An Overview of CNC Machining Tolerances

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Tolerances are a form of measurement intended to designate the level of precision that is needed for a certain part to be manufactured. In layman’s terms, tolerance tells you how much room there is to mess up before the quality inspection is classified as a failure.

Tolerances are usually measured by a ± symbol. For example, let’s use a normal 2X4. Now, let’s say you want to cut the 2X4 so that it measures 2.350 inches in length. Because of how specific the cut is, if it is .007 inches longer or shorter it would not work, therefore the tolerance would be ± .007 inches. The 2X4 would have to fall between 2.343” and 2.357” to pass a quality inspection.

Common Tolerance Types for CNC

Standard Tolerances: These are mainly used for the widest selection of parts. The typical tolerance at most milling services is ± 0.1 mm. This is the standard or default tolerance.

Bilateral Tolerances: Mostly used for the exterior dimensions. The displacement from the given dimension can be a little bigger or a little smaller. An example would be ± 0.09 mm. The part can be either 0.09 mm longer, or 0.09 mm shorter than the given dimension.

Geometric Dimensioning and Tolerancing:  A type of CNC machining tolerance that outlines specific geometric dimensions. Things such as flatness, concentricity, the true position, etc. This tolerance is much more detailed than the rest. It is often used when a part requires extremely precise dimensions.

Unilateral Tolerances:  Unilateral tolerances only deviate in one direction, so it is either positive or negative, but it cannot be both. An example is +0.00/-0.03mm. Meaning that the part can be smaller by 0.03mm but cannot exceed the specific measurement. Unilateral tolerances are often used when you are designing a part that goes into another part.

Limit Tolerances:  This tolerance is expressed as a range, so that as long as the part measurement falls between that value range, it is fine. For example, a limit tolerance would look like this: 10-10.3mm. Here, the lower limit is 10mm and the upper limit is 10.3mm. As long as the measurement is in between 10mm and 10.3mm, then it will work.

If you didn’t understand tolerances before, we hope that this blog helps! At Gerber Wood Products we want to exceed your expectations, that’s why we craft each product in an excellent and personalized fashion. Make sure you head over to to look at the excellent services we can offer you!

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