CNC Machining

In CNC machining, precision in tolerances is imperative to ensure optimal part performance. The criticality of dimensional variations impacting a part’s functionality underscores the importance of understanding tolerance thresholds, particularly in components like critical engine parts where precision is paramount. Conversely, some parts may tolerate marginally greater deviations without compromising functionality.

When to Opt for Tight Tolerances

The necessity for tighter tolerances varies across different features of a component. While the overall structure may not require strict tolerances, specific areas such as fitting or joining points demand high precision. Industries requiring precision, like aerospace and medical devices, necessitate complex tolerances to ensure flawless functionality within intricate assemblies. Enhanced accuracy in machining processes reduces error margins, vital for components interacting closely with others, where mismatches can lead to operational inefficiencies or safety hazards.

Cost Implications of Tight Requirements

However, achieving precision tolerances can escalate machining time and costs. Maintaining exacting standards, like ±0.001”, strains CNC tools and increases wear, particularly in mass production scenarios. While a new tool may meet these standards initially, sustaining precision over repeated use becomes challenging, necessitating frequent tool changes or adjustments. This extends production times and elevates costs, especially for projects requiring numerous parts to stringent standards.

Tolerances Depend on Material Characteristics

Different materials exhibit unique properties that directly impact the achievable tolerance levels in CNC machining. These factors include:


Achieving high precision in machining softer materials presents challenges due to their propensity to undergo dimensional changes upon contact with cutting tools. Thus, meticulous attention is necessary when working with softer materials to maintain accuracy.


Materials with rough textures pose challenges during machining, as they accelerate tool wear and compromise dimensional accuracy. Managing abrasive materials often entails frequent tool changes to uphold precision throughout the machining process.

Heat Stability

Primarily affecting non-metallic materials, heat buildup during machining can distort shapes, limiting the applicable processes. Understanding heat stability is crucial in selecting appropriate machining techniques for specific materials.

Choice of Manufacturing Methods

The selection of CNC machines significantly influences manufacturing outcomes, as each machine possesses distinct capabilities and limitations. Understanding a machine’s tolerance capacity is paramount, as it determines its suitability for producing different parts. Complex designs or tight tolerance requirements may necessitate supplementary operations to achieve desired precision.


Given the intricacy of parts with stringent tolerances, meticulous inspection procedures are indispensable. Comprehensive inspection involves utilizing specialized measurement tools such as calipers, micrometers, optical comparators, and coordinate measuring machines (CMMs). This rigorous verification process ensures the quality and reliability of the final product, albeit at the expense of additional time and resources.

How to Determine Optimal Tolerance Levels

Sometimes, identifying the suitable degree of precision poses a nuanced challenge. Particularly for product or part designers, it is imperative to establish the acceptable variation range for dimensions, as this knowledge profoundly influences manufacturing decisions, project timelines, and costs. The following guidance aims to facilitate your decision-making process:

Consider the Functional Requirements of Your Part

The functional role of your part typically dictates the requisite precision during machining processes. Parts operating autonomously, without integration into larger systems, often necessitate less stringent tolerances in manufacturing. Given the increased expense associated with tighter tolerances, it is prudent to prioritize them only when indispensable. If your part’s functionality does not mandate exceptional precision, selecting standardized tolerance levels

Seeking a Reputable CNC Machining Partner

Achieving precise tolerances in manufacturing necessitates collaboration with a trusted CNC machining company. Engaging in comprehensive discussions with manufacturing specialists regarding project objectives and specifications is essential to determine the optimal tolerances. Typically, engineers or part designers delineate these tolerances before engaging a CNC machining or rapid prototyping service, facilitating smoother production processes and cost efficiencies.

It’s imperative to note that the absence of specified tolerances when submitting a part for production often results in CNC services defaulting to their standard tolerance, typically around ±0.1 mm (±0.004 in.). While this variance may seem negligible to the untrained eye, it can significantly impact the assembly fit of the final product. For instance, a hole smaller by merely 0.005 inches can complicate the integration of its corresponding part unnecessarily.

BOYI: Your Premier Partner for CNC Machining Solutions

BOYI upholds machining standards in line with ISO 2768, adhering meticulously to ISO 2768-m for metals and ISO 2768-c for plastics. We excel in meeting specialized precision requirements, ensuring exact compliance with your specified parameters through our cutting-edge machining technologies.

CNC Workshop:

Situated in Shenzhen, China, our expansive manufacturing facility boasts a diverse array of CNC machines, including 3-axis, 4-axis, and 6-axis variants. Furthermore, we offer a variety of surface finishes for our CNC machined components. Beyond CNC machining, we extend a suite of manufacturing capabilities to our global clientele.

Comprehensive Quality Assurance:

Our CNC machining services encompass thorough quality inspections, material certifications, and comprehensive dimensional analyses accompanied by detailed reports. Leveraging state-of-the-art metrology tools, our full-dimensional CNC inspection process guarantees that prototypes and final parts precisely meet your specified requirements and tolerances. In essence, we meticulously verify and validate adherence to your specifications, ensuring unparalleled accuracy in every component we deliver.

Contact Us:

Should you have inquiries regarding CNC machining tolerances or seek further insights into BOYI’s CNC machining services, feel free to reach out to a member of our manufacturing team without hesitation.

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