CNC Machining Tolerances and How to Specify Them

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In the world of precision manufacturing, CNC machining stands out for its ability to produce highly accurate and repeatable parts. Central to this capability is the concept of tolerances. Understanding and correctly specifying machining tolerances is not just a technical detail—it's a critical business decision that impacts cost, functionality, and lead time for your projects.


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What are CNC Machining Tolerances?

A tolerance defines the permissible limit of variation in a physical dimension. It is the total amount a specific dimension is permitted to vary, expressed as a range (e.g., ±0.1 mm) or as a unilateral/bilateral deviation. Tighter tolerances mean the part must be manufactured to a dimension closer to the exact nominal value.

Why Tolerances Matter

Specifying tolerances correctly is a balancing act. Unnecessarily tight tolerances can drastically increase costs due to the need for specialized equipment, additional machining steps, longer cycle times, and increased part inspection and potential scrap rates. Conversely, tolerances that are too loose can lead to parts that do not fit or function as intended in their final assembly, causing failures and costly rework.

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How to Specify Tolerances Effectively

1. Apply Critical Thinking: Not every feature on a part requires a tight tolerance. Identify the critical features—such as mating surfaces, bore diameters for bearings, or alignment pins—that are essential for the part's function. Apply standard tolerances (e.g., ±0.125 mm or ±0.005") to noncritical features to keep costs down.



2. Understand Standard Machining Capabilities: For most CNC milling and turning operations, a standard tolerance of ±0.125 mm (±0.005") is readily achievable and costeffective. Specifying within this range by default is an excellent practice for controlling costs without sacrificing quality for noncritical applications.

3. Use GD&T for Complex Relationships: When parts have complex geometries or the relationship between features is more important than the features themselves, consider using Geometric Dimensioning and Tolerancing (GD&T). GD&T provides a more comprehensive and functional way to control form, profile, orientation, location, and runout.

4. Communicate with Your Manufacturing Partner: This is perhaps the most crucial step. Provide clear and comprehensive drawings with all tolerances marked. Discuss the part's function and application with your CNC machining supplier. An experienced partner, like our team specializing in onestop CNC part processing, can offer valuable insights. We can advise on design for manufacturability (DFM), suggesting adjustments that maintain functionality while simplifying machining and reducing cost.

By strategically specifying tolerances and collaborating with a trusted manufacturing expert, you can optimize your designs for performance, reliability, and costefficiency. This approach ensures you get the highquality parts you need to drive your product's success and your business growth.