CNC machines can achieve very high levels of accuracy. Some machines can achieve accuracies as high as +/-0.0025mm. However, running the milling of a part on a CNC machine’s highest level of accuracy is however an expensive and time-intensive undertaking. This is where machining tolerances come in. Since different parts require different degrees of accuracy, it is economical for a designer to specify the specific tolerance needed for that part.
What are tolerances?
In short, tolerances are measurements that signify the level of precision needed for a part that you want to manufacture. Specifically, machining tolerances indicate the degree of variation permitted in a part’s final dimensions or measured value.
Machinists measure machining tolerances by numerical values, typically preceded with a ± symbol. For example, you may assign a tolerance of ±0.001″ to a part measuring 2.550 inches in length. This would indicate that the manufactured part would have a variable length measuring between 2.549” and 2.551”. If a part measuring 1.5 inches in height needs a tolerance of ±0.005”, the final part should fall into the range of 1.495” and 1.505” to pass quality inspection.
Specified machining tolerances tell manufacturers and CNC machining services what degree of precision to use when producing a part. The smaller the tolerance—in the manufacturing world this is a tighter tolerance—the more precision required. The larger the tolerance—also called looser—the less precision you need.