Computer numerical control (CNC) machining can achieve incredibly high levels of precision with accuracies as high as ±0.0025mm.
While highly accurate, no two machined parts are completely uniform. To help bridge these deviations and uphold strict industry standards, cnc machining services rely on machining tolerances for an acceptable degree of variation to exist between any two parts.
With so many different parts requiring varying degrees of accuracy, knowing the machine tolerance of a specific component often contributes to a more economical manufacturing process.
For the ultimate crash course on CNC machining tolerance, our guide covers:
- What is Machining Tolerance and How to Find It
- Common Types of CNC Machining Tolerances
- The Roles of CNC Machining in Creating Tighter Tolerances
What are Tolerances in Machining?
Part tolerances are often denoted with a ± symbol preceding a set of values (measurements) that dictate the level of precision required for a specified component.
Since no two parts are identical, machining tolerances define the acceptable degree of variation (as a measurable value) that can exist between any two parts’ final dimensions. For example, if a part measuring 2.750 inches in length has a tolerance of ± 0.001”, then the acceptable measurements of that end part may have a variable length between 2.749” to 2.751”.
Here’s another example:
- An aerospace component measuring 2.5 inches in height has a tolerance of ± 0.005.” As a result, the final component should reside between 2.495” and 2.505” to meet quality specifications.
How to Find the Right CNC Machining Tolerance
Maximum and minimum tolerances for CNC machined parts significantly influence production turnaround and the overall costs of produced parts. With precision being the key driving factor behind CNC machining processes, obtaining high levels of accuracy that are near original specifications is to be expected – therefore, paying an additional fee for “tight tolerances” isn’t always required (as it’s understood to be built into the original service).
As a result, it’s not uncommon for customers hiring CNC machining services to unintentionally overpay when requesting the “highest quality” parts and components. In most cases, product designers and engineers already possess the machining tolerances required for machinists to automatically execute their manufacturing process to those set tolerances.
Struggling to Find a Machine Shop that Understands the Extreme Importance of Precision?
Moseys does everything with precision. That’s one of our basic guarantees.
All the same, discussing your project needs with a trusted precision machining company can help you:
- Eliminate unneeded tolerances that aren’t adding value
- Discover materials better suited for your manufacturing and tolerance needs
- Save time and money by finding the best tolerances for multiple upcoming projects
Main Types of Tolerances in CNC Machining
1. Standard Tolerances
General tolerances are often defined for linear or angular dimensions along with chamfers and other parts. When customers don’t provide set tolerances, standard machining tolerances are used to reach the required specifications.
2. Bilateral Tolerances
Mostly used for exterior dimensions, bilateral tolerances provide deviations that can be either negative or positive. For example, if the bilateral tolerance for a machined part is +/- 0.05mm, it indicates that the final part can be either 0.05mm shorter or longer than the specifications.
3. Unilateral Tolerances
Unilateral tolerances only support deviations occurring in one direction and are generally used when machining parts designed to fit into another.
4. Limit Tolerances
Limit tolerances are expressed as a minimum and maximum permissible value for the dimension of a finished part. So, for example, if the limit tolerance is 12.5mm – 13mm, then the part’s measurement must fall within those two limits.
5. Geometrical Tolerances
Compared to what other tolerances require, geometrical tolerancing is far more extensive and often used for parts that demand incredibly high levels of precision. In addition to outlining the geometric characteristics of a machined part, this form of tolerancing helps machinists to determine how flat a piece should be, its true position, and its concentricity.
CNC Machining Tolerance Chart & Specifications
The following are standard mechanical tolerances as defined in ISO 2768:
Linear Dimensions
| Linear Deviation Range (inches) | Tolerance Category | |||
| f (fine) | m (medium) | c (coarse) | v (very coarse) | |
| 0.019 up to 0.118 | ±0.0019 | ±0.0039 | ±0.0078 | – |
| over 0.118 up to 0.236 | ±0.0019 | ±0.0039 | ±0.0118 | ±0.0196 |
| over 0.236 up to 1.181 | ±0.0039 | ±0.0078 | ±0.0196 | ±0.0393 |
| over 1.181 up to 4.724 | ±0.0059 | ±0.0118 | ±0.0314 | ±0.0590 |
| over 4.724 up to 15.748 | ±0.0078 | ±0.0196 | ±0.0472 | ±0.0984 |
| over 15.748 up to 39.370 | ±0.0118 | ±0.0314 | ±0.0787 | ±0.1574 |
| over 39.370 up to 78.740 | ±0.0196 | ±0.0472 | ±0.1181 | ±0.2362 |
| over 78.740 up to 157.480 | – | ±0.0787 | ±0.1574 | ±0.3149 |
Chamfer Heights & External Radii
| Permissible Deviations (inches) | Tolerance Class | |||
| f (fine) | m (medium) | c (coarse) | v (very coarse) | |
| 0.019 up to 0.118 | ±0.0007 | ±0.0078 | ±0.0157 | ± 0.0157 |
| over 0.118 up to 0.236 | ±0.0019 | ±0.0196 | ±0.0393 | ±0.0393 |
| over 0.236 | ±0.0393 | ±0.0393 | ±0.0787 | ±0.0787 |
How CNC Machining is Used for Creating Tighter Tolerances
CNC machining tolerance varies depending on the material and its characteristics. Three characteristics that significantly influence tolerance requirements are the hardness, heat stability, and abrasiveness of the material being machined.
Since different materials possess individual characteristics, not all materials require tighter tolerances. Therefore, machining metallic parts, aluminum alloys, sheet metal, or polymer-based plastic parts, often require different degrees of tolerances that may result in a shorter or longer cycle depending on the accuracy of tolerance machining processes.
Use Professional Machining to Meet Tolerance Standards
High tolerance machining is often necessary for achieving industry standards for many components over many industries. But, paying for needlessly tight tolerances can drive up manufacturing costs without necessarily adding value.
That’s why knowing the right machining tolerance standards can keep your manufacturing processes more economical while helping to protect your business from overpaying. If you’re not sure what CNC machining tolerances you need, consider using Moseys.
From helping your engineers with design recommendations to providing complete supply chain fortification, our high-precision CNC machining services consistently achieve tight tolerances and unmatched uniformity. Serving more than a dozen high-demand industries like aerospace, defense, and medical, we’ve been manufacturing quality since 1975.
To discover what Mosey’s can do for you, connect with one of our machining specialists today.
