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.

Standard Tolerances

Machinists use standard machining tolerances for the most widely fabricated parts. There are standard machining tolerances for parts such as threads, pins, pipes, and so on. Some milling services offer typical tolerances of +/-0.1mm. Machinists usually apply these tolerances when the customer doesn’t specify tolerance levels. These typical tolerances can be found on a CNC machining tolerance chart.

Unilateral Tolerances

Unilateral tolerances accommodate deviations in only one direction. The deviation is either positive only or negative only. An example of such tolerance is +0.00/-0.06mm. This means that the finished part can be at most smaller by measurement of 0.06 mm but must not exceed the specified measurement. You usually implement unilateral tolerance when designing a part that goes into another. The part must not be bigger than the specified measurement as that will mean it will be unable to go into its position.

Bilateral Tolerances

When you use bilateral tolerance, the deviation from the given dimension can be either negative or positive. This means it can be a tiny bit bigger or a tiny bit smaller. An example of a bilateral tolerance is +/- 0.06mm. This indicates that the machined part can be 0.06mm shorter or longer than the specified measurement. Bilateral tolerances are used mostly for exterior dimensions.

Limit Tolerances

A limit tolerance is a type of CNC machining tolerance expressed as a range of values, where the part is fine as long as the measurement falls between that range. 13 – 13.5mm for example is a limit tolerance indicating that the part must have a measurement that falls between the upper and the lower limit. (13mm is the upper limit and 13.5mm is the lower limit)

Geometric Dimensioning and Tolerancing

Geometric dimensioning and tolerancing are much more thorough than the other systems of machining tolerances. It is a type of CNC machining tolerance that highlights the measurements and allowable deviations. It also outlines specific geometric characteristics for the machined part such as how flat it should be, its concentricity, and its true position. Geometric dimensioning and tolerancing are often used for parts that have extremely precise dimensions.