CNC Machining Tolerance

There are two definitions of tolerance in CNC machining: one refers to the tolerance of CNC machine tools, and the other refers to the design tolerance of CNC machining.
CNC machine tool tolerance refers to the degree of dimensional accuracy that a machine tool can achieve when machining parts. The precision of CNC machining technology is extremely high, and some machine tools can achieve a manufacturing accuracy of ± 0.0025mm, which is about a quarter of the size of our hair. However, different machine tools have different tolerances, which are usually defined by the machine tool manufacturer. The commonly used 0.02mm tolerance on the market is a widely used average. CNC machining service providers will also provide customers with the tolerances of their CNC machine tools.
In design and processing, tolerance refers to the allowable range of variation in part dimensions. As long as the part meets this range, it can achieve all functional requirements. At this point, the tolerance standard is formulated by the part designer based on the function, fit, and shape of the part. Design and machining tolerances are crucial for the fit and assembly of parts. You can imagine that electric engine parts clearly require stricter tolerances compared to simple door and handle fits. After all, the components of the engine are both numerous and complex, requiring each feature to match well. Usually, corresponding tolerance markings will appear next to the applicable dimensions of the parts.
There are many different types of tolerances, such as limit tolerances, unidirectional tolerances, bidirectional tolerances, and geometric tolerance systems called geometric tolerances (GD&T).
Extreme tolerance
The limit tolerance includes the upper and lower deviation values, defining the acceptable range of dimensions: the upper deviation is the maximum acceptable size, and the lower deviation value is the minimum acceptable size. Any value within this range is acceptable. For example, if the limit tolerance is marked as 0.55-0.65mm, it means that the dimension reference will be allowed as long as it does not exceed this tolerance zone.
Example of extreme tolerances for shafts and holes
Unidirectional tolerance
Unidirectional tolerance refers to the basic dimension that can only have one direction change, that is, the tolerance can only change in the positive or negative direction (compared to the reference value+or -). For example, the tolerance is written as 1.5 mm+. 000/-. 005, which is a unilateral tolerance that only allows negative direction variation. This means that the dimensional deviation can increase by 0.005 millimeters in the defined negative direction, but no deviation is allowed in the opposite direction.
Example of Unidirectional Tolerance
Bidirectional tolerance
In CNC machining technology, if the tolerance is symmetrical around the basic dimension, it is a bidirectional tolerance. That is to say, the upper and lower deviation values of the basic dimensions are the same. Unlike single-phase tolerances, the deviation of bidirectional tolerances occurs simultaneously in both positive and negative directions.
Geometric tolerances (GD&T)
Geometric tolerance is a more advanced and complex geometric tolerance system than standard dimensions and tolerances (SD&T). It not only provides the dimensions and tolerances of the parts, but also specifies the deviation values of features such as points, lines, and surfaces of the parts. SD&T specifies the shape of the part, while GD&T also specifies the geometric features of the part, such as flatness, fit position, concentricity, and so on.