Home / Resources / Blog /

CNC Machining Tolerances Standard: A Comprehensive Guide

423   |   Published by VMT at Aug 28 2021


In modern manufacturing, CNC machining is widely used to produce high-precision parts for various industries such as aerospace, automotive, and medical. CNC (Computer Numerical Control) machines are capable of accurately translating computer-generated designs into physical objects. To ensure the parts meet the required specifications, tolerances play a crucial role in the CNC machining process.


Since CNC machining is a subtractive machining, CNC milling parts are run with the highest precision. CNC machining machines can reach an ultra-high level of accuracy, and even some machines can reach an accuracy of +/-0.0025mm, and there will be CNC machining tolerances. Because each CNC machined part has a different degree of material and shape, the required accuracy also has a different degree. The tolerances of CNC machined parts are also different.



What is a Tolerance?


Tolerance is the accuracy level of the measured value required by CNC machined parts. In short, it is the allowable interval value of the final size of the CNC machined part or the measured value expressed by the machining tolerance. It usually starts with the ± symbol. For example, a CNC machined part with a height of 1.5 inches requires a tolerance of ±0.005"", so the final CNC machined part should be within the range of 1.495"" and 1.505"" to pass the quality inspection.



Clarify the tolerances of CNC machining parts so that the CNC machining service manufacturers can control the tolerances of the parts. The smaller the tolerance value, the higher the accuracy requirements of the manufacturing industry. Conversely, the larger the tolerance, the looser and the lower the required accuracy.



The Concept of Tolerances


Tolerances in CNC machining refer to the allowable deviation from the ideal dimensions specified in the design. They define the acceptable range within which the final part dimensions must fall to meet the desired functionality. Tolerances are typically expressed as a plus/minus value or a range of values.



Types of Tolerances


1. Geometric Tolerances


Geometric tolerances define the allowable variation in the form, orientation, and location of features on a part. These tolerances ensure the proper fit and assembly of multiple parts, as well as the overall performance of the product. Common geometric tolerances include flatness, parallelism, concentricity, and perpendicularity.



2. Dimensional Tolerances


Dimensional tolerances control the acceptable range of linear dimensions such as length, width, and height. These tolerances are crucial for achieving the desired fit, functionality, and interchangeability of parts. Examples of dimensional tolerances include diameter, thickness, and depth.



3. Importance of CNC Machining Tolerances


Accurate tolerances are vital in CNC machining as they directly impact the functionality, performance, and safety of the final product. Properly defined tolerances ensure the desired fit and assembly of parts, minimize the risk of failure or malfunction, and enhance the overall quality of the manufactured components.





CNC machining parts tolerance standard



Standard Tolerance Classes


International Tolerance Grades (IT Grades)


International Tolerance Grades, also known as IT grades or IT numbers, provide a standardized system for specifying the general tolerance requirements for parts. The IT grades range from IT01 (extremely tight tolerance) to IT16 (very loose tolerance). Manufacturers determine the appropriate IT grade based on factors such as the part's function, material, and manufacturing process.



Tolerance Zones


Tolerance zones establish the acceptable limits within which the actual dimensions of a part can vary. Two common tolerance zones used in CNC machining are the Maximum Material Condition (MMC) and the Least Material Condition (LMC).



1. Maximum Material Condition (MMC)


The MMC represents the condition of a feature where it contains the maximum amount of material within the specified tolerance zone. It ensures the maximum fit or clearance between mating parts.



2. Least Material Condition (LMC)


The LMC represents the condition of a feature where it contains the least amount of material within the specified tolerance zone. It ensures the minimum fit or clearance between mating parts.





Common CNC Machining Parts Tolerance Categories


1. Standard tolerance


We know that tolerance is the control of the accuracy of CNC machined parts. CNC machined parts such as threads, cuts, pipes have standard tolerances. Numerically controlled machined parts for a wide range of uses require standard tolerances. Most CNC milling services provide ±0.1MM, which is also the general CNC machining part tolerance standard specified by the mechanical engineer when the customer does not specify the tolerance level. Standard CNC machining tolerances are usually set by various international standard organizations, the most common are (ISO) International Organization for Standardization, (ASME) American Society of Mechanical Engineers, etc.




2. Geometrical dimensions and tolerances


This is a more standard and thorough tolerance standard than other tolerance systems. Geometric tolerances and dimensions are the tolerances of CNC machining parts, highlighting the measured value and deviation interval. For example, the flatness, true positioning position and concentricity of CNC machined parts. Geometric dimensions and tolerances are usually used for high-precision CNC machined parts.




3. Unilateral tolerance


The so-called single-sided tolerance refers to the deviation value in only one direction. The deviation is only positive or negative. Also listed as ﹢0.01/﹣0.05mm. In other words, the finished CNC machining parts can be expanded to 0.01mm or reduced -0.05mm. This means that it cannot be larger than the specified size, otherwise it cannot enter the corresponding location.




4. Bilateral tolerance


When the CNC machining parts require bilateral tolerances, the given dimensions must be positive or negative. This means it can be larger or smaller. For example, ±0.05mm means that the length of the specified measurement value of the CNC machined part is 0.05mm. Bilateral tolerances are mainly used for external dimensions.



5. Limit tolerance


The limit tolerance is a CNC machining tolerance, a series of tolerance values that represent the limit. As long as the CNC machined part is within the measurement interval, this type of tolerance value can be used. For example, the minimum tolerance is 11MM and the maximum tolerance is 11.5MM, which is a limit tolerance value. This means that 11-11.5mm is only between 11mm and 11.5. 





Factors Influencing Tolerances in CNC Machining



Several factors influence the achievable tolerances in CNC machining. Manufacturers must consider these factors to optimize the manufacturing process and achieve the desired level of precision.



1. Material Selection


Different materials have varying properties and behaviors during machining. Material selection plays a critical role in determining the achievable tolerances. Harder materials may require more precise tooling and cutting parameters to maintain the desired tolerances.



2.Machine Capability


The capabilities and accuracy of the CNC machine used for machining operations significantly affect the achievable tolerances. Factors such as machine rigidity, positional accuracy, and repeatability contribute to the overall precision of the process.



3.Tooling and Fixturing


The selection and condition of cutting tools, as well as the proper fixturing of the workpiece, influence the dimensional accuracy and surface finish of the machined part. Well-maintained tools and robust fixturing are essential for achieving tight tolerances.



4. Cutting Parameters


Optimizing cutting parameters such as cutting speed, feed rate, and depth of cut can impact the dimensional accuracy and surface finish of the machined part. Adjusting these parameters based on the material and geometry of the part is crucial to maintain the desired tolerances.



5.Environmental Factors


Environmental conditions, including temperature, humidity, and vibration, can affect the machining process. These factors can cause material expansion or contraction, tool deflection, or instability, leading to deviations from the desired tolerances.





Precautions for Tolerance of CNC Machining Parts



This is one of the keys to determine the correct tolerances of CNC machining parts, and it is the key to clearly understand whether the size or dimensional change of the part is suitable for the performance and function of the part.



1. Material tolerance


When specifying the tolerances of CNC machining parts, the use of materials is generally considered, which is related to the achievable tolerance level. Includes the following functions:


a. The hardness of metal: the hardness of metal is one of the factors that affect the tolerance of CNC machining parts. Since cutting tools change their dimensions during the machining process, it is difficult for softer materials to be CNC machined with strict tolerances. Therefore, processing softer materials tends to consume more time.


b. Corrosiveness: Rough materials are usually hard during CNC machining and may even cause more wear. Therefore, it is more difficult to specify tolerances for CNC machined parts for such materials.



2. Selection of manufacturing method


Due to different CNC machine tools, the machining accuracy is also different. CNC machine tool tolerance standards determine the degree of CNC machining that the machine tool can handle. In this case, the machinist must perform additional operations on the CNC machined parts to achieve the fine tolerance range of the CNC machined parts you need.



3. Inspection


This is a process that takes a lot of time to check precision CNC machined parts. Must have professional inspection tools or equipment, such as two-dimensional inspection, three-dimensional inspection, etc.



4. Cost control


The accuracy of tolerances directly affects the quality of CNC machined parts and also affects the cost. The higher the accuracy, the more expensive it is, and the speed of CNC machining parts will be slower, which requires more precise CNC machining tools. High-precision CNC machining parts inspection requires more comprehensive CNC machining inspection equipment. Another factor that affects the cost of tolerances of CNC machining parts is the failure rate, which tends to be higher. High-precision tolerance machining will increase the production cost of CNC machining and the cost of quality inspection.




Achieving Tighter Tolerances


To achieve tighter tolerances, advanced machining techniques and stringent quality control measures are employed.



1. Advanced Machining Techniques


Advanced machining techniques such as high-speed machining, precision grinding, and electrical discharge machining (EDM) can enhance the precision and accuracy of CNC machining processes. These techniques are suitable for complex geometries or parts with stringent tolerance requirements.



2. Inspection and Quality Control


Regular inspection and quality control checks during and after the machining process help identify any deviations from the desired tolerances. The use of coordinate measuring machines (CMM), optical measurement systems, and other metrology tools ensures the parts meet the specified tolerances.




Design Considerations for CNC Machining Tolerances



Designing parts with manufacturability and tolerance requirements in mind is crucial to optimize the CNC machining process and minimize variations.



1. Design for Manufacturability (DFM)


Applying DFM principles involves considering the machining process, material selection, and part geometry during the design phase. Designers can incorporate features that simplify machining operations and ensure better tolerance control.



2.Tolerance Stack-Up Analysis


Performing tolerance stack-up analysis helps identify potential cumulative variations that may arise from the assembly of multiple parts. By understanding how individual tolerances stack up, manufacturers can design parts with appropriate tolerances to ensure proper fit and functionality.



3. Balancing Cost and Tolerance Requirements


Tolerance requirements impact manufacturing costs. Striking a balance between the desired tolerances and the associated production costs is essential. Manufacturers must evaluate the cost implications of tighter tolerances and determine the necessary level of precision based on the part's function and requirements.




Common Challenges and Troubleshooting


Despite best efforts, challenges may arise during CNC machining that affect tolerances. Understanding these challenges and implementing effective troubleshooting techniques is crucial for maintaining quality.



1. Dimensional Deviation


Dimensional deviations can occur due to factors such as tool wear, machine deflection, or improper tool paths. To address this, regular tool inspection and maintenance, machine calibration, and optimizing tool paths can help minimize dimensional deviations.



2. Surface Finish Issues


Surface finish problems, such as roughness or waviness, can impact the functionality and aesthetics of the machined part. Proper selection of cutting tools, appropriate machining parameters, and utilizing finishing operations can improve surface finish and maintain tolerances.



3. Tool Wear and Breakage


Tool wear and breakage can result in variations in dimensions and surface finish. Employing robust tool life management strategies, using high-quality tool materials, and monitoring cutting conditions can help reduce tool-related issues.




How to Choose the Right Tolerance of CNC Machining Parts?



1. Find a reputable CNC machining parts company


If you are looking for a suitable CNC machining parts company, the first thing you should consider is a trustworthy CNC machining parts service company, which will help you better communicate and understand. Professional manufacturing experts will discuss the specifications and goals of the project with you, and at the same time give you the best advice and the best solution.


Under normal circumstances, engineers or designers will plan their tolerance range, and then submit the specified manufacturing tolerance request to the CNC machining parts service company. This provides a shortcut for CNC machining services and saves production and time costs.


Also note that if no tolerance is asked or specified when producing a CNC machined part, most CNC machining services will automatically use its standard tolerance, which is usually about ±0.001 inches (±0.0.0254 mm). For example, if the hole of a CNC machined part is too small, even if it is 0.127mm, installing its adjacent CNC machined part in the hole may be more challenging than necessary.




2. Think about the tolerances of CNC machining parts you need


Not all CNC machined parts need to be designed with strict tolerances. The specific usage scenarios of the parts usually determine the required tolerance accuracy of the CNC machining parts. For example, creating parts that are not combined with other parts usually requires lower CNC milling accuracy. Considering how much it costs to achieve strict CNC machining parts tolerances, if you don't need it, please don't try.







Regarding the tolerances of CNC machined parts, we have a series of ISO standard requirements. We will formulate according to the needs of customers, while meeting the needs, we can also provide high-quality surface treatment. We have multiple 5-axis, 4-axis, and 3-axis CNC machining machines. In terms of detection, we also have high-precision measuring instruments such as two-dimensional and three-dimensional. In addition, we also provide different CNC machining and manufacturing services for more than 60 corporate customers around the world. If you have any questions about the tolerances of CNC machined parts or want to know more about VMT's CNC machined parts service standard tolerances, please feel free to contact our CNC machining and manufacturing team.


CNC machining tolerances play a critical role in ensuring the quality, functionality, and interchangeability of manufactured parts. By understanding the concept of tolerances, implementing appropriate tolerance classes, considering influencing factors, and employing effective troubleshooting techniques, manufacturers can achieve precise and reliable results in CNC machining processes.




CNC Machining Tolerance Standard FAQs



1. Why are tolerances important in CNC machining?

Tolerances are crucial in CNC machining as they ensure the desired fit, functionality, and interchangeability of parts, ultimately impacting the quality and performance of the final product.


2. What are geometric tolerances?

Geometric tolerances define the acceptable variation in the form, orientation, and location of features on a part, ensuring proper fit and assembly.


3. How do material selection and machine capability influence tolerances?

Material selection affects machining behavior, while machine capability determines the accuracy and precision of the manufacturing process, both influencing the achievable tolerances.


4. What are some advanced machining techniques for achieving tighter tolerances?

High-speed machining, precision grinding, and electrical discharge machining (EDM) are examples of advanced techniques used to achieve tighter tolerances in CNC machining.


5. How can designers optimize tolerances in CNC machining?

Designers can optimize tolerances through design for manufacturability (DFM) principles, tolerance stack-up analysis, and balancing cost and tolerance requirements during the design phase.





> <

Latest posts

+86 15099911516