Standard Tolerances for CNC Machining

Standard Tolerances for CNC Machining

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CNC machining suppliers computer numerical control (commonly known as CNC) machining refers to the use of computer programming and electromechanical equipment to automatically process metal parts (and non-metals) as needed. The CNC machine tool performs all operations on the workpiece according to the program so as to provide us with the final product.



1. Common tolerances in CNC machining


When defining product measurements, engineering tolerances are a necessary requirement. Unless the customer specifies any specific tolerances, the parts are usually manufactured according to the general tolerance level.


Therefore, it is in the designer’s interest to define the tolerances of the parts with appropriate features before the parts are handed over to the CNC service company for CNC processing. This ensures customer satisfaction and reduces the cost of re-machining in the CNC machining workshop if there is any non-compliance.


With proper features, we mean that there is no need to define tolerances for each dimension. This actually has the opposite effect, because adding requirements to each measurement will result in very expensive components.


CNC machining suppliers usually define it only for those features in parts that mate with other parts. When we do this, it can ensure that the component is well installed in the components and works as expected.



2. The basis of CNC tolerance


Now that we understand why tolerance limits are needed and their various types, let us see why it is important to choose the correct limit. The appropriate tolerance limits of CNC service companies can ensure perfect part performance without incurring unnecessary high costs.



3. Standardized tolerances of CNC machining


The standard prototype and production tolerances for custom CNC machining are ±0.005 inches (0.13 mm). This means that the position, width, length, thickness, or diameter of any part feature will not deviate from the nominal value by more than this amount.


For example, the planned 1-inch (25.4 mm) wide bracket has a width of 0.995 to 1.005 inches (25.273 to 25.527 mm), and one leg of the bracket has a 0.25 inch (6.35 mm) hole. The diameter of the bracket is 0.245 to 0.255 inches (6.223 to 6.477 mm).


This is very close, but if higher accuracy is required, the standard accuracy or production tolerance of the CNC machining supplier is ±0.002 inches (0.051 mm). We can also maintain ±0.0002 inches (0.00508 mm) on the reamed hole and ±0.002 (0.051 mm) on the feature position, provided that these features are machined on the same side of the part.


However, depending on the geometry and material of the part, if you let us know your requirements, we can usually achieve higher accuracy. For these and other exceptions, please be sure to indicate in the part design when uploading the file for reference.



4. Guide on CNC machining tolerance


Please note that these are bilateral tolerances. If expressed in unilateral terms, the standard tolerance should be +0.000/- 0.010 inches (or +0.010/- 0.000 inches), while in the CNC service company’s bracket example, the tolerance based on the limit is 1.005/0.995 inches.


All are acceptable, as are metric values, provided you spell them out in your design. And to avoid confusion, please follow the “three-digit” dimensions and tolerances shown, and avoid extra zeros in 1.0000 or 0.2500 inches, unless there is an absolute reason to do so.



5. Matters needing attention in surface roughness of machining tolerances


Surface roughness definition: the surface accuracy of the machined surface. The surface roughness is usually expressed by the maximum height of the profile Rz. The figure below shows that the difference between the top of the highest peak and the bottom of the lowest valley is 12.5μm.



Rz is required to be less than 12.5μm


In addition, it can also be expressed by “ten-point height of microscopic unevenness” and “arithmetic mean deviation of contour”.

The measurement of surface roughness requires the use of specific measuring instruments.

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