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What is the Tolerance Standard of CNC Machined Parts?

278   |   Published by VMT at Aug 28 2021



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.




CNC machining parts tolerance standard





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. 





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.





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.






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