Manufacturers across industries often use different tolerances for CNC parts machining because of the different types of machining processes and parts geometries. We’ll discuss typical CNC tolerances.
Standard Tolerances
Standard CNC tolerances are the typical tolerances machinists adopt for commonly manufactured products. Most machining experts use CNC milling tolerances of about +/- 0.1mm whenever customers do not indicate preferred tolerance levels. Various international organizations, such as the American Society of Mechanical Engineers (ASME)) the American National Standards Institute (ANSI) and the Organization for Standardization (ISO) define the scope of these standards.
Limit Tolerances
A limit tolerance is indicated as a range of values where the machined part is acceptable so far its measurement is within the preferred range. For instance, 15 – 15.5mm is a limit tolerance for CNC machining, showing that the CNC part’s dimension must be between the lower and upper limit (15.5mm as the upper limit and 15mm as the lower limit).
Unilateral Tolerances
Unilateral tolerances allow variation in one direction, either negative or positive only. For instance, a tolerance of +0.00/-0.03mm shows that the machined part can be about 0.03mm smaller but not bigger than the indicated measurement.
Product designers often use these tolerances in designs where a component must fit perfectly with another. Besides, a part will not serve its intended purpose if it is too large or small. Therefore, ensure the finished part does not exceed the preferred dimensions.
Bilateral Tolerances
The variation from the defined dimension can be positive or negative when using bilateral tolerances, allowing a minor increase or decrease in the size of the part. For example, +/- 0.05mm tolerance indicates that the part can be 0.05mm longer or shorter than the specified measurement. These specifications are often applicable to exterior dimensions.
Geometric Dimensioning and Tolerance (GD&T)
GD&T is a versatile system of detailing and communicating the standard machining tolerances. It is more complex than the typical tolerance systems. It uses feature control frames to indicate specific forms and dimensional tolerances of parts.
Geometric dimensioning and tolerancing keep the final measurements of a part within the specified boundaries. Similarly, it indicates the part’s geometric properties, including its flatness, concentricity, and true position. More notably, GD&T symbols ensure the dimensional accuracy of part features in some parts with higher processing requirements.