CNC Machining Tolerance in Your Designs


Mating parts isn’t as easy as it seems. CNC machining tolerance is one of the most crucial specs needed to ensure the right fit. Tolerance describes the range by which your part could be bigger or smaller than expected.

Why is CNC Machining Tolerance so Important?

The world would be amazing if every 1” diameter hole was exactly 1” wide. In reality, even the most accurate cutters and mills are always a little bit off. The difference may not seem like much but sometimes just thousands of an inch dictate the success of your part or assembly.

Best Design Practices to Get the Parts You Need

Here’s the way to ensure that your mating parts play well together. Your designs should include an upper and lower error limit that you’ll tolerate. It’s essentially a margin of error. In some cases, it’s driven by the manufacturing facility and the results they can repeat with confidence. Want to vary from your manufacturers tolerance? Include upper and lower limits on a mechanical drawing. Engineers Edge offers a good resource page on the subject.

The truth is, every shop has its own CNC machining tolerance standards for various machines and methods. Protolabs maintains results within +/- 0.005”. Smaller tolerances are considered special cases and sometimes we can work with you to achieve that goal.

Machining Tolerance in Stock Parts

While most designers focus their attention on their part designs, they often forget about the stock parts used in their models. When you order a 1/4” dowel pin to use in your assembly, it probably isn’t exactly 0.250”. If you expect a pin to fit securely in a hole, a small difference becomes huge. The manufacturer of the dowel pin typically provides the tolerance. You can even include it in your models. Some designers find it helpful to use that figure so they can create a complete representation.

Regardless of your preferences, with assemblies, always consider the tolerances associated with your stock parts, your manufacturer, and your anticipated result.


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For more helpful machining tips, download our CNC Machining Design Guide.

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